30 HYDROGEN ION CONCENTRATION 



The constant k is a definite characteristic of every acid. It varies 

 with the temperature although, as a rule, not very greatly. It is the 

 dissociation constant or the affinity constant of the acid. Its magni- 

 tude is, as Wilhehn Ostwald pointed out, the only rational measure 

 of the strength of an acid. 



|"TT+1 [-Q— 1 



In order to avoid misunderstandings the value |^ will be 



[Hb] 



henceforth taken as the "dissociation constant" and its reciprocal 



rxTOl 



rTT4.i ro-1 ^s the "affinity constant." The dissociation constant is 

 l-n J Lb J 



a measure of the force which tends to decompose (ionically) the 

 acid; the affinity constant is the force which tends to reunite its 

 ions. By the terms "force" and "affinity" of a chemical reaction 

 is understood that amount of maximal work C which may be ob- 

 tained from the decomposition of one mol of the molecular com- 

 pound involved, as for instance, in the case when 60 grams of acetic 

 acid decompose into 59 grams of acetate ions + 1 gram H+ ions. 

 The necessary condition however is that the molecular compound 

 involved be present in one molar concentration, and that a suffi- 

 ciently large reaction volume be chosen so that the concentration 

 is not measurably affected by the decomposition. 



The relation of this amount of work C to the constant k is ex- 

 pressed 



C = RT . Ink 



where k has its above given meaning. If the reacting molecular 

 species are in solution in any concentration such as [AcOH], [Ace- 

 tate"], [11+] then the maximal work W obtained from the decom- 

 position of one mole is expressed as: 



W = C + RTlnX '^"°«' 



[Acetate-] [H+] 



* See following section. 



t The figure given is that found by Michaelis and Garmendia (Biochem. 

 Zeitschr. 67, 431 (1914). Abbott and Bray (J. Amer. Chem. Soc. 31, 760 

 (1909) had previously found the value 1.95 X lO"'. It must be stated at 

 once that in the light of some recent developments (cf. Chapter III) some 

 of the above figures must be revised, as well as the figures in the tables of 

 dissociation constants given on pages 59 and 72. But none of these newer 

 corrections are large. 



