ISO 3431:1976

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