THE PROCESS , 155 



cipitate is thoroughly washed with water, both by decantation 

 and on the filter. The filter with its precipitate is transferred to 

 a beaker and dissolved with an excess of standard alkali in the 

 presence of phenolphthalein. The residual alkali is determined 

 by titration with standard acid. Each cubic centimeter of alkali 

 employed should correspond to one milligram of phosphorus pent- 

 oxid (P 2 O 5 ). 



The reagents have the composition indicated below : 



Ammonium Molybdate. Ninety grams of the crystals of am- 

 monium molybdate are placed in a large beaker and dissolved in 

 a little less than one liter of water. The beaker is allowed to stand 

 over night and the clear liquor decanted. Any undissolved acid 

 is brought into solution in a little ammonia water and added to 

 the clear liquor. If a trace of phosphoric acid be present a little 

 magnesium sulfate is added and enough ammonia to produce a 

 slight alkaline reaction. The volume of the solution is then 

 made up to one liter. It is to be observed that nitric acid is not 

 used in the preparation of this reagent, which is known as the 

 aqueous solution. Each cubic centimeter of this solution is capa- 

 ble of precipitating three milligrams of phosphorus pentoxid. 



Standard Potassium Hydroxid. This solution is made of such 

 strength that one cubic centimeter is equivalent to one milli- 

 gram of phosphorus pentoxid. Treated with acid of normal 

 strength, 100 cubic centimeters are required to neutralize 32.65 

 cubic centimeters thereof. The strength of the solution can not 

 be safely assumed from the weight and purity of the material, but 

 is to be ascertained by comparison with a solution of a phosphate 

 of known composition. 



Standard Acid. This should have the same strength, volume 

 for volume, as the standard alkali solution. It is made by dilut- 

 ing 326.5 cubic centimeters of normal nitric acid to one liter. 



Ammonium Nitrate. A saturated aqueous solution of the salt 

 is used. 



Indicator. The indicator to be used is an alcoholic solution of 

 phenolphthalein, one gram in 100 cubic centimeters of 60 per 

 cent, alcohol, and half a cubic centimeter of this should be used 

 for each titration. 



