346 M. R. Schneider on the Volumetric 



In using it the above method may be modified as follows. 



The same apparatus is used for decomposing the sulphide of 

 antimony and collecting the sulphuretted hydrogen. A solution 

 of arsenious acid, prepared by dissolving arsenious acid (purified 

 by resublimation) in water, pure soda being added until a neu- 

 tral or feebly alkaline reaction is set up, is placed in the receiver. 

 The solution is so strong that 1 cubic centim. contains about 

 0*005 — 0*006 grm. of arsenious acid, and it is compared with a 

 solution of iodine which contains a known quantity of iodine*. 

 According to the quantity of sulphide of antimony, 50, 100, or 

 200 cubic, centims. are used. The latter quantity would be 

 quite sufficient for the sulphuretted hydrogen disengaged from 

 1*5 grm. sulphuret of antimony. 



From the hydrochloric acid gas which passes along with the 

 liberated sulphuretted hydrogen into the receiver the liquid soon 

 becomes acid, and then sulphuret of arsenic is deposited. The 

 decomposition of the sulphuretted hydrogen is rapid and com- 

 plete. Chloride of antimony does not distil over if the distilla- 

 tion be not continued too long. Nor are any organic substances 

 formed by the action of the boiling hydrochloric acid on the paper, 

 which might subsequently exert a reducing action on the solution 

 of iodine f. 



As soon as the liquid in the retort is cold, it is transferred to 

 a measuring-flask, some solution of tartaric acid is added J, and 

 the flask filled up to the mark. After this liquid has been filtered 

 an aliquot part is measured off, and after saturation with bicar- 

 bonate of soda the quantity of arsenious acid is determined by 

 solution of iodine. 



The calculation of the result is very simple. If the volume 

 of solution of iodine corresponding to the arsenical solution 

 taken be called V, the volume of iodine solution corresponding 

 to the arsenical solution after the distillation be called o, and the 



* This is best effected by adding to a known quantity of the arsenical 

 solution a few drops of hydrochloric acid till it is feebly acid, and then 

 excess of bicarbonate of soda, some solution of starch, and finally solution 

 of iodine. 



t A paper filter, 2 inches in diameter, was boiled in hydrochloric acid, 

 and the vapour disengaged collected in water to which 8 cubic centims. 

 solution of arsenic (10 cubic centims. = 23*3 cubic centims. solution of 

 iodine) had been added. After the distillation, the arsenical solution (after 

 the addition of bicarbonate of soda and of starch) required 18*7 cubic cen- 

 tims. solution of iodine. Now 10 : 8 = 23*3 : 18*64. 



X The error in the volume caused by the suspended sulphide of arsenic 

 is so small, that in the majority of cases, and where unusually large quan«» 

 tities of substance are not operated upon, it may be neglected. 



The sulphide of arsenic precipitated from a hydrochloric acid solution 

 persistently retains some chlorine, doubtless as chloride of arsenic, even 

 after washing. This is not the case when the solution contains tartaric acid. 



