Dissociation constants of malonic acid in its sodium-salt solutions at 25 degrees C from electrometric titration measurements

Abstract

Electrometric titration of 0.1 M malonic acid by 0.1007 N sodium hydroxide was completed at 25° C for the determination of the pH titration curve and the calculations of the primary and secondary dissociation constants. The treatment of the data embodied the application of the Henderson equation for liquidjunction potential, the Debye-Huckel theory of inter ionic attraction, and the Guggenheim-Huckel expression for the variation of the ionic activity coefficients with concentration. The dissociation constants found for malonic acid at 25° C are K, = 2.06XlO-3 and K2= 2.94 X IO-6.

Dissociation constants and pH-titration curves at constant ionic strength from electrometric titrations in cells without liquid junction: titrations of formic acid and acetic acid

“…The titration:cell is a modification of a cell for titrations with liquid junction previously described by Burton, Hamer, and Acree [13].…”

Calculation of the concentration and dissociation constant of each acid group in a mixture from the pH titration curve of the mixture

“…These symbols can be replaced by activities [2] to secure the thermodynamic-activity constant instead of the classical dissociation constant, [{I.…”

“…Since the values of a are obtained from a series of simultaneous equations discussed below and the ionic strength remains nearly constant, thus giving an approximately constant value [2] for the activity-coefficient product, jnjnn_IAn, either the classical or the thermodynamic method may be employed. The former is used here because of its simplicity and direct adaptability to a simple analytical treatment of mixed acids.…”

“…If the acid is a single polybasic one, the salts formed in sequence on addition of NaOH are NaHn_IAn, Na2Hn_2An, etc., whereas a mixture of monobasic acids in any proportions forms, in succession, the salts represented by N aAn, NaAn', NaAn", etc. A logical application of the law of mass action leads to no confusion in the calculation of the value of a for each successively weaker acid in any of these mixtures, and hence we may point out the general principles by discussing the particular application of the law to malonic acid, for which the data are available [2]. Equation 1 gives the values of the primary dissociation constant; [{I, of a polybasic acid when [HnAn] is the equivalent concentration 0 total acid, determined by titration with standard alkali, a is the fraction corresponding to the strongest or primary acid group, [H+] is the hydrogen-ion concentration, al is the degree of ionization of the primary salt, and [NaHn_IAn], [Na2H n_2An], etc., are the molar concentrations of the salts.…”

“…General equations for calculating the values of the two dissociation constants, Kl and K 2, from the pH titration curves for dibasic acids were then developed [2], and when applied to malonic acid gave Kl=2. 06X10-3 and K2=2.…”