kSIST FprEN ISO 4935:2026

• Abstract:

  The method is applicable to sulphur contents between 0,002 % (m/m) and 0,10 % (m/m). Specifies principle, reagents and material, apparatus, sampling, procedure, expression of results and test report. Annex A gives additional information on the international cooperative tests. Annex B represents the precision date graphically. Annex C deals with induction furnaces and infrared sulfur analysers.

• Status: Not Published
• Public Enquiry End Date: 30-Jan-2026
• ICS: 77.040.30 - Chemical analysis of metals
• ICS: 77.080.01 - Ferrous metals in general
• Technical Committee: IFEK - Ferrous metals
• Current Stage: 5020 - Formal vote (FV) (Adopted Project)
• Start Date: 26-Oct-2026
• Due Date: 14-Dec-2026
• Completion Date: 27-Oct-2026
• Ref Project: FprEN ISO 4935 - Steel and iron - Determination of sulfur content - Infrared absorption method after combustion in an induction furnace (ISO/FDIS 4935:2026)

• Revises: SIST EN 24935:1997 - Steel and iron - Determination of sulphur content - Infrared absorption method after combustion in an induction furnace (ISO 4935:1989)
• Effective Date: 25-Oct-2023

kSIST FprEN ISO 4935:2026 is a SIST draft standard for determining sulfur content in steel and iron by infrared absorption after combustion in an induction furnace. It specifies a laboratory method for sulfur contents from 0,002 % to 0,10 % (mass fraction) and is intended for chemical analysis of ferrous metals.

What does kSIST FprEN ISO 4935:2026 specify?

kSIST FprEN ISO 4935:2026 specifies the complete workflow for measuring sulfur in steel and iron by combusting a test portion in a high-frequency induction furnace and measuring the sulfur dioxide by infrared absorption. The document is written as an analytical method standard, so it tells the laboratory how to prepare samples, run the analysis, calculate the result and report it.

The standard is organized into:

• Clause 1 Scope
• Clause 2 Normative references
• Clause 4 Principle
• Clause 5 Reagents and materials
• Clause 6 Apparatus
• Clause 7 Sampling and sample preparation
• Clause 8 Procedure
• Clause 9 Expression of results
• Clause 10 Test report

Annex	What it covers
Annex A	Features of commercial high-frequency induction furnaces and infrared sulfur analysers
Annex B	Additional information on the international interlaboratory test
Annex C	Graphical representation of precision data

In practice, this means the document is not only about the measurement principle. It also tells users what equipment and reference solutions to use, how to set up calibration, and what information belongs in the test report.

What are the key requirements of kSIST FprEN ISO 4935:2026?

kSIST FprEN ISO 4935:2026 is built around combustion, calibration and blank correction. The most important requirements are the preparation of the sample, control of contamination, calibration in the right sulfur range, and calculation of sulfur content from the analyser reading.

Combustion principle and measurement

Clause 4 states that the test portion is burned in oxygen in a high-frequency induction furnace with an accelerator present. The sulfur in the sample is converted to sulfur dioxide, and the sulfur dioxide is measured by infrared absorption.

In practice, the laboratory needs equipment that can produce a stable combustion and a stable infrared signal. The quality of the measurement depends on complete combustion and reliable gas measurement.

Reagents, gases and materials

Clause 5 requires analytical-grade reagents, water meeting ISO 3696 or equivalent purity, oxygen of at least 99,5 % (volume fraction), pure iron of known very low sulfur content, an accelerator, quartz wool, magnesium perchlorate, inert ceramic material impregnated with sodium hydroxide, and sulfur standard solutions prepared from potassium sulfate.

This matters because sulfur contamination at very low levels can distort the result. The document also warns about handling magnesium perchlorate and quartz wool, so the laboratory must control both analytical purity and safe handling.

Apparatus and furnace setup

Clause 6 requires ordinary laboratory apparatus plus a micro pipette, tin capsules and ceramic crucibles suitable for combustion in an induction furnace. All volumetric glassware is required to be class A, and the furnace and analyser are to be used according to the manufacturer’s instructions.

In practice, the method depends on stable instrument operation and consistent consumables. Annex A gives additional features of commercial furnaces and analysers, which helps users check whether their equipment matches the intended setup.

Sampling, sample preparation and blank control

Clause 7 requires sampling and sample preparation in accordance with ISO 14284 or appropriate national standards. Clause 8.2 calls for degreasing the test sample before weighing it, and Clause 8.3 requires duplicate blank tests before determination.

This is important because surface contamination, oil and atmospheric contamination can affect sulfur results. The blank test is used to detect contamination in the reagents, crucibles, accelerator and handling steps before the real samples are measured.

Calibration ranges and curve preparation

Clause 8.5 sets three calibration ranges:

• below 0,005 % sulfur
• from 0,005 % to 0,04 % sulfur
• from 0,04 % to 0,10 % sulfur

For each range, the document specifies how to prepare calibration members from sulfur standard solutions, dry them in tin capsules, measure them with the same type of setup used for the samples, and plot a calibration curve from the net readings.

In practice, the laboratory must choose the calibration range that matches the expected sulfur level in the sample. That makes the calibration curve more relevant and improves the reliability of the final result.

Calculation, precision and reporting

Clause 9 converts the analyser reading to milligrams of sulfur by means of the calibration curve and then calculates sulfur content as a mass percentage after subtracting the blank value. Clause 9.2 also gives precision information based on an interlaboratory test, with repeatability and reproducibility data across several sulfur levels.

Clause 10 requires the report to identify the sample, the laboratory and the date, state the method as ISO 4935, give the result with its unit, and note any unusual features or any operation not specified in the document. For users, this means the report must show not just the answer, but also the conditions that could have influenced it.

Who uses kSIST FprEN ISO 4935:2026?

kSIST FprEN ISO 4935:2026 is used by laboratories that analyze steel and iron for sulfur content, especially in chemical composition control. It is relevant to testing laboratories, quality control teams, producers of ferrous metals, and buyers who need a defined analytical method for incoming inspection or product verification.

It also supports users who operate combustion analysers and need a repeatable procedure for calibration, blank correction and reporting. Annex A is particularly useful for laboratories selecting or checking induction furnaces and infrared sulfur analysers.

What changed in kSIST FprEN ISO 4935:2026 from the previous edition?

kSIST FprEN ISO 4935:2026 replaces ISO 4935:1989. The foreword states that this second edition has been technically revised, with the following changes:

• normative references have been revised
• precision data have been updated
• the text has been improved editorially

Which standards are used with kSIST FprEN ISO 4935:2026?

Standard	What it contributes
ISO 648	Single-volume pipettes used for accurate liquid transfer
ISO 1042	One-mark volumetric flasks used to prepare sulfur standard solutions
ISO 3696	Water quality for analytical laboratory use
ISO 14284	Sampling and preparation of steel and iron samples for chemical composition analysis
ISO 5725-2	Statistical treatment of precision data in the collaborative test
ISO 5725-3	Statistical treatment of intermediate precision and collaborative study design
ISO 5725-1	Definitions used in the precision notes for repeatability conditions

These references support the method’s measurement quality. In practice, they define the glassware, water purity, sampling approach and precision treatment needed to make the sulfur results consistent and defensible.

What does the kSIST FprEN ISO 4935:2026 document contain?

The document contains the operating principle, reagent list, apparatus requirements, sampling instructions, step-by-step procedure, calculation formula and test report content for sulfur determination in steel and iron. It also contains calibration instructions for three sulfur ranges, including the preparation of sulfur standard solutions and the use of blank tests.

The annexes add practical support:

• Annex A describes typical furnace and analyser features, including oxygen supply, purification, flow control and infrared gas analysis
• Annex B summarizes the interlaboratory test behind the precision data
• Annex C presents the precision relationships graphically

The document also includes tables for preparing sulfur standard solutions, calibration members and precision data. These tables are used to set up the method, check performance and interpret the quality of the result.

SLOVENSKI STANDARD
oSIST prEN ISO 4935:2026
01-januar-2026
Jeklo in železo - Določevanje žvepla - Infrardeča absorpcijska metoda po
zgorevanju v indukcijski peči (ISO/DIS 4935:2025)
Steel and iron - Determination of sulfur content - Infrared absorption method after
combustion in an induction furnace (ISO/DIS 4935:2025)
Stahl und Eisen - Bestimmung des Schwefelgehalts - Verfahren mit Infrarotabsorption
nach Verbrennung im Induktionsofen (ISO/DIS 4935:2025)
Aciers et fontes - Détermination du soufre - Méthode par absorption dans l'infrarouge
après combustion dans un four à induction (ISO/DIS 4935:2025)
Ta slovenski standard je istoveten z: prEN ISO 4935
ICS:
77.040.30 Kemijska analiza kovin Chemical analysis of metals
77.080.01 Železne kovine na splošno Ferrous metals in general
oSIST prEN ISO 4935:2026 en,fr,de
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

oSIST prEN ISO 4935:2026
oSIST prEN ISO 4935:2026
DRAFT
International
Standard
ISO/DIS 4935
ISO/TC 17/SC 1
Steel and iron — Determination
Secretariat: JISC
of sulfur content — Infrared
Voting begins on:
absorption method after
2025-11-12
combustion in an induction furnace
Voting terminates on:
2026-02-04
Aciers et fontes — Dosage du soufre — Méthode par absorption
dans l'infrarouge après combustion dans un four à induction
ICS: 77.080.01
THIS DOCUMENT IS A DRAFT CIRCULATED
FOR COMMENTS AND APPROVAL. IT
IS THEREFORE SUBJECT TO CHANGE
AND MAY NOT BE REFERRED TO AS AN
INTERNATIONAL STANDARD UNTIL
PUBLISHED AS SUCH.
This document is circulated as received from the committee secretariat.
IN ADDITION TO THEIR EVALUATION AS
BEING ACCEPTABLE FOR INDUSTRIAL,
TECHNOLOGICAL, COMMERCIAL AND
USER PURPOSES, DRAFT INTERNATIONAL
STANDARDS MAY ON OCCASION HAVE TO
ISO/CEN PARALLEL PROCESSING
BE CONSIDERED IN THE LIGHT OF THEIR
POTENTIAL TO BECOME STANDARDS TO
WHICH REFERENCE MAY BE MADE IN
NATIONAL REGULATIONS.
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 SUPPORTING DOCUMENTATION.
Reference number
ISO/DIS 4935:2025(en)
oSIST prEN ISO 4935:2026
DRAFT
ISO/DIS 4935:2025(en)
International
Standard
ISO/DIS 4935
ISO/TC 17/SC 1
Steel and iron — Determination
Secretariat: JISC
of sulfur content — Infrared
Voting begins on:
absorption method after
combustion in an induction furnace
Voting terminates on:
Aciers et fontes — Dosage du soufre — Méthode par absorption
dans l'infrarouge après combustion dans un four à induction
ICS: 77.080.01
THIS DOCUMENT IS A DRAFT CIRCULATED
FOR COMMENTS AND APPROVAL. IT
IS THEREFORE SUBJECT TO CHANGE
AND MAY NOT BE REFERRED TO AS AN
INTERNATIONAL STANDARD UNTIL
PUBLISHED AS SUCH.
This document is circulated as received from the committee secretariat.
IN ADDITION TO THEIR EVALUATION AS
BEING ACCEPTABLE FOR INDUSTRIAL,
© ISO 2025
TECHNOLOGICAL, COMMERCIAL AND
USER PURPOSES, DRAFT INTERNATIONAL
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
STANDARDS MAY ON OCCASION HAVE TO
ISO/CEN PARALLEL PROCESSING
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
BE CONSIDERED IN THE LIGHT OF THEIR
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
POTENTIAL TO BECOME STANDARDS TO
WHICH REFERENCE MAY BE MADE IN
or ISO’s member body in the country of the requester.
NATIONAL REGULATIONS.
ISO copyright office
RECIPIENTS OF THIS DRAFT ARE INVITED
CP 401 • Ch. de Blandonnet 8
TO SUBMIT, WITH THEIR COMMENTS,
CH-1214 Vernier, Geneva
NOTIFICATION OF ANY RELEVANT PATENT
Phone: [phone]
RIGHTS OF WHICH THEY ARE AWARE AND TO
PROVIDE SUPPORTING DOCUMENTATION.
Email: [e-mail]
Website: www.iso.org
Published in Switzerland Reference number
ISO/DIS 4935:2025(en)
ii
oSIST prEN ISO 4935:2026
ISO/DIS 4935:2025(en)
Contents Page
Foreword .iv
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Principle . 1
5 Reagent and materials . 1
6 Apparatus . 2
7 Sampling and sample preparation . 3
8 Procedure . 3
8.1 General operating instructions .3
8.2 Test sample and sample preparation .3
8.3 Blank test .4
8.3.1 Low standard .4
8.4 Determination .5
8.5 Establishment of the calibration curve .5
8.5.1 Samples having sulfur contents less than 0,005 % (mass fraction) .5
8.5.2 Samples having sulfur contents between 0,005 % (mass fraction) and 0,04 %
(mass fraction) .6
8.5.3 Samples having sulfur contents between 0,04 % (mass fraction) and 0,1 %
(mass fraction) .6
9 Expression of results . 7
9.1 Method of calculation .7
9.2 Precision . . .7
10 Test report . 8
Annex A (informative) Features of commercial high-frequancy induction furnaces and infrared
sulfur analysers . 9
Annex B (informative) Additional information on the international interlaboratory test .11
Annex C (informative) Graphical representation of precision data . 14
Bibliography .15

iii
oSIST prEN ISO 4935:2026
ISO/DIS 4935:2025(en)
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
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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)
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rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
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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 4935:1989), which has been technically
revised.
The main changes compared to the previous edition are as follows:
— 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
oSIST prEN ISO 4935:2026
DRAFT International Standard ISO/DIS 4935:2025(en)
Steel and iron — Determination of sulfur content — Infrared
absorption method after combustion in an induction furnace
1 Scope
This document specifies an infrared absorption method after combustion in an induction furnace for
determination of sulfur in steel and iron.
The method is applicable to sulfur contents between 0,002 % (mass fraction) and 0,10 % (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 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
Combustion of a test portion in presence of an accelerator at a high temperature in a high-frequency
induction furnace in a current of pure oxygen. Transformation of sulfur into sulfur dioxide.
Measurement by infrared absorption of the Sulfur dioxide carried by a current of oxygen.
5 Reagent and materials
During the analysis, unless otherwise stated, use only reagents of recognized analytical grade and only
distilled water in accordance with ISO 3696 or water of equivalent purity.
5.1 Water, free from sulfur dioxide.
Boil water for 30 min, cool to room temperature and bubble with oxygen (5.2) for 15 min. Prepare just before use.

oSIST prEN ISO 4935:2026
ISO/DIS 4935:2025(en)
5.2 Oxygen, 99,5 % (volume fraction) minimum.
An oxidation catalyst [copper(II) oxide or platinum] tube heated to a temperature above 450 °C shall be used
prior to a purifying unit (see Annex A), when the presence of organic contaminants is suspected in the oxygen.
5.3 Pure iron, of known low sulfur content less than 0,000 5 % (mass fraction).
5.4 Appropriate solvent, suitable for washing greasy or dirty test samples, for example, high-purity acetone
5.5 Magnesium perchlorate [Mg(ClO ) ], particle size: from 0,7 mm to 1,2 mm.
4 2
WARNING — Magnesium perchlorate is a highly potent reagent and shall be opened as short as
possible and kept in a dry and organic free environment. It also pulverises during storage and using
too small particles will cause changes or stop the flow. Using a grid to separate small particles is not
recommended but just vibrate will cause larger particles on top.
5.6 Accelerator, tungsten-tin mixture or tungsten of known low sulfur content less than 0,000 5 % (mass
fraction). The particle size of the accelerator may be dependent on the type of apparatus used.
5.7 Quartz wool, fine or course shall be used as glass wool will absorb sulfur (SO ).
WARNING — Quartz is hazardous to inhale so protective breathing filter and a hood shall be used
when handling quartz wool.
5.8 Sulfur, standard solutions
Weigh, to the nearest 0,1 mg, the mass given in Table 1 of potassium sulfate [minimum purity: 99,9 % (mass
fraction)] previously dried at 105 °C to 110 °C for 1 h or to constant mass and cooled in a desiccator.
Table 1 — Preparation of sulfur standard solutions
Reference of sulfur standard solution Mass of potassium sulfate Concentration of sulfur
g mg/ml
5.7.1 0,217 4 0,40
5.7.2 0,380 4 0,70
5.7.3 0,543 4 1,00
5.7.4 1,086 9 2,00
5.7.5 1,902 2 3,50
5.7.6 2,717 2 5,00
5.7.7 4,347 5 8,00
Transfer quantitatively into seven 100 ml one-mark volumetric flasks. Dilute to the mark with water and mix.
5.9 Inert ceramic, (attapulgus clay) impregnated with sodium hydroxide, particle size: from 0,7 mm
to 1,2 mm.
6 Apparatus
During the analysis, unless otherwise stated, use only ordinary laboratory apparatus.
All volumetric glassware shall be class A, in accordance with ISO 648 or ISO 1042 as appropriate.
The apparatus required for combustion in a high-frequency induction furnace and the subsequent infrared
absorption measurement of the evolved sulfur dioxide may be obtained commercially from a number of
manufacturers. Follow the manufacturer’s instructions for the operation of the instrument.

oSIST prEN ISO 4935:2026
ISO/DIS 4935:2025(en)
Features of commercial instruments are given in Annex A.
6.1 Micro pipette, 50 μl and 100 μl, the limit of error shall be less than 1 μl.
6.2 Tin capsule, about 6 mm in diameter, 18 mm in height, 0,3 g in mass and approximately 0,4 ml in volume.
6.3 Ceramic crucible, capable of withstanding combustion in an induction furnace
Ignite crucibles in an electric furnace in air or in a current of oxygen for not less than 2 h at 1 100 °C and
store in a desiccator before use.
NOTE For the determination of low sulfur contents it is advisable to ignite crucibles at 1 350 °C in a current of oxygen
7 Sampling and sample preparation
Sampling and preparation of the samples shall be carried out in accordance with ISO 14284 or appropriate
national standards for steel and iron.
8 Procedure
WARNING — The risks related to combustion analysis are mainly burns in pre-igniting the ceramic
crucibles and in the fusions. Use crucible tongs at all times and suitable containers for the used
crucibles. Normal precautions for handling oxygen cylinders shall be taken. Oxygen from the
combustion process shall be removed effectively from the apparatus since a high concentration of
oxygen in a confined space can present a fire hazard.
8.1 General operating instructions
Purify the oxygen supply using tubes packed with the inert ceramic (attapulgus clay) impregnated with
sodium hydroxide (5.8) and magnesium perchlorate (5.5), and maintain a quiescent flow rate whilst on
standby. Maintain a quartz wool (5.7), filter or a stainless steel net as a dust collector. Clean and change as
necessary. The furnace chamber, pedestal post and filter trap shall be cleaned frequently to remove oxide
build-up.
Allow each item of equipment to stabilize for the time recommended by the manufacturers of the equipment
when the main supply is switched on after being out of action for any length of time.
After cleaning the furnace chamber and/or changing filters or after the equipment has been inoperative for
a period, stabilize the apparatus by burning several samples of similar type to the samples to be analyzed
prior to setting up for analysis.
Flush oxygen through the apparatus and adju
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