ISO 9441:2026

• Abstract:

  This document specifies a spectrophotometric method by using 4-(2-pyridylazo)-resorcinol (PAR) for the determination of niobium in steel. The method is applicable to all types of steel with niobium contents between 0,005 % (mass fraction) and 1,3 % (mass fraction).

• Status: Published
• Publication Date: 02-Aug-2026
• ICS: 77.080.20 - Steels
• 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: 03-Aug-2026
• Due Date: 19-Oct-2026
• Completion Date: 03-Aug-2026

• Revises: ISO 9441:1988 - Steel — Determination of niobium content — PAR spectrophotometric method
• Effective Date: 21-Oct-2023

Overview

ISO 9441: Steel - Determination of niobium content - 4-(2-Pyridylazo)-resorcinol (PAR) spectrophotometric method is an international standard published by ISO. This standard outlines a precise and reliable spectrophotometric procedure using 4-(2-pyridylazo)-resorcinol (PAR) for determining niobium content in steel. Applicable to a range of steel types, it covers niobium concentrations from 0.005% to 1.3% (mass fraction), addressing analytical needs for both routine and specialized quality control in steel manufacturing and testing.

The standard specifies the fundamental principles, required reagents, apparatus, sampling and sample preparation, test procedures, calculation methods, and requirements for reporting results. It is widely recognized for improving accuracy and comparability of niobium determination in both research and industrial laboratories.

Key Topics

• Scope: Determination of niobium in steel within the 0.005% to 1.3% range by mass.
• Analytical Principle: The steel sample is dissolved and treated to isolate niobium, which is then reacted with PAR to form a colored complex. This complex is measured spectrophotometrically at around 550 nm for quantification.
• Reagents and Apparatus:
  • High-purity chemicals, including pure iron (low niobium), acids, and PAR reagent.
  • Spectrophotometer capable of absorbance measurements at 550 nm and various cell path lengths (typically 10 mm and 40 mm).
  • Calibration standards and glassware compliant with ISO norms.
• Sampling and Preparation: Guidance for representative sample selection, preparation, and handling, referencing ISO 14284 for steel and iron chemistry.
• Procedure: Step-by-step method for dissolution, separation, formation of the niobium-PAR complex, and measurement. Includes blank tests for accuracy.
• Expression of Results: Detailed calculation formula for expressing niobium content, consideration of blank corrections, and requirements for precision.
• Precision and Reporting:
  • Annex A shares outcomes from international interlaboratory comparisons.
  • Annex B presents precision data in graphical format, serving as a reference for laboratory performance.
  • The test report should include sample identification, reference to ISO 9441, results, any deviations or issues, and all important procedural details.

Applications

ISO 9441 is widely used in various metallurgical and materials science contexts, including:

• Steel Production Quality Control: Ensures controlled niobium levels for optimal mechanical properties in construction, automotive, and engineering steels.
• Research and Development: Provides reliable data for the development of advanced steel alloys and performance studies.
• Material Certification: Supports certification and traceability in line with international and customer-specific requirements.
• Interlaboratory Consistency: Facilitates harmonized results between laboratories worldwide, supporting global trade and compliance.

Related Standards

For comprehensive application and understanding, ISO 9441 should be used alongside the following standards:

• ISO 14284: Steel and iron - Sampling and preparation of samples for the determination of chemical composition: Methods for sampling and sample preparation.
• ISO 648: Laboratory glassware - Single-volume pipettes: Pipette specification for volumetric measurements.
• ISO 1042: Laboratory glassware - One-mark volumetric flasks: Standard for accuracy in solution preparation.
• ISO 3696: Water for analytical laboratory use - Specification and test methods: Specifies water quality for laboratory analyses.
• ISO 5725 series: Guidance on accuracy (trueness and precision) of measurement methods and results.

Ensuring conformance to ISO 9441 strengthens quality control in steel manufacturing and enhances the reliability of niobium content determination in laboratory and industrial environments.

International
Standard
ISO 9441
Second edition
Steel — Determination
2026-08
of niobium content —
4-(2-Pyridylazo)-resorcinol (PAR)
spectrophotometric method
Aciers — Détermination de la teneur en niobium — Méthode
spectrophotométrique au moyen de 4-(2-Pyridylazo)-résorcinol
(PAR)
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: [phone]
Email: [e-mail]
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 . 1
6 Apparatus . 2
7 Sampling and preparation of the test samples . 3
8 Procedure . 3
8.1 Test portion .3
8.2 Blank test .3
8.3 Determination .3
8.3.1 Dissolution of the test portion .3
8.3.2 Separation of the niobium .3
8.3.3 Preparation of the test solution .4
8.3.4 Formation of the coloured complex .4
8.3.5 Spectrophotometric measurements .4
8.4 Establishment of the calibration curve .4
8.4.1 Preparation of the calibration solutions .4
8.4.2 Spectrophotometric measurements .5
8.4.3 Plotting the calibration curve .5
9 Expression of results . 5
9.1 Method of calculation .5
9.2 Precision . . .6
10 Test report . 7
Annex A (informative) Additional information on the international interlaboratory test . 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 9441:1988), which has been technically
revised.
The main changes are as follows:
— the normative references have been revised;
— the precision data has been updated;
— the text has been improved editorially.
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 9441:2026(en)
Steel — Determination of niobium content —
4-(2-Pyridylazo)-resorcinol (PAR) spectrophotometric
method
1 Scope
This document specifies a spectrophotometric method by using 4-(2-pyridylazo)-resorcinol (PAR) for the
determination of niobium in steel. The method is applicable to all types of steel with niobium contents
between 0,005 % (mass fraction) and 1,3 % (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 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
Dissolution of a test portion in hydrochloric acid followed by oxidation with hydrogen peroxide.
Precipitation of niobium and tantalum with phenylarsonic acid, using zirconium as a carrier.
Formation of a complex of niobium with 4-(2-pyridylazo)-resorcinol (PAR) in a sodium tartrate medium
buffered by a sodium acetate solution adjusted to pH 6,3.
Spectrophotometric measurement of the coloured complex at a wavelength of about 550 nm.
5 Reagents
During the analysis, unless otherwise stated, use only reagents of recognized analytical grade and only
distilled water or grade 2 water as specified in ISO 3696.
5.1 Pure iron, in flake or powder form, with a niobium content less than 0,000 5 % (mass fraction).

5.2 Potassium hydrogen sulfate (KHSO ).
5.3 Hydrochloric acid, ρ approximately 1,19 g/ml.
5.4 Hydrochloric acid, ρ approximately 1,19 g/ml, diluted (1 + 9).
5.5 Sulfuric acid, ρ approximately 1,84 g/ml, diluted (1 + 1).
5.6 Sulfuric acid, ρ approximately 1,84 g/ml, diluted (1 + 4).
5.7 Hydrogen peroxide, 300 g/l.
5.8 Sodium hydroxide, 120 g/l.
Store in a polyethylene bottle.
5.9 Zirconium nitrate, 3 g/l solution in hydrochloric acid medium.
Dissolve 0,3 g of zirconium nitrate in 50 ml of hydrochloric acid, ρ approximately 1,19 g/ml, diluted 1 + 4.
Filter through a fine filter paper, dilute to 100 ml with water and mix.
5.10 Sodium acetate buffer, pH-value 6,3.
Dissolve 350 g of sodium acetate trihydrate in 700 ml of water, add 5,5 ml of glacial acetic acid, ρ approximately
1,05 g/ml, dilute to 1 000 ml with water and mix. Adjust the pH-value to 6,3 with small additions of acetic
acid or sodium hydroxide (5.8), using a pH-meter for measurement.
5.11 Tartaric acid, 100 g/l.
5.12 Phenylarsonic acid [C H AsO(OH) ], 40 g/l.
6 5 2
5.13 Phenylarsonic acid [C H AsO(OH) ], 0,5 g/l.
6 5 2
5.14 Disodium dihydrogen ethylenediamine tetraacetic acid (EDTA·Na ), 15 g/l.
Dissolve 15 g of disodium dihydrogen ethylenediamine tetraacetate dihydrate (C H O N Na ·2H O) in
10 14 8 2 2 2
water, dilute to 1 000 ml with water and mix.
Store in a polyethylene bottle.
5.15 4-(2-pyridylazo)-resorcinol (PAR) (C H N O ), mono- or di-sodium salt, 0,6 g/l.
11 9 3 2
5.16 Niobium standard solution, 0,200 g/l.
Weigh, to the nearest 0,000 1 g, 0,143 1 g of niobium(V) oxide. (99,5 % minimum) and transfer into a
platinum crucib
...