10 



L. F. Nilson, 



their solutions, previously acidulated with hydrochloric acid. But the method 

 was judged unfit; for, when the sulphide of selenium seemed to be com- 

 pletely deposited, there was still, on evaporating, obtained a precipitate of 

 the same compound; and, on further evaporation to dryness, the residue, 

 when heated, smelled strongly of selenium, and the crucible was attacked by it. 



Muspkatt has, concerning this matter: "The determination of sele- 

 nious acid is very difficult; fusion with nitrate of potash is the first step 

 to convert the acid into selenic acid, which latter can then be precipitated 

 by nitrate of baryta;" he also points out some precautions that must be 

 observed on heating the selenate of barium previously to the weighing, but 

 he only once in his analyses applies this method of determination. In other 

 cases, a water estimation, of the details of which, except in a single in- 

 stance, (diselenite of sodium), we are left wholly ignorant, is most often 

 considered to be sufficient; more rarely is the quantity of the base deter- 

 mined, but so much the oftener a formula assumed, without any kind of 

 analytical basis. 



Rose in a detailed memoir on the quantitative determination of sele- 

 nium, also mentions the methods of determining selenious acid. His re- 

 searches proved, in the first place, with regard to its precipitating and weigh- 

 ing as selenate of barium, that tiiis method of proceeding suffers from two 

 great faults, rendering it completely unavailable. Selenate of barium is 

 namely soluble in water and still more so in acids; it shares, moreover, 

 with the corresponding sulphate the property of precipitating, along with 

 it, other salts present in the solution, which, though soluble by themselves, 

 cannot be washed out from the precipitate with boiling water. If a good 

 result happens to be obtained, the only cause is, that the two faults com- 

 pensate each other. 



The method of precipitating selenious acid with hydrosulphuric 

 acid, stated to be unavailable by Berzelius, is considered by Rose as 

 leading to the complete precipitation of the acid, in form of selenium 

 mixed with sulphur — Se + S 2 — ; the quantity of selenium is cal- 

 culated from the weight of the precipitate dried at 100°. Should there be 

 any reason to suppose that the precipitate contains excess of sulphur 



— which easily takes place on precipitating with hydrosulphuric acid 



— he recommends treatment with oxidating agents and to reduce the 

 selenious acid then formed with sulphurous acid. It seems to me, however, 

 to be a shorter way to treat the precipitate, obtained by means of hydro- 



') Ann. d. Ph. u. Ch. CXIII S. 472. 



