196 ELECTROCHEMISTRY 



tween the liquid (charged positively) and the metal (charged 

 negatively) which is characteristic for each metal and each liquid. 



The force which tends to drive the metal ions back into the 

 metal again is the osmotic pressure of the dissolved ions. If, 

 through the introduction of a dissociable salt of the metal, the 

 concentration (osmotic pressure) of the metal ions in the liquid 

 is increased, this may equal or even exceed the solution pressure 

 of the metal itself. In the former case the potential difference 

 between the metal and the solution is abolished; in the latter it 

 is renewed again, but in the opposite sense, the metal being now 

 charged positively and the solution negatively. 



If, now, we build up a galvanic element as follows: Metallic 

 silver in contact with dilute AgNOa, in conducting communi- 

 cation with strong AgNOs, the latter solution being again in con- 

 tact with metallic silver, it is evident that there must be a differ- 

 ence in potential between the two portions of metallic silver, a 

 potential which will be greater the greater the difference between 

 the concentrations of the two silver nitrate solutions. Provided 

 the potential between the two silver nitrate solutions themselves 

 (due to the unequal migration-velocities of silver and NO 3 ions) 

 can be neglected or made to vanish (and we will shortly explain 

 how this may be approximately achieved), it has been shown 

 by Nernst that the potential of this "concentration chain" can 

 be expressed by the formula: 



where R is the gas constant, T the absolute temperature, F the 

 Faraday constant, r? the valency of the ion (in this case 1), d the 

 concentration of the stronger solution of silver nitrate, Ci that of 

 the weaker and tt the potential in volts. 



In this way the absolute concentrations of two different solu- 

 tions of an ion can be accurately computed, provided only a 

 minute amount of current be allowed to pass during the potential 

 determination, without any significant alteration in the compo- 

 sition of the solutions. The measurement is, in other words, 

 static; and static methods of measurement must be employed in 

 determining the combining capacities of proteins because the 

 combining capacity of proteins is not possessed of one fixed value 

 but of fixed minimum and maximum values and, between these 



