When a solution of pure protosulphate of iron thus obtained is used for the preparation of carbonate of iron, care must be taken to use for precipitating, a solution of carbonate of soda which is free from silica, phosphoric acid, etc., which if present would surely go down with the precipitate. The precipitated carbonate should be washed with water which has been freed from _air_ by previous boiling and better with water which is still boiling hot, dried as quickly as possible, first by pressure between folds of paper and then in a water bath, and preserved in well closed vessels containing lumps of quicklime as recommended above.
The formulas given in the Pharmacopœias for the preparation of the sesquioxide of iron, which besides being employed as a remedy itself, is used in preparing all the other sesquicompounds of iron used in Pharmacy, appear to be open to great objection on the ground of affording, instead of a pure sesquioxide of iron, an _indefinite mixture_ of sesquioxide with carbonate of the protoxide. No necessity whatever exists for this; the following _modus operandi_, besides being much less troublesome in its execution than those given by the Pharmacopœias, will furnish a product of constant composition, being an anhydrous sesquioxide of iron free from protoxide, and either chemically pure or very nearly so. The materials required are, five parts of commercial copperas which has been recrystallized once or twice, six parts of crystallized pure carbonate of soda, (Na O, C O^2 + 10 HO) or two parts of dry carbonate of soda, and one part of nitrate of soda. (Chili saltpetre). The carbonate and nitrate of soda are dissolved together in one portion of hot water and the copperas in another portion, and the two solutions, after filtrating mixed together, evaporated to dryness and the dry mass exposed to the lowest possible red heat for a few minutes. On pouring water upon the mass thus obtained, sulphate of soda and nitrate of soda dissolve and sesquioxide of iron separates as a heavy powder very easily washed { 233} by decantation. When thoroughly washed and dried it appears as a dark reddish brown _perfectly impalpable_ powder, which is perfectly and easily soluble in dilute acids, and even in acetic acid and the composition of which is Fe^2 O^3.
