ISO 15355:2026

• Abstract:

  This document specifies a potentiometric titration method for the determination of chromium content in steel and iron. The method is applicable to chromium contents between 1 % (mass fraction) and 35 % (mass fraction). This document applies to steel and iron with vanadium contents less than 1 % (mass fraction) for chromium contents higher than 10 % (mass fraction) and less than 0,2 % (mass fraction) for chromium contents less than 10 % (mass fraction).

• Status: Published
• Publication Date: 14-Sep-2026
• ICS: 77.080.01 - Ferrous metals in general
• Technical Committee: ISO/TC 17/SC 1 - Methods of determination of chemical composition
• Drafting Committee: ISO/TC 17/SC 1 - Methods of determination of chemical composition
• Current Stage: 6060 - International Standard published
• Start Date: 15-Sep-2026
• Due Date: 20-Dec-2026
• Completion Date: 15-Sep-2026

• Revises: ISO 15355:1999 - Steel and iron — Determination of chromium content — Indirect titration method
• Effective Date: 07-Jan-2025

ISO 15355:2026, published by ISO, specifies an indirect potentiometric titration method for determining chromium content in steel and iron. It is written for laboratories that need to measure chromium between 1 % and 35 % mass fraction, and it also sets the vanadium limits that affect the calculation.

The document tells the user how to prepare the test solution, carry out the titration, correct for vanadium interference, calculate the chromium result, and report the test. It is aimed at routine analytical work where the result must be traceable and comparable between laboratories.

What does ISO 15355:2026 specify?

ISO 15355:2026 specifies the full analytical procedure for chromium determination in steel and iron by potentiometric back titration. The scope states the usable chromium range and the vanadium conditions that apply within that range.

The document is organized into clauses on scope, normative references, principle, reagents, apparatus, sampling, determination, expression of results, interpretation of results, and test report. It also includes two informative annexes that support the precision data.

Annex	What it covers
Annex A	Additional information on the international interlaboratory precision tests
Annex B	Graphical representation of precision data

What are the key requirements of ISO 15355:2026?

ISO 15355:2026 requires a controlled redox sequence: the sample is fused, oxidized, reduced with an excess of iron(II) salt, and then back-titrated potentiometrically. In practice, this means the analyst must follow the chemistry and the end-point detection closely, because the final chromium value depends on the titration curve and the vanadium correction.

• Principle, Clause 4 - The sample is fused with sodium peroxide, acidified with sulfuric acid, oxidized to dichromate with peroxodisulfate using silver as a catalyst, then reduced and back-titrated. This is the core measurement route.
• Reagents, Clause 5 - The method uses analytical-grade reagents and grade 2 water from ISO 3696. Potassium dichromate is used for solution preparation and standardization, and the iron(II) salt batch has to be checked for reduction grade. That matters because reagent strength directly affects the result.
• Apparatus, Clause 6 - Volumetric glassware must be class A, and the titration setup uses a Pt-Ag/AgCl redox electrode, a burette, a high-impedance voltmeter, a muffle furnace at 650 °C under argon, and clean zirconium crucibles. This means the method depends on both accurate volume delivery and stable electrochemical measurement.
• Sampling and sample preparation, Clause 7 - Sampling follows ISO 14284 or appropriate national standards, and the test sample must be chips or powder smaller than 2 000 µm. Good sampling reduces segregation and helps the fused sample behave consistently.
• Preparation of the test solution, Clause 8.1 - A 0.25 g test portion is fused with sodium peroxide at 650 °C for 2 h to 3.5 h under argon, then dissolved and boiled. The temperature control matters because the document warns that overheating can cause positive interference.
• Titration, Clause 8.2 - Silver nitrate and ammonium peroxodisulfate are added, the solution is boiled, hydrochloric acid is added, and the calculated amount of iron(II) salt is introduced before titration with potassium dichromate. The end-point is taken from the titration curve or interpolation, so the analyst must record the potential carefully.
• Expression of results, Clause 9 - Chromium content is calculated from the titration data, with a vanadium correction when vanadium is between 0.03 % and 1 %. If vanadium is below 0.03 %, the correction is negligible.
• Precision and reporting, Clauses 9.3 and 11 - Table 1 gives repeatability and reproducibility limits across the chromium range, and the test report must identify the sample, the laboratory, the date, the method reference, the result, unusual features, and any deviations.

Who uses ISO 15355:2026?

ISO 15355:2026 is used by chemical testing laboratories, steel and iron producers, and quality control teams that need chromium results for incoming material, process control, or product release. It is also relevant to buyers, inspectors, and certification bodies that compare chromium test results from different laboratories.

What changed in ISO 15355:2026 from the previous edition?

ISO 15355:2026 cancels and replaces ISO 15355:1999 and is technically revised. The stated changes are updated normative references, addition of mandatory Clause 3 with renumbering of later clauses, and re-assessed precision data.

Which standards are used with ISO 15355:2026?

ISO 15355:2026 relies on several referenced standards for glassware, water, and sample preparation:

• ISO 385 - burettes for the titration setup
• ISO 648 - single-volume pipettes
• ISO 1042 - one-mark volumetric flasks
• ISO 3696 - water for analytical laboratory use
• ISO 14284 - sampling and preparation of steel and iron samples

It also points to related standards for vanadium determination and precision evaluation:

• ISO 4942, ISO 4947, ISO 9647 - used to determine vanadium content when a correction is needed
• ISO 5725-1, ISO 5725-2, ISO 5725-3 - used in the interlaboratory precision work behind the repeatability and reproducibility data

What does the ISO 15355:2026 document contain?

ISO 15355:2026 contains the analytical principle, reagent list, apparatus requirements, sampling instruction, detailed test procedure, calculation formulas, vanadium correction, precision summary, and reporting requirements. The precision material is supported by Table 1 in the main text and by Annex A and Annex B.

Annex A gives the interlaboratory study background, including the test samples and the experimental results used to assess precision. Annex B presents the precision relationships graphically, so users can see how repeatability and reproducibility vary with chromium content.

The document also includes formulae for preparing the potassium dichromate solution, checking the reduction grade of the iron(II) salt, calculating chromium content, and correcting vanadium interference. In practice, these parts are used by laboratories that calibrate the reagents, validate the method, and document a test result.

International
Standard
ISO 15355
Second edition
Steel and iron — Determination
2026-09
of chromium content — Indirect
titration method
Aciers et fontes — Détermination de la teneur en chrome —
Méthode par titration indirecte
Reference number
© ISO 2026
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
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or ISO’s member body in the country of the requester.
ISO copyright office
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Phone: [phone]
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Website: www.iso.org
Published in Switzerland
ii
Contents Page
Foreword .iv
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 of the test sample . 4
8 Determination . 4
8.1 Preparation of the test solution .4
8.2 Titration.4
9 Expression of results . 5
9.1 Method of calculation .5
9.2 Correction of the interference of vanadium .5
9.3 Precision . . .5
10 Interpretation of the results . 6
11 Test report . 7
Annex A (informative) Additional information on the international interlaboratory precision
tests . 8
Annex B (informative) Graphical representation of precision data .10
Bibliography .11

iii
Foreword
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This document was prepared by Technical Committee ISO/TC 17, Steel, Subcommittee SC 1, Methods of
determination of chemical composition.
This second edition cancels and replaces the first edition (ISO 15355:1999), which has technically been
revised.
The main changes are as follows:
— updated the normative references;
— added mandatory Clause 3 and renumbered subsequent clauses;
— re-assessed the precision data.
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
International Standard ISO 15355:2026(en)
Steel and iron — Determination of chromium content —
Indirect titration method
1 Scope
This document specifies a potentiometric titration method for the determination of chromium content in
steel and iron.
The method is applicable to chromium contents between 1 % (mass fraction) and 35 % (mass fraction).
This document applies to steel and iron with vanadium contents less than 1 % (mass fraction) for chromium
contents higher than 10 % (mass fraction) and less than 0,2 % (mass fraction) for chromium contents less
than 10 % (mass fraction).
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 3696, Water for analytical laboratory use — Specification and test methods
ISO 14284, Steel and iron — Sampling and preparation of samples for the determination of chemical composition
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/
4 Principle
Fusion of the sample with sodium peroxide followed by acidification with sulfuric acid.
Oxidization of chromium (III) to dichromate with peroxodisulfate using silver as a catalyst.
Reduction of the dichromate with an excess of a solid iron (II) salt and potentiometric back titration of the
excess with a dichromate solution.
Any interference from vanadium is corrected mathematically.

5 Reagents
During the analysis, unless otherwise specified, use only reagents of recognized analytical grade and only
grade 2 water as specified in ISO 3696.
5.1 Sulfuric acid, ρ approximately 1,84 g/ml.
5.2 Sulfuric acid, diluted 1 + 6.
5.3 Sulfuric acid, diluted 1 + 1.
5.4 Hydrochloric acid, ρ approximately 1,19 g/ml, diluted 1 + 1.
5.5 Sodium peroxide, Na O .
2 2
It is recommended to use a powder as fine as possible.
5.6 Silver nitrate solution, approximately 1 %.
Dissolve 5 g of AgNO in 300 ml of water. Dilute to 500 ml and mix.
5.7 Ammonium peroxodisulfate solution, approximately 20 %
Dissolve 100 g of (NH ) S O in 300 ml of water. Dilute to 500 ml and mix.
4 2 2 8
5.8 Potassium dichromate, K Cr O , with known purity minimum 99,9 % (mass fraction).
2 2 7
Immediately prior to use dry the salt at 105 °C for at least 2 h. Allow to cool to room temperature in a
desiccator.
5.9 Potassium dichromate solution, 0,017 mol/l.
Weigh, to the nearest 0,1 mg, 4,9 g of the salt (5.8) and dissolve it in 500 ml of water in a 1 000 ml volumetric
flask. Let the flask stand for 6 h to dissolve the salt completely. Temper the solution to 20 °C, dilute to the
mark with water and mix carefully.
The molarity of the solution, C , is given by Formula (1).
KCrO
22 7
mp
a
C  (1)
KCrO
22 7
M 100
KCrO
22 7
where
C is the concentration, expressed in mol/l, of the potassium dichromate solution;
KCrO
22 7
m is the mass, expressed in grams, of potassium dichromate;
a
p is purity, expressed as a percentage in mass fraction, of potassium dichromate;
M is the molar mass for potassium dichromate (294,184 6 g/mol).
KCrO
22 7
5.10 Ammonium iron(II) sulfate hexahydrate, Fe(NH ) (SO ) ·6H O.
4 2 4 2 2
The reduction grade of the salt shall be determined for each batch that is to be used.
Thoroughly grind a portion of 300 g to 400 g of the salt so that no large crystals are left. Store the salt in a
jar that can be tightly closed. Transfer, to five 400 ml glass beakers, about 300 mg (to the nearest 0,1 mg) of
the potassium dichromate (5.8) and dissolve in 200 ml of water and 35 ml of sulfuric acid (5.2). Add to each
beaker about 2 700 mg (to the nearest 0,1 mg) of the ground iron salt. Stir with a glass rod to dissolve all
crystals. Titrate the solutions with the potassium dichromate solution (5.9).

The reduction grade, R, is given by Formula (2).
()AB()CD
R (2)
E
where
Fpm
b
A is the factor, expressed in milligrams, given by the formula: A ;
M 100
KCrO
22 7
B is the factor for conversion of chromium to ammonium iron(II) sulfate hexahydrate (22,625 4);
C is the factor, expressed in mg/ml, given by the formula: CL6 C ;
KCrO
22 7
D is the volume, expressed in millilitres, of added potassium dichromate solution (5.9);
E is the mass, expressed in milligrams, of ammonium iron(II) sulfate hexahydrate;
F is twice the molar mass for chromium (103,992 g/mol);
p is purity, expressed as a percentage (mass fraction) of potassium dichromate (5.8);
m is the mass, expressed in milligrams, of potassium dichromate;
b
M is the molar mass for potassium dichromate (294,184 6 g/mol);
KCrO
22 7
L is the factor for conversion of potassium dichromate to ammonium iron(II) sulfate hexahydrate
(392,144 8 g/mol);
C is the concentration, expressed in mol/l, of the potassium dichromate solution (5.9).
KCrO
22 7
Calculate the average and the relative standard
...