Determination of Antimony. 843 



however, there has been no method for directly determining sul- 

 phuret of antimony by the method of volumetric analysis. This 

 is attempted to be solved by the following process. The sul- 

 phide of antimony, whether precipitated by sulphuretted hy- 

 drogen from solutions containing antimonic acid or oxide of 

 antimony, is decomposed by boiling hydrochloric acid in such 

 a manner that for every equivalent of antimony three equivalents 

 of sulphuretted hydrogen are liberated. Consequently the de- 

 termination of this gas is a measure for the antimony, and its 

 accurate estimation affords an indirect method of determining 

 that body. Hence the real point is an accurate volumetric 

 estimation of sulphuretted hydrogen. This can be effected in 

 several ways. 



The determination of sulphuretted hydrogen liberated from 

 sulphide of antimony by treatment with boiling hydrochloric 

 acid, by passing it into solution of sesquichloride of iron, and 

 then estimating the protoxide of iron formed by means of per- 

 manganate of potash, gives very inaccurate results. The error 

 is caused by the fact that part of the sulphur which separates 

 from the sulphuretted hydrogen is oxidized by the excess of ses- 

 quichloride of iron to sulphuric acid, in consequence of which 

 some protoxide of iron is formed, which is added to that reduced 

 by the sulphuretted hydrogen. H. Rose observed a long time 

 ago, that when sulphuretted hydrogen was passed into a warm 

 solution of sesquichloride of iron, a small quantity of sulphuric 

 acid was formed. The present case offers these conditions ; for 

 the solution of sesquichloride becomes heated by the hydro- 

 chloric acid vapours which pass into it. The quantity of sul- 

 phuric acid formed is never considerable ; it varies with the tem- 

 perature, and small quantities of lower oxides of sulphur appear 

 also to be produced. In no case can the resulting protoxide of 

 iron serve as an accurate measure for the sulphuretted hydrogen. 



More accurate results are obtained when the sulphuretted 

 hydrogen is determined by means of iodine. 



The accuracy of this method has been doubted on many hands. 

 Bunsen himself* does not appear to consider it particularly 

 accurate, and Mohrf obtained results differing with the concen- 

 tration of the solutions employed. 



It must be admitted that the direct measurement of sulphur- 

 etted hydrogen by means of solution of iodine is by no means so 

 accurate as that of sulphurous acid by the same process ; yet, 

 when certain precautions are used, the method gives accurate 

 results. These precautions are, that the solution of sulphuretted 

 hydrogen must be greatly diluted, that water free from air must 



* Bunsen, Iodometrische Bestimmungen, p. 26. 

 t Lehrbuch der Titrirmethode, vol. i. p. 302. 



