30 METHODS OF QUANTITATIVE ANALYSIS. 



in solution are collected in the beaker and treated with 

 ammonium chloride (about one-sixth its volume). To 

 this is added while stirring constantly magnesia mixture 

 as long as a precipitate is formed. Finally a slight excess 

 of magnesia mixture is ruri in and the solution allowed to 

 stand for 12 hours in the cold. The precipitate of am- 

 monium magnesium phosphate is then filtered ofif and 

 washed with diluted ammonia (i to 3) until a sample of 

 the filtrate acidified with nitric acid gives no reaction with 

 silver nitrate solution. The filter and contents are then 

 dried at 100 degrees and as much as possible of the pre- 

 cipitate is removed from the paper. The filter is inciner- 

 ated in a weighed porcelain crucible and the precipitate 

 is then added and" the whole ignited at a bright red heat. 

 By this ignition magnesium pyrophosphate is formed, 

 ammonia and water being expelled. If on cooling the 

 residue appears gray, moisten with a few drops of nitric 

 acid, then drive off the acid carefully by heating and again 

 ignite. By this treatment a pure white mass should 

 remain. One part of magnesium pyrophosphate is 

 equivalent to 0.6375 part of phosphoric acid (PjOj). 



Ammonium molybdate solution is prepared in the fol- 

 lowing manner: 100 grams of pure molybdic acid are dis- 

 solved in 400 grams of ammonia (specific gravity 0.960, 

 equivalent to 10 per cent. NH3). This solution is poured 

 cautiously into 1500 grams of nitric acid (specific grav- 

 ity 1.2). The mixture is warmed for about an hour at 

 a temperature not to exceed 50 degrees C. and then kept 

 for two or three days in a moderately warm place. In 

 case molybdic acid precipitates out, the solution should 

 be filtered. 



Preparation of magnesia mixture: 55 grams of mag- 

 nesium chloride and 105 grams of ammoniarn chloride 

 are dissolved in water, 350 c.c. of ammonia (24 per cent., 

 specific gravity 0.91) added and the solution diluted to 



