ISO/TS 20917:2023

Fertilizers, soil conditioners and beneficial substances — Determination of ammonium citrate, disodium-EDTA soluble phosphorus and potassium by ICP-OES in inorganic fertilizers

This document specifies an inductively coupled plasma-optical emission spectrometer (ICP-OES) method for the determination of the ammonium citrate, disodium-etheylenediamine tetraacetic acid (EDTA) soluble phosphorus and potassium content in fertilizer extract solutions. This document is applicable to all inorganic fertilizer products, whether compound or complex fertilizer products or blends thereof, and straight concentrated fertilizer products.

Engrais, amendements et substances bénéfiques – Détermination de la teneur en phosphore et potassium solubles dans le citrate d’ammonium et l’EDTA disodique par ICP-OES dans les engrais inorganiques

General Information

• Status: Published
• Publication Date: 26-Jul-2023
• ICS: 65.080 - Fertilizers
• Technical Committee: ISO/TC 134 - Fertilizers, soil conditioners and beneficial substances
• Drafting Committee: ISO/TC 134 - Fertilizers, soil conditioners and beneficial substances
• Current Stage: 6060 - International Standard published
• Start Date: 27-Jul-2023
• Due Date: 01-Aug-2022
• Completion Date: 27-Jul-2023

Standards Content (Sample)

TECHNICAL ISO/TS
SPECIFICATION 20917
First edition
2023-07
Fertilizers, soil conditioners
and beneficial substances —
Determination of ammonium citrate,
disodium-EDTA soluble phosphorus
and potassium by ICP-OES in inorganic
fertilizers
Engrais, amendements et substances bénéfiques – Détermination
de la teneur en phosphore et potassium solubles dans le citrate
d’ammonium et l’EDTA disodique par ICP-OES dans les engrais
inorganiques
Reference number
ISO/TS 20917:2023(E)
© ISO 2023

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ISO/TS 20917:2023(E)
COPYRIGHT PROTECTED DOCUMENT
© ISO 2023
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: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland
ii
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ISO/TS 20917:2023(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 1
5 Reagents . 2
6 Apparatus . 3
7 Sampling and sample preparation . 4
8 Procedure .4
8.1 Extraction of P and K . 4
8.2 Instrument conditions . 4
8.3 Preparation of standards . 5
8.3.1 Standards from reagent salts . 5
8.3.2 Standards from commercial stock solutions . 5
8.3.3 Calibration and wavelength splitting . 7
8.3.4 Analysis . 7
8.4 Quality control . 7
8.4.1 Reference material . 7
8.4.2 Duplicates . 7
9 Calculations and expression of results . 7
9.1 Calculations . 7
9.2 Precision . . . 8
9.2.1 General . 8
9.2.2 Repeatability, r . 9
9.2.3 Reproducibility, R . 9
10 Test report .10
Annex A (informative) Ring test background and results .11
Annex B (informative) Critical factors .24
Bibliography .26
iii
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ISO/TS 20917:2023(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 document 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).
ISO draws attention to the possibility that the implementation of this document may involve the use
of (a) patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed
patent rights in respect thereof. As of the date of publication of this document, ISO had not received
notice of (a) patent(s) which may be required to implement this document. However, implementers are
cautioned that this may not represent the latest information, which may be obtained from the patent
database available at www.iso.org/patents. ISO shall not be held responsible for identifying any or all
such patent rights.
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 134, Fertilizers, soil conditioners and
beneficial substances.
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
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ISO/TS 20917:2023(E)
Introduction
Inductively coupled plasma-optical emission spectrometer (ICP-OES) instrumentation is becoming
increasingly popular in fertilizer testing laboratories (see References [1], [2] and [3]). Because these
instruments are automated and can determine multiple elements simultaneously, they offer significant
productivity gains. Also, the method does not generate hazardous wastes. The ammonium citrate
disodium etheylenediamine tetraacetic acid (EDTA) organic solvent and samples containing high
phosphorus and/or potassium concentrations pose some analytical challenges for the instrumentation,
which must be addressed. Due to the increasing use of this technology, this document was created.
For organic fertilizers and for liquid fertilizers containing phosphorous acid (H PO or phosphite), an
3 3
alternative method that specifically measures PO is preferred since the ICP-OES measures all elemental
4
phosphorus, which can result in a high bias for these types of fertilizer materials (see Reference [1]).
Due to the inability of the ICP-OES to differentiate between phosphorus species, under normal
operating conditions, this method is primarily intended for fertilizer materials in which the source(s)
of phosphorus is known. While this document can be used with minimal bias, the repeatability and
reproducibility is not consistent with other standards routinely used for inspection at this time, such as
[4] [5] [6]
EN 15959 , EN 15477 or ISO 22018 .
As a result, this document is intended for screening purposes, where a large number of fertilizer
phosphorus and potassium results must be obtained as efficiently as possible.
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TECHNICAL SPECIFICATION ISO/TS 20917:2023(E)
Fertilizers, soil conditioners and beneficial substances —
Determination of ammonium citrate, disodium-EDTA
soluble phosphorus and potassium by ICP-OES in inorganic
fertilizers
1 Scope
This document specifies an inductively coupled plasma-optical emission spectrometer (ICP-OES)
method for the determination of the ammonium citrate, disodium-etheylenediamine tetraacetic acid
(EDTA) soluble phosphorus and potassium content in fertilizer extract solutions.
This document is applicable to all inorganic fertilizer products, whether compound or complex fertilizer
products or blends thereof, and straight concentrated fertilizer products.
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 8157, Fertilizers, soil conditioners and beneficial substances — Vocabulary
ISO 14820-1, Fertilizers and liming materials — Sampling and sample preparation — Part 1: Sampling
ISO 14820-2, Fertilizers and liming materials — Sampling and sample preparation — Part 2: Sample
preparation
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 8157 apply.
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/
4 Principle
Phosphorus and potassium contained in inorganic fertilizers that is soluble in a heated, dilute
ammonium citrate and disodium-EDTA solvent is determined. The neutral ammonium citrate is the
primary solvent for phosphate, while the EDTA is mainly included to chelate calcium, magnesium and
other soluble metallic cations to minimize their interaction with phosphate and eliminate the water-
wash step (see Reference [7]). Also, since this solvent is a relatively weak solution, it extracts primarily
the highly soluble potassium compounds and has been used for fertilizer potassium determination
since 1993. It compares very favourably with other soluble potash solvents such as ammonium oxalate.
The user of this document is not obligated to determine both phosphorus and potassium; however, one
of the main advantages of this approach is to measure both simultaneously in a single extract in a more
productive, cost-effective way.
The resulting extract solution is tested by ICP-OES to quantify the levels of phosphorus and potassium,
which can be converted to their equivalent P O and K O concentrations. The extract solution is
2 5 2
1
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ISO/TS 20917:2023(E)
nebulized to produce a fine aerosol, which is introduced to a plasma or high energy source to ionize
or excite electrons to higher energy orbitals. As these electrons return to ground state, they emit
characteristic ultraviolet or visible wavelengths that are unique to each element. The intensity of the
wavelength emission is proportionate to the concentration, which can be quantified by various detector
types including a photomultiplier tube (PT), a charge injection device (CID) or a charge coupled device
(CCD).
5 Reagents
5.1 Ammonium citrate, dibasic, (NH ) HC H O , with a molecular weight (MW) of 226,19 and a
4 2 6 5 7
purity greater than 98 % or 980,0 g/kg.
5.2 EDTA, disodium salt, dihydrate, C H N Na O ·2H O, with a MW of 372,24 and a purity greater
10 14 2 2 8 2
than 98 % or 980,0 g/kg.
5.3 Ammonium hydroxide, NH OH, with a MW of 35,05 and a purity equal to 28,0 % to 30,0 % as
4
NH or ρ = 0,91 g/ml.
3
5.4 Nitric acid, HNO , 67 % to 70 %, with a ρ = 1,40 g/ml.
3
5.5 Potassium dihydrogen phosphate, KH PO , with a MW of 136,09, with a purity greater than
2 4
1)
99 % or 990,0 g/kg .
5.6 Potassium chloride, KCl, with a MW of 74,55 and a purity greater than 99 % or 990 g/kg.
2)
5.7 Potassium nitrate, KNO , with a MW of 101,10 with a purity greater than 99 % or 990,0 g/kg .
3
3)
5.8 Octyl phenol ethoxylate .
5.9 Beryllium stock solution, of 10 000 μg/ml.
5.10 Scandium stock solution, of 10 000 μg/ml.
5.11 Cesium chloride, CsCl, MW of 168,36, with a purity greater than 99,99 % or 999,99 g/kg.
5.12 Lithium nitrate, LiNO , MW of 68,95, with a purity greater than 99 % or 990,0 g/kg.
3
5.13 Citrate-EDTA extraction solution (0,11 M ammonium citrate and 0,033 M disodium-EDTA).
Weigh and transfer 25 g disodium-EDTA (5.2) and 50 g dibasic ammonium citrate (5.1) to a 2 l volumetric
flask containing approximately 500 ml of deionized (or equivalent) water. Adjust the pH to near neutral
by adding 30 ml of ammonium hydroxide and water (1:1, volume fraction) solution in a fume hood. Adjust
the final pH to 7,00 (±0,02) using a pH electrode and meter while adding the ammonium hydroxide and
water (1:1, volume fraction) solution drop-by-drop and stirring. After obtaining a stable pH of 7,00 ±
1) SRM 194a is the trade name of a product supplied by National Institute of Standards and Technology (NIST).
This information is given for the convenience of users of this document and does not constitute an endorsement by
ISO of the product named. Equivalent products may be used if they can be shown to lead to the same results.
2) SRM 193 is the trade name of a product supplied by NIST. This information is given for the convenience of users
of this document and does not constitute an endorsement by ISO of the product named. Equivalent products may be
used if they can be shown to lead to the same results.
3) Triton X-100 is the trade name of a product supplied by Sigma-Aldrich. This information is given for the
convenience of users of this document and does not constitute an endorsement by ISO of the product named.
Equivalent products may be used if they can be shown to lead to the same results.
2
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ISO/TS 20917:2023(E)
0,02, bring the flask to volume with deionized water (or equivalent) and mix. Larger volumes of this
solution can be prepared; however, it is susceptible to microbial degradation resulting in a shelf life of
no more than three weeks, if stored in a dark location.
5.14 Octyl phenol ethoxylate, 0,5 % or 5 ml/l.
Add 1 ml of octyl phenol ethoxylate (5.8) to a 200 ml volumetric flask and dilute to volume with
deionized water.
5.15 Internal standard/ionization buffer (10 µg/ml Sc in 0,018 M CsCl and 4 % nitric acid).
Add 1,00 ml of 10 000 μg/ml scandium stock standard, 3 g cesium chloride, 40 ml nitric acid, and 1 ml
of 0,5 % octyl phenol ethoxylate (5.14) to a 1 l volumetric flask containing approximately 500 ml of
deionized (or equivalent) water. Bring flask to volume with deionized (or equivalent) water and mix.
If beryllium is used as an internal standard, add 4,00 ml of 10 000 μg/ml beryllium stock standard to
obtain a concentration of 40 μg/ml beryllium.
Alternatively, lithium from LiNO can be substituted for cesium as an ionization buffer at 7 g/l LiNO .
3 3
5.16 Nitric acid rinse solution, 1 %.
Add 10 ml of nitric acid (5.4) to approximately 500 ml of deionized or equivalent water contained in a
1 l volumetric flask and bring to volume.
5.17 2 500 µg/ml phosphorus from phosphate (PO ).
4
Add 2,746 1 g of potassium dihydrogen phosphate (5.5) to a 250 ml volumetric flask, then bring to
volume with deionized (or equivalent) water. This standard also contains 3 156,5 μg/ml K; also, this
standard can take several hours to solubilize and stabilize. This standard should be prepared fresh
each time. Alternatively, a commercial custom standard from a phosphate source prepared in a water
matrix and preserved with a biocide is acceptable.
The weight added should be adjusted for the purity of the reagent used.
NOTE A commercial stock standard preserved in acid is not acceptable as the acid changes the matrix of the
pH neutral ammonium citrate EDTA solution and produces erroneous results.
5.18 7 500 µg/ml potassium from potassium chloride.
Add 1,430 0 g of potassium chloride (5.6) to a 100 ml volumetric flask, then bring to volume with
deionized (or equivalent) water. Alternatively, 1,939 4 g of potassium nitrate (5.7) can be used rather
than potassium chloride. This standard should be prepared fresh each time. Alternatively, a commercial
custom standard from a potassium source prepared in a water matrix and preserved with a biocide is
acceptable.
The weight added should be adjusted for the purity of the reagent used.
NOTE A commercial stock standard preserved in acid is not acceptable as the acid changes the matrix of the
pH neutral ammonium citrate EDTA solution and produces erroneous results.
6 Apparatus
6.1 Analytical balance, capable of weighing to 0,1 mg.
6.2 pH meter, with a readability to 0,01.
6.3 pH combination electrode.
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ISO/TS 20917:2023(E)
6.4 Constant temperature water bath, capable of maintaining a temperature of 65 °C ± 2 °C.
6.5 Shaking water bath, capable of maintaining a temperature of 65 °C ± 2 °C and set to approximately
200 reciprocations/min.
A water bath is preferred, but alternatively an enclosed heated air shaker is also acceptable; however,
ensure the added citrate-EDTA solution (5.13) is at 65 °C ± 2 °C before starting.
6.6 ICP-OES instrument.
A radial view instrument is preferred for potassium, while phosphorus can be determined in either
radial or axial view.
6.7 Rotating riffle splitter, or comparable.
6.8 Grinding mill, capable of grinding fertilizer material to pass a 0,50 mm screen or sieve.
7 Sampling and sample preparation
7.1 Collect a field sample using a recognized sampling procedure such as that given in ISO 14820-1 or
another comparable one (see References [8] and [9]).
7.2 Prepare solid and liquid fertilizer materials using a recognized sample preparation procedure
such as that given in ISO 14820-2 or another comparable one.
8 Procedure
8.1 Extraction of P and K
For samples containing less than 45 % P O or K O, weigh 0,5 g ± 0,01 g of material and completely
2 5 2
transfer to a 250 ml class A volumetric flask. For samples containing 45 % or more P O or K O, weigh
2 5 2
0,45 g ± 0,01 g of ground fertilizer material and completely transfer to a 250 ml class A volumetric flask.
Dispense 100 ml of 65 °C ± 2 °C preheated (6.4) ammonium citrate, disodium-EDTA extraction solution
(5.13) into each flask and insert a rubber stopper. Shake test solutions in a 65 °C ± 2 °C preheated water
bath set to approximately 200 reciprocations per minute for exactly 60 min. Then, remove from the
water bath; allow to cool to room temperature; bring to volume with deionized (or equivalent) water;
stopper and mix. Filter any test solution containing suspended debris using P and K free filters. Some
slowly soluble phosphate compounds can continue to solubilize over time resulting in a high bias, so
test solutions should be run as soon as possible, but no more than 24 h after extraction.
8.2 Instrument conditions
Optimal instrument conditions identified during robustness testing are listed in Table 1. ICP-OES’ differ
in their design and options, so adjustment to the conditions listed in Table 1 can be necessary; however,
any adjustments to these conditions must be performance-based and validated. Special attention
should be paid to the recovery of P in fertilizer concentrates such as concentrated superphosphate,
diammonium phosphate (46 % of P O ), and monoammonium phosphate (50 % or 52 % of P O ), since
2 5 2 5
these materials require optimal instrument performance.
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ISO/TS 20917:2023(E)
a
Table 1 — Recommended ICP-OES conditions
Factor Setting
Power, kW 1,4
Plasma flow, l/min 18,0
Auxiliary flow, l/min 2,25
Nebulizer pressure, l/min 0,7
Nebulizer Concentric type
Spray chamber Double path cyclonic
Sample pump tube black/black
Buffer/internal standard pump tube gray/gray
CsCl ionic buffer concentration, M 0,018
Buffer matrix 4 % nitric acid
Exposure length, s 10
Number of exposures 2
Rinse time, s 35
Total analysis time, min 2
a
Other instrument conditions that produce equivalent test results are
an option provided the test conditions are validated.
8.3 Preparation of standards
8.3.1 Standards from reagent salts
Calibration standards can be made from potassium dihydrogen phosphate (5.17) and potassium
chloride (5.18). Several calibration standards are required for the following reasons:
a) multiple ICP-OES wavelengths are utilized to expand the dynamic calibration range,
b) some wavelengths are split into multiple calibration segments, and
c) a minimum of 5 points per curve is recommended.
The standards used and their P and K concentrations expressed as μg/ml and as their percent oxide
forms are listed in Table 2. These standards are susceptible to microbial degradation and have a shelf
life of no more than three weeks, if stored in the dark.
8.3.2 Standards from commercial stock solutions
Alternatively, custom stock standards can also be purchased but must be in a water matrix and
preserved with a biocide as standards preserved in acid will change the pH of the matrix and produce
erroneous results.
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ISO/TS 20917:2023(E)
6
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Table 2 — ICP P and K calibration standards
Standard ID Volume Citrate-EDTA Volume of P Volume of K P P O P O solution P O ferti- K K O K O solution K O ferti-
2 5 2 5 2 5 2 2 2
ml ml standard standard   lizer µg/ml lizer
a b e e
ml ml µg/ml µg/ml % % µg/ml % %
Blank 250 100 0 0 0 0 0,000 00 0 0 0 0,000 00 0
d c
1 250 100 10 of Std 7 NA 12 27,5 0,002 75 1,4 15,2 18,3 0,001 83 0,9
d c
2 250 100 20 of Std 7 NA 24 55,0 0,005 50 2,7 30,3 36,5 0,003 65 1,8
c
3 250 100 5 NA 50 114,6 0,011 46 5,7 63,1 76,0 0,007 60 3,8
c
4 250 100 10 NA 100 229,1 0,022 91 11,5 126,3 152,1 0,015 21 7,6
c
5 250 100 15 NA 150 343,7 0,034 37 17,2 189,4 228,1 0,022 81 11,4
c
6 250 100 22 (15+7) NA 220 504,1 0,050 41 25,2 277,8 334,6 0,033 46 16,7
c
7 250 100 30 NA 300 687,4 0,068 74 34,4 378,8 456,3 0,045 63 22,8
c
8 250 100 40 NA 400 916,5 0,091 65 45,8 505 608,4 0,060 84 30,4
c c c c c
9 250 100 NA 20 NA NA NA NA 600 722,7 0,072 27 36,1
c c c c c
10 250 100 NA 25 NA NA NA NA 750 903,4 0,090 34 45,2
c c c c c
11 250 100 NA 30 NA NA NA NA 900 1 084,1 0,108 41 54,2
a
P standard concentration is equal to 2 500 µg/ml P as PO or 2,746 1 g of potassium dihydrogen phosphate KH PO is brought to 250 ml, which also contains 3 156,5 µg/ml K.
4 2 4
b
K standard concentration is equal to 7 500 µg/ml K from KCl or 1,430 0 g of potassium chloride or 1,939 4 g of potassium nitrate is brought to 100 ml.
c
NA : not applicable.
d
Serial dilution from another standard; i.e. 10 of Std 7 signifies: pipet 10 ml from standard 7.
e
Equivalent final result when based upon a 0,5 g sample brought to a final volume of 250 ml.
NOTE 1 A 52 % P O fertilizer material weighed at 0,45 g rather than 0,5 g results in a concentration of 46,8 % P O , which is a 2 % extrapolation from the top standard.
2 5 2 5
NOTE 2 A 62 % K O fertilizer material weighed at 0,45 g rather than 0,5 g results in a concentration of 55,8 % K O, which is a 3 % extrapolation from the top standard.
2 2

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ISO/TS 20917:2023(E)
8.3.3 Calibration and wavelength splitting
As the higher points on the calibration curve have a much greater influence, this can adversely impact
the accuracy of the lower points. It is therefore recommended that the broad calibration range be split
into two segments, which are listed in Table 3. By splitting the same wavelength into two separate
curves (see Reference [1]) or a low and high concentration range and/or by using two separate
wavelengths, the quality of the lower P and K concentrations will improve.
Table 3 — Calibration criteria for ammonium citrate, disodium-EDTA soluble P and K by
a
ICP-OES
Element Wavelength Preferred view Calibration Standards used Curve fit Spectral
b
nm range (see Table 1) deconvolution
μg /ml
P 213,618 (1) Radial or axial 0 to 100 Blank, 1, 2, 3, 4 Linear Cu 213,598
P 213,618 (2) Radial or axial 100 to 400 4, 5, 6, 7, 8 Linear Cu 213,598
P 214,914 (1) Radial or axial 0 to 100 Blank, 1, 2, 3, 4 Linear Cu 214,898
P 214,914 (2) Radial or axial 100 to 400 4, 5, 6, 7, 8 Linear Cu 214,898
c
P 177,434 (1) Axial 0 to 100 Blank, 1, 2, 3, 4 Linear None
c
P 177,434 (2) Axial 100 to 400 4, 5, 6, 7, 8 Linear None
c
P 178,222 (1) Axial 0 to 100 Blank, 1, 2, 3, 4 Linear None
c
P 178,222 (2) Axial 100 to 400 4, 5, 6, 7, 8 Linear None
K 766,485 (1) Radial 0 to 126 Blank, 1, 2, 3, 4 Linear None
K 766,485 (2) Radial 126 to 505 4, 5, 6, 7, 8 Linear None
K 766,485 (3) Radial 379 to 900 7, 8, 9, 10, 11 Linear None
a
Other instrument conditions that produce equivalent test results are an option provided the test conditions are validated.
b
(1), (2) and (3) are used to distinguish between the same wavelength used multiple times to cover separate concentration ranges.
c
Optional, less intense wavelengths without any spectral interference that can be considered.
8.3.4 Analysis
A test portion is prepared as described in Clause 7 and extracted as described in 8.1 and presented to
the ICP-OES configured according to Tables 1 and 3 using calibration standards outlined in Table 2.
8.4 Quality control
8.4.1 Reference material
Within each analytical batch of samples, inclusion of one or more certified or consensus fertilizer
materials for quality assurance purposes is recommended, especially for the fertilizer concentrates (i.e.
P O and K O greater than 40 %).
2 5 2
8.4.2 Duplicates
Duplicate pours from each extract solution are recommen
...

ISO/TC 134
Date: 2021-10-20
ISO TSDTS 20917.3:2022(E)
ISO/TC 134/WG 1
Secretariat: INSO
Fertilizers, soil conditioners and beneficial substances -— Determination of ammonium
citrate, disodium-EDTA soluble phosphorus and potassium by ICP-OES in inorganic
fertilizers
First edition
Date: 2022-01-20

---------------------- Page: 1 ----------------------
© ISO 2022
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: [phone]
Email: [e-mail]
Website: www.iso.org
Technical Specification

i

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Published in Switzerland
ii © ISO 2022 – All rights reserved

---------------------- Page: 3 ----------------------
Contents
Forward . iii
Introduction . iv
1 Scope ….1
2 Normative eferences ….1
3 Terms and definitions ………………………………………….…….2
4 Principle .2
5 Reagents ……………….2
6 Apparatus . 4
7 Sampling nd Preparation .4
8 Procedure . 5
8.1 Extraction of P and K ………………………………………………………….5
8.2 Instrument Conditions ………………………………………………………….…5
8.3 Preparation of Standards .6
8.3.1 Standards from reagent salts .6
8.3.2 Standards from commercial stock solutions .6
8.3.3 Calibration and wavelength splitting .7
8.3.4 Analysis . 8
8.4 Reference Material . 8
8.5 Duplicates .8
9 Calculation and expression of results . 8
9.1 Calculations .8
9.2 Precision .9
9.2.1 Repeatability, r .9
9.2.2 Reproducibility, R .9
10 Special .11
11 Test Report ………………………………………………………………………….11
Annex A (informative) Ring test results .12
Annex B (informative) Critical factors . 25
Bibliography ………………………………………………………………….……….….27
iii

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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 proposed technical specificationdocument 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 proposed technical specificationdocument 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 proposed technical
specificationdocument 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 proposed technical specificationdocument is information given for
the convenience of users and does not constitute an endorsement.
For an explanation onof 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.htmlthe following URL: .
This proposed technical specificationdocument was prepared by Technical Committee ISO/TC
134, Fertilizers, soil conditioners and beneficial substances.
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 2022 – All rights reserved

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Introduction
Inductively coupled plasma – -optical emission spectrometer (ICP-OES) instrumentation is
becoming increasingly popular in fertilizer testing laboratories (1,4,5). see References [1], [2]
and [3]). Because these instruments are automated and can determine multiple elements
simultaneously, they offer significant productivity gains. Also, the method does not generate
hazardous wastes. The ammonium citrate disodium etheylenediamine tetraacetic acid (EDTA)
organic solvent and samples containing high phosphorus and/or potassium concentrations
pose some analytical challenges for the instrumentation, which must be addressed. Due to the
increasing use of this technology, a Technical Specificationthis document was created at this time
with the goal of generating a Final Draft International Standard.

v

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ISO/DTS 20917:2022(E)
Fertilizers, soil conditioners and beneficial substances – Determination of
Ammonium Citrate, Disodium-EDTA Soluble Phosphorus and Potassium
by ICP-OES in Inorganic Fertilizers

1 Scope

This Technical Specification specifies an ICP-OES method for the determination of the ammonium
citrate, disodium-EDTA soluble phosphorus and potassium content in fertilizer extract solutions. This
specification is suitable for all inorganic fertilizer products, whether compound or complex fertilizer
products or blends thereof, and straight concentrated fertilizer products. For organic fertilizers and
for liquid fertilizers containing phosphorous acid (H PO or phosphite), an alternative method
3 3
that specifically measures PO is preferred since the ICP-OES measures all elemental
4
phosphorus, which can result in a high bias for these types of fertilizer materials (1). see
Reference [1]).
Due to the inability of the ICP-OES to differentiate between phosphorus species, under normal
operating conditions, this methmethod is primarily intended for fertilizer materials in which the
source(s) of phosphorus is known. While the trueness of this technical specification is
gooddocument can be used with minimal bias, the repeatability and reproducibility is not
consistent with other standards routinely used for governative inspection at this time, such as
[4] [5] [6]
EN15959, EN15477EN 15959 , EN 15477 or ISO 22018. .
As a result, this technical specificationdocument is intended for screening purposes, where a
large number of fertilizer phosphorus and potassium results must be obtained as efficiently as
possible. Where governative inspection is critical, such as where fertilizer tolerances or desired
statistical certainty criteria have been exceeded by this technical specification, then standards
such as EN15959, EN15477 or ISO 22018 can be used for confirmation.

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ISO/DTS 20917:2022(E)
Fertilizers, soil conditioners and beneficial substances
— Determination of ammonium citrate, disodium-
EDTA soluble phosphorus and potassium by ICP-OES
in inorganic fertilizers
1 Scope
This document specifies an inductively coupled plasma-optical emission spectrometer (ICP-
OES) method for the determination of the ammonium citrate, disodium-etheylenediamine
tetraacetic acid (EDTA) soluble phosphorus and potassium content in fertilizer extract
solutions.
This document is applicable to all inorganic fertilizer products, whether compound or complex
fertilizer products or blends thereof, and straight concentrated fertilizer products.
12 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 proposed technical specification. document. For dated
references, only the edition cited applies. For undated references, the latest edition of the
referenced document (including any amendments) applies.

EN 15959:2011ISO 8157, Fertilizers – Determination of extracted phosphorus., soil conditioners and
beneficial substances — Vocabulary
EN 15477:2009, Fertilizers – Determination of the water-soluble potassium content.
ISO 14820--1 2016, Fertilizers and liming materials –— Sampling and sample preparation –—
Part 1: Sampling
ISO 14820--2 2016, Fertilizers and liming materials –— Sampling and sample preparation –—
Part 2: Sample preparation
ISO 22018:2021 Fertilizers, soil conditioners and beneficial substances – Determination of
EDTA soluble phosphorus content in inorganic fertilizers.
ISO 13528:2015, Statistical methods for use in proficiency testing by interlaboratory comparison.
43 Terms and definitions
For the purposes of this technical specificationdocument, the terms and definitions given in ISO
8157 apply.
ISO and IEC maintain terminologicalterminology databases for use in standardization at the
following addresses:
© ISO 2022 – All rights reserved 1

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ISO/DTS 20917:2022(E)
IEC Electropedia: available at — ISO Online browsing platform: available at
https://www.iso.org/obp
— IEC Electropedia: available at https://www.electropedia.org/
54 Principle
Phosphorus and potassium contained in inorganic fertilizers that is soluble in a heated, dilute
ammonium citrate and disodium-EDTA solvent is determined. The neutral ammonium citrate is
the primary solvent for phosphate, while the EDTA is mainly included to chelate calcium,
magnesium and other soluble metallic cations to minimize their interaction with phosphate and
eliminate the water-wash step (6). see Reference [7]). Also, since this solvent is a relatively
weak solution, it extracts primarily the highly soluble potassium compounds and has been used
for fertilizer potassium determination since 1993. It compares very favorablyfavourably with
other soluble potash solvents such as ammonium oxalate. The user of this technical
standarddocument is not obligated to determine both phosphorus and potassium; however, one
of the main advantages of this approach is to measure both simultaneously in a single extract in
a more productive, cost-effective way.
The resulting extract solution is tested by ICP-OES to quantify the levels of phosphorus and
potassium, which can be converted to their equivalent P O and K O concentrations. The
2 5 2
extract solution is nebulized to produce a fine aerosol, which is introduced to a plasma or high
energy source to ionize or excite electrons to higher energy orbitals. As these electrons return
to ground state, they emit characteristic ultraviolet or visible wavelengths that are unique to
each element. The intensity of the wavelength emission is proportionate to the concentration,
which can be quantified by various detector types including a photomultiplier tube (PT), a
charge injection device (CID) or a charge coupled device (CCD).
65 Reagents
5.1 Ammonium citrate, dibasic, (NH ) HC H O , Formula Weight (FW) with a molecular
4 2 6 5 7
weight (MW) of 226,19, and a purity >greater than 98 % or 980,0 g/kg.
5.2 EDTA, disodium salt, dehydrate, C H N Na O ·2H O, FWwith a MW of 372,24, and a
10 14 2 2 8 2
purity >greater than 98 % or 980,0 g/kg.
5.3 Ammonium hydroxide, NH OH; FW , with a MW of 35,05; and a purity =equal to 28,0-
4
30,0 % as NH or ρ = = 0,91 g/ml.
3
5.4 Nitric acid, HNO , 67-70%, %, with a ρ = 1,40 g/ml.
3
5.5 Potassium dihydrogen phosphate, KH PO , FWwith a MW of 136.,09, certified at 26,93%
2 4
1
P or 269,3 with a purity greater than 99 % or 990,0 g/kg ; National Institute of Standards and
Technology (NIST) SRM 194a (Gaithersburg, MD), or comparable.
5.6 Potassium chloride, KCl, FWwith a MW of 74,55; and a purity >greater than 99 % or
990 g/kg.

1 SRM 194a is the trade name of a product supplied by National Institute of Standards and Technology (NIST). This information is given for the convenience of

users of this document and does not constitute an endorsement by ISO of the product named. Equivalent products may be used if they can be shown to lead to the
same results.

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ISO/DTS 20917:2022(E)
5.7 Potassium nitrate, KNO , FWwith a MW of 101.,10; NIST SRM193 with a purity greater
3
2
than 99 % or comparable, certified at 38,66% K or 386,6 990,0 g/kg .
3
Triton X-100, 5.8 Octyl phenol ethoxylate .
5.9 Beryllium, Be, stock solution – 10000 of 10 000 μg/ml;, commercially available.
5.10 Scandium, Sc, stock solution – 10000 of 10 000 μg/ml;, commercially available.
5.11 Cesium chloride, CsCl, FWMW of 168,36, with a purity >greater than 99.,99 % or
999,99 g/kg.
5.12 Lithium nitrate, LiNO , FWMW of 68,95, with a purity >greater than 99 % or 990,0 g/kg.
3
5.13 Citrate-EDTA extraction solution (0,11 M ammonium citrate and 0,033 M disodium-
EDTA).
Weigh and transfer 25 g disodium-EDTA and 50 g dibasic ammonium citrate to a 2 l volumetric
flask containing approximately 1500 500 ml of deionized (or equivalent) water. Adjust the pH
to near neutral by adding 30 ml of (1 + 1, v/v) ammonium hydroxide +and water (1:1, volume
fraction) solution in a fume hood. Adjust the final pH to 7,00 (±0,02) using a pH electrode and
meter while adding the ammonium hydroxide +and water (1 + :1, v/vvolume fraction) solution
drop-by-drop and stirring. After obtaining a stable pH of 7,00 (±± 0,02),, bring the flask to
volume with deionized water (or equivalent) and mix. Larger volumes of this solution can be
prepared; however, it is susceptible to microbial degradation resulting in a shelf life of no more
than 3three weeks, if stored in a dark location.
5.14 0,5 % or 5 mL/L Triton-X – ml/l octyl phenol ethoxylate.
Add 1 mL Triton X-100 ml of octyl phenol ethoxylate (5.8 above) to a 200 ml volumetric flask
and dilute to volume with deionized water.
5.15 Internal standard/ionization buffer (10 µg/ml Sc in 0,018 M CsCl and 4 % nitric acid).
– Add 1 mL 10000 ml 10 000 μg/ml Sc stock standard, 3 g cesium chloride, 40 ml nitric acid,
and 1 ml 0,5% Triton-X % octyl phenol ethoxylate to a 1 l volumetric flask containing
approximately 500 ml of deionized (or equivalent) water. Bring flask to volume with deionized
(or equivalent) water and mix. If Be is used as an internal standard, add 4 ml of 10000
10 000 μg/ml Be stock standard to obtain a concentration of 40 μg/ml Be.
5.15.1 Alternatively, Li from LiNO maycan be substituted for Cs as an ionization buffer at 7 g
3
LiNO3 per liter. litre.
2% Nitric Acid Rinse Solution – 5.16 4 % nitric acid rinse solution.
Add 20 40 ml of nitric acid (5.4) to approximately 500 ml of deionized or equivalent water
contained in a 1 l volumetric flask and bring to volume.

2 SRM 193 is the trade name of a product supplied by NIST. This information is given for the convenience of users of this document and does not constitute an

endorsement by ISO of the product named. Equivalent products may be used if they can be shown to lead to the same results.

3 Triton X-100 is the trade name of a product supplied by Sigma-Aldrich. This information is given for the convenience of users of this document and does not

constitute an endorsement by ISO of the product named. Equivalent products may be used if they can be shown to lead to the same results.


© ISO 2022 – All rights reserved 3

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ISO/DTS 20917:2022(E)
2500 5.17 2 500 µg/ml phosphorus from phosphate (PO4) – ).
Add 2,7461 746 1 g of potassium dihydrogen phosphate (5.5 above) to a 250 ml volumetric
flask, then bring to volume with deionized (or equivalent) water. This standard also contains
31563 156,5 μg/ml K; also, this standard maycan take several hours to solubilize and stabilize.
This standard should be prepared fresh each time. Alternatively, a commercial custom standard
from a phosphate source prepared in a water matrix and preserved with a biocide is acceptable.
NOTE : A commercial stock standard preserved in acid is not acceptable as the acid will
changechanges the matrix of the pH neutral ammonium citrate EDTA solution and produceproduces
erroneous results.
7500 5.18 7 500 µg/ml potassium from potassium chloride – .
Add 1,4300 430 0 g of potassium chloride (5.6 above) to a 100 ml volumetric flask, then bring to
volume with deionized (or equivalent) water. Alternatively, 1,9394 939 4 g of potassium nitrate
(5.7 above) can be used rather than potassium chloride. This standard should be prepared fresh
each time. Alternatively, a commercial custom standard from a potassium source prepared in a
water matrix and preserved with a biocide is acceptable.
NOTE : A commercial stock standard preserved in acid is not acceptable as the acid will
changechanges the matrix of the pH neutral ammonium citrate EDTA solution and produceproduces
erroneous results.
76 Apparatus
6.1 Analytical balance, with a readability to 0,1 mg.
6.2 pH meter, with a readability to pH 7,00.
6.3 pH combination electrode.
6.4 Constant temperature water bath, capable of maintaining temperaturesa temperature of
65 ± 2°C ± 2 °C.
6.5 Heated shaking water bath, capable of maintaining bath temperaturesa temperature of
65 ± 2°C, ± 2 °C and set to approximately 200 reciprocations/min. NA
A water bath is preferred, but alternatively an enclosed heated air shaker is also acceptable;
however, ensure the added citrate-EDTA solution (5.13) is at 65 ± °C ± 2 °C before starting.
6.6 ICP-OES instrument – .
A radial view instrument is preferred for potassium, while phosphorus can be determined in
either radial or axial view.
6.7 Rotating riffle splitter, or comparable.
6.8 Grinding mill –, capable of grinding fertilizer material to pass a 0,50 mm screen or sieve.
87 Sampling and sample preparation
7.1 Collect a field sample using a recognized sampling procedure (such as ISO 14820-1; or
comparable (see References [8] and [9]).
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ISO/DTS 20917:2022(E)
7.1 7.2 ,3).
Prepare solid and liquid fertilizer materials using a recognized sample preparation procedure
such as ISO 14820-12 or comparable.
98 Procedure
9.18.1 Extraction of P and K
For samples containing less than 45 % P2O5 or K2O, weigh 0,5 ± g ± 0,01 g of material and
completely transfer to a 250 ml class A volumetric flask. For samples containing 45 % or more
P O or K O, weigh 0,45 ± ± 0,01 g of ground fertilizer material and completely transfer to a
2 5 2
250 mL class A volumetric flask. Dispense 100 ml of 65 ± °C ± 2 °C preheated ammonium
citrate, disodium-EDTA extraction solution (5.13 above) into each flask and insert a rubber
stopper. Shake test solutions in a 65 ± °C ± 2 °C preheated water bath set to approximately
200 reciprocations per minute for exactly 60 min; then. Then, remove from the water bath,;
allow to cool to room temperature,; bring to volume with deionized (or equivalent) water,;
stopper, and mix. Filter any test solution containing suspended debris using P and K free filters.
Some slowly soluble phosphate compounds maycan continue to solubilize over time resulting in
a high bias, so test solutions should be run as soon as possible, but no more than 24 hoursh after
extraction.

NOTE: If a homogeneous ground laboratory sample is not attainable, the weights and
volumes listed should be at least doubled to improve representation.

9.58.2 Instrument conditions
Optimal instrument conditions identified during robustness testing are listed in Table 1. ICP-
OES’ differ in their design and options, so adjustment to the conditions listed in Table 1 maycan
be necessary; however, any adjustments to these conditions must be performance-based and
validated. Special attention should be paid to the recovery of P in fertilizer concentrates such as
(40% TSP
MAP + S concentrated superphosphate, diammonium phosphate (46 % of P O ), and
2 5
DAP (46% P O ), and MAP %
2 5 monoammonium phosphate (50 % or 52 % of P O ), since these
2 5
.
materials require optimal instrument performance

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ISO/DTS 20917:2022(E)
a
a
Table 1 – — Recommended ICP-OES conditions conditions
Factor Setting
Power, kW 1,4
Plasma flow, l/min 18,0
Auxiliary flow, l/min 2,25
Nebulizer pressure, l/min 0,7
Nebulizer Concentric type
Spray chamber Double path cyclonic
Sample pump tube black/black
Buffer/internal standard pump tube gray/gray
CsCl ionic buffer concentration, M 0,018
Buffer matrix 4 % nitric acid
Exposure length, s 10
Number of exposures 2
Rinse time, s 35
Total analysis time, min 2
a
NOTE: Other instrument conditions that produce equivalent test results
are an option provided the test conditions are validated.
9.68.3 Preparation of standards
9.6.18.3.1 Standards from reagent salts
Calibration standards can be made from potassium dihydrogen phosphate (5.17) and
potassium chloride (5.18). Several calibration standards are required for the following reasons:
a) multiple ICP-OES wavelengths are utilized to expand the dynamic calibration range, b) some
wavelengths are split into multiple calibration segments, and c) a minimum of 5 points per
curve is recommended. The standards used and their P and K concentrations expressed as
ug/mL and as their percent oxide forms are listed in Table 2. These standards are susceptible to
microbial degradation and have a shelf life of no more than 3 weeks, if stored in the dark.
9.6.28.3.2 Standards from commercial stock solutions
Alternatively, custom stock standards can also be purchased but must be in a water matrix and
preserved with a biocide as standards preserved in acid will change the pH of the matrix and
produce erroneous results.
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ISO/DTS 20917:2022(E)
Table 2 –— ICP P and K calibration standards
Citrate- P O
2 5
Standard ID Volume Volume of P Volume of K P P 2O 5 solution P 2O 5 K K 2O K 2O solution
K O
EDTA 2

standard standard  fertilizer  Fertilizer
,
(mL)
a b e
ml ml ml µg/ml % % µg/ml µg/ml %
fertilizer
µg/ml
e
%
ml
Blank 250 100 0 0 0 0 0,00000000 00 0 0 0 0,00000000 00 0
d c
1 250 100 10 of Std 7 NA 12 27,5 0,00275002 75 1,4 15,2 18,3 0,00183001 83 0,9
d c
2 250 100 20 of Std 7 NA 24 55,0 0,00550005 50 2,7 30,3 36,5 0,00365003 65 1,8
c
3 250 100 5 NA 50 114,6 0,01146011 46 5,7 63,1 76,0 0,00760007 60 3,8
c
4 250 100 10 NA 100 229,1 0,02291022 91 11,5 126,3 152,1 0,01521015 21 7,6
c
5 250 100 15 NA 150 343,7 0,03437034 37 17,2 189,4 228,1 0,02281022 81 11,4
c
6 250 100 22 (15+7) NA 220 504,1 0,05041050 41 25,2 277,8 334,6 0,03346033 46 16,7
c
7 250 100 30 NA 300 687,4 0,06874068 74 34,4 378,8 456,3 0,04563045 63 22,8
c
8 250 100 40 NA 400 916,5 0,09165091 65 45,8 505 608,4 0,06084060 84 30,4
c c c c c
9 250 100 NA 20 NA NA NA NA 600 722,7 0,07227072 27 36,1
c c c c c
10 250 100 NA 25 NA NA NA NA 750 903,4 0,09034090 34 45,2
c c c c c
11 250 100 NA 30 NA NA NA NA 900 10841 084,1 0,10841108 41 54,2
a
P standard concentration = 2500 ugis equal to 2 500 µg/ml P as PO4 or 2,7461 746 1 g of potassium dihydrogen phosphate KH2PO4 is brought to 250 ml, which also
contains 31563 156,5 ug/ml K.
b
K standard concentration = 7500 ugis equal to 7 500 µg/ml K from KCl or 1,4300 430 0 g of potassium chloride or 1,9394 939 4 g of potassium nitrate is brought to 100 ml.
c
na = NA : not applicable.
d
Serial dilution from another standard; 10 of Std 7 = pipet 10 ml from standard 7.
e
Equivalent final result when based upon a 0.,5 g sample brought to a final volume of 250 ml.
NOTE 1 : A 52 % P2O5 fertilizer material weighed at 0,45 g rather than 0,5 g results in a concentration of 46,8 % P2O5, which is a 2 % extrapolation from the top
standard.
NOTE 2 : A 62 % K2O fertilizer material weighed at 0,45 g rather than 0,5 g results in a concentration of 55,8 % K2O, which is a 3 % extrapolation from the top standard.
© ISO 2022 – All rights reserved 1

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ISO/DTS 20917:2022(E)
9.6.38.3.3 Calibration and wavelength splitting
BecauseAs the higher points on the calibration curve have a much greater influence, this can
adversely impact the accuracy of the lower points. It is therefore recommended that the broad
calibration range be split into two segments, which are listed in Table 3. By splitting the same
wavelength into two separate curves [i.e. ((see Reference [1) and (2)]) or a low and high
concentration range] and/or by using two separate wavelengths, the quality of the lower P and
K concentrations will improve.
Table 3 – — Calibration criteria for ammonium citrate, disodium-EDTA soluble P and K
by
a a
ICP-OES OES
Element Wavelength, nm Preferred view Calibration Standards used Curve fit Spectral
b
range (see Table 1) deconvolution

, μg /ml
b
nm
P 213.,618 (1) Radial or axial 0 to 100 Blank, 1, 2, 3, 4 Linear Cu 213.,598
P 213.,618 (2) Radial or axial 100 to 400 4, 5, 6, 7, 8 Linear Cu 213.,598
P 214.,914 (1) Radial or axial 0 to 100 Blank, 1, 2, 3, 4 Linear Cu 214.,898
P 214.,914 (2) Radial or axial 100 to 400 4, 5, 6, 7, 8 Linear Cu 214.,898
c
P 177.,434 (1) Axial 0 to 100 Blank, 1, 2, 3, 4 Linear None
c
P 177.,434 (2) Axial 100 to 400 4, 5, 6, 7, 8 Linear None
c
P 178.,222 (1) Axial 0 to 100 Blank, 1, 2, 3, 4 Linear None
c
P 178.,222 (2) Axial 100 to 400 4, 5, 6, 7, 8 Linear None
K 766.,485 (1) Radial 0 to 126 Blank, 1, 2, 3, 4 Linear None
K 766.,485 (2) Radial 126 to 505 4, 5, 6, 7, 8 Linear None
K 766.,485 (3) Radial 379 to 900 7, 8, 9, 10, 11 Linear None
a
Other instrument conditions that produce equivalent test results are an option provided the test conditions are
validated.
b
(1), (2) and (3) are used to distinguish between the same wavelength used multiple times to cover separate
concentration ranges.
c
Optional, less intense wavelengths without any spectral interference that maycan be considered.
9.6.48.3.4 Analysis
A test portion is prepared as described in section Clause 7 and extracted as described in section
8.1 and presented to the ICP-OES configured according to Tables 1 and 3 using calibration
standards outlined in Table 2.

8.4     8.4 Quality control
9.7.18.4.1 Reference material
Within each analytical batch of samples, inclusion of one or more certified or consensus
fertilizer materials for quality assurance purposes is recommended, especially for the fertilizer
concentrates (i.e., P2O5 and K2O > 40%). Some sources of these materials include Laboratory
© ISO 2022 – All rights reserved 1

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ISO/DTS 20917:2022(E)
Quality Services International (LQSI) (), and the Magruder () or AFPC () check sample programs. P2O5
and K O greater than 40 %).
2
9.7.28.4.2 Duplicates
Duplicate pours from each extract solution are recommended. Duplicate phosphate results that
agree within 0.,183 and duplicate potash results that agree within 0.,148 are recommended as
outlined
...

FINAL
TECHNICAL ISO/DTS
DRAFT
SPECIFICATION 20917
ISO/TC 134
Fertilizers, soil conditioners
Secretariat: INSO
and beneficial substances —
Voting begins on:
2023-02-03 Determination of ammonium citrate,
disodium-EDTA soluble phosphorus
Voting terminates on:
2023-03-31
and potassium by ICP-OES in inorganic
fertilizers
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/DTS 20917:2023(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 2023

---------------------- Page: 1 ----------------------
ISO/DTS 20917:2023(E)
FINAL
TECHNICAL ISO/DTS
DRAFT
SPECIFICATION 20917
ISO/TC 134
Fertilizers, soil conditioners
Secretariat: INSO
and beneficial substances —
Voting begins on:
Determination of ammonium citrate,
disodium-EDTA soluble phosphorus
Voting terminates on:
and potassium by ICP-OES in inorganic
fertilizers
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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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OF ANY RELEVANT PATENT RIGHTS OF WHICH
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BEING ACCEPTABLE FOR INDUSTRIAL, TECHNO­
ISO/DTS 20917:2023(E)
Website: www.iso.org
LOGICAL, COMMERCIAL AND USER PURPOSES,
DRAFT INTERNATIONAL STANDARDS MAY ON
Published in Switzerland
OCCASION HAVE TO BE CONSIDERED IN THE
LIGHT OF THEIR POTENTIAL TO BECOME STAN­
DARDS TO WHICH REFERENCE MAY BE MADE IN
ii
  © ISO 2023 – All rights reserved
NATIONAL REGULATIONS. © ISO 2023

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ISO/DTS 20917:2023(E)
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 1
5 Reagents . 2
6 Apparatus . 3
7 Sampling and sample preparation . 4
8 Procedure .4
8.1 Extraction of P and K . 4
8.2 Instrument conditions . 4
8.3 Preparation of standards . 5
8.3.1 Standards from reagent salts . 5
8.3.2 Standards from commercial stock solutions . 5
8.3.3 Calibration and wavelength splitting . 7
8.3.4 Analysis . 7
8.4 Quality control . 7
8.4.1 Reference material . 7
8.4.2 Duplicates . 7
9 Calculations and expression of results . 7
9.1 Calculations . 7
9.2 Precision . . . 8
9.2.1 General . 8
9.2.2 Repeatability, r . 9
9.2.3 Reproducibility, R . 9
10 Test report .10
Annex A (informative) Ring test background and results .11
Annex B (informative) Critical factors .24
Bibliography .26
iii
© ISO 2023 – All rights reserved

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ISO/DTS 20917:2023(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 134, Fertilizers, soil conditioners and
beneficial substances.
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.

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ISO/DTS 20917:2023(E)
Introduction
Inductively coupled plasma-optical emission spectrometer (ICP-OES) instrumentation is becoming
increasingly popular in fertilizer testing laboratories (see References [1], [2] and [3]). Because these
instruments are automated and can determine multiple elements simultaneously, they offer significant
productivity gains. Also, the method does not generate hazardous wastes. The ammonium citrate
disodium etheylenediamine tetraacetic acid (EDTA) organic solvent and samples containing high
phosphorus and/or potassium concentrations pose some analytical challenges for the instrumentation,
which must be addressed. Due to the increasing use of this technology, this document was created.
For organic fertilizers and for liquid fertilizers containing phosphorous acid (H PO or phosphite), an
3 3
alternative method that specifically measures PO is preferred since the ICP­OES measures all elemental
4
phosphorus, which can result in a high bias for these types of fertilizer materials (see Reference [1]).
Due to the inability of the ICP-OES to differentiate between phosphorus species, under normal
operating conditions, this method is primarily intended for fertilizer materials in which the source(s)
of phosphorus is known. While this document can be used with minimal bias, the repeatability and
reproducibility is not consistent with other standards routinely used for inspection at this time, such as
[4] [5] [6]
EN 15959 , EN 15477 or ISO 22018 .
As a result, this document is intended for screening purposes, where a large number of fertilizer
phosphorus and potassium results must be obtained as efficiently as possible.
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TECHNICAL SPECIFICATION ISO/DTS 20917:2023(E)
Fertilizers, soil conditioners and beneficial substances —
Determination of ammonium citrate, disodium-EDTA
soluble phosphorus and potassium by ICP-OES in inorganic
fertilizers
1 Scope
This document specifies an inductively coupled plasma-optical emission spectrometer (ICP-OES)
method for the determination of the ammonium citrate, disodium-etheylenediamine tetraacetic acid
(EDTA) soluble phosphorus and potassium content in fertilizer extract solutions.
This document is applicable to all inorganic fertilizer products, whether compound or complex fertilizer
products or blends thereof, and straight concentrated fertilizer products.
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 8157, Fertilizers, soil conditioners and beneficial substances — Vocabulary
ISO 14820­1, Fertilizers and liming materials — Sampling and sample preparation — Part 1: Sampling
ISO 14820­2, Fertilizers and liming materials — Sampling and sample preparation — Part 2: Sample
preparation
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 8157 apply.
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/
4 Principle
Phosphorus and potassium contained in inorganic fertilizers that is soluble in a heated, dilute
ammonium citrate and disodium­EDTA solvent is determined. The neutral ammonium citrate is the
primary solvent for phosphate, while the EDTA is mainly included to chelate calcium, magnesium and
other soluble metallic cations to minimize their interaction with phosphate and eliminate the water­
wash step (see Reference [7]). Also, since this solvent is a relatively weak solution, it extracts primarily
the highly soluble potassium compounds and has been used for fertilizer potassium determination
since 1993. It compares very favourably with other soluble potash solvents such as ammonium oxalate.
The user of this document is not obligated to determine both phosphorus and potassium; however, one
of the main advantages of this approach is to measure both simultaneously in a single extract in a more
productive, cost-effective way.
The resulting extract solution is tested by ICP-OES to quantify the levels of phosphorus and potassium,
which can be converted to their equivalent P O and K O concentrations. The extract solution is
2 5 2
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ISO/DTS 20917:2023(E)
nebulized to produce a fine aerosol, which is introduced to a plasma or high energy source to ionize
or excite electrons to higher energy orbitals. As these electrons return to ground state, they emit
characteristic ultraviolet or visible wavelengths that are unique to each element. The intensity of the
wavelength emission is proportionate to the concentration, which can be quantified by various detector
types including a photomultiplier tube (PT), a charge injection device (CID) or a charge coupled device
(CCD).
5 Reagents
5.1 Ammonium citrate, dibasic, (NH ) HC H O , with a molecular weight (MW) of 226,19 and a
4 2 6 5 7
purity greater than 98 % or 980,0 g/kg.
5.2 EDTA, disodium salt, dehydrate, C H N Na O ·2H O, with a MW of 372,24 and a purity
10 14 2 2 8 2
greater than 98 % or 980,0 g/kg.
5.3 Ammonium hydroxide, NH OH, with a MW of 35,05 and a purity equal to 28,0-30,0 % as NH or
4 3
ρ = 0,91 g/ml.
5.4 Nitric acid, HNO , 67-70 %, with a ρ = 1,40 g/ml.
3
5.5 Potassium dihydrogen phosphate, KH PO , with a MW of 136,09, with a purity greater than
2 4
1)
99 % or 990,0 g/kg .
5.6 Potassium chloride, KCl, with a MW of 74,55 and a purity greater than 99 % or 990 g/kg.
2)
5.7 Potassium nitrate, KNO , with a MW of 101,10 with a purity greater than 99 % or 990,0 g/kg .
3
3)
5.8 Octyl phenol ethoxylate .
5.9 Beryllium, Be, stock solution of 10 000 μg/ml, commercially available.
5.10 Scandium, Sc, stock solution of 10 000 μg/ml, commercially available.
5.11 Cesium chloride, CsCl, MW of 168,36, with a purity greater than 99,99 % or 999,99 g/kg.
5.12 Lithium nitrate, LiNO , MW of 68,95, with a purity greater than 99 % or 990,0 g/kg.
3
5.13 Citrate-EDTA extraction solution (0,11 M ammonium citrate and 0,033 M disodium­EDTA).
Weigh and transfer 25 g disodium-EDTA and 50 g dibasic ammonium citrate to a 2 l volumetric flask
containing approximately 500 ml of deionized (or equivalent) water. Adjust the pH to near neutral by
adding 30 ml of ammonium hydroxide and water (1:1, volume fraction) solution in a fume hood. Adjust
the final pH to 7,00 (±0,02) using a pH electrode and meter while adding the ammonium hydroxide and
water (1:1, volume fraction) solution drop-by-drop and stirring. After obtaining a stable pH of 7,00 ±
1)  SRM 194a is the trade name of a product supplied by National Institute of Standards and Technology (NIST).
This information is given for the convenience of users of this document and does not constitute an endorsement by
ISO of the product named. Equivalent products may be used if they can be shown to lead to the same results.
2) SRM 193 is the trade name of a product supplied by NIST. This information is given for the convenience of users
of this document and does not constitute an endorsement by ISO of the product named. Equivalent products may be
used if they can be shown to lead to the same results.
3) Triton X-100 is the trade name of a product supplied by Sigma-Aldrich. This information is given for the
convenience of users of this document and does not constitute an endorsement by ISO of the product named.
Equivalent products may be used if they can be shown to lead to the same results.
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ISO/DTS 20917:2023(E)
0,02, bring the flask to volume with deionized water (or equivalent) and mix. Larger volumes of this
solution can be prepared; however, it is susceptible to microbial degradation resulting in a shelf life of
no more than three weeks, if stored in a dark location.
5.14 0,5 % or 5 ml/l octyl phenol ethoxylate.
Add 1 ml of octyl phenol ethoxylate (5.8) to a 200 ml volumetric flask and dilute to volume with
deionized water.
5.15 Internal standard/ionization buffer (10 µg/ml Sc in 0,018 M CsCl and 4 % nitric acid).
Add 1 ml 10 000 μg/ml Sc stock standard, 3 g cesium chloride, 40 ml nitric acid, and 1 ml 0,5 % octyl
phenol ethoxylate to a 1 l volumetric flask containing approximately 500 ml of deionized (or equivalent)
water. Bring flask to volume with deionized (or equivalent) water and mix. If Be is used as an internal
standard, add 4 ml of 10 000 μg/ml Be stock standard to obtain a concentration of 40 μg/ml Be.
Alternatively, Li from LiNO can be substituted for Cs as an ionization buffer at 7 g LiNO per litre.
3 3
5.16 4 % nitric acid rinse solution.
Add 40 ml of nitric acid (5.4) to approximately 500 ml of deionized or equivalent water contained in a
1 l volumetric flask and bring to volume.
5.17 2 500 µg/ml phosphorus from phosphate (PO ).
4
Add 2,746 1 g of potassium dihydrogen phosphate (5.5) to a 250 ml volumetric flask, then bring to
volume with deionized (or equivalent) water. This standard also contains 3 156,5 μg/ml K; also, this
standard can take several hours to solubilize and stabilize. This standard should be prepared fresh
each time. Alternatively, a commercial custom standard from a phosphate source prepared in a water
matrix and preserved with a biocide is acceptable.
NOTE A commercial stock standard preserved in acid is not acceptable as the acid changes the matrix of the
pH neutral ammonium citrate EDTA solution and produces erroneous results.
5.18 7 500 µg/ml potassium from potassium chloride.
Add 1,430 0 g of potassium chloride (5.6) to a 100 ml volumetric flask, then bring to volume with
deionized (or equivalent) water. Alternatively, 1,939 4 g of potassium nitrate (5.7) can be used rather
than potassium chloride. This standard should be prepared fresh each time. Alternatively, a commercial
custom standard from a potassium source prepared in a water matrix and preserved with a biocide is
acceptable.
NOTE A commercial stock standard preserved in acid is not acceptable as the acid changes the matrix of the
pH neutral ammonium citrate EDTA solution and produces erroneous results.
6 Apparatus
6.1 Analytical balance, with a readability to 0,1 mg.
6.2 pH meter, with a readability to pH 7,00.
6.3 pH combination electrode.
6.4 Constant temperature water bath, capable of maintaining a temperature of 65 °C ± 2 °C.
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ISO/DTS 20917:2023(E)
6.5 Heated shaking water bath, capable of maintaining a temperature of 65 °C ± 2 °C and set to
approximately 200 reciprocations/min.
A water bath is preferred, but alternatively an enclosed heated air shaker is also acceptable; however,
ensure the added citrate­EDTA solution (5.13) is at 65 °C ± 2 °C before starting.
6.6 ICP-OES instrument.
A radial view instrument is preferred for potassium, while phosphorus can be determined in either
radial or axial view.
6.7 Rotating riffle splitter, or comparable.
6.8 Grinding mill, capable of grinding fertilizer material to pass a 0,50 mm screen or sieve.
7 Sampling and sample preparation
7.1 Collect a field sample using a recognized sampling procedure such as ISO 14820-1 or comparable
(see References [8] and [9]).
7.2 Prepare solid and liquid fertilizer materials using a recognized sample preparation procedure
such as ISO 14820­2 or comparable.
8 Procedure
8.1 Extraction of P and K
For samples containing less than 45 % P O or K O, weigh 0,5 g ± 0,01 g of material and completely
2 5 2
transfer to a 250 ml class A volumetric flask. For samples containing 45 % or more P O or K O, weigh
2 5 2
0,45 ± 0,01 g of ground fertilizer material and completely transfer to a 250 mL class A volumetric flask.
Dispense 100 ml of 65 °C ± 2 °C preheated ammonium citrate, disodium-EDTA extraction solution
(5.13) into each flask and insert a rubber stopper. Shake test solutions in a 65 °C ± 2 °C preheated water
bath set to approximately 200 reciprocations per minute for exactly 60 min. Then, remove from the
water bath; allow to cool to room temperature; bring to volume with deionized (or equivalent) water;
stopper and mix. Filter any test solution containing suspended debris using P and K free filters. Some
slowly soluble phosphate compounds can continue to solubilize over time resulting in a high bias, so
test solutions should be run as soon as possible, but no more than 24 h after extraction.
8.2 Instrument conditions
Optimal instrument conditions identified during robustness testing are listed in Table 1. ICP­OES’ differ
in their design and options, so adjustment to the conditions listed in Table 1 can be necessary; however,
any adjustments to these conditions must be performance-based and validated. Special attention
should be paid to the recovery of P in fertilizer concentrates such as concentrated superphosphate,
diammonium phosphate (46 % of P O ), and monoammonium phosphate (50 % or 52 % of P O ), since
2 5 2 5
.
these materials require optimal instrument performance
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ISO/DTS 20917:2023(E)
a
Table 1 — Recommended ICP-OES conditions
Factor Setting
Power, kW 1,4
Plasma flow, l/min 18,0
Auxiliary flow, l/min 2,25
Nebulizer pressure, l/min 0,7
Nebulizer Concentric type
Spray chamber Double path cyclonic
Sample pump tube black/black
Buffer/internal standard pump tube gray/gray
CsCl ionic buffer concentration, M 0,018
Buffer matrix 4 % nitric acid
Exposure length, s 10
Number of exposures 2
Rinse time, s 35
Total analysis time, min 2
a
Other instrument conditions that produce equivalent test results are
an option provided the test conditions are validated.
8.3 Preparation of standards
8.3.1 Standards from reagent salts
Calibration standards can be made from potassium dihydrogen phosphate (5.17) and potassium
chloride (5.18). Several calibration standards are required for the following reasons: a) multiple ICP-
OES wavelengths are utilized to expand the dynamic calibration range, b) some wavelengths are split
into multiple calibration segments, and c) a minimum of 5 points per curve is recommended. The
standards used and their P and K concentrations expressed as ug/mL and as their percent oxide forms
are listed in Table 2. These standards are susceptible to microbial degradation and have a shelf life of no
more than 3 weeks, if stored in the dark.
8.3.2 Standards from commercial stock solutions
Alternatively, custom stock standards can also be purchased but must be in a water matrix and
preserved with a biocide as standards preserved in acid will change the pH of the matrix and produce
erroneous results.
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ISO/DTS 20917:2023(E)
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Table 2 — ICP P and K calibration standards
Standard ID Volume Citrate-EDTA Volume of P Volume of K P P O P O solution P O ferti- K K O K O solution K O ferti-
2 5 2 5 2 5 2 2 2
ml ml standard standard   lizer µg/ml lizer
a b e e
ml ml µg/ml µg/ml % % µg/ml % %
Blank 250 100 0 0 0 0 0,000 00 0 0 0 0,000 00 0
d c
1 250 100 10 of Std 7 NA 12 27,5 0,002 75 1,4 15,2 18,3 0,001 83 0,9
d c
2 250 100 20 of Std 7 NA 24 55,0 0,005 50 2,7 30,3 36,5 0,003 65 1,8
c
3 250 100 5 NA 50 114,6 0,011 46 5,7 63,1 76,0 0,007 60 3,8
c
4 250 100 10 NA 100 229,1 0,022 91 11,5 126,3 152,1 0,015 21 7,6
c
5 250 100 15 NA 150 343,7 0,034 37 17,2 189,4 228,1 0,022 81 11,4
c
6 250 100 22 (15+7) NA 220 504,1 0,050 41 25,2 277,8 334,6 0,033 46 16,7
c
7 250 100 30 NA 300 687,4 0,068 74 34,4 378,8 456,3 0,045 63 22,8
c
8 250 100 40 NA 400 916,5 0,091 65 45,8 505 608,4 0,060 84 30,4
c c c c c
9 250 100 NA 20 NA NA NA NA 600 722,7 0,072 27 36,1
c c c c c
10 250 100 NA 25 NA NA NA NA 750 903,4 0,090 34 45,2
c c c c c
11 250 100 NA 30 NA NA NA NA 900 1 084,1 0,108 41 54,2
a
P standard concentration is equal to 2 500 µg/ml P as PO or 2,746 1 g of potassium dihydrogen phosphate KH PO is brought to 250 ml, which also contains 3 156,5 ug/ml K.
4 2 4
b
K standard concentration is equal to 7 500 µg/ml K from KCl or 1,430 0 g of potassium chloride or 1,939 4 g of potassium nitrate is brought to 100 ml.
c
NA : not applicable.
d
Serial dilution from another standard; 10 of Std 7 = pipet 10 ml from standard 7.
e
Equivalent final result when based upon a 0,5 g sample brought to a final volume of 250 ml.
NOTE 1 A 52 % P2O5 fertilizer material weighed at 0,45 g rather than 0,5 g results in a concentration of 46,8 % P2O5, which is a 2 % extrapolation from the top standard.
NOTE 2 A 62 % K2O fertilizer material weighed at 0,45 g rather than 0,5 g results in a concentration of 55,8 % K2O, which is a 3 % extrapolation from the top standard.

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ISO/DTS 20917:2023(E)
8.3.3 Calibration and wavelength splitting
As the higher points on the calibration curve have a much greater influence, this can adversely impact
the accuracy of the lower points. It is therefore recommended that the broad calibration range be split
into two segments, which are listed in Table 3. By splitting the same wavelength into two separate
curves (see Reference [1]) or a low and high concentration range and/or by using two separate
wavelengths, the quality of the lower P and K concentrations will improve.
Table 3 — Calibration criteria for ammonium citrate, disodium-EDTA soluble P and K by
a
ICP-OES
Element Wavelength Preferred view Calibration Standards used Curve fit Spectral
b
nm range (see Table 1) deconvolution
μg /ml
P 213,618 (1) Radial or axial 0 to 100 Blank, 1, 2, 3, 4 Linear Cu 213,598
P 213,618 (2) Radial or axial 100 to 400 4, 5, 6, 7, 8 Linear Cu 213,598
P 214,914 (1) Radial or axial 0 to 100 Blank, 1, 2, 3, 4 Linear Cu 214,898
P 214,914 (2) Radial or axial 100 to 400 4, 5, 6, 7, 8 Linear Cu 214,898
c
P 177,434 (1) Axial 0 to 100 Blank, 1, 2, 3, 4 Linear None
c
P 177,434 (2) Axial 100 to 400 4, 5, 6, 7, 8 Linear None
c
P 178,222 (1) Axial 0 to 100 Blank, 1, 2, 3, 4 Linear None
c
P 178,222 (2) Axial 100 to 400 4, 5, 6, 7, 8 Linear None
K 766,485 (1) Radial 0 to 126 Blank, 1, 2, 3, 4 Linear None
K 766,485 (2) Radial 126 to 505 4, 5, 6, 7, 8 Linear None
K 766,485 (3) Radial 379 to 900 7, 8, 9, 10, 11 Linear None
a
Other instrument conditions that produce equivalent test results are an option provided the test conditions are validated.
b
(1), (2) and (3) are used to distinguish between the same wavelength used mult
...