Detection and Quantitative Determination of Arsenic. '251 



whole liquid thus obtained is then concentrated in the water- 

 bath to the weight of 1 pound (during their operation the 

 bright yellow colour of the fluid in most cases changes into a 

 brownish tint), and a saturated solution of sulphurous acid in 

 water is added to the residuary, still very acid, fluid, with 

 stirring, until the smell of the sulphurous acid becomes clearly 

 perceptible. The whole mixture is then again heated for 

 about an hour, until the excess of sulphurous acid is com- 

 pletely expelled. 



B. Separation of the Arsenic as Sulphuret of Arsenic. 



The fluid thus obtained (and which is about double the 

 amount of the hydrochloric acid employed) is allowed to cool, 

 and then poured into a beaker glass, and a slow stream of 

 washed sulphuretted hydrogen gas transmitted through it for 

 the space of about twelve hours; the gas conducting tube is 

 rinsed with some solution of ammonia, and the ammoniacal 

 solution thus obtained added to the fluid in the beaker glass ; 

 this glass is then lightly covered with printing paper and kept 

 at a very moderately warm heat (30° C.) until the smell of 

 sulphuretted hydrogen has completely disappeared. The 

 precipitate obtained in this manner is collected upon a filter, 

 which ought not to be too largej and washed. 



C. Purification of Sulphuret of Arsenic so obtained. 



The precipitate obtained in B, and which besides organic 

 matter, contains, as sulphuret of arsenic, all the arsenic origi- 

 nally present in the substance under investigation, and pos- 

 sibly moreover other metallic sulphurets, is then, together with 

 the filter, dried in a small porcelain basin, heated in the water- 

 bath ; fuming nitric acid is added drop by drop until the 

 whole is moistened ; the moist mass is then evaporated to dry- 

 ness in the water-bath. Pure, previously heated, hydrated sul- 

 phuric acid is then added to the residue, so as to moisten it 

 uniformly ; the mass is heated in a water-bath for the space 

 of from two to three hours, and finally in the sand-bath at a 

 somewhat higher but still moderate temperature (150° C), 

 until the charred mass begins to crumble. The residue is 

 treated with from 10 to 20 parts of distilled water, until the 

 last drops running off" no longer manifest any acid reaction : 

 the water employed in the washing is added to the filtrate. 



D. Quantitative Determination of the Sulphuret of Arsenic. 



The limpid fluid obtained in C is mixed with some hydro* 

 chloric acid and precipitated by sulphuretted hydrogen, as 

 directed in B. The precipitate, after the supernatant fluid has 



Phil, Mag. S. 3. Vol. 25. No. 166. Oct. 1844, S 



