Overview
ISO 2993:1974 specifies a titrimetric method for determining the free acidity in ammonium sulphate for industrial use. Applicable to products with free acidity (expressed as H2SO4) of ≥ 0.01% (m/m), the standard describes dissolving a test portion in water and titrating with 0.1 N sodium hydroxide in the presence of a suitable pH indicator. ISO 2993 is also approved by IUPAC, ensuring international recognition for laboratory and quality-control use.
Key topics and technical requirements
- Scope: Determination of free acidity in industrial ammonium sulfate when ≥ 0.01% H2SO4.
- Test portion: Approximately 100 g of sample (weighed to 0.1 g).
- Sample preparation: Dissolve in about 500 ml of water at 20–25 °C; filter if turbid and collect filtrate and washings.
- Titrant: 0.1 N standard volumetric sodium hydroxide solution.
- Indicator: Methyl purple (10%) or any indicator with a colour change in the pH range 5.2–5.6. Endpoint described as red‑violet to brilliant green.
- Alternate endpoint detection: Titration may be performed using a pH meter instead of a visual indicator.
- Calculation: Free acidity expressed as % H2SO4 is calculated from the volume (mL) of 0.1 N NaOH used and the mass of the test portion, using the factor that 1 mL of 0.1 N NaOH corresponds to 0.0049 g H2SO4.
- Apparatus: Ordinary laboratory glassware and equipment; no specialized instruments required.
Applications and who uses it
ISO 2993 is intended for:
- Manufacturers of ammonium sulphate (fertilizer and industrial grades) for product specification and quality control.
- Analytical and QC laboratories performing routine acidity checks on incoming or final product batches.
- Procurement and compliance teams specifying acceptable acidity limits in contracts and technical specifications.
- Regulatory bodies and certification labs verifying product conformity with international testing practices.
Practical benefits include simple, reproducible titrimetric testing that supports process control, product quality assurance, and supplier verification.
Related standards
The standard is part of a series of test methods for industrial ammonium sulphate:
- ISO 2992 – Iron content (2,2’-bipyridyl photometric method)
- ISO 2994 – Matter insoluble in water (gravimetric)
- ISO 3332 / 3333 – Related determinations (ammoniacal nitrogen, copper content) - see annex for status
Keywords: ISO 2993, ammonium sulphate, free acidity, titrimetric method, H2SO4 determination, 0.1 N sodium hydroxide, methyl purple, industrial quality control.
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