Heat to boiling the filtrate from the insoluble residue, oxidize with
concentrated nitric acid or bromine, and concentrate to about 25 cc. Add
ammonium hydroxide in slight excess, boil one minute, and filter.
Dissolve the precipitate on the filter in a small amount of hot dilute
hydrochloric acid. Reprecipitate with ammonium hydroxide, filter, and
wash. Unless very accurate results are necessary this solution and
reprecipitation may be omitted. Unite the two filtrates for
determination of calcium. Ignite and weigh the precipitate. It will
comprise oxides of iron and aluminium and phosphate. If much phosphate
is present it should be determined in a separate sample and a correction
for the amount applied; otherwise it may be neglected. Determine the
iron in the ignited precipitate by fusion with sodium or potassium
pyrosulfate, reduction with zinc, and titration with potassium
permanganate. Aluminium (Al) is calculated as follows: