Iron ores — Determination of total iron content using the EDTA photometric titration method — Part 1: Microwave digestion method

This document specifies a photometric titration method using EDTA for the determination of the total iron content of iron ores. This method is applicable to a concentration range of a mass fraction of 37,00 % to 72,00 % of total iron in natural iron ores, iron ore concentrates and agglomerates, including sinter products. This method does not apply as a referee method.

Minerais de fer — Dosage du fer total par la méthode titrimétrique photométrique à l'EDTA — Partie 1: Méthode de digestion par micro-ondes

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Status
Published
Publication Date
20-Jul-2022
Current Stage
6060 - International Standard published
Due Date
21-Mar-2023
Completion Date
21-Jul-2022
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INTERNATIONAL ISO
STANDARD 21826-1
First edition
2022-07
Iron ores — Determination of
total iron content using the EDTA
photometric titration method —
Part 1:
Microwave digestion method
Minerais de fer — Dosage du fer total par la méthode titrimétrique
photométrique à l'EDTA —
Partie 1: Méthode de digestion par micro-ondes
Reference number
ISO 21826-1:2022(E)
© ISO 2022
---------------------- Page: 1 ----------------------
ISO 21826-1:2022(E)
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© ISO 2022

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

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Published in Switzerland
© ISO 2022 – All rights reserved
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ISO 21826-1:2022(E)
Contents Page

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

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

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

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

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

5 Reagents ........................................................................................................................................................................................................................ 2

6 Apparatus .................................................................................................................................................................................................................... 5

7 Sampling and samples ........................................................................................................................................... .........................................6

7.1 Laboratory sample .............................................................................................................................................................................. 6

7.2 Preparation of test samples ........................................................................................................................................................ 6

7.2.1 General ........................................................................................................................................................................................ 6

7.2.2 Ores having significant contents of combined water or oxidizable

compounds .............................................................................................................................................................................. 6

7.2.3 Ores outside the scope of 7.2.2 ............................................................................................................................... 6

8 Procedures ................................................................................................................................................................................................................. 6

8.1 Number of determinations .......................................................................................................................................................... 6

8.2 Test portion ............................................................................................................................................................................................... 6

8.3 Blank test ..................................................................................................................................................................................................... 7

8.4 Determination ......................................................................................................................................................................................... 7

8.4.1 Decomposition of the test portion ...................................................................................................................... 7

8.4.2 Titration ..................................................................................................................................................................................... 7

8.5 Procedure of blank test ................................................................................................................................................................... 7

9 Expression of results ....................................................................................................................................................................................... 8

9.1 Calculation of total iron content ............................................................................................................................................. 8

9.2 General treatment of results ...................................................................................................................................................... 8

9.2.1 Repeatability and permissible tolerance ...................................................................................................... 8

9.2.2 Methods for checking the acceptability of the test results and determining

the final quoted result ................................................................................................................................................... 9

9.2.3 Between-laboratories precision ........................................................................................................................... 9

9.2.4 Check for trueness ............................................................................................................................................................ 9

9.2.5 Calculation of final result ........................................................................................................................................ 10

9.3 Oxide factors ........................................................................................................................................... ............................................... 10

10 Test report ...............................................................................................................................................................................................................11

Annex A (informative) Flowsheet of the procedure for the acceptance of analytical values

for test samples ..................................................................................................................................................................................................12

Annex B (informative) Typical titration curves ....................................................................................................................................13

Annex C (informative) Titration parameters for reference .....................................................................................................15

Bibliography .............................................................................................................................................................................................................................26

iii
© ISO 2022 – All rights reserved
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ISO 21826-1:2022(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 102, Iron ore and direct reduced iron,

Subcommittee SC 2, Chemical analysis.

This first edition of ISO 21826-1 cancels and replaces ISO/TS 21826:2020, which has been technically

revised.
The main changes are as follows:
— “This method does not apply as a referee method” has been added to the Scope.
A list of all parts in the ISO 21826 series can be found on the ISO website.

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.
© ISO 2022 – All rights reserved
---------------------- Page: 4 ----------------------
INTERNATIONAL STANDARD ISO 21826-1:2022(E)
Iron ores — Determination of total iron content using the
EDTA photometric titration method —
Part 1:
Microwave digestion method

WARNING — This document can involve hazardous materials, operations and equipment. It does

not address all of the potential safety concerns associated with its use. It is the responsibility of

the user of this method to establish appropriate health and safety practices and determine the

applicability of regulatory limitations prior to use.
1 Scope

This document specifies a photometric titration method using EDTA for the determination of the total

iron content of iron ores.

This method is applicable to a concentration range of a mass fraction of 37,00 % to 72,00 % of total iron

in natural iron ores, iron ore concentrates and agglomerates, including sinter products.

This method does not apply as a referee method.
2 Normative references

The following documents are referred to in the text in such a way that some or all of their content

constitutes requirements of this document. For dated references, only the edition cited applies. For

undated references, the latest edition of the referenced document (including any amendments) applies.

ISO 385, Laboratory glassware — Burettes
ISO 648, Laboratory glassware — Single-volume pipettes
ISO 1042, Laboratory glassware — One-mark volumetric flasks

ISO 2596, Iron ores — Determination of hygroscopic moisture in analytical samples — Gravimetric, Karl

Fischer and mass-loss methods
ISO 3082, Iron ores — Sampling and sample preparation procedures
ISO 7764, Iron ores — Preparation of predried test samples for chemical analysis
3 Terms and definitions
No terms and definitions are listed in this document.

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

— ISO Online browsing platform: available at https:// www .iso .org/ obp
— IEC Electropedia: available at https:// www .electropedia .org/
© ISO 2022 – All rights reserved
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ISO 21826-1:2022(E)
4 Principle

The samples are digested in closed vessels in a microwave oven in a mixture of nitric acid, hydrochloric

acid and hydrofluoric acid. The resulting solution is diluted with water and adjusted to pH 1 with sodium

hydroxide. Sulphuric acid is added to the solution to eliminate titanium hydroxide precipitation. Boric

acid is added to the solution to reduce the interference of fluoride. Iron is determined by automatic

photometric titration with EDTA.
5 Reagents

During the analysis, use only recognized analytical grade reagents and only distilled water or water of

equivalent purity.
5.1 Iron powder (purity > 99,98 %).
5.2 Hydrochloric acid, ρ 1,16 g/ml.
5.3 Nitric acid, ρ 1,42 g/ml.
5.4 Hydrofluoric acid solution, 1+9.
5.5 Sulphuric acid solution, 1+1.
5.6 Boric acid solution, 10 %.

Weigh 100 g of boric acid into a 1 000 ml beaker. Add 25 g of sodium hydroxide and dilute to 1 000 ml

with water. The solution should be made at the time of use.
5.7 Sodium hydroxide solution, 200 g/l.
5.8 Iron standard solution, 0,100 0 mol/l.

Weigh 0,558 5 g of iron powder (5.1) into a 250 ml beaker. Add 5 ml of hydrochloric acid (5.2) and 5 ml

of nitric acid (5.3). Cover the beaker with a watch glass and heat at 80 °C (hotplate) until completely

dissolved. Cool and transfer to a 100 ml volumetric flask and dilute to the volume mark with water.

5.9 Disodium dihydrogen ethylenedinitrilotetraacetate dihydrate (EDTA) solution,
C(C H N O Na ·2H O) = 0,1 mol/l.
10 14 2 8 2 2
5.9.1 Preparation of the EDTA solution

Dissolve 40 g of disodium EDTA dihydrate in water and dilute to 1 l with water. Store this solution in a

polyethylene bottle.
5.9.2 Standardization of the EDTA solution

5.9.2.1 Weigh, to the nearest 0,000 2 g, approximately 0,20 g (m ) of iron powder (5.1) to a 250 ml

beaker. Add 5 ml of hydrochloric acid (5.2) and 5 ml of nitric acid (5.3). Cover the beaker with a watch

glass and heat at 80 °C (hotplate) until completely dissolved. Add 2 ml of sulphuric acid solution (5.5)

and dilute to 100 ml with water.

5.9.2.2 While stirring the solution with a glass rod, add 15 ml of sodium hydroxide (5.7), 1 ml of

sulfosalicylic acid (5.10) and dilute to 150 ml with water. Wash the glass rod. Cover the beaker with

a watch glass and heat the solution at 210 °C (hotplate) to near boiling. Wash the watch glass and the

inner wall of the beaker.
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ISO 21826-1:2022(E)

5.9.2.3 Place the beaker on the titration stand (6.3). (The temperature of the solution is above 80 °C

at this point.) Place the combined glass pH electrode (6.4), stir the solution with a magnetic bar (6.6)

or plastic propeller stirrer/rod stirrer. Adjust the acidity of the solution to pH (1 ± 0,05) with sodium

hydroxide (5.7) by manual operation through a dropper.
There should be no temperature compensation at the time of pH adjustment.

5.9.2.4 Place the photometric sensor (6.5) and the dispensing tube tip into the beaker. The detection

window of the photometric sensor and the dispensing tube tip shall be immersed. Ensure there are no

bubbles between the light path in the detection window of the photometric sensor.

5.9.2.5 Titrate with the EDTA solution (5.9) using the potentiometer titration apparatus (6.2). The

colour of the solution changes to light yellow at the end point. The end point is detected automatically.

Record the volume of the end point as V .

The temperature of the solution shall be above 50 °C at the end of the titration with EDTA.

NOTE The photometric sensor is immersed into the solution after the solution is adjusted to pH 1 in order to

avoid the sensor being corroded by acids.
Typical titration curves are given in Annex B.
Titration parameters for reference are given in Annex C.

5.9.2.6 Carry out the analysis in quadruplicate and record the volume of the end point as V , V , V

1 2 3
and V .

5.9.2.7 The un-blank calibration concentration of the EDTA solution, C (n = 1, 2, 3, 4), expressed in

n(u)
mole per litre, is calculated to four decimal places using Formula (1):
C = (1)
n u
0,055847×V
where
n is 1, 2, 3, 4;
m is the mass, in grams, of the iron powder;
V is the volume at end point, in millilitres, of EDTA;
0,055 847 is the multiple of the atomic mass of iron.

If the range (C ‒ C ) of the four test results (C , C , C , C ) is equal to or less than

max min 1(u) 2(u) 3(u) 4(u)

0,000 2 mol/l, the arithmetic mean of the four test results shall be reported as the final quoted result,

C .
EDTA(u)

If the range of the four test results is greater than 0,000 2 mol/l, the median of the four test results shall

be reported as the final quoted result, C .
EDTA(u)
5.9.3 Blank test for the standardization of EDTA

5.9.3.1 In a 250 ml beaker, add 1,00 ml of iron standard solution (5.8), 5 ml of hydrochloric acid (5.2),

5 ml of nitric acid (5.3), 2 ml of sulphuric acid solution (5.5) and 100 ml of water. Follow the procedure

given in 5.9.2.2 to 5.9.2.5. Record the volume of the end point as V , V , V and V .

01 02 03 04
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ISO 21826-1:2022(E)

5.9.3.2 If the range of the four test results (V , V , V V ) is equal to or less than 0,02 ml, the

01 02 03, 04

arithmetic mean of the four test results shall be reported as the final quoted result, V .

If the range of the four test results is greater than 0,02 ml, the median of the four test results shall be

reported as the final quoted result, V .

5.9.3.3 The blank value of the titration, V , is calculated to two decimal places using Formula (2):

VV=−A (2)
0 B
where
V is the final quoted result;

A is the volume of EDTA (5.9), in millilitres, equivalent to 1,00 ml of the iron standard solution (5.8).

A is calculated to two decimal places using Formula (3):
0,1000
A=×10, 0 (3)
EDTA u
where
1,00 is the volume, in millilitres, of the iron standard solution;
0,100 0 is the concentration, in mole per litre, of the iron standard solution;

C is the un-blank calibration concentration (5.9.2.7), in mole per litre, of the EDTA solution.

EDTA(u)

NOTE In the absence of iron, there is no iron sulfosalicylate in the solution and the photometric titration

cannot be carried out. The addition of the iron solution is therefore necessary to promote an indicator response

in the blank solution and thus allow a suitable correction for the blank in terms of its equivalent in millilitres of

the EDTA standard solution.

The 1 ml one-mark pipette shall be previously calibrated by weighing the mass of water delivered and

converting to volume.
5.9.4 Calculation of the concentration of EDTA solution

The concentration of the EDTA solution, C (n = 1, 2, 3, 4), expressed in mole per litre, is calculated to

four decimal places using Formula (4):
C = (4)
0,055847×−VV
n 0
where
n is 1, 2, 3, 4;
m is the mass, in grams, of the iron powder;
V is the volume at end point, in millilitres, of EDTA;

V is the blank value as defined in 5.9.3.3, in millilitres, of the EDTA solution;

0,055 847 is the multiple of the atomic mass of iron.

If the range (C ‒ C ) of the four test results (C , C , C , C ) is equal to or less than 0,000 2 mol/l,

max min 01 02 03 04

the arithmetic mean of the four test results shall be reported as the final quoted result, C

EDTA.
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ISO 21826-1:2022(E)

If the range of the four test results is greater than 0,000 2 mol/l, the median of the four test results shall

be reported as the final quoted result, C .
EDTA
5.10 Sulfosalicylic acid, 100 g/l.
6 Apparatus

Use one-mark volumetric flasks that conform to the requirements of ISO 1042, Class A. Use single-

volume pipettes that conform to the requirements of ISO 648, Class A.
6.1 Analytical balance, designed for weighing to the nearest 0,1 mg.
6.2 Potentiometric titration apparatus.

An automatic potentiometric titrimeter capable of titrating to a fixed volume using either variable or

fixed titrant increments.
Use burettes that conform to the requirements of ISO 385, Class A.
6.3 Titration stand.
The accessories of the automatic potentiometric titrimeter should be used.
6.4 Combined glass pH electrode, with a measuring range of pH 0 to pH 14.

6.5 Photometric sensor, specifically designed for colour-indicated titrations in acidic solution. The

wavelength 520 nm shall be used. The photometric sensor shall be connected to the potentiometer by

means of a suitably screened cable. Before used, the photometric sensor output potential should be

optimized to 1 000 mV.
6.6 Magnetic stir bar, which should have a chemically inert surface.

6.7 Weighing spatula, of a non-magnetic material or demagnetized stainless steel.

6.8 Weighing bottle, of approximate volume 10 ml.
6.9 Modified polytetrafluoroethylene (PTFE-TFM) pressure vessel, of 100 ml.

6.10 Microwave digestion oven, equipped with temperature sensors, enabling direct temperature

control in a single reference vessel up to 250 °C. The feedback from the sensors and terminal controller

can provide accurate control and reproducibility of all digestion parameters such as microwave power,

time and temperature. Temperature sensors should be accurate to (210 ± 5) °C.

The temperature measurement system should be periodically calibrated at an elevated temperature.

Follow the microwave manufacturer’s instructions for the specific temperature sensor calibration

procedure.
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ISO 21826-1:2022(E)
7 Sampling and samples
7.1 Laboratory sample

For analysis, use a laboratory sample of 100 µm nominal top size, which has been taken and prepared

in accordance with ISO 3082. For ores with significant contents of combined water or oxidizable

compounds, use a particle size of less than 160 µm nominal top size.

NOTE See ISO 7764 for guidance on the significant contents of combined water and oxidizable compounds.

7.2 Preparation of test samples
7.2.1 General
Depending on the ore type, proceed in accordance with either 7.2.2 or 7.2.3.

7.2.2 Ores having significant contents of combined water or oxidizable compounds

Prepare an air-equilibrated test sample in accordance with ISO 2596 with the following types of ore:

a) processed ores containing metallic iron;

b) natural or processed ores in which the sulfur content is higher than 0,2 % mass fraction;

c) natural or processed ores in which the content of combined water is higher than 2,5 % mass

fraction.
7.2.3 Ores outside the scope of 7.2.2
Prepare a predried test sample as follows.

Thoroughly mix the laboratory sample and, taking multiple increments, extract a test sample in

such a manner that it is representative of the whole contents of the container. Dry the test sample at

(105 ± 2) °C as specified in ISO 7764.
8 Procedures
8.1 Number of determinations

Carry out the analysis at least in duplicate and independently on one prepared test sample (see 7.2).

NOTE The expression “independently” means that the second, and any subsequent, result is not affected by

the previous result(s).
8.2 Test portion

Taking several increments, weigh, to the nearest 0,000 2 g, approximately 0,25 g of the predried air

equilibrated test sample obtained in accordance with 7.2.2. Concurrently, determine the hydroscopic

moisture content in accordance with ISO 2596 and calculate the dried mass of the test portion (m).

Where the test sample is obtained in accordance with 7.2.3, transfer 0,25 g of the test portion to a

weighing bottle (6.8). Dry the open weighing bottle with the test portion and the lid for 2 h at (105 ± 2) °C.

Close the weighing bottle with the lid, transfer to a desiccator and cool to room temperature (about

20 min).

Weigh the bottle (with the lid on) to the nearest 0,000 2 g (m ). Transfer the test portion to the vessel

and then weigh the weighing bottle with the lid on (m ). The dried mass of the test portion (m) is the

difference between the two weighings, m and m .
1 2
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ISO 21826-1:2022(E)
8.3 Blank test

8.3.1 In each run, at least one blank test shall be carried out in parallel with the analysis of the ore

sample(s) under the same conditions.

8.3.2 Where the analysis is carried out on several samples at the same time, the blank value may be

represented by one test, provided that the procedure is the same and the reagents used are from the

same reagent bottles.
8.4 Determination
8.4.1 Decomposition of the test portion

Transfer the test portion (see 8.2) to the 100 ml PTFE-TFM vessel (6.9). Add 5 ml of hydrochloric acid

(5.2), 5 ml of nitric acid (5.3) and 0,5 ml of hydrofluoric acid solution (5.4).
For TFe < 40 %, add 1 ml of hydrofluoric acid solution (5.4).

If part of the sample stays on the inner wall of the vessel, wet it by adding acids drop by drop, then

gently swirl the solution to homogenize the sample with the acids. Visually look for a reaction. If a

reaction occurs, allow the reaction to subside completely before capping the vessel.

Close the vessel and introduce it into the rotor segment, then tighten by using a torque wrench. Insert

the segment into the microwave cavity and connect the temperature sensor. Set the digest temperature

to 210 °C and hold this temperature for 30 min. Run the microwave programme to completion.

Cool the rotor by air or by water until the temperature of the solution is below 50 °C. Open the vessels

carefully. Wash the caps and collect the solution to the 250 ml beakers. Add 2 ml of sulphuric acid

solution (5.5) to the vessels and transfer the solution of the vessels to the 250 ml beakers. Add 30 ml of

boric acid solution (5.6) and dilute to 100 ml with water.

The microwave oven should be operated according strictly to the supplier’s instructions.

To allow for a feedback process control, a reference vessel shall be prepared containing a chemistry

identical to any other sample vessel being processed during the same run.

For TFe < 40 %, if white precipitate (SiO ) in the solution is found, filtration should be applied to

remove the precipitate before adding 30 ml of boric acid solution (5.6). Transfer the solution and the

white precipitate on to the fast filter paper. Wash the beaker and the white precipitate with warm

hydrochloric acid (2 %) until the yellow colour of iron(III) chloride is no longer observed, and then

wash in warm water six to eight times. Collect the filtrate and washings in a 250 ml beaker.

8.4.2 Titration

Follow the procedure given in 5.9.2.2 to 5.9.2.5 and record the volume of the end point as V.

8.5 Procedure of blank test

8.5.1 Using 1,00 ml of the iron standard solution (5.8) instead of the test portion, add the same

amounts of all reagents in 8.4.1 and run the remaining steps specified in 8.4.1 and 8.4.2. Carry out the

analysis in duplicate and record the results as V and V .
S1 S2

8.5.2 If |V –V | is equal to or less than 0,02 ml, the arithmetic mean of V and V shall be reported

S1 S2 S1 S2
as the final quoted result, V .

If |V –V | is greater than 0,02 ml, the blank test shall be repeated and the median of the four test

S1 S2
results shall be reported as the final quoted result, V .
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ISO 21826-1:2022(E)

8.5.3 The blank test value of the titration (V ) is calculated to two decimal places using Formula (5):

0,1000
VV=−10, 0× (5)
sS0
EDTA
where
V is the volume at end point, in millilitres, of EDTA;
1,00 is the volume, in millilitres, of iron standard solution;
0,100 0 is the concentration, in mole per litre, of iron standard solution;
C is the concentration, in mole per litre, of EDTA solution.
EDTA
9 Expression of results
9.1 Calculation of total iron content

The total iron content, w , as a percentage by mass, is calculated to two decimal places using

Formula (6):
CV×−V ×0,055847
EDTA S
w = ×100 (6)
where
C is the concentration, in mole per litre, of EDTA solution;
EDTA

V is the end point volume for the test portion, in millilitres, of EDTA solution;

V is the blank test value as defined in 8.5.3, in millilitres, of EDTA solution;

m is the dried mass, in grams, of the test portion;
0,055 847 is the multiple of the atomic mass of iron.
9.2 General treatment of results
9.2.1 Repeatability and permissible tolerance
The precision of this analytical method is expressed by Formulae (7) to (10):
σ =0,084 (7)
σ =0,067 (8)
R =02, 3 (9)
P=02, 7 (10)
where
σ is the indepe
...

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