Overview
ISO 6839:1982 - "Anionic surface active agents - Determination of solubility in water" specifies a simple, reproducible laboratory method to determine the solubility behavior of anionic surfactants as a function of temperature. The procedure establishes the temperatures at which solutions of known concentration change from cloudy to clear on heating and from clear to cloudy on cooling, allowing construction of a solubility curve and, where applicable, estimation of the Krafft temperature.
Key topics and technical requirements
- Principle: Identify the limiting temperatures of solubility by heating and cooling solutions of fixed concentration and observing cloudiness/clarity transitions.
- Scope: Applicable to pure anionic surfactants and technical liquid formulations provided solutions are optically clear and not strongly coloured.
- Sample preparation: Laboratory samples prepared per ISO 667. Use distilled or equivalent purity water only.
- Apparatus: standard lab glassware, borosilicate test tubes (20 mm × 200 mm), precision thermometers complying with ISO 653, and a thermostatically controlled water bath capable of −5 °C to +90 °C with ±0.1 °C control.
- Procedure highlights:
- Perform a preliminary determination by heating ~10 mL aliquots to clear, then cooling to cloud point to bracket a temperature range.
- In a controlled bath, place two tubes of the same concentration (one cloudy, one clear) at equilibrium and adjust bath temperature until one remains clear and the other remains cloudy; record limiting temperatures to 0.1 °C.
- Repeat across a series of concentrations (typically 1–50% w/w) to plot the solubility curve.
- Results: Express solubility as percentage by mass at a given temperature. The standard provides precision/reproducibility information from interlaboratory tests for common surfactants.
Practical applications and users
ISO 6839:1982 is used to:
- Determine formulation limits and storage conditions for detergents, emulsifiers, and anionic surfactant products.
- Support R&D and quality control when selecting surfactant grades or concentrations.
- Identify the Krafft temperature of ionic surfactants in product development.
Primary users include analytical chemists, formulation scientists, QC laboratories in chemical and detergent manufacturing, and technical regulatory teams.
Related standards
- ISO 607 - Surface active agents and detergents - Methods of sample division
- ISO 653 - Thermometer specifications
- ISO 667 - Preparation and storage of laboratory samples
Keywords: ISO 6839:1982, anionic surface active agents, solubility in water, solubility curve, Krafft temperature, surfactants, detergents, sodium laurate, sodium lauryl sulfate.
