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.
