Ammonium Magnesium Phosphate of Analysis. 193 



cosmic salt, the solution of the precipitate, and reprecipitation 

 tends to reduce the higher indications. Thus, for example, the 

 error in (2) and (3) is +0*0124 grams in terms of magnesium 

 pyrophosphate, while in (13) and (14), similarly carried out 

 except the decantation of the excess of the precipitant, solu- 

 tion and reprecipitation, the error is +0*0036 grm. 



The special influence of free ammonia during precipitation, 

 was investigated in the following experiments. Definite 

 volumes of the magnesium nitrate solution were drawn from a 

 burette into a platinum dish, ammonium chloride — 10 grm. — 

 was added, the magnesium was brought down by dilute 

 ammonia in presence of microcosmic salt, and strong ammonia 

 equal to one-third the volume of the solution was added. The 

 solutions, after standing, were filtered off on asbestos under 

 pressure in a perforated crucible, and the precipitates were 

 washed with ammonia diluted to the proportion of three parts 

 of water to one of ammonia, dried after moistening with a 

 drop of saturated solution of ammonium nitrate, ignited and 

 weighed. The results are given in experiments (1) and (2) of 

 Table IV. In these determinations the mean error reaches 

 +0*0183 grm. in terms of magnesium pyrophosphate ; while 

 in experiments (3) and (4), made similarly excepting that the 

 supernatant liquid was decanted from the precipitate first 

 thrown down, the precipitate dissolved in hydrochloric acid, 

 and after dilution reprecipitated by dilute ammonia imme- 

 diately supplemented by enough strong ammonia to make one- 

 fourth the volume of the entire solution, the error amounts in 

 the mean to +0*0061 in terms of the pyrophosphate. 



Table IV. 



Mg 2 P 2 7 



corresponding 



to Mg(N0 3 ) 2 



taken. 



grm. 



(1) 0-5311 



(2) 0-5311 



Mg 2 P 2 0, 

 found, 

 grm. 



0-5503 



05505 



(3) 0-5311 0-5393 



(4) 0-5311 05351 



Error in 

 terms of 

 Mg 2 P 2 7 



grm. 

 + 0-0192 

 + 0-0194 



+ 0-0082 

 + 0-0040 



Error 

 in terms 

 of MgO. 



grm. 

 + 0-0069 

 + 0-0070 



+ 0-0029 10,- 

 + 0-0017 10,- 



Volume. 

 cm 3 . 

 200 

 200 



200,100 

 200,100 



In experiments (1) and (2) the precipitate was influenced by an 

 excess of microcosmic salt, ammonium chloride, and free 

 ammonia in large amount ; in experiments (3) and (4), by 

 decanting in the manner previously described, by dissolving 

 the precipitate, and reprecipitating, the effects of an excess of 

 microcosmic salt and ammonium chloride are reduced to a mini- 

 mum, and, in a comparison of the results with those of experi- 

 ments (11) and (12) of Table III the tendency of the free 



