266 EXAMINATION FOB ARSENIC 



well washed. The filtrate, before being thrown away, 

 should, for greater certainty, again be saturated with 

 sulphuretted hydrogen gas and set aside for some time 

 in a closed vessel. 



This precipitate always contains, in addition to sul- 

 phide of arsenic, certain sulphuretted organic matters 

 which are precipitated with it, and must be completely 

 destroyed; this is best effected in the following man- 

 ner: 



The filter containing the precipitate is placed in a 

 somewhat capacious crucible of genuine porcelain, and 

 digested with concentrated nitric acid until the whole 

 is converted into a homogeneous mass. The free ni- 

 tric acid, of which more may be added if necessary, is 

 neutralized with pure carbonate of soda, and the solu- 

 tion carefully evaporated to dryness. It is important 

 that the mass should contain a sufficient quantity of 

 nitrate of soda, which is easily insured. It is gradu- 

 ally heated over a large spirit-lamp, or gas-burner, 

 until the salt fuses ; it blackens at first, but afterwards 

 fuses, quietly and without deflagration, to a clear color- 

 less liquid. The whole of the organic matter is now 

 burnt, and the arsenic converted into arsenate of soda. 



Pure concentrated sulphuric acid is then gradually 

 dropped upon the cooled saline mass in the crucible, 

 and a gentle heat applied, until, after addition of an 

 excess of acid, the nitric and nitrous acids are com- 

 2>letely expelled, and the mass is converted into bisul- 

 phate of soda. If nitric acid containing hydrochloric 

 acid had been originally employed for the oxidation 

 of the sulphuretted hydrogen precipitate, a loss of 

 arsenic might now result, from its volatilization as 

 chloride of arsenic. The purity, in this respect, of the 

 nitric acid and carbonate of soda, must therefore have 

 been previously ascertained. 



The acid saline mass is now dissolved, in the crucible 

 itself, with the smallest quantity of hot water, and the 

 solution introduced into Marsh's apparatus. 



