TOTAL PHOSPHATE AND POTASH 99 



is again taken up with hydrochloric acid, filtered, again 

 evaporated to dryness, and heated for half an hour at 

 ids'" C. The residue is now dissolved, as far as possible, 

 in dilute nitric acid, filtered, and made up to about 50 

 C.C. To the solution is added several grams of ammonium 

 nitrate, and 50 c.c. of ammonium-molybdate solution. 

 The mixture is allowed to stand in a warm place 

 for twenty-four hours, the yellow precipitate filtered 

 off, and washed thoroughly with i per cent, nitric 

 acid solution. 



The precipitate is dissolved in dilute ammonia, and 

 the filtered solution reprecipitated by the addition of 

 excess of nitric acid (i part of acid to 2 parts of water). 

 A further 5 c.c. of ammonium molybdate are also added, 

 and the mixture allowed to stand in a warm place for 

 some time, as before. The precipitate thus obtained 

 is filtered in a Gooch crucible, washed well with i per 

 cent, nitric acid, dried in the oven, and ignited until of 

 a uniform dark blue colour and of constant weight. 

 During ignition the base of the Gooch should be pro- 

 tected by a cap or by placing it inside another crucible. 



The residue contains 3-96 per cent, of PgOg, which 

 corresponds to the formula P2O5.M03O8.21M0O3. 



79. Potassium. 



For the estimation of the potash, 50 c.c. of the 

 original solution are treated as in the case of the 

 phosphoric acid, except that the residue is taken up 

 after the second evaporation with dilute hydrochloric 

 instead of nitric acid. To the solution is added 5 c.c. 

 of a 5 per cent, solution of platinic chloride, and the 

 mixture carefully evaporated on the water-bath almost 

 to dryness. The residue is transferred to a filter paper 

 or a Gooch crucible, washed first with 80 per cent, 

 alcohol, and then with ammonium chloride solution 



