Appendix 233 



filter, and the filtrate if not clear passed through the same paper 

 again. 50 c.c. of the filtrate are then taken and treated as directed 

 below. 



(c) Total phosphoric acid. The nitric acid solution of a weighed 

 quantity of the sample, after destruction of the organic matter if 

 necessary, and removal of the silica by suitable means, is treated as 

 below. 



(d) The molybdate method. To the solution obtained in (a), (6) or 

 (c), which should contain 0-1 to 0-2 grm. of P2O5, 100 to 150 c.c. of 

 molybdic acid solution are added, the whole warmed to 70 C. in a 

 water-bath for 15 minutes, allowed to cool, and filtered. The pre- 

 cipitate is washed first by decantation and afterwards on the filter 

 paper with 1 per cent, nitric acid; the filtrate and washings are set 

 aside and tested with more molybdic acid. The precipitate is dis- 

 solved in cold 2 per cent, ammonia solution, about 100 c.c. being 

 used for the purpose. 15 to 20 c.c. of magnesia mixture are then 

 added, drop by drop with constant stirring. After standing two 

 hours, with occasional stirring, the precipitate is filtered oi3, washed 

 with 2 per cent, ammonia, dried, ignited, and weighed as magnesium 

 pyrophosphate. 



The molybdic acid solution. 125 grm. of molybdic acid and 100 c.c. 

 of water are placed in a litre flask, and the acid dissolved by the 

 addition, while shaking, of 300 c.c. of 8 per cent, ammonia. 400 grm. 

 of ammonium nitrate are added, the solution is made up to the mark 

 with water, and the whole added to 1 litre of nitric acid (sp. gr. 1-19). 

 It is maintained at about 35 C. for twenty-four hours and then 

 filtered. 



Magnesia mixture. 110 grm. of crystallised magnesium chloride 

 and 140 grm. of ammonium chloride are dissolved in 1300 c.c. of 

 water. 700 c.c. of 8 per cent, ammonia are added, and the whole 

 allowed to stand for several days and filtered. 



Ammonia solutions. (1) 8 per cent. 1 volume of ammonia solution 

 of sp. gr. 0-880 is mixed with 3 volumes of water and the solution 

 adjusted by addition of more water or ammonia till the sp. gr. is 

 0-967. 



(2) 2 per cent. 1 volume of 8 per cent, ammonia is mixed with 

 3 volumes of water. 



(This is the official method: the titration method given on p. 229 

 is simpler and equally accurate.) 



