Overview
ISO 6331:1983 specifies a titrimetric method for the determination of chromium content in chromium ores and concentrates having a chromium content greater than 7% (m/m). The standard defines the analytical principle, required reagents and apparatus, detailed procedure (including sample decomposition and titration), and the expression of results. ISO 6331 is intended to be read in conjunction with ISO 6629 (general instructions for chemical analysis of chromium ores).
Key topics and requirements
- Scope: Applicable to products with >7% chromium (m/m).
- Analytical principle:
- Decomposition of a test portion by fusion with sodium peroxide.
- Leaching, acidification and removal of excess peroxide.
- Oxidation of Cr(III) to chromate using ammonium peroxodisulfate with silver nitrate as catalyst.
- Reduction of chromate and titration of resulting Cr(VI) equivalents with ammonium iron(II) sulfate.
- End-point obtained either by visual back-titration with potassium permanganate (KMnO4) or by potentiometric titration using a platinum indicator and calomel/tungsten reference electrode.
- Reagents explicitly specified: sodium peroxide, sulfuric/orthophosphoric acids, manganese(II) sulfate, silver nitrate, ammonium peroxodisulfate, ammonium iron(II) sulfate (standardized), potassium permanganate, potassium nitrite, urea, sodium chloride.
- Sample and decomposition: Typical test portion 0.5 g; fusion heating up to ~800–850 °C; leaching and acid treatment steps described.
- Calculation and reporting:
- Formulae for chromium content by mass and conversion to Cr2O3 using factor 1.4615.
- Permissible tolerance between duplicate determinations: ~0.2% (Cr 7–15%), ~0.3% (15–30%), ~0.4% (>30%).
- Quality controls: Standardization of titrants (KMnO4 and ammonium iron(II) sulfate) and use of blanks.
Applications and users
- Routine laboratory analysis for mining companies, mineral assay laboratories, and quality control departments in processing plants to verify chromium content of ores and concentrates.
- Use in product specification, trade/contract verification, and compliance testing where standardized, comparable chromium assay results are required.
- Useful to regulatory bodies and certification laboratories that require a standardized titrimetric procedure for chromium determination.
Related standards
- ISO 6629 - Chromium ores and concentrates: methods of chemical analysis - general instructions (to be read with ISO 6331).
- Note: ISO 6331 references sampling standards in preparation for chromium ores; users should ensure appropriate sampling standards are applied alongside this analytical method.
Keywords: ISO 6331, chromium ores, chromium content determination, titrimetric method, sodium peroxide fusion, ammonium iron(II) sulfate, potassium permanganate, potentiometric titration, Cr2O3 conversion.
