Acidify with hydrochloric acid the filtrate from the separation of
calcium and concentrate it to about 100 cc. Add 20 cc. of a saturated
solution of microcosmic salt, cool, and make slightly but distinctly
alkaline by adding ammonium hydroxide, drop by drop. Allow the solution
to stand four hours, then filter and wash with 3 per cent ammonium
hydroxide. Dissolve the precipitate, especially in the presence of large
amounts of sodium or potassium, in a slight excess of dilute
hydrochloric acid and reprecipitate the magnesium with ammonium
hydroxide and a few drops of microcosmic salt solution. If great
accuracy is not required this solution and reprecipitation may be
omitted. Ignite the precipitate and weigh it as magnesium pyrophosphate
(Mg_{2}P_{2}O_{7}), 21.9 per cent of which is the equivalent of
magnesium (Mg.). If manganese is present[64a] it is precipitated with
the magnesium and a correction for it should be applied after having
determined manganese in a separate sample. The weight of manganese
pyrophosphate (Mn_{2}P_{2}O_{7}) is 2.58 times the weight of manganese.