ISO 4941:2024

Steel and iron — Determination of molybdenum content — Thiocyanate spectrophotometric method

ISO 4941:2024

Name:ISO 4941:2024   Standard name:Steel and iron — Determination of molybdenum content — Thiocyanate spectrophotometric method
Standard number:ISO 4941:2024   language:English language
Release Date:22-Jul-2024   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   ICS number:77.080.01 - Ferrous metals in general

International
Standard
ISO 4941
Third edition
Steel and iron — Determination of
2024-07
molybdenum content — Thiocyanate
spectrophotometric method
Aciers et fontes — Détermination des teneurs en molybdène —
Méthode spectrophotométrique au thiocyanate
Reference number
© ISO 2024
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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 . 3
7 Sampling and preparation of the test samples . 3
8 Procedure . 3
8.1 Test portion .4
8.2 Determination .4
8.2.1 Preparation of the test solution .4
8.2.2 Preparation of the compensating solution .4
8.2.3 Formation and extraction of the coloured complex .4
8.2.4 Spectrophotometric measurements .5
8.3 Establishment of the calibration curve .5
8.3.1 Preparation of the calibration solutions, related to spectrophotometric
measurements performed using cells with an optical path length of 10 mm .5
8.3.2 Spectrophotometric measurements .5
8.3.3 Plotting the calibration curve .6
9 Expression of results . 6
9.1 Method of calculation .6
9.2 Precision . . .7
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
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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 third edition cancels and replaces the second edition (ISO 4941:1994), which has been technically
revised.
The main changes are as follows:
— the normative references have been revised;
— the precision data has been updated.
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 4941:2024(en)
Steel and iron — Determination of molybdenum content —
Thiocyanate spectrophotometric method
1 Scope
This document specifies a thiocyanate spectrophotometric method for the determination of molybdenum
contents in steel and iron. The method is applicable to molybdenum mass fractions between 0,005 % and
0,125 %.
Vanadium and tungsten interfere with the measurement if, because of their contents, the V/Mo ratio is
greater than 16 or the W/Mo ratio is greater than 8.
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 4800, Laboratory glassware — Separating funnels and dropping funnels
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 terminological 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 an appropriate mixture of acids and decomposition of the carbides by
oxidation.
Quantitative formation of a coloured complex of molybdenum, in the presence of thiocyanate, iron(II) and/
or copper(II) ions and extraction of this compound using butyl acetate.
Spectrophotometric measurement of the coloured complex at a wavelength of about 470 nm.

5 Reagents
During the analysis, unless otherwise stated, use only reagents of recognized analytical grade and grade 2
water as specified in ISO 3696.
5.1 Pure iron, in flake or powder form, with a molybdenum content less than 0,000 5 % (mass fraction)
and free from tungsten and vanadium.
5.2 Butyl acetate.
5.3 Nitric acid, ρ approximately 1,40 g/ml.
5.4 Hydrochloric acid, ρ approximately 1,19 g/ml.
5.5 Hydrochloric acid, ρ approximately 1,19 g/ml, diluted (3 + 1).
5.6 Hydrochloric acid, ρ approximately 1,19 g/ml, diluted (1 + 1).
5.7 Acid mixture I.
Add 2 volumes of hydrochloric acid (5.4) to 1 volume of nitric acid (5.3) and mix well. Prepare this mixture
immediately before use.
5.8 Acid mixture II.
Add 150 ml of orthophosphoric acid (ρ approximately 1,70 g/ml) to 300 ml of water, and add 360 ml of
perchloric acid (ρ approximately 1,67 g/ml) to this diluted acid (see NOTE). Transfer the solution into a
1 000 ml one-mark volumetric flask. Dilute to the mark with water and mix.
NOTE In the preparation of this acid mixture, 360 ml of perchloric acid (ρ approximately 1,67 g/ml) can be
replaced by 150 ml of sulfuric acid (ρ approximately 1,84 g/ml).
5.9 L(+)- ascorbic acid solution, 100 g/l.
Prepare this solution just before use.
5.10 Ammonium thiocyanate solution, 320 g/l.
Store this solution away from light.
5.11 Copper(II), solution corresponding to 70 mg of Cu(II) per litre in a hydrochloric acid medium.
Dissolve 0,188 g of copper(II) chloride di-hydrate (CuCl ·2H O) or 0,275 g of copper(II) sulfate penta-
2 2
hydrate (CuSO ·5H O) into 125 ml of hydrochloric acid (5.4). Transfer the solution into a 1 000 ml one-mark
4 2
volumetric flask, dilute to the mark with water and mix.
5.12 Tin(II) copper(II) chloride, solution in a hydrochloric acid medium.
Dissolve 80 g of tin(II) chloride di-hydrate (SnCl ·2H O) in 155 ml of hydrochloric acid (5.4). Add 100 ml
2 2
of copper(II) solution (5.11). Transfer the solution into a 1 000 ml one-mark volumetric flask, dilute to the
mark with water and mix.
Prepare this solution just before use.

5.13 Iron, 10 g/l solution
Weigh, to the nearest 0,01 g, 10,0 g of the pure iron (5.1). Place the portion into a 1 000 ml beaker, and
dissolve by careful addition of 500 ml of the acid mixture II (5.8). After cooling, transfer into a 1 000 ml one-
mark volumetric flask, dilute to the mark with water and mix.
5.14 Molybdenum, standard solutions.
5.14.1 Molybdenum standard solution, corresponding to 500 mg/l of Mo.
Weigh, to the nearest 0,1 mg, 0,500 g of molybdenum [minimum purity 99,95 % (mass fraction), see NOTE].
Transfer the portion into a 1 000 ml beaker. Add 500 ml of hydrochloric acid (5.4) and heat; add two drops of
nitric acid (5.3) to aid dissolution. Cool the solution.
Transfer the solution into a 1 000 ml one-mark volumetric flask. Dilute to the mark with water and mix.
1 ml of this standard solution contains 500 µg of Mo.
NOTE Pure metals, especially those in powder form, are subject to changes in their stoichiometry as a result of
oxidation, therefore check the oxygen content prior to use.
5.14.2 Molybdenum standard solution, corresponding to 5 mg/l of Mo.
Transfer 10,0 ml of the molybdenum standard solution (5.14.1) into a 1 000 ml, one-mark volumetric flask.
Add 500 ml of hydrochloric acid (5.4) dilute to the mark with water and mix.
Prepare this solution immediately before use.
1 ml of this standard solution contains 5 µg of Mo.
6 Apparatus
Dur
...

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