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XL On the Relative Strengths or " Avidities" of Weak 

 Acids. By John Shields, D.Sc, Ph.D.* 



WHEN the sodium salt of an acid A is mixed with the 

 equivalent quantity of another acid B, the base will 

 in general be distributed between the two acids, and the ratio 

 of distribution will depend on the relative strengths of the 

 competing acids A and B. 



If | represent that fraction of the neutral sodium salt 

 which is decomposed on the addition of the acid B, then 1— J 

 will represent the quantity of the original salt which remains 

 undecomposed. 



By applying Guldberg and Waage's law, after equilibrium 

 has taken place, we get 



or 



(rfi)H 



where c and c x denote the velocities of the opposed reactions. 

 The ratio of distribution of the base between the two acids 

 is therefore proportional to the square root of the ratio of the 

 coefficients of velocity, 



i-?~Vv 



This ratio of distribution gives us the measure of the 

 relative strengths or, as Julius Thomsen has called it, the 

 " avidities " of the acids for any given dilution. 



As regards the measurement of the ratio of distribution, 

 Thomsen employed a calorimetric method ; but Ostwald, 

 Gladstone, Jellet, Wiedemann, and Lowenthal and Lenssen 

 employed various other methods, which, however, are not 

 particularly well adapted for measuring the relative strengths 

 of very weak acids. In the present paper I propose to show 

 how the avidities of the weakest acids may be determined 

 from the rate at which their salt-solutions are hydrolyzed. 

 At this stage it will be convenient to point out that water is 

 here regarded as a weak acid, and potash and soda as the same 

 base since they are equally strong (cf. Reicher, Annalen, 

 ccxxviii. p. 257). 



Arrhenius (Zeitsch. f. physikal. Chem. v. p. 13, 1890) 

 has shown that when two weak acids compete for the same 

 base, the ratio of distribution is very nearly proportional to 



* Communicated by the Author. 



