198 Gooch and Austin — Magnesium Phosphate, etc. 



amounts of ammonium chloride this process yields a phosphate 

 of nearly ideal constitution if only the boiling be prolonged 

 from three to five minutes. The greater part of the ammonium 

 magnesium phosphate— about 90 per cent — forms in this process 

 before free ammonia is added, and the ammonium which enters 

 the phosphate thus formed is derived from the microcosmic 

 salt, which must become correspondingly acidic. Under these 

 conditions, the tendency to form an insoluble ammonium mag- 

 nesium phosphate richer in ammonia and poorer in magnesia 

 than the normal salt, does not develop. In the process of Dr. 

 Gibbs, as well in the modified precipitation process in the cold, 

 the use of the faintly ammoniacai solution and wash-water is 

 sufficient and advantageous. 



In the precipitation of a soluble phosphate by magnesia mix- 

 ture the tendency of the precipitant and that of the ammonium 

 salt are antagonistic, so that the effect of the latter salt is some- 

 what masked, though manifest. This opposition of effects has 

 been noted by Mahon,* who, though regarding the actual 

 attainment of an exact balance as uncertain, ventures the 

 opinion that accurate results should be attainable by the care- 

 ful relative adjustment of the proportions of the precipitant 

 and ammonium salt. Mahon claims to get the best results by 

 a very gradual addition of magnesia mixture to the ammoniacai 

 solution of the phosphate containing about sixteen per cent of 

 ammonium chloride, strong ammonia being added subsequently. 

 From our observations, however, recorded in section C of Table 

 VI, it appears that the method of introducing the magnesia 

 mixture gradually into the ammoniacai phosphate (taken in 

 quantity sufficiently large to give unmistakable indications) 

 produces a precipitate deficient in magnesium and so leads to 

 errors of deficiency in the phosphorus indicated. The use of 

 strong ammonia, moreover, we have shown to be both unnec- 

 essary and disadvantageous. Our experiments go to show that 

 good results may be expected when the solution of the phos- 

 phate containing a moderate excess of the magnesium salt and 

 not more than five to ten per cent of ammonium chloride is 

 precipitated by making it slightly ammoniacai, the precipitate 

 being washed in slightly ammoniacai wash-water. In general, 

 however, and especially when more ammonium chloride than 

 this proportion, or more magnesium salt than twice the amount 

 theoretically necessary, is present, it is safer to * decant the 

 supernatant liquid from the precipitate (through the filter to 

 be used subsequently to hold the phosphate), to dissolve the 

 precipitate in a little hydrochloric acid and reprecipitate by 

 dilute ammonia, washing with faintly ammoniacai wash- water. 



* Jour. Amer. Chem. Soc. xx, 445. 



