Digestion
The food sample to be analyzed is weighed into a digestion flask and then digested by
heating it in the presence of sulfuric acid (an oxidizing agent which digests the food), anhydrous
sodium sulfate (to speed up the reaction by raising the boiling point) and a catalyst, such as
copper, selenium, titanium, or mercury (to speed up the reaction). Digestion converts any
nitrogen in the food (other than that which is in the form of nitrates or nitrites) into ammonia,
and other organic matter to C02
and H20. Ammonia gas is not liberated in an acid solution
because the ammonia is in the form of the ammonium ion (NH4+) which binds to the sulfate ion
(SO2-) and thus remains in solution: