Overview
ISO 4321:1977 specifies a titrimetric method for the determination of active oxygen in commercial washing powders. The method targets peroxyhydrates (for example, sodium perborate) and is based on the co‑reduction of the peroxyhydrate and potassium permanganate in an acid solution with liberation of oxygen. The standard addresses dissolution challenges specific to powdered detergents and defines a dissolution procedure suitable for accurate active oxygen measurement.
Keywords: washing powders, active oxygen, titrimetric method, peroxyhydrates, sodium perborate, potassium permanganate, quality control, ISO 4321:1977.
Key Topics
- Principle: Co‑reduction of peroxyhydrate and KMnO4 in a 5 N sulfuric acid medium; titration endpoint indicated by a pale pink coloration persisting for at least 15 s.
- Specific reagents: aluminium sulphate octadecahydrate, bismuth nitrate pentahydrate, manganese sulphate monohydrate, freshly standardized potassium permanganate (~0.1 N), and 5 N sulfuric acid solutions.
- Apparatus: ordinary laboratory equipment including a 1 000 ml one‑mark volumetric flask, 500 ml conical flask, 2 000 ml beaker and mechanical stirrer (referenced to ISO glassware requirements where applicable).
- Sampling and test portion: laboratory sample prepared according to ISO 607; typically about 10 g weighed to 0.01 g for analysis.
- Procedure highlights:
- Dissolve the test portion in warm water (35–40 °C) using a one‑mark 1 000 ml volumetric flask; agitation and a specific dissolution approach are used to minimize loss of oxygen.
- Prepare Solution L with added bismuth and manganese in 5 N sulfuric acid; take 100.0 ml aliquot of Solution L for titration.
- Titrate with standard potassium permanganate until a persistent pale pink end‑point is reached; if the end‑point is not sharp, aluminium sulphate or adjusted acid volumes may be used.
- Expression of result: Active oxygen content is calculated from the volume (V, mL) and normality (T) of KMnO4 and the mass (m, g) of test portion using the formula given in the standard.
- Performance: Repeatability and reproducibility limits are specified (maximum within‑lab difference ~1.3% of mean at ~2% active oxygen; inter‑laboratory up to ~5%).
Applications
- Routine quality control of detergents and washing powders containing peroxyhydrates.
- Verification of label claims for bleaching or oxidizing capacity.
- Process control in manufacturing to ensure consistent active oxygen content and compliance with product specifications.
Related Standards
- ISO 607 - Surface active agents: sampling and sample division methods (referenced for laboratory sampling).
Notes: The method should not be used for washing powders that contain components reactive with acid permanganate under analytical conditions. EDTA‑type chelating agents may be present up to 1% (m/m) without invalidating the procedure. For full procedural detail and official equations, consult the ISO 4321:1977 document.