274 EXAMINATION FOR ARSENIC 



and dried at a very gentle heat, the arsenic becomes 

 yellow, the antimony orange. The yellow spots of 

 sulphide of arsenic are not affected by hydrochloric 

 acid, while those of sulphide of antimony disappear on 

 gently heating. 



If sulphuretted hydrogen gas be passed through the 

 tube containing the metallic mirror, and heat applied, 

 the metal is converted into a sulphide. If the mirror 

 consist of antimony, black, or partly orange-red, sulphide 

 of antimony is produced, while arsenic gives a yellow 

 sulphide. The color, however, is not the only distinc- 

 tion between these compounds, another is afforded by 

 their unequal volatility, sulphide of arsenic being far 

 more volatile than that of antimony. 



Moreover, antimony, and arsenic, in the form of sul- 

 phides, may be separated by cyanide of potassium, 

 according to the method given in No. 62. 



The presence of antimony in the precipitated sul- 

 phides may also be ascertained by oxidizing them as 

 directed at p. 266. In that case, the fused mass, before 

 treatment with sulphuric acid, should be dissolved in 

 water, when the antimony would remain undissolved 

 in the form of antimonate of soda. 



Or the precipitate by sulphuretted hydrogen may 

 be washed with a concentrated solution of carbonate 

 of ammonia, which is poured over it several times. The 

 sulphide of arsenic is dissolved while the sulphide of 

 antimony remains undissolved. If there is a consider- 

 able quantity of the precipitate, a portion may be 

 dissolved in aqua regia, the solution treated with sul- 

 phurous acid to reduce the arsenic acid to arsenious, 

 concentrated by evaporation, a piece of bright copper foil 

 placed in it, and then warmed. Antimony and arsenic 

 are reduced and cover the copper with a steel-colored 

 coating, which is easily removed if the copper is heated 

 with caustic ammonia. It is then easily determined 

 which of the metals is present. If both, they may be 



