164 Soil Analysis [273-275 



Details of the Analysis 



273. Oxide of Iron and Alumina.— Boil the filtrate 

 from the insoluble portion over a Bunsen, then add dilute am- 

 monia in slight excess. Boil for a few seconds, then allow the 

 precipitate to settle. Filter rapidly, wash once by decantation, 

 then re-dissolve in HCland re -precipitate with ammonia. Boil 

 again and filter, collecting the filtrates from both precipitations 

 in the same beaker. Wash well with hot water, dry, ignite, 

 and weigh. 



The object of this double precipitation is to prevent the 

 precipitate from being contaminated with CaC0 3 . Should 

 there be any considerable quantity of lime in the soil, 

 the precipitate first formed by ammonia is sure to contain 

 some. 



After weighing the mixed oxides, dissolve them by digesting 

 with a few c.c. of strong HC1. When the digestion has gone 

 on for about half-an-hour, decant the liquid into a 250-c.c. flask 

 and add a further quantity of acid. When it is all dissolved, 

 make up to 250 c.c. with water. Reduce the iron in 50 c.c. of 

 this, and titrate exactly as described in paragraph 77. 



The percentage of A1 2 3 is obtained by subtracting the 

 sum of the percentages of Fe 2 3 and P 2 5 from the percentage 

 of ammonia precipitate. 



274. Lime. — Boil the filtrate and washings from the 

 ammonia precipitate, and add ^ gram of solid ammonium oxalate. 

 Allow to settle, and test the supernatant liquid for lime with 

 a drop of ammonium oxalate solution. Should lime be present 

 in the liquid, another \ gram of solid oxalate must be added. 

 Filter. Wash well with hot water. Dry the precipitate, ignite 

 in a Fletcher furnace (see paragraph 46), and weigh as CaO. 



275. Magnesia and Alkalis.— Transfer the filtrate 



