DISSOCIATION OF STRONG ELECTROLYTES 121 



33. The three deviation-coefficients, £„, f^, fa, according to 



Bjerrum 



In order to calculate the relative osmotic pressure of an ion from 

 its concentration, c (assuming the osmotic pressure of a solution which 

 approaches in character an ideal gas to be equal to 1) the concen- 

 tration must be multiplied by a factor fo, the osmotic coefficient, which 

 is always less than 1. The relative conductivity is similarly obtained 

 (assuming the conductivity when it is not affected by electrostatic 

 forces to be equal to 1) bj^ multiplying by the factor f^, the con- 

 ductivity coefficient. Also, the active mass is obtained by multiplying 

 the concentration by the factor fa, the activity coefficient. 



In the preceding sections it has been attempted to attribute a 

 further significance to the coefficients, f^u and fo, and now we come to 

 the activity coefficient, which is the most significant for our purposes. 

 We shall first outline briefly Bjerrum's attempt^'* to derive theoreti- 

 cally fa from fo, and then the procedure for the practical determina- 

 tion of fa. 



1, According to Bjerrum's thermodynamic derivation the follow- 

 ing relationship exists between the osmotic factor fo, determined by 

 the freezing point method, and the activity factor fa and also the 

 concentration c: 



f. + ^°-i + -^ (1) 



dc dc 



which is to be taken by all means only as an approximation formula 

 for dilute solutions. From this, therefore, one can calculate fa from 

 fo, at least under certain simple conditions, and in that particular 

 case when the osmotic coefficient is given with sufficient accuracy by 

 the freezing point method. 



Instead of the above complicated function Bjerrum has also 

 worked out a simpler approximate method for the calculation of the 

 activity coefficient in a solution of a strong electrolyte. Thus for a 

 solution of a strong electrolyte whose molar concentration is c and 

 which is composed of two equivalent ions (i.e., two univalent or two 

 divalent etc. ions, as in the case of KCl, MgS04, but not Na2S04) 

 he obtained the expression 



26 3/- 

 - log fa = — n2 Vc (la) 



