168 HYDROGEN ION CONCENTRATION 



difference between a hydrogen electrode (Pt — H2) under one atmos- 

 phere pressure of hydrogen-gas and a solution normal with respect 

 to the H-ion as equal to zero. In this way it was possible to arrive 

 at the electromotive series of the metals and some non-metals shown 

 in table 24, which could be used just as hydrogen in the form of a 

 platinum electrode covered with gas.^ 



With the help of Nernst's equation and these values one can 

 calculate the potential between a metal and a solution containing 

 ions of this metal in any concentration. Conversely, the E.M.F. 

 of the concentration chain being known, the unknown concentration 



* t These two values are in reference to a solution 1 A'' with respect to 0H~- 

 ions. They are frequently given in tables in reference to a 1 A' H"*" solution, 

 in which case they are +1.23 and +1.68 respectively. 



of the metal ions in the solution can be calculated. This is partic- 

 ularly important for the cases where complex ions are formed and 

 of the relatively insoluble metal salts. Thus, for example, the 

 solubility of AgCl can be estimated by determining the concentration 

 of the Ag+-ions in the solution in the chain 



Ag 



AgNos 

 1 molar 



AgCl, sat. 

 solution 



Ag 



3 Some of these values were not determined in the direct experimental way, 

 but were indirectly calculated. 



