loo CHEMICAL ANALYSIS OF SOIL 



which has been saturated with the double ammonium 

 platinum chloride, and finally again with diluted 

 alcohol until the washings are colourless. The 

 residual precipitate is dried at ioo°C. It consists of 

 KgPtClg, and the weight of KgO is obtained by 

 multiplying by the factor 0-1937. 



80. Iron and Aluminium. 



One hundred c.c. of the original extract, equivalent 

 to 5 grams of soil, are heated in a beaker nearly to the 

 boiling point, with a few c.c. of nitric acid, in order 

 to oxidise any ferrous iron to the ferric condition. 

 Strong ammonia solution is then added until pre- 

 cipitation is complete. The mixture is boiled for 

 five minutes or so, allowed to settle, and the clear 

 solution decanted off through a filter. The precipitate 

 is again washed by decantation with 50 c.c. or so of 

 boiling water, as much of the clear solution poured off 

 as possible, and the residue dissolved in a few 

 c.c. of hydrochloric acid, and precipitated with 

 ammonia solution as before, excess of ammonia being 

 avoided. The precipitate is transferred to the filter, 

 washed free from chlorides with hot water, and then 

 dried and ignited in the usual way. It consists of the 

 ferric oxide, alumina, and phosphoric acid. 



The iron is estimated separately, the phosphoric acid 

 having been estimated as above, and the alumina 

 determined by difference. 



81. Iron. 



One hundred c.c. of the original extract are boiled 

 with nitric acid if any ferrous iron is present ; if not, 

 the solution is precipitated forthwith by strong ammonia, 

 the precipitate filtered and washed, and then dissolved 



