ISO 15355:2026

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
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: +41 22 749 01 11
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii
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
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 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
...