Thiosulphate with Iodic Acid. 



239 



postponing the appearance of the starch blue until a quantity 

 of iodic acid had been added considerably in excess of the 

 theoretical. These experiments were executed with entirely 

 different reagents, and under varied conditions of concentra- 

 tion, the results in every case exactly confirming those already 

 observed. 



For the purpose of more particularly investigating this sub- 

 ject, there were prepared and standardized an approximately 

 decinormal solution of sodium thiosulphate, and an approxi- 

 mately one-fiftieth normal solution of iodic acid. Measured 

 portions of the sodium thiosulphate solution were titrated with 

 the iodic acid in the presence of starch emulsion under vary- 

 ing conditions of mass, time and dilution. 



To determine the variability of the end-point of the reaction 

 when the titration was conducted as directed by Riegler, a 

 series of experiments was made. Measured amounts of the 

 sodium thiosulphate solution were drawn from a burette into 

 an Erlenmeyer beaker of suitable capacity, the sides of the 

 beaker were carefully washed down with a small amount of 

 water, 5 cm3 of starch emulsion were added, and the iodic acid 

 was slowly dropped into the small. bulk of acid and starch, with 

 constant agitation of the mixture, until the first tint of blue 

 coloration appeared. The results obtained are given in Table 



Table III. 



N 



Variation of the End .Reaction betioeen — Sodium Thiosulphate 



N 



10 



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

 50 ' J 



NaaSjjOa taken. 



HI0 3 introduced. 

 cm 3 . 



( 1) 



1 



2) 



3) 



4) 



5) 



6) 



<) 



8) 



( 9) 

 (10) 



(11) 

 (12) 



28 

 27 

 28 

 28 

 28 

 28 

 28 

 28 

 18 

 18 

 18 

 18 



13 



79 

 03 

 32 

 32 

 71 I 

 83 I 

 43 j 

 94 1 

 67 I 

 50 f 

 60 J 



Mean value. 

 cm 3 . 



28*32 



Yariation. 

 cm 3 . 



0*19 — 

 0-53 — 

 0-29 — 

 0-00 



o-oo 



0-39 + 

 0-51 + 

 0-11 + 

 0*26 + 

 0-01 — 

 0-18— 

 0'08 — 



These experiments indicate that the constancy of the end 

 reaction in different titrations of equal volumes of the same 

 solution depends to a certain degree on the volume of sodium 

 thiosulphate taken. The results in the case of the maximum 

 amounts vary within a range of l*04 cm3 , which corresponds to 



