Overview
ISO 673:1981 specifies a standardized laboratory method for determining the content of ethanol‑insoluble matter in commercial soaps (excluding compounded products). The procedure identifies additives and foreign matter that are poorly soluble or insoluble in 95% (V/V) ethanol - for example, alkali carbonates, chlorides, borates, perborates, sulphates, silicates, phosphates and organic constituents such as starches, dextrins, caseins, sugars, cellulose derivatives and alginates.
This method provides a repeatable approach for quality control, supplier verification and batch release testing.
Key Topics
- Principle: Dissolve a measured soap sample in 95% ethanol, reflux, filter the undissolved residue, dry and weigh the residue; report as percentage by mass.
- Test portion: About 5 g of laboratory sample (weighed to 0.01 g).
- Reagent: Ethanol 95% (V/V).
- Apparatus: 500 ml conical flasks (ground glass), water-cooled reflux condenser, water bath, oven controllable at 103 ± 2 °C, analytical balance accurate to 0.001 g, filter paper, desiccator.
- Drying and weighing: Dry filter paper in oven at 103 ± 2 °C for 1 h, cool in desiccator and weigh to 0.001 g; repeat drying/cooling/weighing until successive weights differ by less than 0.001 g.
- Calculation: Content (%) = (m1 × 100) / m0, where m1 = mass of residue (g) and m0 = mass of test portion (g).
- Special cases: For residues strongly adhering to the flask (e.g., silicated soaps), dissolve the residue in hot distilled water, evaporate, dry at 103 ± 2 °C and add this mass to the filter residue.
- Reproducibility: Inter-laboratory difference shall not exceed 0.05% for contents ≤ 1% and 0.1% for contents > 1%.
Applications
- Quality control: Quantify low‑solubility or insoluble additives and contaminants in soap production.
- Raw material verification: Confirm supplier claims and detect foreign matter (inorganic salts, fillers, organic binders).
- Batch release & troubleshooting: Monitor production consistency and investigate unexpected residues or formulation issues.
- Regulatory and specification compliance: Provide documented, repeatable data for product specifications and regulatory records.
Related Standards
- Other ISO methods and national standards for surfactants and soap testing, and standards governing sampling and laboratory sample preparation (sampling procedures may be covered by separate standards).
Notes for practitioners: follow the specified apparatus and temperatures precisely, document any deviations or incidents in the test report (sample identification, method reference, results and test conditions) and ensure careful washing with hot ethanol to remove soluble soap components before final drying and weighing.