240 0. F. Walker— Titration of Sodium 



0*0035 grm. of iodic acid, while the average variation is 0*25 cm3 , 

 corresponding to 0*0009 grm. The variation in the analyses of 

 the smaller amounts is less, the range being 0*44 cm3 , correspond- 

 ing to 0*0015 grm., and the average variation being 0*1 3 cm3 , or 

 0-0005 grm. The probable error which these irregularities 

 would introduce in any series of practical analyses by this 

 method is obviously greater than can ordinarily be permitted 

 in iodometric work. 



The experiments detailed in Table IV were performed 

 exactly similarly to those of the last series except that two 

 grams of potassium iodide were added to the sodium thiosul- 

 phate before the titration was commenced. 



Table IV. 



Variation of the End Reaction betioeen — Sodium Thiosulphate 



N 

 and — Iodic Acid, in the presence of Potassium Iodide. 





tfa 2 S 2 03 taken. 



HI0 3 introduced. 



Mean value. 



Variation 





cm 3 . 



cm 3 . 



cm 3 . 



cm 3 . 



( 1) 



6 



32-53 ") 





0*05 + 



( 2) 



6 



32-45 | 





0-03 — 



( 3) 



6 



32*67 [ 



32*48 



0*19 + 



(4) 



6 



32-37 





o-ii- 



( 5) 



6 



32-36 







0-12 — 



( 6) 



6 



32-50 







0*02 + 



( 7) 



4 



22-30 } 





o-n + 



( 8) 



4 



21-98 I 

 22-17 f 



22-19 



0-21- 



( 9) 



4 



0*02 — 



(10) 



4 



22-30 







0-11 + 



These experiments indicate plainly that in the presence of 

 potassium iodide the end reaction of different titrations of 

 equal volumes of the same solution is practically independent 

 of the amount taken for analysis. The results in the case of 

 the maximum amounts vary within a range of 0*31 cm3 , or 

 0'OOLl grm. of iodic acid, while the average variation is 0*09 cm3 , 

 corresponding to 0*0003 grm. The variation in the analyses of 

 the smaller amounts is practically the same as that of the larger, 

 the range being 0*32 cm3 , corresponding to 0*0011 grm., and the 

 average variation being 0*ll cm3 , or 0*0004 grm. It is therefore 

 evident that the presence of potassium iodide in the sodium 

 thiosulphate to be titrated will bring the variation of the forma- 

 tion of the reading tint within permissible limits. 



A series of experiments was made to determine the nature 

 and effect of the " after coloration " observed to take place 

 when a solution of sodium thiosulphate, free from potassium 

 iodide, was titrated with iodic acid to blue coloration, and then 

 bleached with sodium thiosulphate. The titrations were per- 



