Overview
ISO 4325:1990 defines a validated titrimetric procedure to determine the chelating agent content in formulated soaps and detergents. The method is designed principally for ethylenediaminetetraacetic acid (EDTA) and its salts, and is suitable for products containing up to about 2% (m/m) chelating agent. It uses copper(II) sulfate as titrant and 1-(2-pyridylazo)-2-naphthol (PAN) as the colour indicator under controlled pH.
Key Topics
- Principle: Dissolve a test portion in water, adjust the solution pH to 4.65, then titrate with a standard copper(II) sulfate solution using PAN as indicator. The endpoint is a colour change from yellow to red-wine.
- Reagents (as cited in the standard):
- Hydrochloric acid solution (c(HCl) = 5 mol/L) for pH adjustment
- Acetate buffer at pH 4.65 (prepared from acetic acid and sodium hydroxide solutions)
- PAN indicator (pyridyl-2-azo-1-naphthol) in ethanol, 0.1% (V/V)
- Standard copper(II) sulfate solution (c(CuSO4) = 0.0100 mol/L) prepared from CuSO4·5H2O
- Apparatus: Typical laboratory glassware, pH meter with electrodes and stirrer, class A burette (50 mL) and heating source to raise sample temperature to about 60 °C during titration.
- Sample preparation: Typical test portion is 10 g dissolved in 100 mL water; electrodes are used to adjust pH to 4.6 ± 0.5; then buffer and PAN are added prior to titration.
- Endpoint and observation: The red-wine colour after titration should persist for at least one minute. A transient or rapidly fading endpoint may indicate the presence of other chelating agents.
- Precision: The standard reports repeatability (same analyst, same lab) and inter-laboratory reproducibility ranges for EDTA up to 2% (m/m). Typical repeatability differences are small (order of 0.01% m/m); reproducibility limits for soaps and detergents are also provided within the document.
Applications
- Quality control of formulated soaps and detergents to verify EDTA content and confirm compliance with formulation specifications.
- Formulation development, enabling chemists to measure chelating agent dosage during product optimisation.
- Regulatory and conformity testing where quantitative determination of sequestrants is required.
Practical considerations:
- The method is principally validated for EDTA; if other chelating agents are present or EDTA is measured in a mixture, confirm method applicability before reporting results.
- Fatty acids or interfering materials can be removed (for example, by filtration of soaps) if necessary to avoid endpoint ambiguity.
Related Standards
- ISO 3696:1987 - Laboratory water quality (analytical reagent grade)
- ISO 385-1:1984 - Laboratory glassware - Burettes - General specifications
- ISO 607:1980 - Sampling and division methods for surface-active agents and detergents
For laboratories implementing ISO 4325:1990, follow the documented reagents, apparatus and operational steps in the standard and report results with full method reference, sample identification and any deviations or observations that could affect the outcome.
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