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XL. Note on the Variation of the Dissociation Coefficient ivith 

 Temperature. By S. Roslington Milner, B.Sc, 1851 

 Exhibition Scholar, University College, Bristol *. 



THE general theory of mass-action, when applied to the 

 dissociation of a binary electrolyte, shows that the rate 

 at which dissociation goes on is proportional to the concentra- 

 tion (amount in gramme molecules per unit volume of solution) 

 of the undissociated part of the salt ; while, along with this, 

 a reverse process, the association of the separated ions to 

 form the original undissociated salt again, proceeds at a rate 

 proportional to the joint product of their concentrations — or, 

 since the ions exist in equal amounts — to the square of the 

 concentration of either. The true velocity of dissociation, 

 which one would measure, is of course the difference between 

 the rates of these two actions, and may be written 



dt ~ kC kC > 



where dc r jdt is the velocity at any stage of the process, at 

 which the concentrations of the undissociated and the disso- 

 ciated parts of the salt are c and c' respectively . When a salt 

 is dissolved in water and dissociates, c continually diminishes, 

 and c' increases, until 



£ C _£V 2 = 0, (1) 



a point being reached at which no further change with time 

 is to be noticed. 



Equation (1), therefore, which may be written 



Kc = c' 2 , (2) 



is the relation which according to mass-action exists, when 

 equilibrium is attained, between the undissociated and the 

 dissociated parts of the solution of a salt, however the total 

 concentration of the salt (i. e. gr. mols. actually present per 

 litre of solution) may vary. The constancy of the coefficient 

 K or c /2 /c was first arrived at by Ostwald, and experimentally 

 proved by him in the case of a large number of organic acids 

 of slight dissociation f ; although in the highly dissociated 

 inorganic acids and salts, formula (2) gives results which are 

 considerably different from the experimental values of the 



* Communicated by the Author. 



t Zeit. Phys. Chem. ii. pp. 36 and 270 (1888). 



