Concentrate the filtrate from the separation of iron and aluminium to
about 100 cc., and add an excess of concentrated solution of ammonium
oxalate, little by little, to the hot ammoniacal solution. Keep the
solution warm and stir at intervals till the precipitate settles readily
and leaves a clear supernatant liquid. Filter, dissolve the precipitate
in a little hot dilute hydrochloric acid, and reprecipitate with
ammonium hydroxide and ammonium oxalate. If great accuracy is not
required this solution and reprecipitation may be omitted, and the first
precipitate may be washed clean with hot water[64a]. Save the filtrate
for determination of magnesium. Ignite the precipitate and weigh it as
calcium oxide, 71.5 per cent of which is the equivalent of calcium (Ca);
or dissolve the precipitate in hot 2 per cent sulfuric acid and titrate
with a standard solution of potassium permanganate.