Overview
ISO 4280:1977 specifies a barium sulphate gravimetric method for determining the sulphate content of cryolite (natural and artificial) and aluminium fluoride intended for industrial use. The procedure applies to materials with sulphate levels expressed as sulphur trioxide (SO3) of 0.1% (m/m) or greater. The method is a classical gravimetric analysis combining alkaline fusion, acid extraction and precipitation of sulphate as BaSO4, followed by calcination and weighing.
Key topics and technical requirements
- Scope and applicability: Intended for cryolite and aluminium fluoride for industrial applications with SO3 ≥ 0.1% m/m.
- Analytical principle: Alkaline fusion of a test portion with sodium carbonate and boric acid → extraction with perchloric acid → precipitation of sulphate as barium sulphate (BaSO4) using barium chloride → filtration, drying, calcination and weighing.
- Sample size and reagents: Typical test portion ~1 g of dried sample; fusion reagents include anhydrous sodium carbonate (3.75 g) and boric acid (1.25 g). Key reagents: perchloric acid (≈70%), HCl (~6 N), BaCl2·2H2O solution, AgNO3 titration check for completeness.
- Apparatus and conditions:
- Platinum dish and crucible for fusion and residue handling.
- Electric furnace for fusion and calcination at 850 ± 20 °C.
- Electric oven for drying at 110 ± 2 °C.
- Filtration on fine-texture, ashless filter paper (pore diameter ≈ 0.4–1.0 µm).
- Calculation: The mass of BaSO4 recovered (blank-corrected) is converted to SO3 using the specified conversion factor (BaSO4 → SO3).
- Safety note: Evaporation of perchloric acid must be done in a well‑ventilated fume hood and away from ammonia or nitrous vapours.
Practical applications and users
- Quality control and analytical laboratories in the aluminium industry and suppliers of cryolite and aluminium fluoride.
- Product certification, incoming material inspection, and process control where accurate quantification of sulphate impurities is required.
- Useful for regulatory compliance, formulation control, and troubleshooting contamination issues in industries using cryolite or aluminium fluoride.
Related standards
- ISO 1619 and ISO 2925 - preparation and storage of test samples for cryolite and aluminium fluoride.
- Other ISO test methods listed in the standard’s annex, e.g., ISO methods for fluorine, aluminium, sodium, silica and moisture determinations, which support complementary chemical analyses in industrial quality control.
Keywords: ISO 4280:1977, cryolite sulphate determination, aluminium fluoride analysis, barium sulphate gravimetric method, SO3 determination, gravimetric analysis, alkaline fusion, perchloric acid extraction.
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