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



ber of grams of potassium iodide used and the second figure 

 the volume of its solution in water. In the fifth column, 

 where two figures are given, the first represents the fall of 

 potential through the cell at the beginning of the experiment 

 and the second figure that at the end of the run. The interval 

 is found in the sixth column. An asterisk following the 

 figures of the sixth column indicates that the current was con- 

 tinued until, and stopped at the moment of, the first appear- 

 ance of gas on the anode. It is evident that the liberation of 

 iodine is no longer quantitative for some time before this gas 

 appears, the safety limit depending upon the current density. 

 Within this safety limit the rise of potential is also not as great 

 as that indicated in the fifth column. The fall of potential 

 through the cell increases continuously and very regularly, 

 after the first few minutes, in which it rises rather more rapidly. 

 Just before the appearance of the gas the potential naturally 

 rises quite rapidly. The gradual change of potential is doubt- 

 less due to the increased resistance of the potassium chloride 

 solution which is formed during the electrolysis at the bound- 

 ary of the two solutions. This part of the cell, especially the 

 small one, is always considerably heated by the current, and a 

 conspicuous line of increased density of the solution, but 

 entirely colorless, gradually creeps up the tube to a distance 

 of several centimeters in an hour's run. In the 7th experi- 

 ment the solution became quite warm and small bubbles of 

 gas appeared on the sides of the glass, long before there was 

 any evidence of gas being evolved at the anode. To test the 

 correctness of the supposition that this was merely dissolved 

 gas, the 8th experiment was made with the cell in a water 

 bath maintained at 9°. Under these conditions no gas appeared 

 until it was evolved at the anode. This experiment also shows 

 that the warming of the solution is, if anything, an advantage ; 

 due, doubtless, to the more rapid diffusion of the iodide or to 

 the greater solubility of the iodine. In experiments 3 and 10 

 the current was varied ; the time of run for each value of the 

 current is given in the sixth column. 



Three determinations with two of the small cells in series, 

 each containing 2 grams of potassium iodide in T*5 CC , with a 

 current density of 0*015 amp./^ 2 showed the following satis- 

 factory agreement in the amount of thiosulphate required : 



, x ( 31-55 cc ,,. 28.60 cc . v 16-62 cc 



^ "(31-58" ^ ' 28.66" ^ 16-60" 



A comparison between the values obtained by two of the 

 iodine voltameters in series with each other and with one or 

 more copper voltameters is shown in Table II. The titrations 



