292 J. T. Norton, Jr.— Influence of Hydrochloric Acid 









Table VI. 











Volume 













Amount 



of H 2 



HC1 



Excess 









of Se0 2 



at begin- 



sp. gr. 



of Na 2 



Se0 2 







taken. 



ning. 



(112). 



S 2 3 . 



taken. 



Error. 





grm T 



cm 3 



cm 3 



cm 3 



grm. 



grm. 



(1) 



•1042 



200 



5 



24-16 



•1041 



•0001 — 



(2) 



•0611 



« 



10 



13'3 



•0611 



•0000 ± 



(3) 



•0850 



(t 



10 



21-9 



•0828 



•0022- 



W 



♦0757 



a 



25 



13 07 



•0749 



•0008 — 



¥) 



•0540 



a 



25 



21-02 



•0522 



•0018 — 



(6) 



•0674 



300 



5 



10-04 



•0679 



•0005 4- 



(?) 



.-2416 



400 . 



5 



15-9 



•2424 



•0008 + 



It is apparent that at the dilution of 200 cm3 we run into difficul- 

 ties, and the greater the excess of thiosulphate present the 

 greater is the error. When the amount of sodium thiosulphate 

 exceeds 20 cm3 a reduction in the amount of acid to 5 cm3 is plainly 

 of advantage, as is shown in a comparison of Exps. (1), (3), 

 and (5), and is not disadvantageous at larger dilutions and 

 with smaller amounts of the thiosulphate, as shown in Exps. 

 (6) and (7). The necessity of placing some limits on the 

 method of Norris and Fay has now, I think, been estab- 

 lished. The excess of the thiosulphate must be carefully regu- 

 lated, as well as the temperature. If one has knowledge of the 

 approximate amount of selenious acid in solution, this is not a 

 matter of great difficulty, and things should be so arranged 



71/ 



that no more than 20 cra3 of the — thiosulphate should ever 



be present in excess of the amount necessary to reduce the 

 selenious acid. If this limit — amounting to O0400 CQ1 of Se() 2 — 

 is placed upon the thiosulphate, so much as 10 cm3 of hydro- 

 chloric acid (sp. gr. 112) may be present without endangering 

 the accuracy of the process, provided the solution is diluted to 

 400 cm3 at the outset ; if only 5 cm3 of hydrochloric acid are 

 present, the volume at the beginning may be reduced with 

 safety to 200 cm3 . At all events, 5 cm3 of the hydrochloric acid 

 are amply sufficient to bring about the reaction between the 

 thiosulphate and the selenium at any dilution within the range 

 of my experiments. With these precautions taken, the process 

 of Norris and Fay is simple, rapid and accurate; without them, 

 as the experimental results indicate, errors of considerable 

 amount may enter. 



According to the method of * Muthmann and Shafer, the 

 determination of selenious acid is effected by the simple addi- 

 tion of potassium iodide to the acidulated solution of selenious 



* Berichte d. d. Chem. Gesell., xxvi, 1008. 



