Overview
ISO 684:1974 - Analysis of soaps - Determination of total free alkali - specifies a volumetric back-titration method to quantify the total free alkali in commercial (uncompounded) soaps. The procedure dissolves a measured soap sample in 95% ethanol, neutralizes free alkali with a known excess of sulphuric acid, then back-titrates the excess acid with an ethanolic potassium hydroxide solution using phenolphthalein as indicator. Results are reported as percentage by mass of NaOH (for sodium soaps) or KOH (for potassium soaps), or as milliequivalents per gram.
Key topics and requirements
- Scope and limitations
- Applicable to commercial soaps excluding compounded products containing additives (e.g., alkali silicates) that decompose with sulphuric acid.
- Not suitable for coloured soaps if colouration interferes with the phenolphthalein end point.
- Reagents and purity
- Use analytical-reagent grade chemicals and distilled or equivalent-purity water.
- Ethanol 95% (v/v), sulphuric acid (approx. normality), and ~0.1 N ethanolic KOH are specified.
- Phenolphthalein indicator in ethanol (1 g per 100 ml).
- Apparatus and sample
- Standard laboratory glassware: 250 ml conical flask with ground joint and a water-cooled reflux condenser.
- Typical test portion: about 5 g of homogenized laboratory sample.
- Procedure essentials
- Dissolve soap in 100 ml ethanol, reflux to ensure dissolution, add exactly 3.0 ml of sulphuric acid, boil ~10 minutes, cool, then back-titrate with ethanolic KOH to phenolphthalein end point.
- Perform duplicate determinations and report the arithmetic mean.
- Calculation and reporting
- Calculate total free alkali from volumes and normalities of acid and KOH and the mass of sample; express as % NaOH or % KOH or meq/g.
- Reproducibility: inter-laboratory difference should not exceed 0.05% (as NaOH or KOH).
Applications and users
- Who uses ISO 684:1974
- Quality control and analytical laboratories in soap and surfactant manufacturing.
- Raw material suppliers, product formulators, and regulatory compliance teams.
- R&D groups validating formulations and monitoring alkaline balance.
- Practical value
- Ensures consistent alkali levels for product safety, skin compatibility, and performance.
- Supports product specifications, batch release testing, and comparative benchmarking.
Related standards
- ISO 685 (referenced): another ISO document related to soap analysis.
- An ISO standard for soap sampling (in preparation at time of publication) is also cited.
Keywords: ISO 684:1974, analysis of soaps, total free alkali, soap testing, alkali content determination, volumetric titration, phenolphthalein, ethanolic titration, soap quality control.