Researches on the Salts of Selenious Acid. 



11 



sulphuric acid, with cyanide of potassium and acidulate the solution with 

 hydrochloric acid; selenium is then, as Rose has found, completely preci- 

 pitated. In analyzing- the salts of mercury, I employed both methods at the 

 same time, and with complete success. 



The best method for the quantitative determination of selenious acid 

 was found by Rose to be reduction with sulphurous acid ; it is then necessary 

 to add hydrochloric acid. This is the way I have chosen for the analysis 

 of most of the selenites here described. The salts, soluble as well as in- 

 soluble, have been acidulated with about 10 c. c. concentrated hydrochloric 

 acid, by means of which even the latter are brought into a soluble state; 

 then a solution of sulphurous acid, saturated at the ordinary temperature, 

 has been added, and the covered vessel, holding only 100 c. c., placed to 

 digest at about 60° for nearly 12 hours, when it was usually impossible to 

 remark any smell of sulphurous acid and the reduction of selenium was 

 completed. If the reduction be accelerated by means of stronger heating, 

 small quantities of selenious acid are carried off with the speedily disappea- 

 ring sulphurous acid gas, as tetrachloride of selenium, by which a loss of 

 selenium, often amounting to several percent, may be caused. By proceeding 

 in the manner just mentioned, the loss, on the contrary, is very slight or 

 none, as the numerous selenium determinations, communicated in the follo- 

 wing pages, will show. Otto's *) statement that selenium thus becomes im- 

 pure with heavy metals, if any such occur in the solution, has not been 

 confirmed in a single instance. Selenium was obtained in shining, black, 

 caked masses at the bottom of the glass; only a small quantity had been 

 deposited on the sides as a yellowish-red, brilliant covering, which was 

 easily removed with a feather. When washed by means of decantation 

 with boiling water, it was collected on a weighed filter, dried at 100° and 

 weighed. The bases, after evaporation of the filtrate and expulsion of the 

 excess of sulphuric acid, have in most cases been determined as waterfree 

 sulphates, but, Avhen this was impossible, as oxides, usually by heating the 

 sulphates to whiteness. Sometimes, as in the cases of the salts of barium, 

 strontium, calcium etc., peculiar methods have been found necessary, which 

 will be accounted for in their proper places. Where the contrary is not 

 particularly remarked, all the selenites have been prepared for analysis by- 

 previous pressing between folds of bibulous paper. 



') Lehrb. d. Chemie 4:te Aufl. I S. 634—635. 



