ARSENIC. 



357 



> 



The glass tube heated, as above mentioned, at one of its wide parts, 

 will show a bluish-black metallic mirror at the constriction beyond. 



If quantitative determination is desired, the glass tube is heated in two 

 places so as to cause all hydrogen arsenide to be decomposed. To collect, 

 however, the arsenic from any gas that might escape, the end of the tube 

 is inverted and placed into solution of nitrate of silver, which is decomposed 

 by the hydrogen arsenide, silver and arsenous acid being formed. The arsenic 

 solution should be introduced into the hydrogen generator in small portions, 

 so as not to produce more hydrogen arsenide at a time than can be decom- 

 posed by the method given. 



The only element which, under the same conditions, forms spots and mirrors 

 similar to arsenic, is antimony ; there are, however, sufficiently reliable tests 

 to distinguish arsenic spots from those of antimony. 



Arsenic spots treated with solution of hypochlorites (solution of bleaching- 

 powder) dissolve readily ; antimony spots are not affected. When nitric acid 

 is added to an arsenic spot and evaporated to dryness and the spot moistened 

 with a drop of silver nitrate, it 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, antimony spots an orange-red, 

 residue. Fig. 47 represents a simpler form of Marsh's appara- 

 tus, which generally will answer for students' tests. 



Preparatory treatment of organic matter for arsenic 

 analysis. If organic matter is to be examined 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 semi-liquid sub- 

 stance is heated in a porcelain dish over a steam bath with 

 hydrochloric acid and potassium chlorate until the mass has a 

 uniform light yellow color and has no longer the odor of 

 chlorine. By this operation all poisonous metals (lead and 

 silver excepted, because insoluble silver chloride and possibly 

 insoluble lead sulphate are formed) are rendered soluble evgn 

 when present as sulphides, and may now be separated by filtra- 

 tion from the remaining solid matter The clear solution is 

 heated and treated with hydrogen sulphide gas for several 

 hours, when arsenic and all metals of the arsenic and lead 

 groups are precipitated as sulphides, a little organic matter 

 also being precipitated generally. 



The precipitate is collected upon a small filter and treated with warm ammo- 

 nium 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-hydrochloric acid, and the solution tested by the 

 methods mentioned for the respective metals. The ammonium sulphide solu- 

 tion is evaporated to dryness, this residue mixed with 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 arsenate, antimony forming antimonic oxide. By 

 treating the mass with warm water, sodium arsenate is dissolved and may be 



