166 THE CHEMICAL ANALYSIS OF SOILS [chap. 



105 . This residue is then taken up with dilute 

 nitric acid, filtered, and made up to about 50 c.c. 

 Five grams of ammonium nitrate are added, and 50 

 c.c. of a solution of ammonium molybdate containing 

 60 grams molybdic acid per litre. The mixture is 

 put aside in a warm place for twenty-four hours, the 

 precipitate is filtered off, and, after washing with 

 ammonium nitrate solution, is dissolved by ammonia 

 into a tared porcelain basin, evaporated to dryness, 

 and gently ignited over an Argand burner. The 

 resulting material contains 3-794 per cent, of phos- 

 phoric acid. For the determination of potash the 

 same procedure is followed, but the residue after the 

 second evaporation is taken up with hot water, breaking 

 up the residue with a glass rod during extraction. 

 To the solution 25 c.c. of a solution of chloroplatinic 

 acid containing 0-005 gram platinum per c.c. is added, 

 and the whole gently evaporated over a water-bath 

 till almost dry. It is then thrown on to a filter and 

 washed with alcohol, then washed again with a solu- 

 tion of ammonium chloride which has been saturated 

 with the double chloride of platinum and ammonium, 

 and finally dissolved off the filter paper with a little 

 hot water in a tared basin, evaporated, and weighed. 

 A Gooch crucible is most convenient for handling both 

 the phosphoric acid and potash precipitates. Instead 

 of chloroplatinic, perchloric acid may be used, with 

 equal, if not greater, accuracy (W. A. Davis). The solu- 

 tion in water from the second evaporation is evaporated 

 nearly to dryness after adding 2-5 c.c. of 20 per cent, 

 solution of perchloric acid, the process being carried 

 to the point of evolution of vigorous white fumes. 

 The soluble perchlorates are now taken up by stirring 

 with 20 c.c. of 95 to 96 per cent, alcohol, and after 

 settling the clear solution is poured through a 9 cm. 



