8 



D. A. Kr eider — Iodine Titration Voltameter. 



pleted, was equal to two of the bulbs, aud no difficulty was 

 experienced in keeping within that limit. In case of acci- 

 dentally overstepping the amount of thiosulphate required, a 

 measured volume of iodine solution may be added and the 

 titration be repeated, subsequent deduction being made for the 

 amount of thiosulphate necessary to bleach the added iodine. 



The end reaction was in all cases taken as the bleaching of 

 the iodine color, without starch. This in itself is quite deli- 

 cate, and I have invariably been able to read it to a small frac- 

 tion of a drop, when the beaker stood on white paper in a 

 good indirect light. As a confirmation of the reading, 5 CC of 

 starch solution was then added and produced a faint purple 

 color. The delicacy of this end reaction was more thoroughly 

 appreciated when, after a number of titrations had been made 

 successively, as in the experiments of Table III, the addition 

 of the starch produced almost precisely the same shade of color 

 in all, despite the fact that a very small fraction of a drop of 

 the thiosulphate produced a distinctly perceptible change in 

 the color. 



Table III. 





Electrodes, 

 cm. 



Dis. bet. 



electrodes, 



cm. 



KI 



grms. 

 in 

 cc. 



02 ""^ 



i o X 

 \ o 



ft ft 



q 



w 6 



Difference. 





cc. 

 0-17 

 0-06 



o-io 



0-23 



% 



(I) 



(2) 

 (S) 



(*) 



j 1-6X2-7 

 1 1-6X2-7 



j 1-6X2-7 

 1 1-6X2-7 



j 1-6X2-7 

 1 2-5X6-0 



( 1-6X2-7 

 \ 1-6X2-7 

 ( 2-5X6-0 



2 

 5 



2 

 5 



2 

 5 



2 

 5 

 5 



5 in 7:5 

 5 " 7-5 



5 " 7-5 

 5 " 8-5 



5 " 7-5 

 10 " 20 



5 " 7-5 

 5 " 7-5 



10 " 20 



I 0-5 

 i 0-5 



M 



H 



0-058 j 



0-058 | 



0-058 

 0-017 



0-058 



0-017 



130-84 

 130-67 



128-49 

 128-55 



127-36 

 127-46 



170-67 

 170-67 

 170-90 



0-13 

 0-047 



0-078 



0135 



Table III is a record of a number of determinations of the 

 constancy of this voltameter. Two or more of the cells were 

 connected in series, and the conditions in each varied as shown. 

 The time of run for 1 to 3 inclusive was about 45 mins., for 

 the 4th, one hour. The amount of hydrochloric acid (1 : 4) 

 was not measured. Enough of the acid was drawn in to insure 

 the covering of the cathode when the iodide solution was 

 drawn in, and to keep the solution acid throughout the experi- 



