Section 8: Procedure
- Weigh duplicate 1.000 ± 0.001 g samples of blended detergent into dry 150 ml beakers.
- Add approximately 90 ml distilled water.
- Add 10 ml of strong acid (310 ml/l) and about 4 g of ammonium persulphate (measured with a scoop). Caution should be taken when adding the acid since some detergents contain large amounts of carbonate and there will be a strong evolution of CO2.
- Heat the beakers on the hot plate at approximately 180°C, for 1½ - 2 hours. To ensure complete and uniform digestion, it is best to digest all samples to a constant volume. (40 ml has been found to be sufficient.)
- Allow the samples to cool and make up to 100 ml in a graduated cylinder.
- Pipette 5 ml of the digested sample and dilute to 100 ml with distilled water in a volumetric flask.
- Run the samples using the manifold shown in the diagram.
- Sodium tripolyphosphate standards can be prepared and treated as above, and then checked against standards of potassium dihydrogen phosphate for complete digestion.