363 Dr. Peuny aud ]\Ir. W. Wallace ow Chloride of Arsenic. 



pipette, and purified by careful rectification. The ])rincipal 

 objection to tbis process is, tbat when the hydrochloric acid is 

 reduced below a certain point, the anhydrous chloride ceases to 

 separate from the other distilled products, and thus a small 

 proportion only is obtained. Our experiments lead us to infer 

 that little, if any, anhydrous chloride can be obtained with acid 

 below twenty per cent. 



The second method consists in passing dry hydrochloric acid 

 gas over powdered arscnious acid. The phajnomena in this pro- 

 cess are exceedingly interesting. The absorption of the gas is 

 immediate, and a considerable elevation of temperature ocems, 

 water and cbloridc of arsenic being simultaneously produced. 

 Almost immediately on contact of the bydrocbloric acid gas, the 

 arscnious acid becomes moist and speedily disappears; and when 

 the action is completed, two liquids are found in its place ; the 

 lower one being anhydrous chloride, and tbe upper one a satu- 

 rated solution of hydrochloric acid in water, with a small quan- 

 tity of dissolved chloride of arsenic. The non-production of 

 hydrated chloride of arsenic in the above circumstances is some- 

 what remarkable, since the quantity of water produced by the 

 double decomposition of the two acids is exactly the amount 

 which is stated to exist in the hydrated chloride ; thus 



As03 + 3HC1 = AsCF -|- 3H0. 



In a particular experiment, we found that 50'9 grains of arsc- 

 nious acid absorbed 63"5 grains of hydrochloric acid gas (dried 

 by oil of vitriol and chloride of calcium), which corresponds very 

 closely to one equivalent of anhydrous chloride, plus three equi- 

 valents of water saturated with hydrochloric acid. This is cer- 

 tainly the more elegant aud more satisfactory process for the 

 preparation of this substance. In both cases the residting chlo- 

 ride is easily purified by distillation. 



The methods for the quantitative estimation of arsenic are too 

 well known to require notice. The new process which we employed 

 for the analysis of the chloride, is based upon the reciprocal action 

 of chromic and arscnious acids in presence of hydrochloric acid. 

 The reaction is exhibited in the following equation : — 



3AsCF + 4Cr03 + 3H0 =3As05 + 2Cr2 CP + 3HC1. 



A weighed quantity of the pure chloride was mixed with water 

 and caustic potash, and excess of hydrochloric acid subsequently 

 added. The mixture being gently heated, a quantity of bichro- 

 mate of potash, barely sufficient to peroxidize the arsenic, was 

 slowly added. ^V. weak solution of a known quantity of the 

 same salt was then cautiously dropped into the liquid, till a por- 

 tion taken out on the end of a rod gave a faint yellow tinge to 

 a solution of acetate of lead spotted on a slab. The delicacy of 



