Ammonium Magnesium Phosphate of Analysis. 197 



though the production of such a salt in presence of an excess 

 of the soluble phosphate compels the combination of a definite 

 amount of magnesium with more than the normal amounts of 

 phosphoric acid and ammonia (as was the case in the former 

 series of experiments), when the supply of the soluble phos- 

 phate is limited the amount of magnesium associated with it 

 must fall below the normal (as is the case in the present series 

 of experiments). Moreover, the behavior of the precipitant is 

 quite in accord with the hypothesis ; for, though the influence 

 of an excess of the soluble phosphate would naturally tend 

 (as was observed) in the same direction as that of the ammo- 

 nium salt and free ammonia, viz., to the production of the phos- 

 phate deficient in magnesium, the tendency of an excess of the 

 magnesium salt must obviously be to increase the amount of 

 magnesium in the phosphate, as was observed in the experi- 

 ments of Table VI. The hypothesis fits the facts, therefore, 

 on both sides ; and, if precipitation is practically complete (as 

 was shown to be the case throughout) the argument for the 

 existence of an ammonium magnesium phosphate — poorer than 

 the normal salt in magnesium — possibly the salt (!N"H 4 ) 4 Mg(P0 4 ) 9 

 — seems to be strong. 



The Practical Determination of Magnesium and Phosphoric Acid. 



In determining magnesium by the procedure in ordinary use, 

 the tendency is strong — as is shown in experiments (1) and (2) 

 of Table IV — toward high plus errors, and the error is 

 due to the combined effects of excesses of the precipitant, 

 the ammonium salt, and free ammonia. The experiments (11) 

 and (12) of B, Table III, show conclusively that such tenden- 

 cies to error may be counteracted effectively by pouring off the 

 supernatant liquid (through the filter to be used subsequently 

 to collect the precipitate) as soon as the precipitate subsides, 

 dissolving the phosphate in the least amount of hydrochloric 

 acid, bringing it down again, after dilution, by a faint excess 

 of ammonia, filtering (best, we think, on asbestos, under pres- 

 sure), washing with faintly ammoniacal water, and igniting as 

 usual. 



Many years ago* a method of precipitating the ammonium 

 magnesium phosphate was advocated by Professor Wolcott 

 Gibbs, which consists, essentially, in boiling the solution of the 

 magnesium salt with microcosmic salt and adding ammonia 

 after cooling, and by which most exact analytical results were 

 obtained. Our experience confirms completely that of Dr. 

 Gibbs, and we desire to direct attention again to a procedure 

 the advantage of which has, unfortunately, not been broadly 

 known and accepted. Even in the presence of considerable 



* This Journal [3], v, 114. 



