ISO 10701:2026

• Abstract:

  This document specifies a methylene blue spectrophotometric method for the determination of sulfur in steel and iron. The method is applicable to sulfur contents between 0,000 3 % (mass fraction) to 0,010 % (mass fraction).

• Status: Published
• Publication Date: 09-Nov-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: 10-Nov-2026
• Completion Date: 10-Nov-2026

• Revises: ISO 10701:1994 - Steel and iron — Determination of sulfur content — Methylene blue spectrophotometric method
• Effective Date: 18-Jan-2025

ISO 10701 is an International Organization for Standardization (ISO) standard for determining sulfur in steel and iron by a methylene blue spectrophotometric method. It specifies a laboratory procedure for low sulfur levels, and it is intended for analysts, testing laboratories, and quality-control teams that need comparable results for steel and iron samples.

What does ISO 10701 specify?

ISO 10701 specifies the scope, the analytical principle, the reagents and apparatus, the procedure, the calculation of results, and the test report for sulfur determination in steel and iron. The document is organized into 11 clauses, followed by two informative annexes.

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

The scope is limited to sulfur contents from 0,0003 % (m/m) to 0,010 % (m/m). The method is affected by niobium, silicon, tantalum, and titanium, so the usable range and the test portion mass depend on how much of those elements is present.

What are the key requirements of ISO 10701?

• Use the method only within its stated sulfur range. ISO 10701 applies to 0,0003 % to 0,010 % sulfur in steel and iron. In practice, this means you first check whether the expected sulfur level falls in the lower or higher part of the range before choosing the test portion and absorbing solution volume.

• Account for interfering elements. Clause 4 states that niobium, silicon, tantalum, and titanium interfere. Table 1 links their maximum allowable contents to the sulfur range and test portion mass. This matters because the sample size and working range are not fixed for every material.

• Prepare the sample according to the sampling standard. Clause 7 requires sampling and sample preparation in accordance with ISO 14284 or appropriate national standards. That keeps the sulfur result tied to a representative test sample rather than to a convenience specimen.

• Follow the wet-chemical sequence in Clause 8. The test portion is dissolved in hydrochloric and nitric acids, treated with perchloric acid, reduced with hydroiodic and hypophosphorous acids in a nitrogen atmosphere, and the evolved hydrogen sulfide is absorbed in zinc acetate. The sulfur is then converted to methylene blue and measured spectrophotometrically at about 665 nm. In practice, this is the core measurement path the laboratory must control.

• Use the correct test portion and absorbing solution. For 0,0003 % to 0,0010 % sulfur, the method uses about 1,00 g of sample and 10 ml of absorbing solution in a 20 ml flask. For 0,0010 % to 0,010 % sulfur, it uses about 0,50 g and 50 ml in a 100 ml flask. This keeps the signal in a measurable range.

• Run a blank and calibrate with sulfur standards. Clause 8 requires a blank test in parallel, and Clause 8.4 builds calibration curves from sulfur standard solutions prepared from potassium sulfate. That means the final result is referenced to the same chemistry as the sample, not to a generic instrument setting.

• Control the temperature and contamination risk. Clause 10 stresses clean laboratory conditions, low-sulfur reagents, and a reduction temperature of 114 °C to 118 °C. It also gives a check for the purification of the reducing mixture: the absorbance should stay below 0,055, corresponding to 1 µg of sulfur.

• Report the result and any deviations. Clause 11 requires identification of the sample, the method reference, the result with units, unusual features, and any operation not specified in the document or any optional operation that could affect the result.

Who uses ISO 10701?

ISO 10701 is used by laboratories that measure sulfur in steel and iron, especially where low sulfur content must be reported with trace-level sensitivity. It is also relevant to steelmakers, foundries, quality assurance teams, and buyers who need a common method for incoming inspection, product release, or dispute testing.

What changed in ISO 10701 from the previous edition?

ISO 10701:2026 replaces ISO 10701:1994 and is described as technically revised. The stated changes are:

• Clause 2, Normative references, was updated.
• Clause 3, Terms and definitions, was added and the later clauses were renumbered.
• The special case that had been in Clause 10 was moved into 8.3.1.
• The precision data were re-evaluated.

Which standards are used with ISO 10701?

ISO 10701 refers to these standards in Clause 2:

• ISO 648 - single-volume pipettes, for accurate liquid handling.
• ISO 1042 - one-mark volumetric flasks, for solution preparation and calibration.
• ISO 3696 - water for analytical laboratory use, for preparing reagents and standards.
• ISO 14284 - sampling and preparation of steel and iron samples, for obtaining the test sample.

The precision study also states that results were treated statistically in accordance with ISO 5725-2 and ISO 5725-3, and the bibliography cites ISO 5725-1, ISO 5725-2, and ISO 5725-3 for accuracy and precision terminology and study design.

What does the ISO 10701 document contain?

ISO 10701 contains the analytical principle, reagent recipes, apparatus descriptions, a sampling clause, a full step-by-step procedure, calibration instructions, the calculation method, and precision data. It includes two apparatus figures for the reduction-distillation setup and the reducing-mixture purification setup, plus tables for application ranges, calibration solutions, and precision data.

Annex A gives the interlaboratory test background and the sample set used for precision evaluation. Annex B turns the precision data into a graph showing the logarithmic relationship between sulfur content and repeatability or reproducibility. The test report clause tells laboratories exactly what information to document so results can be traced and compared.

International
Standard
ISO 10701
Second edition
Steel and iron — Determination of
2026-11
sulfur content — Methylene blue
spectrophotometric method
Aciers et fontes — Détermination du soufre — Méthode
spectrophotométrique au bleu de méthylène
Reference number
© ISO 2026
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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 . 4
7 Sampling and sample preparation . 5
8 Procedure . 5
8.1 Test portion .6
8.2 Blank test .6
8.3 Determination .6
8.3.1 Preparation of the test solution .6
8.3.2 Reduction and distillation .6
8.3.3 Colour development .7
8.3.4 Spectrophotometric measurements .7
8.4 Establishment of the calibration curves .7
8.4.1 Preparation of the calibration solutions .7
8.4.2 Spectrophotometric measurements .7
8.4.3 Plotting the calibration curve .7
9 Expression of results . 8
9.1 Method of calculation .8
9.2 Precision . . .8
10 Notes on procedure . 9
11 Test report . 10
Annex A (informative) Additional information on the international precision test .11
Annex B (informative) Graphical representation of precision data .12
Bibliography .13

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 10701:1994), which has been technically
revised.
The main changes are as follows:
— Clause 2, Normative references has been updated;
— the mandatory Clause 3, terms and definitions has been added and subsequent clauses have been
renumbered;
— Clause 10, Special case, has been moved into 8.3.1;
— the precision data has been re-evaluated.
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 10701:2026(en)
Steel and iron — Determination of sulfur content —
Methylene blue spectrophotometric method
1 Scope
This document specifies a methylene blue spectrophotometric method for the determination of sulfur in
steel and iron.
The method is applicable to sulfur contents between 0,000 3 % (mass fraction) to 0,010 % (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 a mixture of hydrochloric and nitric acids.
Evaporation with perchloric acid until white fumes appear to remove hydrochloric and nitric acids.
Dissolution of the salts in hydrochloric acid. Evolution of hydrogen sulfide by reducing with a mixture of
hydroiodic and hypophosphorous acids in a nitrogen atmosphere, distillation, and absorption into a zinc
acetate solution.
Formation of methylene blue by reacting with N,N-dimethyl-p-phenylenediamine and iron(III) solution.
Spectrophotometric measurement at a wavelength of about 665 nm.
Niobium, silicon, tantalum and titanium interfere in the determination of sulfur.
Depending on the contents of the interfering elements, the application ranges and test portion masses given
in Table 1 apply.
Table 1 — Application ranges and test portions
Maximum allowable content of the interfering ele-
ments
Test portion Application ranges ∆w
s
% (mass fraction)
g
% (mass fraction)
Nb Si Ta Ti
0,5 1,0 0,3 1,0 1,0 0,000 3 to 0,001 0
1,0 2,0 0,6 2,0 0,50 0,001 0 to 0,010
5 Reagents
During the analysis, unless otherwise stated, use only reagents of recognized analytical grade with a very
low sulfur content, and only freshly prepared grade 2 water as specified in ISO 3696.
5.1 Hydrochloric acid, ρ about 1,19 g/ml.
5.2 Hydrochloric acid, ρ about 1,19 g/ml, diluted 1 + 15.
5.3 Perchloric acid, ρ about 1,54 g/ml.
5.4 Hydrobromic acid, ρ approximately 1,48 g/ml.
5.5 Mixture of hydrochloric and nitric acids.
Mix one volume of hydrochloric acid (5.1) and one volume of nitric acid, ρ about 1,40 g/ml.
Prepare immediately before use.
5.6 Reducing reagent solution.
Transfer 200 ml of hydriodic acid, ρ about 1,70 g/ml and 50 ml of hypophosphorous acid, ρ about
1,49 g/ml into the purifying apparatus (see Figure 1). Purge with nitrogen (5.12) at a flow rate of 100 ml/
min for 10 min, to mix the acids and expel air from the system. Switch on the electric heating mantle. Heat
to boiling and boil gently for about 120 min at a temperature of about 115 °C in a current of nitrogen. When
purification is completed (see 10.3), switch off the electric heating mantle. Then cool the solution and keep
it in a brown bottle.
5.7 Absorbing solution.
Dissolve 5 g of zinc acetate dihydrate [Zn(CH COO) .2H O] in 400 ml of water. Add 200 ml of sodium
3 2 2
hydroxide, 30 g/l, and 70 g of ammonium chloride, and then dilute to 1 000 ml with water.
5.8 Iron solution, 10 g/l.
Weigh, to the nearest 0,01 g, 1,00 g of pure iron, free from sulfur. Transfer to a 300 ml beaker, cover with a
watch glass, dissolve by heating with an addition of 20 ml of hydrochloric acid (ρ about 1,19 g/ml, diluted
1 + 1) and boil gently for about 10 min. Then add 2 ml of nitric acid (ρ about 1,40 g/ml) drop by drop to
oxidize iron. Remove the oxides of nitrogen by boiling, and cool to room temperature. Transfer to a 100 ml
one-mark volumetric flask, dilute to the mark with water and mix.
5.9 Iron (lll) chloride solution.
Dissolve 1 g of iron (Ш) chloride hexahydrate (FeCl .6H O) in about 40 ml of water. Add 10 ml of hydrochloric
3 2
acid (5.1) and dilute to 100 ml with water.

5.10 N,N-dimethyl-p-phenylenediamine in hydrochloric acid medium.
Dissolve 0,5 g of N,N-dimethyl-p-phenylenediamine dihydrochloride [NH C H N(CH ) .2HCl] in about
2 6 4 3 2
100 ml of water. Add 230 ml of hydrochloric acid (5.1) and dilute to 500 ml with water.
5.11 Sulfur standard solutions.
5.11.1 Sulfur standard solution, 1 g/l.
Weigh, to the nearest 0,000 1 g, 5,435 2 g of potassium sulfate [minimum purity: 99,5 % (mass fraction)],
previously dried at 110 °C for 2 h and cooled to room temperature in a desiccator. Dissolve in water, transfer
to a 1 000 ml one-mark volumetric flask quantitatively, dilute to the mark with water and mix.
1 ml of this standard solution contains 1 mg of sulfur.
5.11.2 Sulfur standard solution, 10 mg/l.
Transfer 10,0 ml of the standard solution (5.11.1) to a 1 000 ml one-mark volumetric flask, dilute to the mark
with water and mix.
1 ml of this standard solution contains 10 µg of sulfur.
5.11.3 Sulfur standard solution, 1 mg/l.
Transfer 10,0 ml of the standard solution (5.11.2) to a 100 ml one-mark volumetric flask, dilute to the mark
with water and mix.
Prepare the solution immediately before use.
1 ml of this standard solution contains 1 µg of sulfur.
5.12 Nitrogen [purity: >99,5 % (volume fraction)].

Key
1 double surface condenser or equivalent (34 cm long)
2 nitrogen inlet
3 hydrogen sulfide trap (18 cm × 2, 5 cm)
4 thermometer
5 three-neck flask (capacity: 500 ml)
Figure 1 — Example of an apparatus for purification of the reducing mixture
6 Apparatus
All volumetric glassware shall be class A, in accordance with ISO 648 or ISO 1042, as appropriate.
Ordinary laboratory apparatus, and the following.
6.1 Apparatus for reduction and distillation
Assemble the apparatus for reduction and distillation as shown i
...