286 HYDROGEN ION CONCENTRATION 



right is in contact with a solution of AgNOs which is saturated with 

 AgCl. Into the same solution on the left side there is immersed 

 another silver electrode, but not du-ectly, being covered with a layer 

 of fused and solidified AgCl. This system is in a state of chemical 

 equilibrium. When the two electrodes are connected by a metal 

 wire there will be no flow of current."' This system, therefore, con- 

 tains three interfaces, a, b, and c. Each of these may be regarded as 

 the source of a potential difference; c is the source of an ordinary 

 metal electrode potential. In order to render this chain currentless 

 the potential at c must be equal to that of a + b. Hence the intro- 

 duction of the layer of solid AgCl has not altered the potential differ- 

 ence between the silver electrode and the solution. This can be 

 summarized by the following generally applicable statement: When 

 between two phases A and C, which are in a stdte of chemical equilibrium^ 



a third phase B is introduced, which is also in 

 a state of equilibrium with the first two phases, 

 the potential difference between A and B is not 

 altered thereby. From this it follows that the 

 total potential from A to C is decomposed into 

 two stages through the introduction of the 



J^^/ flgNO, ^ third phase. 

 5<y/c/ Sdution Now if we employ a piece of sUver covered 



a b c with solid AgCl in one case as a pole of a gal- 



FiG. 31 vanic chain, and in a second case in the form of 



a colloidal solution in a cataphoresis experi- 

 ment, we shall find that in the first case the total potential of the 

 silver is manifested towards the solution, while in the second case 

 there is manifested only the potential difference between the ad- 

 jacently moving surfaces, i.e., between the solution and the solid 

 AgCl. In this way it becomes clear why the superficially con- 

 sidered, identical interface (Ag-aqueous solution) will show a differ- 

 ent electromotive potential e depending upon other conditions than 

 the electrokinetic potential f . The cleanliness of the surface of the 

 silver is immaterial for the electromotive potential, while for the elec- 

 trokinetic potential it becomes, on the contrary, very noticeable 

 when the silver becomes coated with the chloride or oxide depending 



^' Solid AgCl has sufficient conductance to permit of the measurement of the 

 potential of this chain at least by means of electrostatic instruments. The 

 conductance is an electrolytic and not a metallic one. 



