Overview
ISO 6387:1983 - "Surface active agents - Determination of the power to disperse calcium soap - Acidimetric method (Modified Schönfeldt method)" - specifies an acidimetric test to quantify the dispersing power of surfactants against calcium soap. The method determines the minimum amount of a dispersing agent required to keep at least 95% of calcium soap dispersed for one hour. Typical test conditions use a 0.5% (m/m) soap solution and controlled temperatures (27 °C or 40 °C).
Key topics and requirements
- Test principle: Prepare a 0.5% aqueous soap solution, equilibrate at the testing temperature for 24 hours, take an aliquot, mix with serial dilutions of the dispersing agent and water of known calcium hardness, hold for 1 hour, then titrate an aliquot of the lower layer to measure dispersed calcium soap.
- Acidimetric titration: Titrate with 0.01 mol·L−1 hydrochloric acid (HCl) using bromocresol green as indicator; compare titrant volumes for initial soap solution (V0) and for samples after treatment (Vs) to calculate dispersing power.
- Reagents: sodium oleate standard soap solution, standard HCl titrant, bromocresol green indicator, and water of known calcium hardness prepared per ISO 2174.
- Apparatus and conditioning: graduated cylinders (ISO 4788), pipettes (ISO 648), thermostatically controlled bath (27 ± 0.5 °C or 40 ± 0.5 °C). Avoid presence of alkaline inorganic salts (phosphates, carbonates, silicates) which interfere with acidimetric endpoints.
- Expression of results: Result expressed as grams of soap dispersed per gram of dispersing agent (dispersing power). Calculation is based on the difference in titrant volumes (V0 and Vs) and the concentration/volume of dispersant used.
- Precision: Interlaboratory comparisons (nine labs) and precision data are provided for typical surfactants at 40 °C.
Applications and users
- Surface-active agent manufacturers performing quality control and batch release testing.
- Detergent and formulation chemists evaluating flocculation resistance and calcium soap dispersion performance in hard water.
- Analytical and R&D laboratories assessing raw materials (e.g., builders, surfactants) and optimizing formulations for hard-water stability.
- Regulatory and standards organizations comparing dispersant effectiveness or verifying supplier claims.
Related standards
- ISO 2174 - Preparation of water with known calcium hardness (referenced for test water).
- ISO 4788 - Graduated measuring cylinders specifications.
- ISO 648 - Pipette specifications.
Keywords: ISO 6387, surfactants, disperse calcium soap, acidimetric method, Schönfeldt method, dispersing power, calcium soap dispersion test, bromocresol green, sodium oleate, water hardness.