Prep. By dissolving one ounce of sulphate of magnesia, and half an ounce of hydrochlorate of ammonia, in eight fluid ounces of distilled water, afterwards adding half a fluid ounce of solution of ammonia, and enough distilled water to make up the bulk to ten fluid ounces.

The composition of the ammonio-sulphate of magnesia is repre-Bented by the formula (Mg O, So3 + Nh4 , O, So3 + 6 HO).

Use. The solution is used for testing phosphate of ammonia. When added to a solution of that salt, a crystalline precipitate of ammonio-magnesian phosphate is formed, this is also known as the triple phosphate, and has the composition (2 Mg O, Nh4 O, Po5 + 12 HO); it is but very sparingly soluble in pure water, and insoluble in water containing chloride of ammonium or ammonia, but readily soluble in acids. When dried and heated to redness, this salt yields 35.7 per cent. of magnesia, and 64.3 of phosphoric acid. According to the direction of the Pharmacopoeia, twenty grains of phosphate of ammonia when precipitated by this test-solution, and the precipitate washed with solution of ammonia diluted with an equal bulk of water, should, when dried and heated to redness, leave a residue of 11.44 grains.

The solution is also made use of to determine the presence of phosphoric acid in the phosphate of iron. For this purpose the salt is dissolved in hydrochloric acid, tartaric acid and excess of ammonia added, and then the test-solution, when the ammonio-magnesian phosphate is precipitated. Ammonia would precipitate the iron if added alone, but with the tartaric acid a soluble compound, ammonio-tartrate of iron, is formed, and the iron is thus held in solution.