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II. The Measurement of the Potential of the Electrodes in 

 Stationary Liquids. The Determination of Changes of 

 Concentration at the Cathode during Electrolysis. By 

 Henry J. S. Sand, Ph.D., M.Sc* 

 [Plate I.] 



IN recent electrochemical investigations accurate data are, as 

 a rule, to be found regarding current-density, voltage, 

 temperature, and concentration of liquid taken, and very often 

 also regarding the potential of one of the electrodes, defined 

 as the drop of potential on the boundary between it and the 

 electrolyte. The actual concentration of the electrolyte in 

 contact with the electrode is, however, very often uncertain, 

 owing to the removal of substance by electrolysis. Some 

 years ago I was able to demonstrate the great importance of 

 this factor in connexion with the electrolysis of acid solutions 

 of copper sulphate |. When small current-densities were 

 employed in a stationary solution, it was possible to show 

 with a fair amount of accuracy that evolution of hydrogen 

 only took place after the complete removal of copper salt 

 from the liquid in contact with the electrode. On the other 

 hand, when large current-densities were employed, this local 

 depletion took place so rapidly that even the most vigorous 

 stirring could not prevent it. The result obtained in this 

 research could be predicted theoretically when the assump- 

 tion was made that the process occurring at the electrode 

 itself was of a reversible nature ; that is, that on the boundary 

 between electrode and electrolyte, chemical equilibrium must 

 be supposed to remain continually established. The matter 

 may also be stated in the following manner corresponding to 

 a form in which it has recently been put forward by Nernst J 

 as a general principle : differences of chemical potential cannot 

 exist for a finite period of time on the boundary of two phases, 

 that is " between infinitely near points," as this would lead 

 to infinitely great forces and reaction velocities. According 

 to this assumption, the actual velocities of electrochemical 

 reactions, and of all others which occur on the boundary of 

 two phases, are infinitely great as soon as conditions of 

 equilibrium have been overstepped by a finite amount, the 

 observed velocities being simply limited by the rate at 

 which diffusion and convection renew the material removed 

 by the reaction. As conditions of equilibrium in electro- 

 chemical reactions are determined by the electrode-potential, 



* Communicated by the Faraday Society. Bead before the Society 

 October 25th, 1904. 



f Phil. Mag. 6th series, i. p. 45. 



t Ztschft. Phys. Chem. xlvii. p. 52 (1904). 



