ELECTROCHEMICAL THERMODYNAMICS 733 



principles just discussed, very important information concern- 

 ing weak acids and bases in solvents containing salt solutions 

 may be obtained. We shall consider the acid case only, since 

 the bases may be investigated in exactly the same manner. 

 Let us construct the cell, 



Ag I AgCl I HCl(wi), MCl(m2) | H2 | HAc(m), MC\{mz) \ 



AgCl I Ag, 



in which HAc is a weak acid, mi is 0.01 molal or less, and the 

 concentrations are such that the total ionic concentration 

 on the two sides is the same or very nearly so, so that 

 Wi 4- W2 = Wh + W3, where m^ is the hydrogen ion concen- 

 tration in the solution of the weak acid. The electromotive 

 force of this cell at 25° is given by 



E = 0.05915 logic ^^5!^^' + 0.05915 logio ^^^^^ , (52) 



where the double accent refers to the hydrochloric acid solution 

 and the single accent to the weak acid solution. Since Wi, W2, 

 and ms are known mn may be evaluated if the first term on the 

 right of this equation is known. Two secondary effects influ- 

 ence this term, which can be completely taken into account if 

 sufficient care is exercised. The first and most important is 

 the effect of the presence of the undissociated molecule of the 

 weak acid which causes th'tci' to differ from its value in pure 

 water even though the concentrations of the ions in the two 

 cell compartments are the same. The second effect is much 

 simpler and merely requires a knowledge of the activity co- 

 efficient of hydrochloric acid in the salt solution. This 

 situation has been investigated very thoroughly by Harned and 

 Robinson, and Harned and Owen, who show that both 7h"tci" 

 and th'tci' as well as mn can be determined without the intro- 

 duction of any inexact considerations. 

 The dissociation of the acid is represented by 



HAc ^ H+ + Ac-, 



