ISO 3704:1976

Overview

ISO 3704:1976 specifies a titrimetric method for determining the acidity of sulphur for industrial use. The method is applicable to products with acidities (expressed as H2SO4) of 0.01% (m/m) or greater. It defines sample preparation, solvent extraction with a water/ propan-2-ol (isopropyl alcohol) mixture, and titration of the extract using standard volumetric sodium hydroxide in the presence of phenolphthalein indicator.

Key Topics and Requirements

• Scope: Measures acidity in industrial sulphur products ≥ 0.01% H2SO4 (m/m).
• Sample preparation: Prepare the test sample by grinding undried material until it passes a 250 µm nominal sieve; use a representative test portion (about 25 g).
• Extraction: Wet the test portion with ~25 ml propan-2-ol, add ~50 ml water, shake and allow to stand (with occasional shaking) to leach acidic substances into the aqueous phase.
• Reagents:
  • Propan-2-ol (previously boiled, cooled and neutralized),
  • Sodium hydroxide, standard volumetric (carbonate-free) - standard in the document is 0.1 N but corrections apply when concentration differs,
  • Phenolphthalein indicator (10 g/L ethanolic solution).
• Titration: Titrate the extract with NaOH to the first permanent pink end point in presence of phenolphthalein.
• Calculation: Acidity is calculated from the volume of NaOH used and the mass of the test portion. The Standard notes that 0.049 g H2SO4 corresponds to 1 ml of exactly 1 N NaOH; apply appropriate correction if the NaOH concentration differs.
• Reporting: Include method reference, result expression, sample details and any deviations or unusual observations.

Applications and Users

• Quality control laboratories in sulphur production and refining.
• Chemical manufacturers and suppliers who must ensure sulphur acidity meets product specifications.
• Industrial end-users (rubber, fertilizers, sulfuric acid production) verifying raw-material compliance.
• Regulatory and testing bodies performing standardized acidity determinations for trade, safety, or specification conformance.

Keywords: ISO 3704:1976, sulphur acidity determination, titrimetric method, propan-2-ol extraction, sodium hydroxide titration, phenolphthalein.

Related Standards

• ISO 2866 - Determination of total carbon (titrimetric).
• ISO 3425 - Ash content at 850–900 °C and residue at 200 °C.
• ISO 3426 - Loss in mass at 80 °C.
• ISO 3705 - Determination of arsenic content.
• ISO 5793 - Determination of chloride content.

This standard provides a clear, reproducible titrimetric procedure for laboratories needing reliable acidity data on industrial sulphur, supporting quality assurance, specification compliance and comparative testing.