LAWS OF ELECTROLYTIC DISSOCIATION 43 



Since kiAi = k2A2, 

 [H+] 



J \ m + n va -\- nl 



and [H+] is therefore unchanged. But this law is only valid for 

 mixtures of the free acids alone in the absence of their salts. It is 

 not applicable to two isohydric solutions of any composition. 



13. Mixtures of weak acids and their alkali salts: Regulators or 



Buffers 



While it is true that acids and bases are capable of presenting all 

 possible gradations of dissociation, this is not the case for their 

 alkali salts. The latter all belong to the class of strong electrolytes 

 to which the law of mass action is not applicable without further 

 amplification. The same is true of the chlorides (bromides, etc.) 

 of the weak bases. For the sake of simplifying calculations we may 

 assume at once that the salts are completely dissociated under all 

 conditions. This assumption is but very little removed from reality, 

 and the results which are to be derived from it possess quite a high 

 degree of accuracy and form the foundation of the theory of regu- 

 lators or buffers.^*^ 



When to a solution of a weak acid its alkali salt is added, the 

 dissociation of the acid becomes greatly depressed, and to a pro- 

 gressively greater extent with increasing amounts of the salt. Thus 

 by adding varying amounts of sodium acetate to acetic acid it is 

 possible to obtain at will any low hydrion concentration. The same 

 results may be obtained with a great variety of other such acid- 

 salt mixtures. The calculation of the [H+] of such mixtures is of 

 special importance and is met with very frequently in experimental 

 procedures. Let us take for example a mixture of acetic acid and 

 sodium acetate. The expression applicable to all such cases is: 



[S-] X [H^] 



[SHI 



^^ Literature: Contributions from Nernst's Laboratory: Fels. Zeitschr. f. 

 Elektrochem. 10,208 (1904) ; Salesski, ibid. 10, 204 (1904). AlsoS.P.L.S0rensen, 

 Biochem. Zeitschr. 21, 131 (1909); Lawrence J. Henderson, Ergebn. d. Physiol. 

 8, 254 (1909); L. Michaelis and P. Rona, Biochem. Zeitschr. 23, 364 (1910). 



