Thermodynamic ionization constants

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]. 

If tautomerism occurs in dilute aqueous acid, then K and K l will be the thermodynamic acid ionization constants and (26) will hold thus... 

Calorimetric results are also subject to medium effects and—when necessary—appropriate corrections must be applied in order to obtain truly thermodynamic ionization enthalpies (and, eventually, ionization constants). 

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. 

Starting from the comparative study of the ionization constants of uracil itself as well as of its several methylated or ethylated derivatives (representing models of tautomeric forms), it may be seen (Table XVII) that uracil and uridine exist in aqueous solution in the diketo form 32. The pX values are not known for the model tautomers 27, 29, and 30, but these forms have been ruled out on the basis of UV studies. Recently the ionization constants of uracil, thymine, their derivatives and nucleotides were determined over the range 10-50°, and thermodynamic enthalpy, entropy, and free energy changes for protonation and depro-tonation of these compounds have been evaluated.93-95,332... 

This ionization constatil in terms of activities is called the true or thermodynamic ionization constant. It docs not differ too much from the K in Eq. (14) for sufficiently low ionic strengths. The two differ more markedly for appreciable ionic strengths. Nuw suppose a salt with no common ion is added to the solution. The ionic strength of the solution will be increased. This increase in ionic strength causes a decrease in the activity coefficients of the ions except in very concentrated solutions. Thus for K of Eq. (18) to stay constant the concentrations of the ions must increase to offset the decrease in their activity coefficients. The ammonia must therefore increase in ionization and K as defined by Eq. (14) must increase. This is known as a salt effect. 

The thermodynamic ionization constants of several hydroxybenzo-[6]thiophenes have been determined by spectroscopic measurements on the anions.196... 

The discussion in the previous sections concerning solvated species indicates that a complete knowledge of the chemical reactions that take place in a system is not necessary in order to apply thermodynamics to that system, provided that the assumptions made are applied consistently. The application of thermodynamics to sulfuric acid in aqueous solution affords another illustration of this fact. We choose the reference state of sulfuric acid to be the infinitely dilute solution. However, because we know that sulfuric acid is dissociated in aqueous solution, we must express the chemical potential in terms of the dissociation products rather than the component (Sect. 8.15). Either we can assume that the only solute species present are hydrogen ion and sulfate ion (we choose to designate the acid species as hydrogen rather than hydronium ion), or we can take into account the weak character of the bisulfate ion and assume that the species are hydrogen ion, bisulfate ion, and sulfate ion. With the first assumption, the effect of the weakness of the bisulfate ion is contained in the mean activity coefficient of the sulfuric acid, whereas with the second assumption, the ionization constant of the bisulfate ion is involved indirectly. 

Thermodynamic functions (entropy, heat capacity, enthalpy and free energy functions) have not been reported for 1,2,4-thiadiazoles. The ionization constants of a number of 1,2,4-thiadiazoIes have been determined potentiometrically or by Hammett s method (65AHC(5)ll9). Polarographic measurements of a series of methylated 5-amino-l,2,4-thiadiazoles show that thiadiazoles are not reducible in methanolic lithium chloride solution, while thiadiazolines are uniformly reduced at E0.s = -1.6 0.02 V. This technique has been used to assign structures to compounds which may exist theoretically as either thiadiazoles or thiadiazolines (65AHC(5)119). 

Thus, the alkoxide ion derived from 112 is the kinetically preferred species, whereas the C-2 alkoxide ion 113 is the thermodynamically more stable species. In this case, the measured equilibrium p/CRO value is actually an apparent ionization constant defined by... 

From a thermodynamic point of view, the ionization constants of a weak acid in two solvents are related to each other by primary medium effect. Thus,... 

The relationship between thermodynamic properties, for example, ionization constants or solubility products of a compound AB, and the coordination chemical properties of solvents can be readily deduced from a consideration of the energy cycles shown in Eqs. (6)-(9). 

From the foregoing considerations it is apparent that thermodynamic properties of compounds such as ionization constants or solubilities do not only depend on heterolytic dissociation energies but may be strongly influenced by free energies of solvation or free-energy contributions associated with changes in the state of aggregation. 

Little is known about the thermodynamics of self-ionization equilibria. It appears that the extent of self-ionization is primarily related to the strength of the bridge bonds. For example, self-ionization constants for HF, HgO, and NHg are decreasing in the order HF K X > HgO K X 10 ) NHg (K x 10 ) which cor-... 

We should recall that the thermodynamic definition of K is in terms of activities. In dilute solutions, the activity of the (nearly) pure H2O is essentially 1. The activity of each dissolved species is numerically equal to its molar concentration. Thus the ionization constant of a weak acid, K, does not include a term for the concentration of water. 

For weak acids, the magnitude of is very small, and as a result the resulting H3O+ and A ions will be produced in small amounts. Under those conditions, both Yjj+ and y will be approximately equal to one, and then one can approximate the thermodynamic equilibrium constant, K, by the concentration-based ionization constant, Ka. 

At a given temperature, the thermodynamic ionization constants are independent of concentration and at a pH value equal to pK, the activity of ionized and neutral forms, are equal. In many measurement techniques, we measure concentration rather than activity, such as in the use of spectroscopic methods. In such instances. 

IS This cun be readily shown to be equivalent to assigning a value of unity to the thermodynamic ionization constant... 

Thermodynamic Ionization Constants from the Potentials of Cells without Liquid Junctions... 

Chap. 11 THERMODYNAMIC IONIZATION CONSTANTS ancl using equation (26c) of Chapter 6, this yields... 

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