ARSENIC. 



191 



Student's appa- 

 ratus for making 

 arsenic spots. 



turns brick-red ; antimony spots treated in like manner remain 

 white. Arsenic spots dissolved in ammonium sulphide and 

 evaporated to dryness show a bright yellow, anti- 

 mony spots an orange-red residue. 



Fig. 10 represents a simpler form of Marsh's 

 apparatus, which will generally answer for stu- 

 dent's tests. 



Preparatory treatment of organic matter for ar- 

 senic analysis. If organic matter is to be exam- 

 ined for arsenic (or for any other metallic poison), 

 it ought to be treated as follows : The substance, 

 if not liquid, is cut into pieces, well mashed and 

 mixed with water ; the liquid or semiliquid sub- 

 stance is heated in a porcelain dish over a steam 

 bath with hydrochloric acid and potassium chlo- 

 rate until the mass has a uniform light yellow 

 color and has no longer an odor of chlorine. By 

 this operation all poisonous metals are rendered 

 soluble, even when present as sulphides, and may now be sepa- 

 rated from some remaining solid matter by filtration. The 

 clear solution is heated and treated with irydrosulphuric acid 

 gas for several hours, when arsenic and all metals of the arsenic 

 and lead groups are precipitated as sulphides, a little organic 

 matter being also generally precipitated. 



The precipitate is collected upon a small filter and treated 

 with warm ammonium sulphide, which dissolves the sulphides 

 of arsenic and antimony, leaving behind the sulphides of the 

 lead group which may be dissolved in nitric, or, if mercury be 

 present, in nitro-muriatic acid and the solution tested by the 

 methods mentioned for the respective metals. The ammonium 

 sulphide solution is evaporated to dryness, this residue mixed 

 with some nitrate and carbonate of sodium and the mixture 

 fused in a small porcelain crucible. By the oxidizing action of 

 the nitrate, both sulphides are converted into the higher oxides, 

 arsenic forming sodium arseniate, antimony forming antimonic 

 oxide. By treating the mass with warm water, sodium arseniate 

 is dissolved and may be filtered off, while antimonic oxide re- 

 mains undissolved and may be dissolved in hydrochloric acid. 



