ISO 6331:2024
(Main)Chromium ores and concentrates — Determination of chromium content — Titrimetric method
Chromium ores and concentrates — Determination of chromium content — Titrimetric method
This document specifies a titrimetric method for the determination of the chromium content of chromium ores and concentrates having a chromium content greater than a mass fraction of 7,00 %.
Minerais et concentrés de chrome — Dosage du chrome — Méthode titrimétrique
General Information
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Standards Content (Sample)
International
Standard
ISO 6331
Second edition
Chromium ores and concentrates —
2024-11
Determination of chromium content
— Titrimetric method
Minerais et concentrés de chrome — Dosage du chrome —
Méthode titrimétrique
Reference number
© ISO 2024
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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 Sample . 4
8 Procedure . 5
8.1 Test portion .5
8.2 Blank test .5
8.3 Number of determinations .5
8.4 Decomposition of the test portion .5
8.5 Determination .6
8.5.1 Visual titration .6
8.5.2 Potentiometric titration.6
9 Expression of results . 6
9.1 Calculation of chromium content in case of visual titration .6
9.2 Calculation of chromium content in case of potentiometric titration.7
9.3 Conversion from chromium content to chromium oxide content (percentage) .8
9.4 General treatment of results .8
9.4.1 Expression of precision .8
9.4.2 Determination of analytical result .8
9.4.3 Between-laboratories precision .9
9.4.4 Check for trueness .9
9.4.5 Calculation of final result .10
10 Test report . 10
Annex A (informative) Additional information on the international interlaboratory test .11
Annex B (normative) Flow chart for the acceptance procedure of test results .18
Bibliography . 19
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
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of ISO document should be noted. This document was drafted in accordance with the editorial rules of the
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This document was prepared by Technical Committee ISO/TC 132, Ferroalloys.
This second edition cancels and replaces the first edition (ISO 6331:1983), which has been technically
revised.
The main changes are as follows:
— in 6.1 (previously in 5.1), the description for the potentiometric titration apparatus has been changed
to be in line with the current status of the development of the instrument, including the description of
electrodes pairs in 6.1.1, titration assembly in 6.1.2 and millivoltmeter in 6.1.3;
— in 6.2, the specifications of crucibles have been added;
— in 8.1, the pre-treatment of test portion and the recommended test portion masses have been adjusted;
— in 8.4 (previously in 6.2), the decomposition conditions of the test portion have been changed including
the specification of beaker, the detailed use of crucibles, the diluted concentration of sulfuric acid, and
the option of whether to filter residues or not;
— in 8.5.2 (previously in 6.3.2), the 60 ml of the sulfuric acid solution has been omitted;
— in 9.4 (previously in 7.4), the paragraph of the "permissible tolerances on results" has been replaced with
the "general treatment of results";
— Clause 10, the test report has been added;
— Annex A (informative), "Additional information on the international interlaboratory test" has been added;
— Annex B (normative), "Flow sheet of the procedure for the acceptance of test results" has been added.
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 6331:2024(en)
Chromium ores and concentrates — Determination of
chromium content — Titrimetric method
1 Scope
This document specifies a titrimetric method for the determination of the chromium content of chromium
ores and concentrates having a chromium content greater than a mass fraction of 7,00 %.
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 6129, Chromium ores — Determination of hygroscopic moisture content in analytical samples —
Gravimetric method
ISO 6153, Chromium ores — Increment sampling
ISO 6154, Chromium ores — Preparation of samples
ISO 80000-1:2022, Quantities and units — Part 1: General
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
Decompose a test portion by fusing with sodium peroxide. Leach the melt with water, acidify it with sulfuric
acid, and remove the excess of hydrogen peroxide by boiling.
Oxidize chromium (III) ions to chromate with ammonium peroxodisulphate in the presence of silver nitrate
as catalyst.
Titrate chromium (VI) with ammonium iron (II) sulfate, the end-point is obtained either visually, by adding
an excess of ammonium iron (II) sulfate and back-titrating with potassium permanganate, or directly by
potentiometric titration.
5 Reagents
During the analysis, unless otherwise stated, use only reagents of recognized analytical grade and only
grade 3 water as specified in ISO 3696.
5.1 Sodium peroxide, in powdered and granulated forms.
5.2 Sulfuric acid, ρ 1,84 g/ml.
5.3 Orthophosphoric acid, ρ 1,70 g/ml.
5.4 Urea.
5.5 Sulfuric acid, diluted 1 + 1.
5.6 Sulfuric acid, diluted 1 + 4.
5.7 Manganese (ll) sulfate, 100 g/l solution.
Dissolve 100 g of manganese (II) sulfate heptahydrate (MnSO ·7H O) in 1 l of water.
4 2
5.8 Manganese (II) sulfate, 1 g/l solution.
Dilute 10 ml of the manganese (II) sulfate solution (see 5.7) to 1 l with water.
5.9 Silver nitrate, 1 g/l solution.
Dissolve 1 g of silver nitrate in 1 l of water. To ensure greater stability of the silver nitrate, add 0,5 ml of
nitric acid for each 1 l of solution. Store the solution in a brown glass bottle.
5.10 Ammonium peroxodisulfate, 250 g/l solution.
Dissolve 250 g of ammonium peroxodisulfate in 1 l of water. Prepare the solution immediately before use.
5.11 Sodium chloride, 50 g/l solution.
Dissolve 50 g of sodium chloride in 1 l of water.
5.12 Potassium permanganate, standard volumetric solution, c(1/5 KMnO ) ≈ 0,1 mol/l.
5.12.1 Preparation of the solution
Dissolve 32 g of potassium permanganate in 1 000 ml of water, transfer to a 10 l brown glass bottle, add 9 l
of water, mix and allow to stand for 7 days to 10 days. Transfer the solution, using a siphon to another brown
glass bottle (insert the siphon pipe in the bottle so that its end is 15 mm above the bottom). Alternatively,
the solution can be filtered through calcined asbestos.
5.12.2 Standardization of the solution
Place 0,2 g of anhydrous sodium oxalate, previously dried at
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