271, 272] General Analysis of the Burned Portion 163 



General Analysis of the Burned Portion 



271. Remove the substance left in the platinum dish, 

 after burning, to a 4-oz. wide-mouthed beaker, and treat with 

 HC1 exactly in the same manner as the portion used for the 

 determination of sulphuric and phosphoric acids. Evaporate 

 to dryness, and moisten with strong HC1. Add 25 c.c. dilute 

 HC1, filter and wash by decantation. Dry, burn, and weigh. 



This weighing operation may cause some little trouble, as 

 the dry silicates are very hygroscopic. The weighing should 

 be performed as rapidly as possible. Directly after weighing, 

 the dish with its contents should be returned to the Argand, 

 and allowed to cool again in the desiccator. Before weighing 

 again, the weights, as found in the first weighing, should be 

 placed on the pan, so that no time may be lost after the dish 

 has been taken from the desiccator. 



272. The filtrate from the insoluble portion is analysed 

 according to the following scheme : 



Add ammonia and filter. 



Precipitate contains 

 Fe 2 O s , A1 2 3 , and 

 P 2 5 . .Weigh, 

 then re -dissolve in 

 HC1, and estimate 

 Fe by K 2 Cr 2 7 

 solution (see para- 

 graph 77). 



Filtrate. Add ammonium oxalate. Filter. 



M 2 



