CHEMISTRY. 101 



poor in iron, only 5 per cent of amuionium nitrate and a slight excess 

 of free nitric acid should be i)resent. Cool the solution to about 50° 

 and add the required amount of molybdic solution, whereupon the 

 temperature sinks to 35 to 40°. Let the solution stand until it ^)er- 

 fectly clear. With solutions rich in iron a high temperature is i . red, 

 and the solution should contain at least 10 per cent of ammonium 

 nitrate and from 5 to 10 cc. of free nitric acid of 1.4 specitic gravity 

 for every 100 cc. of solution. Heat the solution nearly to the boiling 

 temperature, add the necessary amount of molybdic solution, and stir 

 vigorously. Allow the precipitate to settle, and filter. For washing 

 the precipitate use a solution containing 100 parts each of ammonia 

 (0.91 specitic gravity) and nitric acid (1.4 specitic gravity), diluting to 

 1 liter. AVash until all iron is removed, tinally using a little cold water 

 and a small amount of 96 per cent alcohol or ether-alcohol, if the pre- 

 cii)itate is large, dry it, and ignite in a flat platinum dish with a plati- 

 num cover at a very low red heat, scarcely perceptible in daylight. 

 The ignited residue sliould be grayish when cooled and should con- 

 tain no yellow or green particles. It should also be free from sublimed 

 molybdic acid, (.'ool in a desiccator and weigh. 



In the method originally proi)osed by the author this residue was 

 given the formula P20:/Mo240c8, which contains 4.018 per cent of phos- 

 ])horic acid. A comparison on phosphorite of this method with that 

 in which phosphoric acid was determined by weighing the magnesium 

 pyrophosphate indicated that this factor was too high. Careful analy- 

 ses of the ignited residue from precipitates obtained with disodium phos- 

 l)l!ate and trisilver jihosphate according to the above directions showed 

 that it had the formula 24M0O34-P2O5, with a phosphoric acid content 

 of 3.944 per cent. By using this factor it was found that the results by 

 the author's method, and by that in which phosphoric acid was deter- 

 mined by weighing the magnesium pyrophosphates, agreed very closely. 



Two methods of determining phosphoric acid based upon the weight 

 of magnesium pyrophosphates were tested, viz, that of Miircker' and 

 that of Wagner.- In the first method the molybdate precipitate is dis- 

 solved in an excess of ammonia which is neutralized by hydrochloric 

 acid before precipitation with a magnesia solution, a sufiicient amount 

 of ammonia being added to bring the final amount of ammonia in the 

 solution up to 2h to 3 per cent. 



In the second method i)hosphoric acid is directly precipitated out of 

 2i to 3 per cent of ammonia solution by means of magnesia mixture. 

 These methods were tested on solutions of disodium and trisilver phos- 

 idiates containing varying amounts of phosphoric acid. The results 

 indicate that for the analysis of ordinary phosphates the Wagner 

 method is sufficiently accurate. For very small amounts of phos]>horic 

 aicd a small plus error was found. The irregularities observed are 



'Ztschr. nnalyt. Cliem., 12 (1873), p. 447. 

 aZtethr. aualyt. Chem., 19 (1880), p. 44. 



