ADSORPTION POTENTIALS AND ELECTROKINETIC PHENOMENA 237 



adsorption of H-ions alone by charcoal, the entire acid molecule 

 (HCl, HNO3, etc.), being adsorbed. The adsorption of the H-ions 

 cannot proceed to their specific adsorption equihbrium, for they are 

 held back in solution by the much less adsorbable Cl-ions (or the 

 corresponding anions of the acid). On the other hand, more Cl-ions 

 are adsorbed than would correspond to their specific adsorption 

 equilibrium since some are carried along by the better adsorbable 

 H-ions. The actual adsorption equihbrium represents an average 

 somewhere between the specific equilibria of the two ions (i.e., the 

 H-ion and the anion). It is of ob\dous interest to determine the 

 adsorption of the H- and OH-ions alone. In order to achieve this end 

 the experiment must be so arranged that the establishment of equilib- 

 rium be hindered as little as possible by opposing forces. This can 

 be approximately accomplished by studjdng the adsorption of HCl 

 in presence of a large excess of KCl. The reason for this is the 

 following : 



Let there be given a chemical system consisting of a dilute aqueous 

 solution of a non-electrolyte (e.g., acetone) and of a solid adsorbent, 

 such as charcoal. The adsorption equilibrium in this system is 

 attained. Further, let the extension of the phases be so great, 

 that the addition to or the removal from the solution of one mol of 

 acetone would cause no appreciable change in concentration. 



Under these conditions the equilibrium attained is characterized by 

 the fact that no work is required to transfer a relatively very small 

 amount, e.g., 1 mol, of acetone from the charcoal surface into the 

 solution. But if we assume that no state of equilibrium has been 

 attained, and that the concentration of acetone is smaller in the 

 solution than it is on the charcoal surface, then there should be 

 present a tendenc}^ to reach equilibrium. If we could render this 

 process reversible, we should be able to gain a certain amount of work. 

 In order to calculate the amount of this work let us imagine the 

 process of attaining equihbrium as occurring in two stages. In the 

 first we shall compress the solution, by means of an osmotic piston, 

 until it has that concentration which wou'd be in equilibrium with 

 the charcoal surface, thus changing the concentration from c to Cq. 



To obtain this result we must expend an amount of work n RT In — , 



c 



where n is the number of mols of acetone in the solution. Now 



let 1 mol of acetone pass from the charcoal surface into the solution. 



