Overview
ISO 457:1983 - Soaps - Determination of chloride content - Titrimetric method specifies an argentometric (silver‑based) titration procedure for measuring chloride in commercial soaps. The method determines chloride as sodium chloride (NaCl) (or expressed as potassium chloride KCl) after decomposition of a soap test portion and separation of fatty acids by filtration. It applies to soaps with chloride content ≥ 0.1% (m/m) and is not applicable to compounded/composite products.
Key topics and technical requirements
- Principle: Chloride is precipitated as silver chloride after sample decomposition; the precipitate is removed by filtration and chloride is quantified by back‑titration of excess silver with ammonium thiocyanate using ammonium iron(III) sulfate as indicator.
- Typical sample/reagents:
- Test portion: ~5 g of soap (weighed to 0.01 g).
- Dissolution in ~50 mL hot water; add ~5 mL concentrated nitric acid.
- Add 25.0 mL of standardized AgNO3 solution (c = 0.10 mol/L).
- Reagents specified in the standard include nitric acid (70–80%), ammonium iron(III) sulfate (~10% w/w), ammonium thiocyanate (standardized), and standardized silver nitrate.
- Glassware & apparatus: 100 mL beaker, 200 mL volumetric flask, boiling water bath, filtration equipment (filter paper), pipettes - with reference to ISO glassware specifications.
- Procedure highlights:
- Heat to separate fatty acids and allow AgCl to aggregate.
- Filter, discard first 10 mL filtrate and collect ≥110 mL; pipette 100.0 mL aliquot for titration.
- Titrate with NH4SCN to a stable red‑brown endpoint.
- Calculation: The standard provides conversion formulas and constants (e.g., factors 0.0585 for NaCl and 0.0746 for KCl) to convert titrant volumes, AgNO3 concentration and sample mass into percentage chloride (m/m).
- Performance: Inter‑laboratory reproducibility guidance - differences between two laboratories on the same sample should not exceed 0.05% (m/m) chloride.
Applications and users
- Who uses it: soap manufacturers, quality control (QC) laboratories, raw material testers, regulatory and standards laboratories, and third‑party testing services.
- Practical uses:
- Routine quality control to monitor salt content (affects soap hardness, solubility and performance).
- Raw material acceptance testing and batch release.
- Troubleshooting production issues related to chloride contamination or formulation.
- Benefits: Established, reproducible volumetric method suitable for routine laboratory workflows where chloride content is ≥0.1% (m/m).
Related standards
- ISO 4323 - potentiometric method (noted as applicable for chloride contents below or above 0.1% m/m).
- ISO 3819 and ISO 1042 - referenced for glassware specifications.
Keywords: ISO 457:1983, soaps chloride determination, titrimetric method, argentometric titration, sodium chloride in soap, soap quality control, chloride content testing.