Soil, treated biowaste, sludge and waste -- Digestion of aqua regia soluble fractions of elements

This document specifies two methods for digestion of soil, treated biowaste, sludge and waste by the use of an aqua regia digestion. Digestion with aqua regia will not necessarily accomplish total decomposition of the sample. The extracted analyte concentrations may not necessarily reflect the total content in the sample but represent the aqua regia soluble metals under the condition of this test procedure. It is generally agreed that for environmental analysis purposes, the results are fit for the intended purpose to protect the environment. This document is applicable for the following elements: Aluminium (Al), antimony (Sb), arsenic (As), barium (Ba), beryllium (Be), boron (B), cadmium (Cd), calcium (Ca), chromium (Cr), cobalt (Co), copper (Cu), iron (Fe), lead (Pb), magnesium (Mg), manganese (Mn), mercury (Hg), molybdenum (Mo), nickel (Ni), phosphorus (P), potassium (K), selenium (Se), silver (Ag), sodium (Na), strontium (Sr), sulfur (S), tellurium (Te), thallium (Tl), tin (Sn), titanium (Ti), vanadium (V), and zinc (Zn). This document can also be applied for the digestion of other elements, provided the user has verified the applicability.

Sols, biodéchets traités, boues et déchets -- Digestion des éléments solubles dans l’eau régale

Le présent document spécifie deux méthodes de digestion de sols, de biodéchets traités, de boues et de déchets en utilisant une digestion ŕ l'eau régale. La digestion ŕ l'eau régale n'aboutit pas nécessairement ŕ une décomposition totale de l'échantillon. Les concentrations des analytes extraits peuvent ne pas nécessairement refléter les teneurs totales dans l'échantillon, mais représentent les métaux solubles dans l'eau régale dans les conditions du présent mode opératoire d'essai. Il est généralement admis qu'aux fins d'analyses environnementales, les résultats sont adaptés ŕ la finalité prévue, ŕ savoir la protection de l'environnement. Le présent document est applicable aux éléments suivants: aluminium (Al), antimoine (Sb), arsenic (As), baryum (Ba), béryllium (Be), bore (B), cadmium (Cd), calcium (Ca), chrome (Cr), cobalt (Co), cuivre (Cu), fer (Fe), plomb (Pb), magnésium (Mg), mangančse (Mn), mercure (Hg), molybdčne (Mo), nickel (Ni), phosphore (P), potassium (K), sélénium (Se), argent (Ag), sodium (Na), strontium (Sr), soufre (S), tellure (Te), thallium (Tl), étain (Sn), titane (Ti), vanadium (V) et zinc (Zn). Le présent document peut également ętre appliqué ŕ la digestion d'autres éléments, ŕ condition que l'utilisateur en ait vérifié l'applicabilité.

General Information

Status
Published
Publication Date
10-Aug-2020
Current Stage
5060 - Close of voting Proof returned by Secretariat
Start Date
18-Jun-2020
Completion Date
17-Jun-2020
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INTERNATIONAL ISO
STANDARD 54321
First edition
2020-08
Soil, treated biowaste, sludge and
waste — Digestion of aqua regia
soluble fractions of elements
Sols, biodéchets traités, boues et déchets — Digestion des éléments
solubles dans l’eau régale
Reference number
ISO 54321:2020(E)
ISO 2020
---------------------- Page: 1 ----------------------
ISO 54321:2020(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2020

All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may

be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting

on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address

below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii © ISO 2020 – All rights reserved
---------------------- Page: 2 ----------------------
ISO 54321:2020(E)
Contents Page

Foreword ........................................................................................................................................................................................................................................iv

Introduction ..................................................................................................................................................................................................................................v

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms and definitions ..................................................................................................................................................................................... 1

4 Principle ........................................................................................................................................................................................................................ 2

5 Interferences and sources of errors ................................................................................................................................................. 2

6 Reagents ........................................................................................................................................................................................................................ 3

7 Apparatus ..................................................................................................................................................................................................................... 3

7.1 General ........................................................................................................................................................................................................... 3

7.2 Method A — Apparatus for thermal heating under atmospheric conditions ................................... 3

7.2.1 Method A1 — Thermal heating under reflux conditions .............................................................. 3

7.2.2 Method A2 — Thermal heating with a heating block with containers ............................. 4

7.3 Method B — Microwave digestion with temperature control, closed vessels ................................. 4

8 Procedure..................................................................................................................................................................................................................... 5

8.1 General ........................................................................................................................................................................................................... 5

8.2 Blank test ..................................................................................................................................................................................................... 5

8.3 Method A — thermal heating under atmospheric conditions ....................................................................... 5

8.3.1 Method A1 — Thermal heating under reflux conditions .............................................................. 5

8.3.2 Method A2 — Thermal heating with a heating block with containers ............................. 6

8.4 Method B — Microwave digestion with temperature control, closed vessels ................................. 7

9 Test report ................................................................................................................................................................................................................... 7

Annex A (informative) Repeatability and reproducibility data for soil, biowaste and sludge

samples ........................................................................................................................................................................................................................... 9

Annex B (informative) Repeatability and reproducibility data for waste samples ..........................................10

Bibliography .............................................................................................................................................................................................................................37

© ISO 2020 – All rights reserved iii
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ISO 54321:2020(E)
Foreword

ISO (the International Organization for Standardization) is a worldwide federation of national standards

bodies (ISO member bodies). The work of preparing International Standards is normally carried out

through ISO technical committees. Each member body interested in a subject for which a technical

committee has been established has the right to be represented on that committee. International

organizations, governmental and non-governmental, in liaison with ISO, also take part in the work.

ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of

electrotechnical standardization.

The procedures used to develop this document and those intended for its further maintenance are

described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the

different types of ISO documents should be noted. This document was drafted in accordance with the

editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).

Attention is drawn to the possibility that some of the elements of this document may be the subject of

patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of

any patent rights identified during the development of the document will be in the Introduction and/or

on the ISO list of patent declarations received (see www .iso .org/ patents).

Any trade name used in this document is information given for the convenience of users and does not

constitute an endorsement.

For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and

expressions related to conformity assessment, as well as information about ISO's adherence to the

World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www .iso .org/

iso/ foreword .html.

This document was prepared by Technical Committee ISO/TC 190, Soil quality, Subcommittee

SC 3, Chemical and physical characterization, in collaboration with the European Committee for

Standardization (CEN) Technical Committee CEN/TC 444, Environmental characterization of solid

matrices, in accordance with the Agreement on technical cooperation between ISO and CEN (Vienna

Agreement).

Any feedback or questions on this document should be directed to the user’s national standards body. A

complete listing of these bodies can be found at www .iso .org/ members .html.
iv © ISO 2020 – All rights reserved
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ISO 54321:2020(E)
Introduction

Regarding the comparability of the procedure described in this document with those of the other

standards mentioned above the next remarks can be made:
— This document describes the digestion of solid samples with aqua regia.

— Differences in the procedures of the different standards are small. An important difference between

the reflux procedures as described in ISO 11466 and EN 13657 and EN 16174 concerns the waiting

time after addition of the acid to the sample, before the digestion starts. ISO 11466 specifies a

waiting time of 16 h, both European standards state that the digestion can start after the first strong

reactions have ceased. In validation work it was proven that the difference between 2 h and 16 h of

waiting was negligible, therefore this document follows the approach of EN 13657 and EN 16174.

— The heating block procedure was added to the reflux and microwave digestion procedures. The

procedure was adopted from the Dutch standard NEN 6961, which specifies a boiling time of 2 h to

4 h. This document specifies a boiling time of 2 h.

The methods specified in this document are providing multi-element aqua regia digestion techniques

for soil, treated biowaste, sludge and waste prior to analysis. It is known that the digestion of

environmental samples with aqua regia will not necessarily lead to complete element recoveries, and

that the extract from a test sample may not reflect the total concentrations of the target analytes.

However, for most environmental applications the result obtained based upon digestion methods

specified in this document are considered to be fit for the intended purpose.

This document is validated for several types of matrices as indicated in Table 1.

Table 1 — Matrices for which this document is validated
Matrix Materials used in the validation test
Municipal sludge
Industrial sludge
Sludge Sludge from electronic industry
Ink waste sludge
Sewage sludge
Compost
Biowaste (Meth-
od A)
Composted sludge
Agricultural soil
Soil
Sludge amended soils
City waste incineration fly ash (“oxidised” matrix)
City waste incineration bottom ash (“silicate” matrix)
Ink waste sludge (organic matrix)
Waste
Electronic industry sludge (“metallic” matrix)
BCR 146R (sewage sludge)
BCR 176 (city waste incineration ash)

WARNING — Persons using this document should be familiar with usual laboratory practice.

Some of the reagents used in this document are highly corrosive and very toxic. Safety

precautions are absolutely necessary, not only due to the strong corrosive reagents, but also to

the high temperature and high pressure.

The use of laboratory-grade microwave equipment with isolated and corrosion resistant safety

devices is required. Domestic (kitchen) type microwave ovens shall not be used, as corrosion

by acid vapours may compromise the function of the safety devices and prevent the microwave

© ISO 2020 – All rights reserved v
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ISO 54321:2020(E)

magnetron from shutting off when the door is open, which could result in operator exposure to

hazardous levels of microwave energy.

All procedures should be performed in a fume hood or in closed force-ventilated equipment. By

the use of strong oxidising reagents, the formation of explosive organic intermediates is possible,

especially when dealing with samples with a high organic content. Do not open pressurized

vessels before they have cooled down. Avoid contact with the chemicals and the gaseous reaction

products.

IMPORTANT — It is absolutely essential that tests conducted according to this document be

carried out by suitably trained staff.
vi © ISO 2020 – All rights reserved
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INTERNATIONAL STANDARD ISO 54321:2020(E)
Soil, treated biowaste, sludge and waste — Digestion of
aqua regia soluble fractions of elements
1 Scope

This document specifies two methods for digestion of soil, treated biowaste, sludge and waste by the

use of an aqua regia digestion.

Digestion with aqua regia will not necessarily accomplish total decomposition of the sample. The

extracted analyte concentrations may not necessarily reflect the total content in the sample but

represent the aqua regia soluble metals under the condition of this test procedure. It is generally agreed

that for environmental analysis purposes, the results are fit for the intended purpose to protect the

environment.
This document is applicable for the following elements:

Aluminium (Al), antimony (Sb), arsenic (As), barium (Ba), beryllium (Be), boron (B), cadmium (Cd),

calcium (Ca), chromium (Cr), cobalt (Co), copper (Cu), iron (Fe), lead (Pb), magnesium (Mg),

manganese (Mn), mercury (Hg), molybdenum (Mo), nickel (Ni), phosphorus (P), potassium (K),

selenium (Se), silver (Ag), sodium (Na), strontium (Sr), sulfur (S), tellurium (Te), thallium (Tl), tin (Sn),

titanium (Ti), vanadium (V), and zinc (Zn).

This document can also be applied for the digestion of other elements, provided the user has verified

the applicability.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.

ISO and IEC maintain terminological databases for use in standardization at the following addresses:

— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
3.1
aqua regia

digestion (3.2) solution obtained by mixing 1 volume of nitric acid (mass fraction of 65 % to 70 %) and

3 volumes of hydrochloric acid (mass fraction of 35 % to 37 %)

Note 1 to entry: These mass percentages agree with the concentrations of 6.2 and 6.3.

3.2
digestion

mineralization of the organic matter of a sample and dissolution of its mineral part, more or less

completely, when reacting with a reagent mixture
3.3
dry residue

dry matter expressed as a percentage by mass after drying at 105 °C ± 5 °C to the constancy of weight

© ISO 2020 – All rights reserved 1
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ISO 54321:2020(E)
3.4
laboratory sample
sample (3.5) intended for laboratory inspection of testing
[SOURCE: ISO 11074:2015, 4.3.7]
3.5
sample
portion of material selected from a larger quantity of material
[SOURCE: ISO 11074:2015, 4.1.17]
3.6
test portion
analytical portion

quantity of material of proper size for measurement of the concentration or other properties of interest,

removed from the test sample (3.7)

Note 1 to entry: The test portion may be taken from the laboratory sample directly if no preparation of sample is

required (e. g. with liquids), but usually it is taken from the prepared test sample.

Note 2 to entry: A unit or increment of proper homogeneity, size and fineness, needing no further preparation,

may be a test portion.
[SOURCE: ISO 11074:2015, 4.3.15]
3.7
test sample
analytical sample

portion of material resulting from the laboratory sample (3.4) by means of an appropriate method of

sample pre-treatment and having the size (volume/mass) necessary for the desired testing or analysis

[SOURCE: ISO 11074:2015, 4.1.3]
4 Principle

A test portion is digested with aqua regia according to one of the following heating procedures:

— Method A: procedure under atmospheric conditions
— A1: reflux for (120 ± 10) min, followed by filtration/centrifugation;

— A2: heating block at (105 ± 5) °C for (120 + 10) min, followed by filtration/centrifugation.

— Method B: microwave digestion

— B1: Temperature controlled procedure: at (175 ± 5) °C for (10 ± 1) min in a closed vessel followed

by filtration/centrifugation.
5 Interferences and sources of errors

The container in which the sample is delivered and stored can be a source of errors. Its material shall

be chosen according to the elements to be determined (e.g. elemental Hg can penetrate polyethylene

walls very fast in both directions. Glass can contaminate samples with its major elements: e.g. B, Na, K,

Si and Al).

Grinding or milling samples includes a risk of contamination of the sample by the environment (air, dust,

wear of milling equipment). Due to elevated temperature losses of volatile compounds are possible.

For the determination of elements forming volatile compounds (e.g. Hg, As) special care has to be taken

during sample pre-treatment.
2 © ISO 2020 – All rights reserved
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ISO 54321:2020(E)

All glassware and plastics ware shall be adequately cleaned and stored in order to avoid any

contamination.

In the case of filtration of the digested solution it is necessary to take care that the filtration procedure

does not introduce contaminants.

Ensure that all of the test portion is brought into contact with the acid mixture in the digestion vessel.

Some elements of interest can be lost due to precipitation with ions present in the final digest solution,

e.g. low soluble chlorides, fluorides and sulfates.
6 Reagents

Use only acids and reagents of recognized analytical grade to avoid high blank values for subsequent

analytical measurements. Use a test blank solution throughout the procedure applying all steps with

the same amount of acids, but without a sample.
6.1 Water, e.g. deionized.
6.2 Hydrochloric acid, c(HCl) ≈ 12 mol/l.
6.3 Nitric acid, c(HNO ) ≈ 15 mol/l.
6.4 Nitric acid, c(HNO ) ≈ 0,5 mol/l.
Dilute 35 ml nitric acid (6.3) to 1 l with water (6.1).
6.5 Antifoaming agent, e.g. n-dodecane (C H ) or n-octanol (C H O) are suitable.
12 26 8 18
7 Apparatus
7.1 General

Usual laboratory apparatus. All glassware and plastics ware shall be adequately cleaned and stored in

order to avoid any contamination.

Depending upon the concentration of the element of interest, particular care should be exercised with

respect to the effective cleaning of the vessels.
7.2 Method A — Apparatus for thermal heating under atmospheric conditions
7.2.1 Method A1 — Thermal heating under reflux conditions

7.2.1.1 Digestion vessel, temperature- and pressure-resistant and capable of containing the mixture

of sample and digest solution, for example a quartz vessel. The digestion vessel shall have a volume of at

least 5 times of the volume of the aqua regia used. The inner wall of the vessel shall be inert and shall not

release substances to the digest in excess of the purity requirements of the subsequent analysis.

NOTE 1 Silica or borosilicate glass vessels can be used instead of quartz vessels.

NOTE 2 It can be necessary to periodically clean the digestion vessels with a suitable surfactant to remove

persistent deposits.
7.2.1.2 Reflux condenser, adaptable to the digestion vessel (7.2.1.1).
© ISO 2020 – All rights reserved 3
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ISO 54321:2020(E)

7.2.1.3 Absorption vessel, volatile species trap, in an open digestion system capable of trapping one

or more volatile measurement species, adaptable to the reflux condenser (7.2.1.2).

7.2.1.4 Heating device, for example a heating mantle, thermostatic controlled, or an aluminium block

thermostat.
7.2.2 Method A2 — Thermal heating with a heating block with containers

7.2.2.1 Digestion tube, 50 ml propylene tube with a screw cap from polypropylene.

The part of the tube not being heated and the screw cap function as a condenser, but are not really

a reflux system. The material of the tube and screw cap need to be tested in order to be sure that

release of elements of interest does not take place. Other materials and vessels with other volumes than

mentioned above are allowed to be used if suitability has been proven.

7.2.2.2 Temperature controlled heating block, heating block able to heat the tube(s) to a temperature

of (105 ± 5) °C.
7.3 Method B — Microwave digestion with temperature control, closed vessels

7.3.1 Digestion vessel, for pressurized microwave digestion, typically 100 ml volume, reagent-,

temperature- and pressure-resistant and capable of containing the mixture of sample and digest solution.

The vessel shall be suitable for the safe application in the temperature and pressure range applied,

capable of withstanding pressures of at least 3 000 kPa.

Digestion vessels made of perfluoroalkoxylalkane (PFA), modified polytetrafluoroethylene (PTFE) or

quartz, and equipped with a safety pressure releasing system to avoid explosion of the vessel, shall

be used. The inner wall of the vessel shall be inert and shall not release contaminations to the digest

solution.

It can be necessary to periodically clean the digestion vessels with a suitable surfactant to remove

persistent deposits.

7.3.2 Microwave digestion system, corrosion resistant and well ventilated. All electronics shall be

protected against corrosion for safe operation.

Use a laboratory-grade microwave oven with temperature feedback control mechanisms.

The microwave digestion system should be able to control the temperature with an accuracy of ±5 °C

and automatically adjust the microwave field output power within 2 s of sensing. Temperature sensors

shall be accurate to ±2 °C, including the final reaction temperature of (175 ± 5) °C. Temperature

feedback control provides the primary performance mechanism for the method. Due to the variability

in sample matrix types and microwave digestion equipment (i.e. different vessel types and microwave

designs), control of the temperature during digestion is important for reproducible microwave heating

and comparable data. Manufacturer specifications of the microwave digestion system must fit these

specifications. The accuracy of the temperature measurement system should be periodically tested

on blank samples at an elevated temperature according to the manufactures instructions. If the

temperature deviates by more than 2 °C from the temperature measured by an external, calibrated

temperature measurement system, the microwave temperature measurement system should be re-

calibrated.
7.4 Sample containers, plastics and glass containers are both suitable.

7.5 Filter paper, usually with a pore size of 0,45 µm and resistant to the diluted aqua regia final

digestion solution.
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ISO 54321:2020(E)
7.6 Volumetric flasks, usually of nominal capacity of 50 ml or 100 ml.
7.7 Analytical balance, with an accuracy of 1 mg or better.

7.8 Boiling aids, anti-bumping granules or glass beads, diameter 2 mm to 3 mm, acid washed.

8 Procedure
8.1 General

Pre-treat, soil, sludge and biowaste samples according to e.g. EN 16179 or ISO 11464 and waste samples

according to e.g. EN 15002.

Determine the dry matter content, depending on the matrix of the sample, e.g. according to EN 15934.

For waste samples the next remarks apply:

— Pre-treatment should include drying or grain size reduction below a particle size of 250 µm for solid

waste or homogenizing by use of a high speed mixer or sonification for liquid waste samples.

— The mass of test portion for a single digestion has to be selected in a way, that:

— it is representative for the laboratory sample;
— it complies with the specifications of manufacturer of the digestion unit.

Referring to the manufacturer’s instructions, the upper limits of mass of the test portion shall be taken

into account.

— For representativeness reasons a mass above 200 mg is to be preferred for the test portion. Follow,

for safety reasons, the manufacturer’s instructions regarding the maximum amount of organic

carbon in the sample.
8.2 Blank test

Carry out a reagent blank test digestion in parallel with the determination, using the same procedure

and the same quantities of all the reagents as in the determination, but omitting the test portion. The

laboratory shall define acceptable limits.

NOTE The measurement of a blank is introduced to determine the contribution of the extracting solution,

glassware, digestion tube and filter paper used to the measured value.
8.3 Method A — thermal heating under atmospheric conditions
8.3.1 Method A1 — Thermal heating under reflux conditions

Weigh approximately 3 g of the test sample (waste samples 1 g to 10 g), with an accuracy of 0,001 g (or

at least three significant figures), and transfer to the digestion vessel (7.2.1.1).

In case of dry samples moisten the test portion with about 0,5 ml to 1,0 ml of water (6.1) and add,

dropwise, if necessary, to reduce foaming, with mixing, (21,0 ± 0,1) ml of hydrochloric acid (6.2)

followed by (7,0 ± 0,1) ml of nitric acid (6.3). Connect the reflux condenser (7.2.1.2) to the digestion

vessel (7.2.1.1). Fill the absorption vessel (7.2.1.3) with approximately 15 ml nitric acid (6.4). Connect

the absorption vessel to the reflux condenser, and let stand at room temperature until any effervescence

almost ceases to allow for slow oxidation of the organic matter in the sample.

The time of standing at room temperature can have an influence on the digestion rate of aqua regia.

For consistency, it is recommended to start heating as soon as possible after the first strong reaction

has ceased.
© ISO 2020 – All rights reserved 5
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ISO 54321:2020(E)

30 ml of aqua regia is only sufficient for the oxidation of about 0,5 g organic carbon. If there is any

doubt of the amount of carbon present, estimate the amount of carbon in the sample or carry out a

determination of TOC. If there is more than 0,5 g of organic carbon in the test portion, proceed as

follows.

Allow first reaction with the aqua regia to subside. Then add an extra 1 ml of nitric acid (6.3) only to

every 0,1 g of organic carbon above 0,5 g. Do not add more than 10 ml of nitric acid at any given time,

and allow any reaction to subside before proceeding further.

Connect the digestion vessel (7.2.1.1) to the heating device (7.2.1.4) and raise the temperature of the

reaction mixture to reflux conditions and maintain for 2 h ensuring that the condensation zone is lower

than 1/3 of the height of the reflux condenser, then allow to cool. Add the content of the absorption

vessel to the reaction vessel via the reflux condenser, rinsing both the absorption vessel and condenser

with further 10 ml of diluted nitric acid (6.4).

Transfer quantitatively the solution content of each vessel into a suitable sized volumetric flask and add

water (6.1) to the volume mark.

Alternatively, another procedure can be applied, such that the adjustment to volume with the solid

residue still present shall be carried out immediately after digestion.

If the measurement solution contains particles due to precipitation which may clog nebulizers or

interfere with an injection of the sample into the instrument, the sample may be centrifuged, allowed to

settle, or filtered (7.5).

The measurement solution is now ready for analysis for elements of interest using appropriate

elemental analysis techniques.
8.3.2 Method A2 — Thermal heating with a heating block with containers

Weigh an amount of not more than 2 g of the test portion (typically 0,5 g to 1 g of dry sample) containing

not more than 0,15 g of organic carbon with an accuracy of 0,001 g (or at least three significant figures)

and transfer it into the digestion vessel (7.2.2.1).

The amount of the test sample depends on the amount of organic matter. The maximum amount of

organic carbon shall not exceed 0,15 g when 8 ml of aqua regia is used. Per additional 0,1 g organic

carbon (more than this 0,15 g), 1 ml of additional concentrated HNO (6.3) shall be added before the

digestion process is started.

N.B.: For some elements, e.g. barium and chromium, the additional volume of HNO is essential in order

to have a sufficient recovery upon digestion.

In case of dry samples moisten the test portion with a few drops of water (6.1). Add (6,0 + 0,1) ml

hydrochloric acid (6.2) followed by (2,0 + 0,1) ml nitric acid (6.3). Let stand at room temperature until

any effervescence almost ceases to allow for slow oxidation of the organic matter in the sample.

The time of standing at room temperature can have an influence on the digestion rate of aqua regia.

For consistency, it is recommended to start heating as soon as possible after the first strong reaction

has ceased.

Loosely screw on the tube cap (not very tight!) and place the digestion vessel on the heating block

(7.2.2.2) and slowly increase the temperature to the boiling point. Keep the temperature on the boiling

point during (120 + 10) min.

Let the vessel cool down to room temperature and fill up with water (6.1) to the volume mark.

If a non-graduated digestion tube is used, transfer quantitatively the solution into a suitable sized

volumetric flask and add water (6.1) to the volume mark. Alternatively, another procedure can be

applied, such that the adjustment to volume with the solid residue still present shall be carried out

immediately after digestion.
6 © ISO 2020 – All rights reserved
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ISO 54321:2020(E)

If the measurement solution contains particles due to precipitation which may clog nebulizers or interfere

with an injection of the sample into the instrument, the sample may be centrifuged or filtered (7.5).

The measurement solution is now ready for analysis for elements of interest using appropriate

elemental analysi
...

NORME ISO
INTERNATIONALE 54321
Première édition
2020-08
Sols, biodéchets traités, boues et
déchets — Digestion des éléments
solubles dans l’eau régale
Soil, treated biowaste, sludge and waste — Digestion of aqua regia
soluble fractions of elements
Numéro de référence
ISO 54321:2020(F)
ISO 2020
---------------------- Page: 1 ----------------------
ISO 54321:2020(F)
DOCUMENT PROTÉGÉ PAR COPYRIGHT
© ISO 2020

Tous droits réservés. Sauf prescription différente ou nécessité dans le contexte de sa mise en œuvre, aucune partie de cette

publication ne peut être reproduite ni utilisée sous quelque forme que ce soit et par aucun procédé, électronique ou mécanique,

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Publié en Suisse
ii © ISO 2020 – Tous droits réservés
---------------------- Page: 2 ----------------------
ISO 54321:2020(F)
Sommaire Page

Avant-propos ..............................................................................................................................................................................................................................iv

Introduction ..................................................................................................................................................................................................................................v

1 Domaine d’application ................................................................................................................................................................................... 1

2 Références normatives ................................................................................................................................................................................... 1

3 Termes et définitions ....................................................................................................................................................................................... 1

4 Principe .......................................................................................................................................................................................................................... 2

5 Interférences et sources d’erreur ....................................................................................................................................................... 2

6 Réactifs ........................................................................................................................................................................................................................... 3

7 Appareillage .............................................................................................................................................................................................................. 3

7.1 Généralités .................................................................................................................................................................................................. 3

7.2 Méthode A — Appareillage de chauffage thermique en conditions atmosphériques ............... 3

7.2.1 Méthode A1 — Chauffage thermique en conditions atmosphériques .............................. 3

7.2.2 Méthode A2 — Chauffage thermique, à l’aide d’un bloc chauffant avec des

récipients ................................................................................................................................................................................ 4

7.3 Méthode B — Digestion par micro-ondes, avec régulation de la température, dans

des récipients fermés ........................................................................................................................................................................ 4

8 Mode opératoire.................................................................................................................................................................................................... 5

8.1 Généralités .................................................................................................................................................................................................. 5

8.2 Essai à blanc .............................................................................................................................................................................................. 6

8.3 Méthode A — Chauffage thermique en conditions atmosphériques ....................................................... 6

8.3.1 Méthode A1 — Chauffage thermique en conditions atmosphériques .............................. 6

8.3.2 Méthode A2 — Chauffage thermique, à l’aide d’un bloc chauffant avec des

récipients ................................................................................................................................................................................ 7

8.4 Méthode B — Digestion par micro-ondes, avec régulation de la température, dans

des récipients fermés ........................................................................................................................................................................ 7

9 Rapport d’essai ....................................................................................................................................................................................................... 8

Annexe A (informative) Données de répétabilité et de reproductibilité relatives aux

échantillons de sol, de biodéchet et de boue .......................................................................................................................... 9

Annexe B (informative) Données de répétabilité et de reproductibilité relatives aux

échantillons de déchets ..............................................................................................................................................................................10

Bibliographie ...........................................................................................................................................................................................................................38

© ISO 2020 – Tous droits réservés iii
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ISO 54321:2020(F)
Avant-propos

L'ISO (Organisation internationale de normalisation) est une fédération mondiale d'organismes

nationaux de normalisation (comités membres de l'ISO). L'élaboration des Normes internationales est

en général confiée aux comités techniques de l'ISO. Chaque comité membre intéressé par une étude

a le droit de faire partie du comité technique créé à cet effet. Les organisations internationales,

gouvernementales et non gouvernementales, en liaison avec l'ISO participent également aux travaux.

L'ISO collabore étroitement avec la Commission électrotechnique internationale (IEC) en ce qui

concerne la normalisation électrotechnique.

Les procédures utilisées pour élaborer le présent document et celles destinées à sa mise à jour sont

décrites dans les Directives ISO/IEC, Partie 1. Il convient, en particulier, de prendre note des différents

critères d'approbation requis pour les différents types de documents ISO. Le présent document a été

rédigé conformément aux règles de rédaction données dans les Directives ISO/IEC, Partie 2 (voir www

.iso .org/ directives).

L'attention est attirée sur le fait que certains des éléments du présent document peuvent faire l'objet de

droits de propriété intellectuelle ou de droits analogues. L'ISO ne saurait être tenue pour responsable

de ne pas avoir identifié de tels droits de propriété et averti de leur existence. Les détails concernant

les références aux droits de propriété intellectuelle ou autres droits analogues identifiés lors de

l'élaboration du document sont indiqués dans l'Introduction et/ou dans la liste des déclarations de

brevets reçues par l'ISO (voir www .iso .org/ brevets).

Les appellations commerciales éventuellement mentionnées dans le présent document sont données

pour information, par souci de commodité, à l’intention des utilisateurs et ne sauraient constituer un

engagement.

Pour une explication de la nature volontaire des normes, la signification des termes et expressions

spécifiques de l'ISO liés à l'évaluation de la conformité, ou pour toute information au sujet de l'adhésion

de l'ISO aux principes de l’Organisation mondiale du commerce (OMC) concernant les obstacles

techniques au commerce (OTC), voir www .iso .org/ avant -propos.

Le présent document a été élaboré par le comité technique ISO/TC 190, Qualité du sol, sous-comité SC 3,

Méthodes chimiques et caractéristiques physiques, en collaboration avec le comité technique CEN/TC 444,

Méthodes d'essai pour la caractérisation environnementale des matrices solides, du Comité européen de

normalisation (CEN) conformément à l’Accord de coopération technique entre l’ISO et le CEN (Accord de

Vienne).

Il convient que l’utilisateur adresse tout retour d’information ou toute question concernant le présent

document à l’organisme national de normalisation de son pays. Une liste exhaustive desdits organismes

se trouve à l’adresse www .iso .org/ fr/ members .html.
iv © ISO 2020 – Tous droits réservés
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ISO 54321:2020(F)
Introduction

En ce qui concerne la comparabilité du mode opératoire décrit dans le présent document avec ceux des

autres normes mentionnées ci-dessus, les remarques suivantes peuvent être faites:

— le présent document décrit la digestion d’échantillons solides par de l’eau régale;

— les différences entre les modes opératoires des différentes normes sont minimes. Une différence

importante entre les modes opératoires de reflux décrits dans l’ISO 11466, l’EN 13657 et l’EN 16174

porte sur le temps d’attente après l’ajout de l’acide à l’échantillon, avant le début de la digestion.

L’ISO 11466 spécifie un temps d’attente de 16 h; les deux Normes européennes indiquent que la

digestion peut commencer après la fin des premières réactions fortes. Il a été prouvé, lors de travaux

de validation, que la différence entre 2 h et 16 d’attente était négligeable, c’est pourquoi le présent

document suit l’approche de l’EN 13657 et de l’EN 16174;

— le mode opératoire avec bloc chauffant a été ajouté aux modes opératoires de digestion par micro-

ondes et à reflux. Le mode opératoire a été tiré de la norme néerlandaise NEN 6961 qui spécifie un

temps d’ébullition de 2 h à 4 h. Le présent document spécifie un temps d’ébullition de 2 h.

Les méthodes spécifiées dans le présent document fournissent des techniques de digestion à l’eau

régale de plusieurs éléments contenus dans les sols, les biodéchets traités, les boues et les déchets avant

l’analyse. Il est connu que la digestion à l’eau régale d’échantillons prélevés dans l’environnement ne

conduit pas nécessairement à une décomposition complète des éléments et que l’extrait obtenu à partir

d’un échantillon pour essai peut ne pas refléter les concentrations totales des analytes cibles. Cependant,

pour la plupart des applications environnementales, les résultats obtenus par les méthodes de digestion

spécifiées dans le présent document sont considérés comme pertinents pour la finalité prévue.

Le présent document est validé pour plusieurs types de matrices, comme indiqué dans le Tableau 1.

Tableau 1 — Matrices pour lesquelles le présent document est validé
Matrice Matériaux utilisés dans l’essai de validation
Boues urbaines
Boues industrielles
Boues Boues produites par l’industrie électronique
Boues d’encres usagées
Boues de curage
Composte
B io dé c h e t s
(méthode A)
Boues compostées
Sols agricoles
Sol
Sols amendés par des boues
Cendres volantes d’incinération des déchets urbains (matrice «oxydée»)
Mâchefers d’incinération des déchets urbains (matrice «silicatée»)
Boues résiduaires d’encre (matrice «organique»)
Déchet
Boues d’industrie électronique (matrice «métallique»)
BCR 146R (boues de curage)
BCR 176 (cendres d’incinération des déchets urbains)

AVERTISSEMENT — Il convient que les utilisateurs du présent document connaissent bien les

pratiques courantes de laboratoire. Certains des réactifs utilisés dans le présent document sont

fortement corrosifs et très toxiques. Il est absolument nécessaire de prendre des mesures de

sécurité, non seulement en raison des réactifs fortement corrosifs, mais aussi de la température

et de la pression élevées.
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ISO 54321:2020(F)

Un matériel à micro-ondes de laboratoire muni de dispositifs de sécurité isolés et résistants

à la corrosion est requis. Les fours à micro-ondes à usage domestique (cuisine) ne doivent pas

être utilisés, car la corrosion par les vapeurs acides peut compromettre le fonctionnement des

dispositifs de sécurité et empêcher l’arrêt du magnétron à l’ouverture de la porte, ce qui pourrait

entraîner une exposition de l’opérateur à des niveaux dangereux d’énergie de micro-ondes.

Il convient que toutes les étapes des modes opératoires soient réalisées sous une hotte

ventilée ou dans un équipement fermé à ventilation forcée. Du fait de l’utilisation de réactifs

fortement oxydants, la formation de produits organiques intermédiaires explosifs est possible,

en particulier lors du traitement d’échantillons à forte teneur en matières organiques. Ne pas

ouvrir les récipients sous pression avant qu’ils n’aient refroidi. Éviter tout contact avec les

produits chimiques et les produits de réaction gazeux.

IMPORTANT — Il est absolument essentiel que les essais réalisés conformément au présent

document soient effectués par du personnel ayant suivi une formation appropriée.
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NORME INTERNATIONALE ISO 54321:2020(F)
Sols, biodéchets traités, boues et déchets — Digestion des
éléments solubles dans l’eau régale
1 Domaine d’application

Le présent document spécifie deux méthodes de digestion de sols, de biodéchets traités, de boues et de

déchets en utilisant une digestion à l’eau régale.

La digestion à l’eau régale n’aboutit pas nécessairement à une décomposition totale de l’échantillon. Les

concentrations des analytes extraits peuvent ne pas nécessairement refléter les teneurs totales dans

l’échantillon, mais représentent les métaux solubles dans l’eau régale dans les conditions du présent

mode opératoire d’essai. Il est généralement admis qu’aux fins d’analyses environnementales, les

résultats sont adaptés à la finalité prévue, à savoir la protection de l’environnement.

Le présent document est applicable aux éléments suivants:

aluminium (Al), antimoine (Sb), arsenic (As), baryum (Ba), béryllium (Be), bore (B), cadmium (Cd),

calcium (Ca), chrome (Cr), cobalt (Co), cuivre (Cu), fer (Fe), plomb (Pb), magnésium (Mg), manganèse

(Mn), mercure (Hg), molybdène (Mo), nickel (Ni), phosphore (P), potassium (K), sélénium (Se), argent

(Ag), sodium (Na), strontium (Sr), soufre (S), tellure (Te), thallium (Tl), étain (Sn), titane (Ti), vanadium

(V) et zinc (Zn).

Le présent document peut également être appliqué à la digestion d’autres éléments, à condition que

l’utilisateur en ait vérifié l’applicabilité.
2 Références normatives
Le présent document ne contient aucune référence normative.
3 Termes et définitions

Pour les besoins du présent document, les termes et définitions suivants s’appliquent.

L’ISO et l’IEC tiennent à jour des bases de données terminologiques destinées à être utilisées en

normalisation, consultables aux adresses suivantes:

— ISO Online browsing platform: disponible à l’adresse https:// www .iso .org/ obp

— IEC Electropedia: disponible à l’adresse http:// www .electropedia .org/
3.1
eau régale

solution de digestion (3.2) obtenue en mélangeant 1 volume d’acide nitrique (fraction massique de 65 %

à 70 %) et 3 volumes d’acide chlorhydrique (fraction massique de 35 % à 37 %)

Note 1 à l'article: Ces pourcentages massiques sont en accord avec les concentrations indiquées en 6.2 et 6.3.

3.2
digestion

minéralisation de la matière organique d’un échantillon et dissolution de sa partie minérale, plus ou

moins complètement, quand elle est mise en réaction avec un mélange de réactifs
3.3
matière sèche

résidu sec exprimé en pourcentage massique après séchage à 105 °C ± 5 °C jusqu’à poids constant

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ISO 54321:2020(F)
3.4
échantillon pour laboratoire

échantillon (3.5) destiné à être utilisé pour un contrôle ou pour des essais en laboratoire

[SOURCE: ISO 11074:2015, 4.3.7]
3.5
échantillon
partie de matériau choisie dans une quantité de matériau plus grande
[SOURCE: ISO 11074:2015, 4.1.17]
3.6
prise d’essai
partie analysée

quantité de matériau, prélevée dans l’échantillon pour essai (3.7), suffisante pour permettre la mesure

de la concentration ou de toute autre propriété étudiée

Note 1 à l'article: Si, en l’absence de toute préparation, la prise d’essai peut être prélevée directement dans

l’échantillon pour laboratoire (par exemple dans le cas de liquides), elle est généralement prélevée dans

l’échantillon préparé pour essai.

Note 2 à l'article: On peut considérer comme prise d’essai une unité ou un prélèvement élémentaire suffisamment

homogène et pur(e), de taille appropriée, et qui ne nécessite aucune autre préparation.

[SOURCE: ISO 11074:2015, 4.3.15]
3.7
échantillon pour essai
échantillon analysé

prise de matériau, issue de l’échantillon pour laboratoire (3.4) au moyen d’une méthode appropriée de

traitement préalable des échantillons, et ayant la taille (volume/masse) nécessaire pour les essais ou

l’analyse souhaités
[SOURCE: ISO 11074:2015, 4.1.3]
4 Principe

Une prise d’essai est digérée à l’eau régale conformément à l’un des modes opératoires de chauffage

suivants:
— méthode A: mode opératoire en conditions atmosphériques:
— A1: reflux pendant (120 ± 10) min, suivi d’une filtration/centrifugation;

— A2: bloc chauffant à (105 ± 5) °C pendant (120 ± 10) min, suivi d’une filtration/centrifugation;

— méthode B: digestion par micro-ondes:

— B1: mode opératoire à température contrôlée: à (175 ± 5) °C pendant (10 ± 1) min dans un

réacteur fermé, suivi d’une filtration/centrifugation.
5 Interférences et sources d’erreur

Le récipient dans lequel l’échantillon est livré et conservé peut être source d’erreurs. Sa matière doit

être choisie en fonction des éléments à analyser (par exemple, le mercure élémentaire peut pénétrer

dans les parois en polyéthylène très rapidement, aussi bien dans un sens que dans l’autre. Le verre peut

contaminer des échantillons avec ses principaux éléments, par exemple B, Na, K, Si et AI).

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ISO 54321:2020(F)

Le broyage des échantillons comporte un risque de contamination de l’échantillon par l’environnement

(par exemple air, poussière, usure de l’équipement de broyage). Des pertes de composés volatils sont

possibles à températures élevées.

En vue de la détermination des éléments formant des composés volatils (par exemple Hg, As), un soin

tout particulier doit être apporté lors du prétraitement de l’échantillon.

L’ensemble de la verrerie et du matériel plastique doit être nettoyé et stocké de façon appropriée, afin

d’éviter toute contamination.

En cas de filtration de la solution après digestion, il est nécessaire de veiller à ce que le mode opératoire

de filtration n’introduise pas de contaminants.

S’assurer que la totalité de la prise d’essai est mise en contact avec le mélange acide dans le flacon de

digestion.

Certains éléments étudiés peuvent être perdus en raison d’une précipitation avec des ions présents

dans la solution de digestion finale, par exemple les chlorures, fluorures et sulfates peu solubles.

6 Réactifs

Utiliser uniquement des acides et des réactifs de qualité analytique reconnue afin d’éviter des valeurs

de blanc élevées pour les mesures analytiques ultérieures. Utiliser une solution d’essai à blanc tout au

long du mode opératoire, en suivant toutes les étapes et en appliquant les mêmes quantités d’acides,

mais sans inclure d’échantillon.
6.1 Eau, par exemple déionisée.
6.2 Acide chlorhydrique, c(HCl) ≈ 12 mol/l.
6.3 Acide nitrique, c(HNO ) ≈ 15 mol/l.
6.4 Acide nitrique, c(HNO ) ≈ 0,5 mol/l.
Diluer 35 ml d’acide nitrique (6.3) à 1 l avec de l’eau (6.1).

6.5 Agent antimoussant, le n-dodécane (C H ) ou le n-octanol (C H O) par exemple, sont

12 26 8 18
appropriés.
7 Appareillage
7.1 Généralités

Matériel courant de laboratoire. L’ensemble de la verrerie et du matériel plastique doit être nettoyé et

stocké de façon appropriée, afin d’éviter toute contamination.

Suivant la concentration de l’élément à analyser, il convient d’apporter un soin tout particulier au

nettoyage efficace des récipients.
7.2 Méthode A — Appareillage de chauffage thermique en conditions atmosphériques
7.2.1 Méthode A1 — Chauffage thermique en conditions atmosphériques

7.2.1.1 Réacteur de digestion, résistant à la température et à la pression et pouvant contenir le

mélange de l’échantillon et de la solution de digestion, par exemple un flacon en quartz. Le récipient

de réaction doit avoir un volume d’au moins cinq fois celui de l’eau régale utilisée. La paroi interne des

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ISO 54321:2020(F)

récipients doit être inerte et ne doit pas libérer de substances dans la solution de digestion à un niveau

supérieur aux exigences de pureté de l’analyse ultérieure.

NOTE 1 Des récipients en silice ou en borosilicate peuvent être utilisés à la place des récipients en quartz.

NOTE 2 Il peut être nécessaire de nettoyer périodiquement les réacteurs de digestion avec un agent tensioactif

approprié afin d’éliminer les dépôts tenaces.

7.2.1.2 Réfrigérant à reflux pouvant s’adapter sur le récipient de digestion (7.2.1.1).

7.2.1.3 Chambre d’absorption, piège pour les substances volatiles servant, dans un système de

digestion ouvert, à piéger une ou plusieurs substances volatiles à mesurer, pouvant s’adapter sur le

réfrigérant à reflux (7.2.1.2).

7.2.1.4 Dispositif de chauffage, par exemple un chauffe-ballon avec thermostat ou un bloc chauffant

en aluminium avec thermostat.

7.2.2 Méthode A2 — Chauffage thermique, à l’aide d’un bloc chauffant avec des récipients

7.2.2.1 Tube de digestion, tube en propylène de 50 ml muni d’un bouchon à vis en polypropylène.

La partie non chauffée du tube et le bouchon à vis fonctionnent comme un réfrigérant, mais ne

constituent pas vraiment un système à reflux. Il est nécessaire de soumettre à essai la matière du tube

et du bouchon à vis pour vérifier l’absence de libération des éléments étudiés. Il est permis d’utiliser

d’autres matières et des récipients de volumes différents que ceux susmentionnés si leur adéquation a

été démontrée.

7.2.2.2 Bloc chauffant avec régulation de la température, bloc chauffant pouvant chauffer le(s)

tube(s) à une température de (105 ± 5) °C.

7.3 Méthode B — Digestion par micro-ondes, avec régulation de la température, dans

des récipients fermés

7.3.1 Réacteur de digestion, pour digestion par micro-ondes sous pression, typiquement d’un

volume de 100 ml, résistant au réactif, à la température et à la pression et pouvant contenir le mélange

de l’échantillon et de la solution de digestion. Le réacteur doit être utilisable en toute sécurité dans la

gamme de températures et de pressions appliquée, et doit pouvoir supporter des pressions d’au moins

3 000 kPa.

Les réacteurs de digestion utilisés doivent être en perfluoroalkoxylalcane (PFA), en

polytétrafluoréthylène modifié (PTFE) ou en quartz, et doivent être équipés d’un système de protection

contre la surpression permettant d’éviter leur explosion. La paroi interne du réacteur doit être inerte et

ne doit pas libérer de contaminants dans la solution de digestion.

Il peut être nécessaire de nettoyer périodiquement les réacteurs de digestion avec un agent tensioactif

approprié afin d’éliminer les dépôts tenaces.

7.3.2 Système de digestion par micro-ondes, résistant à la corrosion et bien ventilé. Toute

l’électronique doit être protégée contre la corrosion pour un fonctionnement en toute sécurité.

Utiliser un four à micro-ondes de laboratoire équipé d’une régulation de la puissance couplée à une

mesure de la température.

Il convient que le système de digestion par micro-ondes soit capable de réguler la température avec une

exactitude de ± 5 °C et de régler automatiquement la puissance de sortie du champ de micro-ondes dans

les 2 s qui suivent la détection. Les capteurs de température doivent avoir une exactitude de ± 2 °C, y

compris pour la température finale de réaction de (175 ± 5) °C. La régulation de la puissance couplée

4 © ISO 2020 – Tous droits réservés
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ISO 54321:2020(F)

à une mesure de la température constitue le principal mécanisme de performance de la méthode.

En raison de la variabilité des types de matrices d’échantillons et de l’équipement de digestion par

micro-ondes (c’est-à-dire différents types de réacteurs et différentes conceptions de micro-ondes),

la régulation de la température pendant la digestion est importante pour la reproductibilité du

chauffage aux micro-ondes et la comparabilité des données. Les spécifications du fabricant du système

de digestion par micro-ondes doivent être conformes à ces spécifications. Il convient de contrôler

périodiquement l’exactitude du système de mesure de la température sur des échantillons à blanc à

une température élevée conformément aux instructions du fabricant. Si la température s’écarte de plus

de 2 °C de la température mesurée par un système de mesure de la température externe étalonné, il

convient d’étalonner à nouveau le système de mesure de la température du four à micro-ondes.

7.4 Récipients à échantillons, des récipients en matière plastique ou en verre sont appropriés.

7.5 Papier-filtre, habituellement d’une taille de pore de 0,45 µm et résistant à la solution de digestion

dans l’eau régale finale diluée.
7.6 Fioles jaugées, habituellement d’une capacité nominale de 50 ml ou 100 ml.
7.7 Balance analytique, ayant une exactitude de 1 mg ou mieux.

7.8 Régulateurs d’ébullition, granules anti-projection ou billes de verre, de 2 mm à 3 mm de diamètre,

lavées à l’acide.
8 Mode opératoire
8.1 Généralités

Prétraiter les échantillons de sols, boues et biodéchets conformément à l’EN 16179 ou à l’ISO 11464 et

les échantillons de déchets conformément à l’EN 15002, par exemple.

Déterminer la teneur en matière sèche, en fonction de la matrice de l’échantillon, par exemple selon les

indications de l’EN 15934.
Pour les échantillons de déchets, les remarques suivantes s’appliquent:

— il convient que le prétraitement comprenne le séchage, la réduction granulométrique en dessous

d’une taille de particule de 250 µm pour des déchets solides, l’homogénéisation à l’aide d’un

mélangeur rapide ou la sonification pour les échantillons liquides;

— la masse de la prise d’essai à analyser pour une seule digestion doit être choisie de façon à être:

— représentative de l’échantillon pour laboratoire;
— conforme aux spécifications du fabricant de l’unité de digestion;

Les masses limites supérieures de la prise d’essai indiquées dans les instructions du fabricant doivent

être prises en compte.

— pour des raisons de représentativité, une masse supérieure à 200 mg est préférable pour la prise

d’essai. Pour des raisons de sécurité, suivre les instructions du fabricant relatives à la quantité

maximale de carbone organique dans l’échantillon.
© ISO 2020 – Tous droits réservés 5
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ISO 54321:2020(F)
8.2 Essai à blanc

Effectuer un essai de digestion à blanc parallèlement à la détermination, en suivant le même mode

opératoire et en utilisant les mêmes quantités de réactifs que pour la détermination, mais sans inclure

la prise d’essai. Le laboratoire doit définir les limites acceptables.

NOTE Le mesurage d’un blanc permet de déterminer la contribution de la solution d’extraction, de la

verrerie, du tube à digestion et du papier-filtre à la valeur mesurée.
8.3 Méthode A — Chauffage thermique en conditions atmosphériques
8.3.1 Méthode A1 — Chauffage thermique en conditions atmosphériques

Peser environ 3 g de l’échantillon pour essai (1 g à 10 g pour les échantillons de déchets) à 0,001 g près

(ou au moins avec trois chiffres après la virgule), et les transférer dans un récipient de digestion (7.2.1.1).

Si les échantillons sont secs, humidifier la prise d’essai avec environ 0,5 ml à 1,0 ml d’eau (6.1) et ajouter

tout en mélangeant, si nécessaire goutte à goutte pour réduire la formation de mousse, (21,0 ± 0,1) ml

d’acide chlorhydrique (6.2) puis (7,0 ± 0,1) ml d’acide nitrique (6.3). Raccorder le réfrigérant à reflux

(7.2.1.2) au réacteur de digestion (7.2.1.1). Remplir la chambre d’absorption (7.2.1.3) avec environ 15 ml

d’acide nitrique (6.4). Raccorder la chambre d’absorption au réfrigérant à reflux et laisser reposer à

température ambiante jusqu’à ce que l’effervescence ait presque cessé afin de permettre l’oxydation

lente des matières organiques dans l’échantillon.

Le temps de repos à température ambiante peut avoir une influence sur la vitesse de digestion de l’eau

régale. Pour des raisons de cohérence, il est
...

FINAL
INTERNATIONAL ISO/FDIS
DRAFT
STANDARD 54321
ISO/TC 190/SC 3
Soil, treated biowaste, sludge and
Secretariat: DIN
waste — Digestion of aqua regia
Voting begins on:
2020­04-22 soluble fractions of elements
Voting terminates on:
Sols, biodéchets traités, boues et déchets — Digestion des éléments
2020­06­17
solubles dans l’eau régale
ISO/CEN PARALLEL PROCESSING
RECIPIENTS OF THIS DRAFT ARE INVITED TO
SUBMIT, WITH THEIR COMMENTS, NOTIFICATION
OF ANY RELEVANT PATENT RIGHTS OF WHICH
THEY ARE AWARE AND TO PROVIDE SUPPOR TING
DOCUMENTATION.
IN ADDITION TO THEIR EVALUATION AS
Reference number
BEING ACCEPTABLE FOR INDUSTRIAL, TECHNO­
ISO/FDIS 54321:2020(E)
LOGICAL, COMMERCIAL AND USER PURPOSES,
DRAFT INTERNATIONAL STANDARDS MAY ON
OCCASION HAVE TO BE CONSIDERED IN THE
LIGHT OF THEIR POTENTIAL TO BECOME STAN­
DARDS TO WHICH REFERENCE MAY BE MADE IN
NATIONAL REGULATIONS. ISO 2020
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ISO/FDIS 54321:2020(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2020

All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may

be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting

on the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address

below or ISO’s member body in the country of the requester.
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Published in Switzerland
ii © ISO 2020 – All rights reserved
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ISO/FDIS 54321:2020(E)
Contents Page

Foreword ........................................................................................................................................................................................................................................iv

Introduction ..................................................................................................................................................................................................................................v

1 Scope ................................................................................................................................................................................................................................. 1

2 Normative references ...................................................................................................................................................................................... 1

3 Terms and definitions ..................................................................................................................................................................................... 1

4 Principle ........................................................................................................................................................................................................................ 2

5 Interferences and sources of errors ................................................................................................................................................. 2

6 Reagents ........................................................................................................................................................................................................................ 3

7 Apparatus ..................................................................................................................................................................................................................... 3

7.1 General ........................................................................................................................................................................................................... 3

7.2 Method A — Apparatus for thermal heating under atmospheric conditions ................................... 3

7.2.1 Method A1 — Thermal heating under reflux conditions .............................................................. 3

7.2.2 Method A2 — Thermal heating with a heating block with containers ............................. 4

7.3 Method B — Microwave digestion with temperature control, closed vessels ................................. 4

8 Procedure..................................................................................................................................................................................................................... 5

8.1 General ........................................................................................................................................................................................................... 5

8.2 Blank test ..................................................................................................................................................................................................... 5

8.3 Method A — thermal heating under atmospheric conditions ....................................................................... 5

8.3.1 Method A1 — Thermal heating under reflux conditions .............................................................. 5

8.3.2 Method A2 — Thermal heating with a heating block with containers ............................. 6

8.4 Method B — Microwave digestion with temperature control, closed vessels ................................. 7

9 Test report ................................................................................................................................................................................................................... 7

Annex A (informative) Repeatability and reproducibility data for soil, biowaste and sludge

samples ........................................................................................................................................................................................................................... 9

Annex B (informative) Repeatability and reproducibility data for waste samples ..........................................10

Bibliography .............................................................................................................................................................................................................................37

© ISO 2020 – All rights reserved iii
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ISO/FDIS 54321:2020(E)
Foreword

ISO (the International Organization for Standardization) is a worldwide federation of national standards

bodies (ISO member bodies). The work of preparing International Standards is normally carried out

through ISO technical committees. Each member body interested in a subject for which a technical

committee has been established has the right to be represented on that committee. International

organizations, governmental and non­governmental, in liaison with ISO, also take part in the work.

ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of

electrotechnical standardization.

The procedures used to develop this document and those intended for its further maintenance are

described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the

different types of ISO documents should be noted. This document was drafted in accordance with the

editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).

Attention is drawn to the possibility that some of the elements of this document may be the subject of

patent rights. ISO shall not be held responsible for identifying any or all such patent rights. Details of

any patent rights identified during the development of the document will be in the Introduction and/or

on the ISO list of patent declarations received (see www .iso .org/ patents).

Any trade name used in this document is information given for the convenience of users and does not

constitute an endorsement.

For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and

expressions related to conformity assessment, as well as information about ISO's adherence to the

World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www .iso .org/

iso/ foreword .html.

This document was prepared by Technical Committee ISO/TC 190, Soil quality, Subcommittee SC 3,

Chemical and physical characterization.

Any feedback or questions on this document should be directed to the user’s national standards body. A

complete listing of these bodies can be found at www .iso .org/ members .html.
iv © ISO 2020 – All rights reserved
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ISO/FDIS 54321:2020(E)
Introduction

Regarding the comparability of the procedure described in this document with those of the other

standards mentioned above the next remarks can be made:
— This document describes the digestion of solid samples with aqua regia.

— Differences in the procedures of the different standards are small. An important difference between

the reflux procedures as described in ISO 11466 and EN 13657 and EN 16174 concerns the waiting

time after addition of the acid to the sample, before the digestion starts. ISO 11466 specifies a

waiting time of 16 h, both European standards state that the digestion can start after the first strong

reactions have ceased. In validation work it was proven that the difference between 2 h and 16 h of

waiting was negligible, therefore this document follows the approach of EN 13657 and EN 16174.

— The heating block procedure was added to the reflux and microwave digestion procedures. The

procedure was adopted from the Dutch standard NEN 6961, which specifies a boiling time of 2 h to

4 h. This document specifies a boiling time of 2 h.

The methods specified in this document are providing multi-element aqua regia digestion techniques

for soil, treated biowaste, sludge and waste prior to analysis. It is known that the digestion of

environmental samples with aqua regia will not necessarily lead to complete element recoveries, and

that the extract from a test sample may not reflect the total concentrations of the target analytes.

However, for most environmental applications the result obtained based upon digestion methods

specified in this document are considered to be fit for the intended purpose.

This document is validated for several types of matrices as indicated in Table 1.

Table 1 — Matrices for which EN 54321 is validated
Matrix Materials used in the validation test
Municipal sludge
Industrial sludge
Sludge Sludge from electronic industry
Ink waste sludge
Sewage sludge
Compost
Biowaste (Meth­
od A)
Composted sludge
Agricultural soil
Soil
Sludge amended soils
City waste incineration fly ash (“oxidised” matrix)
City waste incineration bottom ash (“silicate” matrix)
Ink waste sludge (organic matrix)
Waste
Electronic industry sludge (“metallic” matrix)
BCR 146R (sewage sludge)
BCR 176 (city waste incineration ash)

WARNING — Persons using this document should be familiar with usual laboratory practice.

Some of the reagents used in this document are highly corrosive and very toxic. Safety

precautions are absolutely necessary, not only due to the strong corrosive reagents, but also to

the high temperature and high pressure.

The use of laboratory-grade microwave equipment with isolated and corrosion resistant safety

devices is required. Domestic (kitchen) type microwave ovens shall not be used, as corrosion

by acid vapours may compromise the function of the safety devices and prevent the microwave

© ISO 2020 – All rights reserved v
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ISO/FDIS 54321:2020(E)

magnetron from shutting off when the door is open, which could result in operator exposure to

hazardous levels of microwave energy.

All procedures should be performed in a fume hood or in closed force-ventilated equipment. By

the use of strong oxidising reagents, the formation of explosive organic intermediates is possible,

especially when dealing with samples with a high organic content. Do not open pressurized

vessels before they have cooled down. Avoid contact with the chemicals and the gaseous reaction

products.

IMPORTANT — It is absolutely essential that tests conducted according to this document be

carried out by suitably trained staff.
vi © ISO 2020 – All rights reserved
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FINAL DRAFT INTERNATIONAL STANDARD ISO/FDIS 54321:2020(E)
Soil, treated biowaste, sludge and waste — Digestion of
aqua regia soluble fractions of elements
1 Scope

This document specifies two methods for digestion of soil, treated biowaste, sludge and waste by the

use of an aqua regia digestion.

Digestion with aqua regia will not necessarily accomplish total decomposition of the sample. The

extracted analyte concentrations may not necessarily reflect the total content in the sample but

represent the aqua regia soluble metals under the condition of this test procedure. It is generally agreed

that for environmental analysis purposes, the results are fit for the intended purpose to protect the

environment.
This document is applicable for the following elements:

Aluminium (Al), antimony (Sb), arsenic (As), barium (Ba), beryllium (Be), boron (B), cadmium (Cd),

calcium (Ca), chromium (Cr), cobalt (Co), copper (Cu), iron (Fe), lead (Pb), magnesium (Mg),

manganese (Mn), mercury (Hg), molybdenum (Mo), nickel (Ni), phosphorus (P), potassium (K),

selenium (Se), silver (Ag), sodium (Na), strontium (Sr), sulfur (S), tellurium (Te), thallium (Tl), tin (Sn),

titanium (Ti), vanadium (V), and zinc (Zn).

This document can also be applied for the digestion of other elements, provided the user has verified

the applicability.
2 Normative references
There are no normative references in this document.
3 Terms and definitions
For the purposes of this document, the following terms and definitions apply.

ISO and IEC maintain terminological databases for use in standardization at the following addresses:

— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at http:// www .electropedia .org/
3.1
aqua regia

digestion (3.2) solution obtained by mixing 1 volume of nitric acid (mass fraction of 65 % to 70 %) and

3 volumes of hydrochloric acid (mass fraction of 35 % to 37 %)

Note 1 to entry: These mass percentages agree with the concentrations of 6.2 and 6.3.

3.2
digestion

mineralization of the organic matter of a sample and dissolution of its mineral part, more or less

completely, when reacting with a reagent mixture
3.3
dry residue

dry matter expressed as a percentage by mass after drying at 105 °C ± 5 °C to the constancy of weight

© ISO 2020 – All rights reserved 1
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ISO/FDIS 54321:2020(E)
3.4
laboratory sample
sample (3.5) intended for laboratory inspection of testing
[SOURCE: ISO 11074:2015, 4.3.7]
3.5
sample
portion of material selected from a larger quantity of material
[SOURCE: ISO 11074:2015, 4.1.17]
3.6
test portion
analytical portion

quantity of material of proper size for measurement of the concentration or other properties of interest,

removed from the test sample (3.7)

Note 1 to entry: The test portion may be taken from the laboratory sample directly if no preparation of sample is

required (e. g. with liquids), but usually it is taken from the prepared test sample.

Note 2 to entry: A unit or increment of proper homogeneity, size and fineness, needing no further preparation,

may be a test portion.
[SOURCE: ISO 11074:2015, 4.3.15]
3.7
test sample
analytical sample

portion of material resulting from the laboratory sample (3.4) by means of an appropriate method of

sample pre-treatment and having the size (volume/mass) necessary for the desired testing or analysis

[SOURCE: ISO 11074:2015, 4.1.3]
4 Principle

A test portion is digested with aqua regia according to one of the following heating procedures:

— Method A: procedure under atmospheric conditions
— A1: reflux for (120 ± 10) min, followed by filtration/centrifugation;

— A2: heating block at (105 ± 5) °C for (120 + 10) min, followed by filtration/centrifugation.

— Method B: microwave digestion

— B1: Temperature controlled procedure: at (175 ± 5) °C for (10 ± 1) min in a closed vessel followed

by filtration/centrifugation.
5 Interferences and sources of errors

The container in which the sample is delivered and stored can be a source of errors. Its material shall

be chosen according to the elements to be determined (e.g. elemental Hg can penetrate polyethylene

walls very fast in both directions. Glass can contaminate samples with its major elements: e.g. B, Na, K,

Si and Al).

Grinding or milling samples includes a risk of contamination of the sample by the environment (air, dust,

wear of milling equipment). Due to elevated temperature losses of volatile compounds are possible.

For the determination of elements forming volatile compounds (e.g. Hg, As) special care has to be taken

during sample pre­treatment.
2 © ISO 2020 – All rights reserved
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ISO/FDIS 54321:2020(E)

All glassware and plastics ware shall be adequately cleaned and stored in order to avoid any

contamination.

In the case of filtration of the digested solution it is necessary to take care that the filtration procedure

does not introduce contaminants.

Ensure that all of the test portion is brought into contact with the acid mixture in the digestion vessel.

Some elements of interest can be lost due to precipitation with ions present in the final digest solution,

e.g. low soluble chlorides, fluorides and sulfates.
6 Reagents

Use only acids and reagents of recognized analytical grade to avoid high blank values for subsequent

analytical measurements. Use a test blank solution throughout the procedure applying all steps with

the same amount of acids, but without a sample.
6.1 Water, e.g. deionized.
6.2 Hydrochloric acid, c(HCl) ≈ 12 mol/l.
6.3 Nitric acid, c(HNO ) ≈ 15 mol/l.
6.4 Nitric acid, c(HNO ) ≈ 0,5 mol/l.
Dilute 35 ml nitric acid (6.3) to 1 l with water (6.1).
6.5 Antifoaming agent, e.g. n­dodecane (C H ) or n­octanol (C H O) are suitable.
12 26 8 18
7 Apparatus
7.1 General

Usual laboratory apparatus. All glassware and plastics ware shall be adequately cleaned and stored in

order to avoid any contamination.

Depending upon the concentration of the element of interest, particular care should be exercised with

respect to the effective cleaning of the vessels.
7.2 Method A — Apparatus for thermal heating under atmospheric conditions
7.2.1 Method A1 — Thermal heating under reflux conditions

7.2.1.1 Digestion vessel, temperature- and pressure-resistant and capable of containing the mixture

of sample and digest solution, for example a quartz vessel. The digestion vessel shall have a volume of at

least 5 times of the volume of the aqua regia used. The inner wall of the vessel shall be inert and shall not

release substances to the digest in excess of the purity requirements of the subsequent analysis.

NOTE 1 Silica or borosilicate glass vessels can be used instead of quartz vessels.

NOTE 2 It can be necessary to periodically clean the digestion vessels with a suitable surfactant to remove

persistent deposits.
7.2.1.2 Reflux condenser, adaptable to the digestion vessel (7.2.1.1).
© ISO 2020 – All rights reserved 3
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ISO/FDIS 54321:2020(E)

7.2.1.3 Absorption vessel, volatile species trap, in an open digestion system capable of trapping one

or more volatile measurement species, adaptable to the reflux condenser (7.2.1.2).

7.2.1.4 Heating device, for example a heating mantle, thermostatic controlled, or an aluminium block

thermostat.
7.2.2 Method A2 — Thermal heating with a heating block with containers

7.2.2.1 Digestion tube, 50 ml propylene tube with a screw cap from polypropylene.

The part of the tube not being heated and the screw cap function as a condenser, but are not really

a reflux system. The material of the tube and screw cap have to be tested in order to be sure that

release of elements of interest does not take place. Other materials and vessels with other volumes than

mentioned above are allowed to be used if suitability has been proven.

7.2.2.2 Temperature controlled heating block, heating block able to heat the tube(s) to a temperature

of (105 ± 5) °C.
7.3 Method B — Microwave digestion with temperature control, closed vessels

7.3.1 Digestion vessel, for pressurized microwave digestion, typically 100 ml volume, reagent-,

temperature- and pressure-resistant and capable of containing the mixture of sample and digest solution.

The vessel shall be suitable for the safe application in the temperature and pressure range applied,

capable of withstanding pressures of at least 3 000 kPa.

Digestion vessels made of perfluoro alkoxyl alkane (PFA), modified polytetrafluoroethene (PTFE) or

quartz, and equipped with a safety pressure releasing system to avoid explosion of the vessel, shall

be used. The inner wall of the vessel shall be inert and shall not release contaminations to the digest

solution.

It can be necessary to periodically clean the digestion vessels with a suitable surfactant to remove

persistent deposits.

7.3.2 Microwave digestion system, corrosion resistant and well ventilated. All electronics shall be

protected against corrosion for safe operation.

Use a laboratory-grade microwave oven with temperature feedback control mechanisms.

The microwave digestion system should be able to control the temperature with an accuracy of ±5 °C

and automatically adjust the microwave field output power within 2 s of sensing. Temperature sensors

shall be accurate to ±2 °C, including the final reaction temperature of (175 ± 5) °C. Temperature

feedback control provides the primary performance mechanism for the method. Due to the variability

in sample matrix types and microwave digestion equipment (i.e. different vessel types and microwave

designs), control of the temperature during digestion is important for reproducible microwave heating

and comparable data. Manufacturer specifications of the microwave digestion system must fit these

specifications. The accuracy of the temperature measurement system should be periodically tested

on blank samples at an elevated temperature according to the manufactures instructions. If the

temperature deviates by more than 2 °C from the temperature measured by an external, calibrated

temperature measurement system, the microwave temperature measurement system should be re-

calibrated.
7.4 Sample containers, plastics and glass containers are both suitable.

7.5 Filter paper, usually with a pore size of 0,45 µm and resistant to the diluted aqua regia final

digestion solution.
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ISO/FDIS 54321:2020(E)
7.6 Volumetric flasks, usually of nominal capacity of 50 ml or 100 ml.
7.7 Analytical balance, with an accuracy of 1 mg or better.

7.8 Boiling aids, anti­bumping granules or glass beads, diameter 2 mm to 3 mm, acid washed.

8 Procedure
8.1 General

Pre­treat, soil, sludge and biowaste samples according to e.g. EN 16179 or ISO 11464 and waste samples

according to e.g. EN 15002.

Determine the dry matter content, depending on the matrix of the sample, e.g. according to EN 15934.

For waste samples the next remarks apply:

— Pre-treatment should include drying or grain size reduction below a particle size of 250 µm for solid

waste or homogenizing by use of a high speed mixer or sonification for liquid waste samples.

— The mass of test portion for a single digestion has to be selected in a way, that:

— it is representative for the laboratory sample;
— it complies with the specifications of manufacturer of the digestion unit.

Referring to the manufacturer’s instructions, the upper limits of mass of the test portion shall be taken

into account.

— For representability reasons a mass above 200 mg is to be preferred for the test portion. Follow, for

safety reasons, the manufacturer’s instructions regarding the maximum amount of organic carbon

in the sample.
8.2 Blank test

Carry out a reagent blank test digestion in parallel with the determination, using the same procedure

and the same quantities of all the reagents as in the determination, but omitting the test portion. The

laboratory shall define acceptable limits.

NOTE The measurement of a blank is introduced to determine the contribution of the extracting solution,

glassware, digestion tube and filter paper used to the measured value.
8.3 Method A — thermal heating under atmospheric conditions
8.3.1 Method A1 — Thermal heating under reflux conditions

Weigh approximately 3 g of the test sample (waste samples 1 g to 10 g), with an accuracy of 0,001 g (or

at least three significant figures), and transfer to the digestion vessel (7.2.1.1).

In case of dry samples moisten the test portion with about 0,5 ml to 1,0 ml of water (6.1) and add,

dropwise, if necessary, to reduce foaming, with mixing, (21,0 ± 0,1) ml of hydrochloric acid (6.2)

followed by (7,0 ± 0,1) ml of nitric acid (6.3). Connect the reflux condenser (7.2.1.2) to the digestion

vessel (7.2.1.1). Fill the absorption vessel (7.2.1.3) with approximately 15 ml nitric acid (6.4). Connect

the absorption vessel to the reflux condenser, and let stand at room temperature until any effervescence

almost ceases to allow for slow oxidation of the organic matter in the sample.

The time of standing at room temperature can have an influence on the digestion rate of aqua regia.

For consistency, it is recommended to start heating as soon as possible after the first strong reaction

has ceased.
© ISO 2020 – All rights reserved 5
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ISO/FDIS 54321:2020(E)

30 ml of aqua regia is only sufficient for the oxidation of about 0,5 g organic carbon. If there is any

doubt of the amount of carbon present, estimate the amount of carbon in the sample or carry out a

determination of TOC. If there is more than 0,5 g of organic carbon in the test portion, proceed as

follows.

Allow first reaction with the aqua regia to subside. Then add an extra 1 ml of nitric acid (6.3) only to

every 0,1 g of organic carbon above 0,5 g. Do not add more than 10 ml of nitric acid at any given time,

and allow any reaction to subside before proceeding further.

Connect the digestion vessel (7.2.1.1) to the heating device (7.2.1.4) and raise the temperature of the

reaction mixture to reflux conditions and maintain for 2 h ensuring that the condensation zone is lower

than 1/3 of the height of the reflux condenser, then allow to cool. Add the content of the absorption

vessel to the reaction vessel via the reflux condenser, rinsing both the absorption vessel and condenser

with further 10 ml of diluted nitric acid (6.4).

Transfer quantitatively the solution content of each vessel into a suitable sized volumetric flask and add

water (6.1) to the volume mark.

Alternatively, another procedure can be applied, such that the adjustment to volume with the solid

residue still present shall be carried out immediately after digestion.

If the measurement solution contains particles due to precipitation which may clog nebulizers or

interfere with an injection of the sample into the instrument, the sample may be centrifuged, allowed to

settle, or filtered (7.5).

The measurement solution is now ready for analysis for elements of interest using appropriate

elemental analysis techniques.
8.3.2 Method A2 — Thermal heating with a heating block with containers

Weigh an amount of not more than 2 g of the test portion (typically 0,5 g to 1 g of dry sample) containing

not more than 0,15 g of organic carbon with an accuracy of 0,001 g ((or at least three significant figures))

and transfer it into the digestion vessel (7.2.2.1).

The amount of the test sample depends on the amount of organic matter. The maximum amount of

organic carbon shall not exceed 0,15 g when 8 ml of aqua regia is used. Per additional 0,1 g organic

carbon (more than this 0,15 g), 1 ml of additional concentrated HNO (6.3) shall be added before the

digestion process is started.

N.B.: For some elements, e.g. barium and chromium, the additional volume of HNO is essential in order

to have a sufficient recovery upon digestion.

In case of dry samples moisten the test portion with a few drops of water (6.1). Add (6,0 + 0,1) ml

hydrochloric acid (6.2) followed by (2,0 + 0,1) ml nitric acid (6.3). Let stand at room temperature until

any effervescence almost ceases to allow for slow oxidation of the organic matter in the sample.

The time of standing at room temperature can have an influence on the digestion rate of aqua regia.

For consistency, it is recommended to start heating as soon as possible after the first strong reaction

has ceased.

Loosely screw on the tube cap (not very tight!) and place the digestion vessel on the heating block

(7.2.2.2) and slowly increase the temperature to the boiling point. Keep the temperature on the boiling

point during (120 + 10) min.

Let the vessel cool down to room temperature and fill up with water (6.1) to the volume mark.

If a non-graduated digestion tube is used, transfer quantitatively the solution into a suitable sized

volumetric flask and add water (6.1) to the volume mark. Alternatively, another procedure can be

applied, such that the adjustment to volume with the solid residue still present shall be carried out

immediately after digestion.
...

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