in Titrations by Sodium Thiosulphate. 



291 



will show that at this degree of dilution the hydrochloric acid 

 present has the least effect. 











Table IT. 













Volume 









Amount 



HC1 



at begin- 



Excess 









Se0 2 



sp. gr. 



ning of 



Na 2 S 2 3 



Se0 2 







taken. 



112 taken. 



titration. 



employed. 



found. 



Error. 





grm. 



cm 3 . 



cm 3 . 



cm 3 . 



grm. 



grm. 



(0 



•0616 



10 



400 



2-28 



0-0625 



0-0009 + 



(2) 



•0628 



a 



(( 



7-11 



0-0631 



0-0003 + 



(3) 



•0508 



a 



a 



11-4 



0-0511 



0-0003 + 



(4) 



'05-87 



a 



a 



12-8 



0-0594 



0-0007+ mean 



(5) 



•0807 



a 



a 



15-3 



0-0813 



0-0006 + -00005 



(6) 



•0633 



a 



a 



20-85 



0-0638 



0-0005 + 



(?) 



•0682 



25 



a 



1-11 



0-0685 



0-0003 + 



(8) 



•0779 



a 



a 



1.35 



0-0788 



0-0009 + 



(9) 



•0465 



u 



a 



18-93 



0-0469 



0-0004 + 



These results, while not so good as those obtained by Morris and 

 Fay, are satisfactory and show that at this degree of dilution the 

 process is accurate. These results accord closely with those 

 contained in Table I. At a dilution of 400 cm3 or in the pres- 

 ence of only 20 cm3 of sodium thiosulphate in excess the 

 hydrochloric acid present had no perceptible effect. Of course, 

 it must be kept in mind that the hydrochloric acid acts only 

 upon the excess of thiosulphate which is not taken up by the 

 selenium dioxide. The slight constant plus error in these 

 results cannot be accounted for by errors in the standards ; 

 they were all carefully determined. Another preparation of 

 selenium dioxide was made, starting with pure selenium care- 

 fully precipitated by sulphurous acid, before putting it through 

 the course of treatment previously described, and the results 

 obtained by the action of the sodium thiosulphate recorded in 

 Table V agree closely with those of the preceding table. 



Table Y. 



Amount 



HC1 



H 2 



Na 2 S 2 3 







Se0 2 



sp. gr. 



at 



in 



Se0 2 





taken. 



112. 



beginning. 



excess. 



found . 



Error. 



grm. 



cm 3 . 



cm 3 . 



cm 3 . 



grm. 



grm. 



(1) -0562 



10 



400 



9-52 



•0566 



•0004 + 



(2) -0651 



25 



a 



11-20 



•0655 



•0004 + 



The next step was to determine the effect of diminishing the 

 dilution and of varying the strength of acid. The following- 

 table gives the results of my experiments. 



