Section 8: Procedure

  1. Weigh duplicate 1.000 ± 0.001 g samples of blended detergent into dry 150 ml beakers.
  2. Add approximately 90 ml distilled water.
  3. 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.
  4. 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.)
  5. Allow the samples to cool and make up to 100 ml in a graduated cylinder.
  6. Pipette 5 ml of the digested sample and dilute to 100 ml with distilled water in a volumetric flask.
  7. Run the samples using the manifold shown in the diagram.
  8. Sodium tripolyphosphate standards can be prepared and treated as above, and then checked against standards of potassium dihydrogen phosphate for complete digestion.

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