6.3 Add a small quantity of anti-bump granules (3.7), 10 g of the catalyst mixture (3.11) and then carefully add 25 ml sulfuric acid (3.9) (see Note). Add 0.5 g paraffin wax (3.8) to reduce foaming and mix. Heat the flask moderately at first, shaking from time to time until frothing ceases and the liquid is practically colourless. Continue the digestion for at least a further 60 minutes. Allow the mixture to cool and then carefully add 250 ml water. Allow to cool to room temperature, and continue as described in 6.4.
Note:If organic matter other than urea exceeds 1.0 g add an additional 1.0 ml sulfuric acid for each 0.1 g organic matter in excess of 1.0 g.