242 C. F. Walker — Titration of Sodium Thiosulphate, etc. 



Table VI. 



Titration 



°^1 



— Sodium Thiosulphate with — Iodic Acid. 









HI0 3 











Na 2 S 2 3 



HI0 3 



required 





KI 







taken. 



introduced. 



by theory. 



Error. 



Error, present. 



Volume 





cm 3 . 



cm 3 . 



cm 3 . 



cm 3 . 



per cent. grm 



cm 3 . 



( 1) 



4 





'18-68 



20*32 



1-64- 



8-0- — 



50 



( 2) 



6 





28-32 



30-48 



2-16 — 



7-0- - 



50 



( 3) 



6 





27-32 



30-48 



3-16 — 



7-0- - 



150 



( 4) 



6 



* ^ 



28-73 



30-48 



1-75- 



6-0- — 



200 



( 5) 



6 





30-77 



30-48 



0-29 + 



0-01+ — 



250 



(6) 



6 





^36-97 



30-48 



C5-49 + 



21-0+ — 



300 



( 1) 



6 





"27-46 



30-48 



3-02 — 



10-0— — 



"50 



( 8) 



6 





26-15 



30-48 



4*33 — 



14-0— — 



350 



( 9) 



6 



t <{ 26-50 



30*48 



3°98 — 



]3-0— - 



200 



(10) 



6 



| 27-16 



30-48 



3-32- 



10-0— — 



250 



(11) 



6 



^ 32-93 



30-48 



2-45 + 



8'0+ — 



300 



(12) 



4 



* j 22-19 



20-32 



1-8.7 + 



9-0+ 0-2 



50 



(13) 



6 





b 32-48 



30-48 



2-00 + 



7-0+ 0-2 



50 



These results show plainly that the amount of iodic acid 

 required to decompose a given amount of sodium thiosulphate 

 may be considerably above or below that required by the terms 

 of Kiegler's equation. Thus, with small volumes, and in the 

 absence of potassium iodide, the thiosulphate is destroyed and 

 the separation of iodine commences when only 93 per cent of 

 the theoretical amount of acid has been titrated. At higher 

 dilutions the action is retarded, so that at 250 cm3 very nearly the 

 theoretical amount of acid is required to produce the first blue 

 color, and at 300 cm3 an excess of 21 per cent over the theoreti- 

 cal amount must be added. If the " after separation " of iodine 

 is considered to be a measure of the excess of iodic acid, and if 

 its amount is accordingly applied as a correction, it appears 

 that for all volumes below 300 cm3 the original thiosulphate is 

 completely destroyed when about 90 per cent of the theoretical 

 amount of iodic acid has been added. The presence of potas- 

 sium iodide in the system retards the action, so that at small 

 volumes an excess of about 8 per cent of iodic acid must be 

 added to completely destroy the thiosulphate and commence 

 the separation of iodine. It is obvious from the preceding 

 experiments that the reaction between iodic acid and sodium 

 thiosulphate is so indefinite in its nature, and so dependent for 

 its completeness on conditions of time, dilution and mass, that 

 its direct application as a means of standardizing solutions 

 must remain impracticable. 



The author is indebted to Professor F. A. Gooch for many 

 valuable suggestions during the course of this investigation. 



* HIOn added to first blue color. 



f Calculated by subtracting from the. amount of iodic acid originally titrated, 

 the volume of thiosulphate of equal strength required to bleach the solution 

 after standing twenty hours. 



