ISO 3425:1975

Overview

ISO 3425:1975 defines standardized laboratory procedures for the determination of ash at 850–900 °C and residue at 200 °C for sulphur intended for industrial use. The methods apply to products containing at least 98% elemental sulphur, expressed as a percentage by mass of the product dried at 80 °C for 2 h. The difference between the ash and the residue corresponds to the non-volatile bituminous matter content at 200 °C.

Key procedural elements include slow combustion in air, controlled high-temperature ashing, and slow evaporation under nitrogen to remove volatile sulphur and volatile matter prior to weighing.

Key Topics

• Scope and applicability: Intended for industrial sulphur with ≥98% elemental sulphur (dry basis).
• Principle for ash (850–900 °C):
  • Slow combustion of a dried test portion in air.
  • Final heating in an electric furnace controlled at 850–900 °C and weighing to constant mass.
• Principle for residue (200 °C):
  • Slow evaporation at 200 ± 10 °C in a nitrogen atmosphere to remove volatile matter and sulphur.
  • Residue weighed after about 10 h until constant mass.
• Sampling & test portions:
  • Dry sample at 80 °C for 2 h before testing.
  • Typical test portion: about 50 g for ash determination; about 2 g for residue determination.
• Apparatus and control:
  • Silica tall-form dishes, electric furnaces and ovens with controlled temperature ranges.
  • Nitrogen flow for residue testing (e.g., adjusted to about 2–3 bubbles/sec through a gas-washing bottle).
• Weighing precision & repeatability:
  • Ash determinations use weighing to 0.001 g and repeat until two successive weighings differ by ≤ 0.005 g.
  • Residue determinations use weighing to 0.0002 g and repeat until two successive weighings differ by ≤ 0.001 g.
• Expression of results:
  • Results expressed as percentage by mass of the dry product, to three decimal places for ash where specified.

Applications

• Quality control: Verifies sulphur purity and quantifies incombustible ash and non-volatile bituminous matter.
• Product specification and acceptance: Provides reproducible test methods for suppliers and purchasers of industrial sulphur.
• Process monitoring: Detects changes in impurity levels during refining or handling that affect downstream processes.
• Regulatory and compliance reporting: Supplies standardized data for conformity with contractual or regulatory requirements.

Related Standards

• ISO 2866 – Determination of total carbon content (titrimetric method).
• ISO 3426 – Determination of loss in mass at 80 °C.
• ISO 3704 – Determination of acidity (titrimetric / photometric methods).
• ISO 3705 – Determination of arsenic content.

For laboratory implementation, follow the detailed apparatus, sampling, procedural steps and result expression specified in ISO 3425:1975 to ensure reproducibility and traceability of ash and residue determinations for industrial sulphur.