REACTIONS IN HOMOGENEOUS SYSTEMS 41 



The potential at the contact of the solution under investi- 

 gation with the standard chloride solution is discussed in the 

 following paragraphs. 



CONCENTRATION CELLS 



At the contact of the solution under investigation with the 

 chloride solution there is a potential due to the diffusion of the 

 ions. As shown in the table on page 35, the rate of diffusion 

 of the different ions varies, and if one of the ions diffuses more 

 rapidly than the others the solution into which it diffuses 

 acquires the charge of the more rapid ion. This difference of 

 diffusion sets up an electrical potential. 



As the positive and negative ions attract each other the 

 oppositely charged ion is dragged after the 'faster one ; . the 

 one is held back and the other pulled onwards so that the rate 

 of movement of the two becomes equal. Nevertheless the 

 more rapid ion confers its charge on the solution into which 

 it is diffusing. 



It has been shown that if two different concentrations of 

 the same salt are in contact the electrical potential is 



0-0235^ v log c l where u is the rate of diffusion of the positive 



ion, v that of the negative ion. C x and C 2 are the concentra- 

 tions of the two solutions.* 



From this equation we see that if a salt is used, with ions 

 which diffuse at about the same velocity the electrical potential 

 becomes very small. Therefore a strong solution of potassium 

 chloride or ammonium nitrate is frequently interposed between 

 the solution under investigation and the chloride solution in 

 the calomel electrode. 



Electrolytes in solution by acting as conductors short 

 circuit the potential between the two solutions, therefore the 

 concentrated solution increases the short circuiting because 

 of the large number of ions in the solutioji. 



The potential due to the contact between the two solutions 

 is reduced to a minimum by the short circuiting and by the 

 nearly equal rate of diffusion of the two ions. 



Differences in potential in the schema shown above are by 

 these means limited to the contact between the hydrogen 

 electrode and the solution under investigation. The potentials 

 have been measured with a series of solutions of known hydro- 

 gen ion concentration. The formula for the electromotive 



* W. Nernst, Zeit. f. physikal. Chem., 1888, vol. n, p. 634. 



