Precipitation of Magnesium Ammonium Phosphate from Homogeneous Solution

Abstract

The application of the technique of precipitation from homogeneous solution, PFHS, to the gravimetric determination of magnesium or phosphate as magnesium ammonium phosphate has been studied as a means of producing purer and larger crystals. For the determination of magnesium, homogeneous precipitation was produced by a slow increase in pH resulting from the hydrolysis of urea. The procedure for the determination of phosphate was to maintain a high pH via the hydrolysis of urea and to release magnesium ions slowly into solution by oxidation of the magnesium-EDTA complex with hydrogen peroxide. Neither procedure produced quantitative homogeneous precipitation and complete recovery of magnesium in the urea-hydrolysis method, and phosphate in the cation-release technique, was only accomplished by a final heterogeneous addition of concentrated ammonia solution. In the case of the latter procedure, abnormal crystallisation, coupled with high results, was obtained with some hydrogen peroxide reagents; this was traced to a phosphate impurity in these reagents. The urea-hydrolysis technique was also applied to the determination of phosphate, but in this case, the method was completely unsuccessful. Quantitative analysis of the precipitates and filtrates indicated that total precipitation occurred and that the magnesium ammonium phosphate precipitates formed by PFHS were mixtures of the monohydrate and hexahydrate, confirmed by X-ray diffraction data, but that conventional precipitates contained negligible amounts of the monohydrate. The poorer precision of the urea-hydrolysis method for the determination of magnesium was attributed to contamination by variable amounts of magnesium and ammonium phosphates, and the high results of the cation-release technique for the determination of phosphate to contamination by trimagnesium phosphate. The conclusion was that precipitation of magnesium ammonium phosphate from homogeneous solution by these techniques offered no distinct advantages over the classical gravimetric method except that the precipitate crystals were larger.