The residue is a white mass of
magnesium pyrophosphate containing 27.92 per cent. of phosphorus, or
63.96 per cent. of phosphoric oxide.
VOLUMETRIC METHOD.
Instead of separating and weighing this compound, the phosphoric oxide
in it can be determined by titration. In many cases the ore may be
dissolved and immediately titrated without previous separation. It is
better, however, to carry the separation so far as to get phosphoric
acid, an alkaline phosphate, or the magnesia precipitate. It may then be
prepared for titration in the following way:--The precipitate in the
last case (without much washing) is dissolved in a little hydrochloric
acid, and the solution in any case rendered fairly acid. Dilute ammonia
is added till it is just alkaline, and then 5 c.c. of the sodic acetate
and acetic acid mixture (as described under the Arsenic Assay). This
should yield a clear distinctly-acid solution. It is diluted to 100 or
150 c.c., heated to boiling, and titrated with the uranium acetate
solution, using that of potassic ferrocyanide as indicator.
The _standard solution_ required is made by dissolving 35 grams of
uranium acetate in water with the aid of 25 c.c. of acetic acid, and
diluting to 1 litre.
An _equivalent solution of phosphoric oxide_ is made by dissolving 25.21
grams of crystallised hydric disodic phosphate (HNa_{2}PO_{4}.
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