180 HYDROGEN ION CONCENTRATION 



at A and B disappear. But if the zone of contact at A were old and 

 the one at B freshly renewed, then a potential difference of up to six 

 millivolts will develop, which will gradually diminish on standing. 

 This fluctuation of the potential with time has also been found by a 

 number of investigators.'' Recently an ingenious apparatus has been 

 devised by Lamb and Larson^ by means of which they succeeded in 

 maintaining a constantly self renewing fresh junction of the solutions, 

 and they obtained potential values of remarkable reproducibility. 

 Without some such arrangement it can scarcely be even expected to 

 attain the sharply defined boundary surfaces which were assumed by 

 Planck. And since the time effect must be such that the potential 

 gradually diminishes, in practice one frequently finds values for 

 potentials which are smaller than those calculated according to 

 Planck's equation. Recently Fales and Vosburgh (1. c.) have even 

 stated that between a saturated (4.1 N) KCl solution and 0.1 to 1.0 

 N HCl-solution there arises no measurable diffusion potential at all. 



Under these circumstances, it must be said that the value of a 

 diffusion potential is not as sharply defined as it should uncondi- 

 tionally be required in concentration chain measurements for anal}d:i- 

 cal purposes, for which it is frequently employed. For this reason 

 it is less important in the practical use of concentration chains to 

 calculate exactly the diffusion potential than it is to arrange the 

 chain in such a way that the diffusion potential should become ex- 

 tremely small and neghgible. The method of achieving this purpose 

 will be described as follows : 



All these diffusion potentials originate in the differences among 

 ion mobilities. Between electrolytes whose individual ions have 

 equal mobilities no diffusion potential develops. The electrolytes 

 in which the difference between the values of u and v is the smallest 

 are KCl (64.6 and 65.5) and a few others such as NH4NO3 (64 and 

 61.7). Solutions of different concentrations of KCl or those of NH4 

 NO3 have therefore extremely small diffusion potentials. Thus 



^ A. Weyl, Messung von Diffusionspotentialen konzentrierter Chlorid- 

 losungen. Dissert. 1905. Bjerrum, Zeitschr. f. Elektrochem. 17, 58, 389 

 (1911). G. N. Lewis and F. F. Rupert, Journ. of the x\mer. Chem. Soc. 33, 

 299 (1911); A. C. Gumming and E. Gilchrist, Transact. Faraday Soc. 9, 174 

 (1913) ; H. A. Fales and W. C. Vosburgh, Journ. of the Amer. Chem. Soc. 40, 

 1291 (1918). 



* A. B. Lamb and A. T. Larson, Journ. of the Amer. Chem. Soc. 42, 229 

 (1920). 



