Dissociation constants of weak bases from electromotive-force measurements of solutions of partially hydrolyzed salts

Abstract

A method for the determination of dissociation constants of weak electrolytes by electromotive-force measuremen ts of solutions of partially hydrolyzed salts of a weak acid and a weak base is described. Although the precision is only half that of the conventional emf m ethod , this procedure has particular advantage in determinin g t he dissociation cons tants of certain bases, for som e of the experim e ntal difficul t ies encountered in adapting the usua l method to solutions containing these bases can be overcome or red uced. The hydrogen-ion concentration of a solution of a salt of this type depends upon the con stants of the weak acid, th e weak base, and water. If two of these are known, th e third can be evaluated by means of e mf m eas ure ments wi t hout the necessity of knowing the exact hydroge n-ion concentration, which is usually difficult to obtain. Hydroge n clectrod es and silver-silver chloride electrodes are used in the cells, and the solut ions are aqueou s mixtures containing the ions of the sa lt and an alkali chloride. The equations for t he calculation of di ssociation constants are deve loped. Th e method was tested by a determination of the basic dissociation constant of tris(hydroxymethyl)aminomcthane at 20 0 , 25 0 , and 30 0 C from three se ri es of meas u reme n ts : (a) by t he con ventio nal e m f m ethod ; (b) by emf studies of the hydrolysis of mixt ures of th e a mine and primary potassium phosphate ; and (c) by emf studies of the h yd rolysis of mixtures of the amine and potassium p-phenolsulfona te. The three determinations were in acceptable agreement. The negative logarithm of the hasic dissociation constant, Pf(b, was found to be 5.946 at 20 0 , 5.920 at 25 0 , and 5.896 at 30 0 C.