Acetic acid thermodynamic ionization values

All equilibrium constants in the present discussion are based on the concentration (not activity) scale. This is a perfectly acceptable thermodynamic scale, provided the ionic strength of the solvent medium is kept fked at a reference level (therefore, sufficiently higher than the concentration of the species assayed). This is known as the constant ionic medium thermodynamic state. Most modern results are determined at 25 °C in a 0.15 M KCl solution. If the ionic strength is changed, the ionization constant may be affected. For example, at 25 °C and 0.0 M ionic strength, the pXj of acetic acid is 4.76, but at ionic strength 0.15 M, the value is 4.55 [24]. 

A potentiometric method for determination of ionization constants for weak acids and bases in mixed solvents and for determination of solubility product constants in mixed solvents is described. The method utilizes glass electrodes, is rapid and convenient, and gives results in agreement with corresponding values from the literature. After describing the experimental details of the method, we present results of its application to three types of ionization equilibria. These results include a study of the thermodynamics of ionization of acetic acid, benzoic acid, phenol, water, and silver chloride in aqueous mixtures of acetone, tetrahydrofuran, and ethanol. The solvent compositions in these studies were varied from 0 to ca. 70 mass % nonaqueous component, and measurements were made at several temperatures between 10° and 40°C. 

As described in the discussion of the computations to determine the ionization constant of acetic acid, if the right-hand side of equation (15) is set equal to (f T/F) In KJ, then the value of should approach the thermodynamic ionization constant Ki as the activity coefficients, which nearly cancel each other, approach unity. The values of given in the fifth column of the table can be expressed by the equation... 

As already stated the limiting value of K is the thermodynamic ionization constant, K, which in this case is 1.753 X 10 c. Another method for obtaining thermodynamic ionization constants is given in Chapter 11, depending on measurements of the electromotive force of concentration cells without liquid junction. Using that method Harned and Ehlers found 1.754 X 10"R for the ionization constant of acetic acid at 25°. However, that constant is based on molalities, m, rather than concentrations, C. The relation between the ionization constants may be readily shown to be... 

At a given temperature, a true ionization constant is a thermodynamic quantity related to the standard free energy change in the reaction. Theoretically this value should be independent of the concentration taken initially for its determination. In practice, however, application of equation (9 7) to the determination of the ionization constant of acetic acid yields values of which vary with concentration as shown in Table 9.2. 

TABLE 8.7 Thermodynamic Values for the Ionizations of Acetic and Chloroacetic Acids in HjO at 25° C... 

TABLE 8.6 Thermodynamic values for the ionizations of acetic and chloroacetic acids in H20 at 25°C157... 

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