60 Gooch and Blumentlial — Use of Selenic Acid. 



addition of 5 cm3 more of the selenic acid after the estimation of 

 the bromine, and repetition of the boiling to almost the limits 

 at which chloride begins to be evolved, resulted in no appreci- 

 able improvement in the results by further evolution of bro- 

 mine. It must be concluded that the reaction by which selenic 

 acid sets free the bromine is complete under the conditions, or 

 else that the supplementary amount of selenic acid is inade- 

 quate to change perceptibly the equilibrium established in the 

 first boiling. 



In experiments to be described the conditions of action were 

 somewhat changed by using sulphuric acid and a selenate, in 

 the place of free selenic acid, to effect the liberation of the 

 bromine. The sodium selenate used was prepared in the 

 laboratory and was contaminated with sodium sulphate, though 

 otherwise pure. The selenate content was, however, deter- 

 mined and a solution of the mixed salts was prepared of such 

 strength that 10 cm3 contained 1 grm. of sodium selenate. A 

 preliminary series of experiments was made to determine the 

 conditions suitable to the use of this reagent. It was found 

 that chlorine is evolved from sodium chloride in presence of 

 sulphuric acid at concentrations much lower than those at 

 which the evolution begins when free selenic acid is the oxidi- 

 zer in absence of sulphuric acid. The effects observed are 

 shown in Table IY. 



Table IV. 



Concentrations of Sulphuric Acid, Selenate, and Chloride at which the Evolu- 

 tion of Chlorine begins. 



NaCl 

 taken 



Na 2 Se0 4 

 taken 



H 2 S0 4 



(1:1) 



taken 



Volume of the 

 reaction liquid 

 at which color 



appears in 

 the receiver 



grm. 



grm. 



cm 3 . 



cm 3 . 



0-1004 



0-6 



12 



35-40 



0*0753 



0-6 



12 



30-35 



0-0500 



0-6 



12 



20-22 



0-0500 



0-6 



6 



16 



0-0500 



0-6 



3 



8-9 



0-0500 



1-8 



3 



9 



The results of similar preliminary experiments with potassium 

 bromide are shown in Table Y. 



A comparison of the results of Tables IV and Y shows that 

 with 3 cm3 of sulphuric acid (1 : 1), 1*8 grm. of sodium selenate, 

 and 0*05 grm. of sodium chloride in the reaction flask (I), the 

 concentration may safely reach a volume of 10 cm3 . Determina- 



