Identifying Magnesium Minerals

Precipitation as Ammonium Magnesium Phosphate. The only common test form magnesium is to precipitate it in the form of ammonium magnesium phosphate, NH₄MgPO₄, by the addition of hydrogen sodium phosphate, HNa₂PO₄, to a strongly ammoniacal solution. The precipitate usually forms somewhat slowly, is white in color, and frequently is granular in texture. In order to make a decisive test certain precautions are necessary. As the precipitation is made in an ammoniacal solution, any precipitates formed by an excess of ammonium hydroxide must be first filtered off. It may be necessary before adding the ammonium hydroxide to add a few drops of nitric acid so as to make certain that any iron in the solution is I n the ferric state.