c. Digestion; Cool and add carefully 50 ml digestion reagent (or substitute 10 ml cone H2SO4, 6-7 g K2SO4, and 1.5 ml mercuric sulfate solution). If large quantities of nitrogen-free organic matter are present, add an additional 50 ml digestion reagent for each gram of solid matter in the sample. After mixing, heat under a hood, or with suitable ejection equipment, to fumes of SO3 and continue to boil briskly until the solution clears (becomes colorless or a pale straw color). Then digest for an additional 30 min. Allows flask and contents to cool; dilute to 300 ml with ammonia-free water and add 0.5 ml phenolphthalein indicator solution and mix. Tilt the flask and add. care-fully. sufficient (approximately 50 mlfor every 50 ml digestion reagent used) hydroxide-thiosulfate reagent to form an alkaline layer at the bottom of the flask.
Connect the flask to the steamed - out distillation apparatus and shake the flask to insure complete mixing. Add more hydroxide-thiosulfate reagent in the prescribed manner if a red phenolphthalein color fails to appear at this stage.