Overview
ISO 1620:1976 specifies a reduced molybdosilicate spectrophotometric method for the determination of silica (SiO2) in natural and artificial cryolite and in materials with a NaF/AlF3 molar ratio of about 3 to 1.7. The method is intended for products with phosphorus pentoxide (P2O5) ≤ 0.02% (m/m) and is widely used in laboratories supporting aluminium and fluorochemical production.
The analytical sequence comprises an alkaline fusion (sodium carbonate and boric acid), dissolution in excess nitric acid to a defined pH, formation of the oxidized molybdosilicate complex, selective reduction in acidic medium with tartaric acid, and spectrophotometric measurement at ~815 nm.
Key Topics
- Scope and applicability: Natural and synthetic cryolite; P2O5 limited to ≤ 0.02% (m/m).
- Sample preparation: Test portion dried and fused with sodium carbonate and boric acid; controlled furnace steps at 550 ± 25 °C (initial reaction) and at 750 ± 25 °C for completion.
- Chemical reagents: Sodium carbonate, boric acid, nitric acid (≈8 N), sodium molybdate (≈0.8 M), tartaric acid, sulphuric acid, and a reducing solution (sodium metabisulphite or sodium sulfite plus 4-amino-3-hydroxy-1-naphthalene sulphonic acid).
- pH control: Dissolution and dilution steps adjusted so aliquot pH lies between ~0.7 and 0.9 (or 0.3–0.5 for different final volumes) to ensure reproducible colour development.
- Color development and measurement: Formation of yellow oxidized molybdosilicate followed by selective reduction in strongly acidic medium; measurement of the reduced coloured complex at about 815 nm against a compensation (blank) solution.
- Calibration: Multi-point standard silica solutions (e.g., 0.500, 0.020, 0.005 g SiO2 per litre stock dilutions) and plotting of a calibration graph for quantification.
Applications
- Quality control in cryolite manufacturing and handling for aluminium smelting alloys and fluxes.
- Process control where reliable SiO2 measurement in fluoride-rich matrices is required.
- Routine laboratory analysis to establish silica content for material specifications, acceptance testing, and trace-level contamination assessment.
Practical benefits include reproducible results through strict pH and temperature control, specificity provided by the molybdosilicate chemistry, and compatibility with standard spectrophotometers.
Related Standards
- ISO 1619 - Preparation and storage of test samples (sample handling prior to analysis).
- ISO 1693 / 1694 - Determination of fluorine and iron content (complementary compositional tests).
- ISO 2366, 2367, 2830 - Sodium and aluminium determination methods for related analyses.
- ISO 3391–3393, 4277, 4280 - Moisture, fluoride and sulphate methods referenced in cryolite testing programs.
This standard provides a robust spectrophotometric procedure for SiO2 determination in cryolite matrices; labs should follow reagent grades, furnace controls and pH checks as specified to ensure accuracy and comparability of results.
Open preview (PDF)Cryolite, natural and artificial — Determination of silica content — Reduced molybdosilicate spectrophotometric method