M' Alpine — Precipitation of Phosphoric Acid. 743 



60 grammes Ammonic Molybdate. 

 500 cc. j^^itric Acid (sp. gr. 1-4). 

 400 cc. Ammonia (sp. gr. -96). 

 400 cc. Water. 



A solution containing these proportions gives an immediate pre- 

 cipitate if added to 10 cubic centimetres of a phospliate solution con- 

 taining '01 grammes of phosphoric acid per litre. 



I next tried the effect of hydi'ochloric acid on the precipitation. 

 For this pui-pose I employed 10 cubic centimetres of a solution con- 

 taining '01 grammes of phosphoric acid per litre, 3 cubic centimetres of 

 the ordinary laboratory solution of ammonic molybdate, and 2 cubic 

 centimetres of strong hydrochloric acid. There was no precipitate 

 after shaking and allowing to stand for some time, but on adding am- 

 monia a precipitate was produced which was not so rapid in its forma- 

 tion, nor so copious as when nitric acid was present. 



I now tried to obtain a yolumetric test for phosphoric acid. To 

 avoid the necessity of adding excess of ammonic molybdate, I added 

 to the solution sodic molybdate, which does not precipitate the phos- 

 phoric acid, and then ammonic molybdate. For example, I took 50 

 cubic centimetres of a solution of phosphate of soda containing excess 

 of sodic molybdate. 5cc. of ammonic molybdate appeared to precipitate 

 the phosphoric acid completely, as the further addition of ammonic 

 molybdate to a portion of the filtered solution gave no precipitate on 

 heating, nor the reaction for ammonic molybdate on adding phosphate 

 solution. Thus it appears that by using an excess of sodic molybdate, 

 the necessity for an excess of ammonic molybdate is avoided. I made 

 various other experiments in this way, and it seems as if there was a 

 point at which the precipitation of the phosphoric acid is complete, 

 and no ammonic molybdate in solution. 



Lastly I substituted for the ammonic molybdate ammonic nitrate, 

 which I added to the phosphate solution containing excess of sodic 

 molybdate, and here also there appeared to be a point where the pre- 

 cipitation of the phosphoric acid was complete, and no ammonia in 

 solution. 



I now attempted to determine the point of complete precipitaftion in 

 various ways, but I could find no convenient way of determining the 

 exact point, when all the ammonia as well as the phosphoric acid had 

 l)een precipitated. 



