146 THE CHEMICAL ANALYSIS OF SOILS [chap. 



manner, but special care must be taken to free the 

 solution from silica and organic matter. For phos- 

 phoric acid a portion of the solution is evaporated to 

 dryness and ignited, the residue is taken up with 

 hydrochloric acid, filtered, again evaporated to dry- 

 ness, and heated in an air - bath for half an hour at 

 1 05 . 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 dilute hydro- 

 chloric instead of nitric acid. 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 solution of ammonium 

 chloride which has been saturated with the double 

 chloride of platinum and ammonium, and finally dis- 

 solved 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. 



The other determinations which may be made in 

 this solution consist of soda, lime, magnesia, iron, 

 alumina, manganese, and sulphuric acid, but in most 



