RATE OF DIFFUSION OF IODINE IN KI 503 



below the "decomposition potential" for the solution. Further, this, 

 current ("Beststrom") does not obey Olun's law, even approximately, for 

 it remains ahnost constant through changes in the e.m.f . of several hundred! 

 per cent, say from 0.1 to 0.3 volt. Nernst and Merriam have shown that 

 with rapid stirring at a given rate the "Reststrom" is practically constant 

 over a very large range of potential differences, and is closely proportional 

 to the rate of diffusion of the depolarizer. They make the assumption 

 that this "Reststrom" is due to the diffusion of the ion (or depolarizer- 

 as the case may be) through the thin unstirred layer which they assvune 

 to cover the electrodes (35). 



Wwk of Harry Heymann. In 1912 Heymann made use of the "Rest- 

 strom" as a means of further establisliing the "Diffusion theory of reaction 

 velocity in heterogeneous media." He worked withplatinmn catalysis of 

 hydrogen peroxide, iodine in KI, and the solution of copper in lo + KI. 

 In the preface to this article Heymann calls attention to the fact that Van 

 Name and Edgar had covered much the same ground, using various metals 

 in I2 + KI solutions. Using a solution of normal KI in which iodine was 

 dissolved, Heymann determined the "Reststrom" using potentials varying 

 from 0.1 to 0.45 volt. The "Reststrom" within these limits was practically 

 constant as it was for Nernst and Merriam. 



He^onann found that the thickness of the "unstirred layer" decreased 

 with more rapid stirring as did Van Name and 'Edgar. Heymami made a 

 single determination of the diffusion constant for iodine in normal KI 

 directly, by a method practically similar to that used in this work. Using 

 his value and exterpolating by Nernst's formula we obtain for the diffusion 

 constant at 25° 1.19. This is in close agreement with our observed data. 

 Exterpolation is hardly fair however over so great a temperature inter- 

 val — ^for Heymann worked at about 8°. In the second part of his article 

 Heymann discusses the work of Van Name and Edgar at length, and 

 quotes much of their data to show that it is in hne with the deductions 

 made by Nernst, Merriam, and himself from experiments with the 

 "Reststrom." He closes by saying, 



Aus an grossen Anzahl von Reststrombestinimingen fand ich, dass der Dif- 

 f usionkoefBzient des Jods mit Steigender KJ-Konzentration stark wachst, und zwar 

 ziemlich analog den von Van Name und Edgar gemessenen Auflosungsgeschwindig- 

 keiten von Metallen in gleieh Konzentrierten Jodkaliumjodidlosungen (36). 



This, of course, is in accord with the results of the present writer also. 



