Overview
ISO 6228:1980 specifies a general reduction and titrimetry method for determination of trace amounts of sulphur compounds expressed as sulphate (SO4 2-) in chemical products for industrial use. The method converts sulphur species to sulphate when necessary, reduces sulphate to hydrogen sulphide (H2S) using a mixture of hydriodic (HI) and phosphinic (hypophosphorous) acids in hydrochloric acid, traps the H2S in a sodium hydroxide/acetone absorber and titrates the resulting sulphide with standard mercury(II) acetate or nitrate using 1,5‑diphenyl‑3‑thiocarbazone (dithizone) as indicator. The procedure is intended for trace sulphate determinations in the low microgram range with an expected precision of about ±5%.
Key topics and technical requirements
- Scope: trace sulphur compounds quantified as sulphate; applicable to solutions and suitably pretreated samples.
- Typical working range: method addresses sulphate contents on the order of tens to a few hundred micrograms (document examples indicate ranges such as ~45–450 µg SO4 2-).
- Reagents of analytical grade: acetone; oxygen‑free nitrogen; NaOH (40 g/L); HI, phosphinic acid, HCl for the reducing solution; dithizone in acetone; standard Hg(II) titrants (0.001 mol/L).
- Apparatus: gas‑tight reduction and entrainment assembly (three‑neck reflux flask, condenser, receiver), microburette for titration.
- Procedure highlights: convert sulphur to sulphate if required (see Annex A for mineralization methods), reflux reduction under N2, entrain H2S into absorber, titrate until colour change with dithizone.
- Quality checks: blank and check tests to confirm gas‑tightness and quantitative recovery; specified acceptable check-test titrant volumes.
- Safety and waste: mercury compounds are toxic - prepare titrants in a fume hood and follow Annex C for mercury removal (oxidize, precipitate with sodium sulfide, recover mercury residues).
Practical applications
- Determination of trace sulphur in industrial chemical products, where control of sulphur content is critical (raw materials, intermediates, laboratory QC).
- Useful in petrochemical, specialty chemicals, and materials industries requiring low-level sulphur analysis.
- Applicable when organic sulphur species require mineralization prior to analysis (Annex A lists combustion, fusion and other mineralization routes).
Who should use this standard
- Analytical chemists and laboratory managers performing trace sulphur analysis.
- Quality assurance and compliance teams in chemical manufacturing and processing.
- Laboratories seeking an ISO‑recognized titrimetric protocol for sulphate expressed as sulphur.
Related information
- ISO 6228:1980 was developed by ISO/TC 47 (Chemistry) and approved in association with IUPAC. Laboratories should also consult product‑specific ISO standards for test‑portion preparation and any additional method modifications.