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Specimens are easily obtainable which are nearly pure and leave on treatment with acid only a slight siliceous residue. For the purpose of gravimetric determination, phosphoric acid is usually precipitated from ammoniacal solutions in the form of magnesium ammonium phosphate which, on ignition, is converted into magnesium pyrophosphate.

The acid will dissolve magnesium pyrophosphate from the surface of the particles of carbon, which will then burn away. Nitric acid also aids as an oxidizing agent in supplying oxygen for the combustion of the carbon. The magnesium ammonium phosphate is readily dissolved by acids, even those which are no stronger than acetic acid.

The glowed substance is weighed after cooling, and is pyrophosphate of magnesia, from which the magnesia or carbonate of magnesia is calculated stoichiometrically. All the ascertained sums must be multiplied by 2, if they are to correspond to the analyzed and weighed quantity of ash.

The solution should be carefully examined at this point and must be filtered if a precipitate has appeared. Heat the clear solution to boiling; remove the burner and add 25 cc. of a solution of disodium phosphate. Remember that the pyrophosphate finally obtained is from one fifth of the original sample.