Overview
ISO 2997:1974 specifies a volumetric titrimetry method for determining the sulphate (SO4) content in industrial phosphoric acid (orthophosphoric acid). The standard applies to products with sulphate contents ≥ 0.0005 % (m/m). The procedure reduces sulphate to sulphide, traps the evolved hydrogen sulphide (H2S), and quantifies sulphur as sulfide by titration with a standard mercury(II) solution using dithizone as indicator. The method has also been approved by IUPAC.
Key topics and requirements
- Scope and detection limit: Applicable to industrial phosphoric acid samples with SO4 ≥ 0.0005 % (m/m).
- Analytical principle: Reduction of sulphate to sulfide using a mixture of hydriodic acid and hypophosphorous acid, followed by absorption of H2S in an acetone + sodium hydroxide solution and titration with mercury(II) acetate in the presence of dithizone indicator.
- Reagents and standards:
- A reference sodium sulfate solution is prepared such that 1 mL = 96 µg SO4 (documented preparation detail).
- Standard titrant: 0.001 M acetate of mercury(II) (or, as a variant, 0.001 M mercury(II) nitrate).
- Fresh preparation of reducing solution (hydriodic + hypophosphorous + hydrochloric acids) under nitrogen and reflux is specified.
- Apparatus: Typical laboratory glassware plus a reduction/distillation assembly (roded joints, reflux condenser), microburette (0.01 mL divisions) and tubing for inert gas (nitrogen) flow.
- Procedure controls: Includes an apparatus leak/function control, a blank determination, and a standard reference test. Formulas to convert titrant volumes to mass-percent SO4 are provided.
- Quality and safety notes: Uses corrosive and toxic reagents (HI, H3PO2, H2S, mercury compounds); appropriate laboratory safety and waste handling are required.
Applications and users
- Who uses this standard:
- Quality control laboratories in phosphoric acid production and phosphate fertilizer industries.
- Chemical manufacturing QC and analytical service labs that monitor sulphate impurities.
- Regulatory and standards laboratories requiring a validated, internationally recognized titrimetric method.
- Practical applications:
- Routine sulphate monitoring to meet product specifications.
- Comparative testing and batch release testing.
- Trace-level sulphate determinations where classical titrimetry is preferred over instrumental methods.
Related standards
- ISO 2997:1974 is noted as IUPAC-approved. The standard mentions an alternate mercury(II) nitrate titrant variant. Laboratories may compare this classical titrimetric method with modern instrumental techniques (ion chromatography, ICP) depending on accuracy, throughput and safety considerations.
Keywords: ISO 2997, phosphoric acid, sulphate determination, sulfate analysis, titrimetry, reduction method, hydriodic acid, hypophosphorous acid, mercury(II) acetate, dithizone, hydrogen sulfide, industrial phosphoric acid analysis.
Open preview (PDF)Phosphoric acid for industrial use — Determination of sulphate content — Method by reduction and titrimetry
