The Estimation of Acidity. 



241 



The production of an e.m.f. in this case, and its direction, 

 depend upon the fact that while both electrodes give off posi- 

 tively charged ions to the solution, and to this extent become 

 negatively charged themselves, the electrode, which is in contact 

 with the more dilute solution, is " f rejer " to do so on account 

 of the lower osmotic pressure of the ions already in solution. 

 For this reason it must become negatively charged to a greater 

 degree, and the electric equilibrium having been upset, an e.m.f. 

 is shown by the galvanometer. 



Hitherto we have dealt with the production of e.m.f. when 

 metals make contact with solutions o-f their salts; but an e.m.f. 

 is similarly produced where certain gaseous elements make con- 

 tact with aqueous solutions containing the same element in an 

 ionised state. This may be demonstrated in the case of oxygen 

 in contact with an alkaline solution, i.e., one containing dissociated 

 OH, but for certain technical reasons a more general application 

 has been found for a method in which hydrogen is brought into 



17 



