Overview
ISO 4318:1989 specifies an azeotropic distillation (Dean–Stark) method for determining the water content of surface active agents and soaps. The procedure removes water from a test portion by azeotropic distillation with boiling xylene or petroleum and measures the collected aqueous phase volume. The method is intended for powders with water content greater than 5 % (m/m), and for products in paste or solution form. It is not applicable to samples containing water-soluble volatile compounds (for example, ethanol).
Key practical points:
- Perform the test in a fume hood because xylene is toxic.
- Read the volume of collected water at a reference temperature of 20 °C.
- Clean apparatus thoroughly (e.g., sulfochromic wash, distilled water, acetone) and dry before use.
Key Topics
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Principle: Azeotropic distillation of water with xylene (boiling range ~130–140 °C) or petroleum (boiling range ~180–220 °C). The distillate is collected in a graduated Dean–Stark trap and the water layer is measured directly.
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Apparatus & reagents:
- 500 ml round-bottom flask (minimum)
- Graduated Dean–Stark (2 ml or 10 ml capacity) - 2 ml scale: 0.1 ml divisions (max error 0.05 ml); 10 ml scale: 0.2 ml divisions (max error 0.1 ml)
- Reflux condenser adapted to the flask and Dean–Stark
- Analytical-grade xylene or petroleum; boiling chips or boiling regulators
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Typical test variables from the standard:
- Sample mass: typically 10–50 g so the trap is at least half full at the end of the run
- Solvent volume: typically 100–300 ml xylene or petroleum
- Heating: maintain distillation (about 2–3 drops/s) until the distillate is clear and no further water separates
- Anti-foaming: add paraffin or dry oleic acid if foaming interferes
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Calculation:
- Water content (mass %) = 100 × V / m
- V = volume of collected water in millilitres (read at 20 °C)
- m = mass of test portion in grams
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Reproducibility: The standard reports inter-laboratory differences should not exceed about 1 % for the same sample.
Applications
- Quality control and routine analysis of surfactants, soaps, and related formulations where water content impacts processing, stability, or performance.
- Suitable for powders (>5 % water), pastes and solutions when volatile water‑soluble compounds are absent.
Related Standards
- ISO 383 - Laboratory glassware (ground joints)
- ISO 607 - Sampling of surfactants and detergents (sample division)
- ISO 8212 - Sampling and sampling techniques for soaps and detergents
This method provides a direct, industry‑accepted approach to quantify free water in surfactant and soap materials using azeotropic distillation, with clear procedural and apparatus requirements to support reproducible QC measurements.
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