Overview
ISO 685:2020 - Analysis of soaps - Determination of total alkali content and total fatty matter content - is a standardized laboratory method for simultaneous measurement of the alkali (NaOH or KOH-equivalent) and the water‑insoluble fatty matter in soaps, including liquid soaps (compounded products are excluded). This second edition (2020) updates the original procedure and supports consistent, comparable soap testing across quality control and certification laboratories.
Key topics and technical requirements
- Scope and applicability: Method applies to solid and liquid soaps but not compounded formulations.
- Principle: Decompose soap with a known volume of standard mineral acid, extract liberated fatty matter with light petroleum, titrate the excess acid in the aqueous phase with standard NaOH to determine total alkali, then recover and titrate/evaporate the fatty extract (converted to potassium soap) to determine total fatty matter.
- Reagents and solvents: Analytical grade reagents (e.g., sulphuric acid ~0.5 mol/L, NaOH, ethanolic KOH, light petroleum 30–60 °C, acetone, ethanol) and indicators (methyl orange, phenolphthalein).
- Apparatus: Common laboratory glassware per ISO 3819 - beakers, separating funnels or extraction cylinder, water bath, oven (103 ± 2 °C), grinder (cheese grater) for bar soaps.
- Sampling: Sampling during manufacture according to ISO 8212; solid bars should be grated and stored airtight to avoid moisture loss.
- Procedure essentials: Duplicate determinations; dissolve sample, acidify, extract fatty matter, wash extracts, titrate aqueous phase for alkali (methyl orange or pH meter where color interferes), evaporate and titrate extract to quantify fatty matter, dry to constant mass.
- Expression of results: Total alkali reported as % mass fraction of NaOH or KOH (or milliequivalents/g); total fatty matter reported as % mass fraction. Inter‑laboratory reproducibility for total alkali: differences should not exceed 0.2 %.
Applications
- Routine soap quality control and batch release testing
- Raw material and formulation verification for soap manufacturers and ingredient suppliers
- Regulatory compliance testing and product specification checks
- Independent testing laboratories and dispute resolution where standardized, traceable soap analysis is required
- R&D when assessing changes in saponification, alkali balance, or fatty matter content
Who uses this standard
- Soap and personal-care product manufacturers
- Quality assurance and QC laboratories
- Third‑party testing and certification bodies
- Raw material suppliers and formulators
- Regulatory agencies and standards organizations
Related standards
- ISO 3819 - Laboratory glassware (beakers)
- ISO 8212 - Soaps and detergents - Techniques of sampling during manufacture
Keywords: ISO 685:2020, analysis of soaps, total alkali content, total fatty matter, soap testing, liquid soaps, titration, extraction, quality control.