Overview
ISO 3431:1976 specifies a turbidimetric method for determining the content of soluble sulphates (SO4) in sodium fluoride primarily used for aluminium production. The standard applies to products with soluble sulphate contents greater than 0.05% (m/m). The procedure precipitates sulphate as barium sulphate (BaSO4) and measures the resulting turbidity using a spectrophotometer or photoelectric absorptiometer at a wavelength of about 470 nm.
Key topics and technical requirements
- Scope: Analytical method for soluble sulphates in industrial sodium fluoride used in aluminium manufacture; applicable when SO4 > 0.05% (m/m).
- Sample preparation: Test portion is dissolved in hydrochloric acid, evaporated to dryness in hydrochloric acid solution, and the residue re-dissolved in hydrochloric acid solution (references ISO 3428 for sampling/drying).
- Reagents and materials:
- Hydrochloric acid (approx. 38% / ~12 N and ~1 N solutions)
- Calibrated barium chloride dihydrate (specified particle size)
- Standard sulphuric acid solution (equivalent to 0.200 g SO4 per litre)
- Glycerine and an acidic sodium chloride solution (240 g/L)
- Turbidimetric reaction:
- Precipitation of BaSO4 by adding a measured amount of BaCl2 to aliquots of the prepared sample solution
- Manual agitation for 1 minute (two turns per second), then letting the suspension rest for 20 minutes at 20 ± 2 °C
- Measurement and calibration:
- Turbidity measured at ~470 nm with matched cuvettes
- Calibration curve constructed from standard SO4 solutions (blank corrections and compensation solution required)
- If turbidity exceeds calibration range, dilution and re‑analysis with appropriate compensation are specified
- Results expression: Sulphate content reported as mg or percentage (m/m) SO4 based on calibration and blank corrections; test report must list method reference, results and any deviations.
Applications and users
- Industrial quality control in aluminium production where sodium fluoride is used as a flux or raw material
- Chemical and analytical laboratories responsible for incoming material inspection and specification compliance
- Producers and suppliers of sodium fluoride who must certify soluble sulphate levels
- Regulatory and standards bodies checking conformity to ISO requirements
Keywords: ISO 3431, sodium fluoride, soluble sulphates, turbidimetric method, BaSO4 precipitation, spectrophotometer, aluminium production, analytical method.
Related standards
- ISO 3428 - Preparation and storage of samples for testing
- ISO 3429 - Iron determination (related analyses)
- ISO 3430 - Silica determination
- ISO 3566 - Chloride determination, turbidimetric method
- ISO 4278 - Carbonate determination (gravimetric)
- ISO 2831 / 2832 / 2833 - Other sodium fluoride test methods and properties
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