Overview
ISO 456:1973 - "Surface active agents - Analysis of soaps - Determination of free caustic alkali" specifies two validated laboratory methods for measuring free caustic alkali in commercial soaps. The standard covers scope, reagents, apparatus, stepwise procedures and expression of results for the ethanol (Method A) and barium chloride (Method B) techniques. It applies to ordinary commercial soaps (not compounded products) and was published by ISO/TC 91 in 1973.
Key topics and technical requirements
- Two alternative methods
- Method A (Ethanol method): dissolve soap in neutralized absolute ethanol, remove CO2 by reflux, then titrate free alkali with an ethanolic hydrochloric acid solution. Applicable mainly to sodium soaps of ordinary quality; not suitable for potassium soaps due to potassium carbonate solubility in ethanol.
- Method B (Barium chloride method): precipitate soap and carbonate with barium chloride in an ethanol/water medium, then titrate the residual alkalinity (representing free KOH) with hydrochloric acid. Recommended for soft potassium soaps and mixed sodium/potassium soaps.
- Reagents and concentrations
- Absolute ethanol (ρ = 0.792 g/ml), ethanolic KOH ~0.1 N, ethanolic HCl ~0.1 N, phenolphthalein indicator in ethanol, and a 60% (V/V) ethanol solution for Method B.
- Apparatus
- Standard laboratory glassware: 500 ml flask (or conical wide-neck flask), reflux condenser, analytical balance.
- Procedure highlights
- Removal of CO2 by gentle reflux, careful neutralization of ethanol prior to sample dissolution (Method A), controlled precipitation with barium chloride (Method B), and endpoint detection with phenolphthalein/thymol-blue indicator mixtures.
- Results and reporting
- Results may be expressed as percentage NaOH (for sodium soaps) or KOH (for potassium soaps), or as milliequivalents/kg. Calculations use titrant volume (V), titrant normality (T) and test portion mass (m). Reproducibility: Method A ±0.02 (absolute), Method B ±0.05 (absolute).
- Test report must state the method used, results and expression, any unusual observations, and any optional operations performed.
Applications
- Quality control in soap manufacturing to quantify residual caustic alkali that affects product safety, skin irritation potential and finished-product quality.
- Regulatory compliance and specification verification for sodium and potassium soap products.
- Troubleshooting production issues such as incomplete neutralization or saponification.
Who should use this standard
- Laboratory analysts in personal care and detergent manufacturing.
- Quality assurance teams in soap production plants.
- R&D chemists developing soap formulations.
- Regulatory bodies and conformity assessment laboratories assessing soap safety and composition.
Related standards
- ISO 456:1973 is part of ISO/TC 91 work on surface active agents. For broader surfactant and soap testing you may consult other ISO documents produced by ISO/TC 91 (refer to your national ISO member body for the latest related standards).