6 D. A. Kr eider — Iodine Titration Voltameter. 



Standard Solutions. 



Sodium thiosulphate is the most convenient medium for the 

 titration of iodine, bnt it is not a very satisfactory standard 

 where great accuracy is desired. The salt is readily obtained 

 quite pure, but there is always some uncertainty as to the 

 amount of extraneous water the crystals may contain. Further, 

 on long standing, especially when exposed to the light, it un- 

 dergoes a slight decomposition, with deposition of sulphur. 

 However, this decomposition is so slow that, if kept in the 

 dark, the solution will remain quite constant for months. 

 In all of my titrations, except those that were merely relative, I 

 have employed an approximately decinormal solution of sodium 

 thiosulphate, standardized against arsenions oxide, by means 

 of an iodine solution. The purest arsenious oxide obtainable 

 was resublimed three times and weighed from a weighing bot- 

 tle. This solution also was only approximately decinormal. 

 The weight of the arsenious oxide was carefully determined 

 and reduced to its weight in vacuo, and the solution accu- 

 rately made up to one liter at 20° in a calibrated flask. The 

 direct employment of the arsenic solution for the titrations 

 is undesirable because it necessitates an excess of bicarbonate 

 and the danger of loss of iodine in the neutralization requires 

 a complicated series of traps. Moreover, the end reaction, 

 when starch is employed, is much slower with arsenic than 

 with the thiosulphate. With the latter it is practically instan- 

 taneous. 



The Titrations. 



The titrations were performed in Ehrlenmeyer beakers. 

 In filling the cell, care was always taken to draw up the last 

 of the iodide solution as far as the ground glass joint, but with- 

 out admitting air which would stir up the solutions. To avoid 

 a possible loss of iodine through a considerable leakage of the 

 joint and vaporization from the concentrated solution, the 

 Ehrlenmeyer beaker containing about 150 cc of water was 

 placed under the cell during the electrolysis. The capillary 

 extension of the cell (fig. 1) reached to the bottom of the 

 beaker. Whenever the leakage of the ground glass joint was 

 sufficient to allow the iodine solution to descend the full length 

 of the capillary during an experiment, the joint was reground 

 with the finest emery. Under these conditions a loss of iodine 

 is impossible. 



After the current was cut off, the ground glass joint was 

 opened slightly and the cell allowed to empty slowly. The 

 great density of the iodine solution keeps it continuously 

 covered with a layer of water of considerable depth in the 



