Ionization constants of weak acids

The ionization of a weak acid, HA, in aqueous solution can be written as 

The molalities and the activity coefficients of the anion and of the nonionized acid must be equal, respectively, in the two expressions. We have chosen also to make m(H + ) and m(H30+) equal. Then the activity coefficients of the hydrogen and hydronium ions must be related by the equation1 

If we choose to combine the activity coefficients of the ions to obtain the mean activity coefficients, Equation (11.87) would become 

1 We have assumed throughout this discussion that the formula for the hydronium ion is H30 +. If the formula were written more generally as H+ -vH20, then the specific equations discussed here would be changed, but the arguments used to develop the equations would be the same. In fact, there is considerable evidence to indicate that the predominant species in water is H+ 4H20. 

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Sarmini, K., and Kenndler, E. (1999). Ionization constants of weak acids and bases in organic solvents. /. Biochem. Biophys. Methods 38, 123—137. 

The same type of cell has been used to determine values of the ionization constants of weak acids and weak bases. We use as an example a weak acid, HA. The representation of the cell is... 

Organic solvents influence the ionization constants of weak acids or bases in several ways (note that they influence the analytes and the buffer as well). Concerning ionization equilibria, an important solvent property is the basicity (in comparison to water), which reflects the interaction with the proton. From the most common solvents, the lower alcohols and acetonitrile are less basic than water. Dimethyl sulfoxide is clearly more basic. However, stabilization of all particles involved in the acido-basic equilibrium is decisive for the pKa shift as well. For neutral acids of type HA, the particles are the free, molecular acid, and the anion, A . In the equilibrium of bases, B, stabilization of B and its conjugated acid, HB, takes place. As most solvents have a lower stabilization ability toward anions (compared to water), they shift the pK values of adds of type HA to higher values in general. No such clear direction of the change is found for the pK values of bases however, they undergo less pronounced shifts. 

Medium Effect and Ionization Constants of Weak Acids and Bases... 

The foregoing example illustrates how equilibrium constants for overall cell reactions can be determined electrochemically. Although the example dealt with redox equilibrium, related procedures can be used to measure the solubility product constants of sparingly soluble ionic compounds or the ionization constants of weak acids and bases. Suppose that the solubility product constant of AgCl is to be determined by means of an electrochemical cell. One half-cell contains solid AgCl and Ag metal in equilibrium with a known concentration of CP (aq) (established with 0.00100 M NaCl, for example) so that an unknown but definite concentration of Kg aq) is present. A silver electrode is used so that the half-cell reaction involved is either the reduction of Ag (aq) or the oxidation of Ag. This is, in effect, an Ag" Ag half-cell whose potential is to be determined. The second half-cell can be any whose potential is accurately known, and its choice is a matter of convenience. In the following example, the second half-cell is a standard H30" H2 half-cell. 

It is possible to estimate, roughly at least, the values of the ionization constants of weak acids and bases by means of potentiometric titration methods. Since the ionization constant K of a weak acid HA is given by... 

The results of recent determinations of the thermodynamic ionization constants of weak acids determined from conductance measurements which have been interpreted substantially as described above, are given in Table VIII. The relations of some of these values to the molecular structures of the corresponding compounds are discussed in Chapter 21. 

Determination of Solubilities. We have mentioned the use of single measurements of conductance in determining the ionization constants of weak acids and bases. In addition, the solubilities of many weakly soluble salts and oxides have been determined by conductance measurements. When the solubility product is less than 10" , a correction must be made for the specific conductance of water. 

The substitution of for H also affects equi-librivim constants, particularly the ionization constants of weak acids and bases dissolved in D2O (10,11). The rates of acid-base catalyzed reactions may be greatly different in H20 as compared to H20 (, 12). Deuterium substitution will tend to increase slightly the strength of hydrogen bonds, and deuterium has a significantly smaller sterio requirement than does 1h. 

Important physicochemical properties (e.g., ionization constants of weak acids and bases) are often not known, which leads to a more or less random experimental approach for the optimization of the resolution. 

Organic solvents influence the ionization constants of weak acids or bases in several ways (note that they in-fluence the analytes and the buffer as well). Concerning ionization equilibria, an important solvent property is the basicity... 

MEDIUM EFFECT AND IONIZATION CONSTANTS OF WEAK ACIDS AND BASES... 

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