Ionization constant of acetic acid

Shedlovsky, T Kay, R. L., The ionization constant of acetic acid in water-methanol mixtures at 25°C from conductance measurements, J. Am. Chem. Soc. 60,151-155 (1956). 

Very precise, classic, measurements of the ionization constant of acetic acid were made many decades ago [2]. The dependence of In K on temperature is illustrated in Eigure 10.2. As the temperature is increased from 273 K, In K and the degree of ionization increases gradually, reaching a maximum just below 298 K, and then... 

Figure 20.3. Extrapolation of ionization constants of acetic acid.

K is the ionization constant of acetic acid and has a numerical value of 0.000018 when concentrations are given in moles per liter. 

Buffers. A mixture of a weak acid and a salt of that acid or a mixture of a weak base and a salt of that weak base serves as a buffer. Let us consider a liter of solution containing 0.1 F.W. of HAc and 0.1 F.W. of NaAc. The ionization constant of acetic acid is given in the expression... 

Using the solubility product of silver acetate, 3.6 X 10—3, and the ionization constant of acetic acid, calculate the solubility of silver acetate in a basic solution, a solution with pH 4.7, and a solution with pH 3.4. 

Table L Data for Obtaining the Ionization Constant of Acetic Acid at 25° from the Potentials of Concentration Cells...

Fig. 1. Plot for the Extrapolation to Zero Ionic Strength for the Ionization Constant of Acetic Acid.

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... 

Although D2O chemically resembles H2O in most respects, it is a toxic substance. The reason is that deuterium is heavier than hydrogen thus, its compounds often react more slowly than those of the lighter isotope. Regular drinking of D2O instead of H2O could prove fatal because of the slower rate of transfer of compared with that of H+ in the acid-base reactions involved in enzyme catalysis. This kinetic isotope effect is also manifest in acid ionization constants. For example, the ionization constant of acetic acid... 

D. Machines and T. Shedlovsky, The determination of the ionization constant of acetic acid, at 25°, from conductance measurements, JACS, 1932,54,1429-1438. 

The equilibrium constant for the forward reaction of (5.23) is /K, where is the acid-dissociation (or ionization) constant of acetic acid. Also, the equilibrium constant for the forward reaction of (5.24) is the ion-product constant for water (K ). Since the equilibrium constant for a net reaction is equal to the product of the equilibrium constants for the individual reactions (see Exercise 1.14c), the equilibrium constant for Reaction (5.20), namely Kf, is given by... 

Weak acid and weak base ionization equilibria have extremely small ionization constants. For example, the ionization constant of acetic acid is 1.8 X 10 . The ionization constant of ammonium hydroxide is 1.7 x 10 This means that there are very few ions present at equilibrium most of the acid or base is present as the un-ionized species. This is the reason they are described as weak acids or bases. The position of ionization equilibrium in each case would be as illustrated in Figure 11.4. 

We will briefly identify now the results for extraction of acetic acid into an organic solvent in the absence of any complexing agent in the organic phase however, the acid can dissociate in the aqueous phase, as shown in Figure 5.2.6. If the dissociation or the ionization constant of acetic acid (/ = 1) is K i (see relation (5.2.61a) and Table 5.2.2) and there is dimerization of the acid in the oil phase, then it can be shown that... 

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. 

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