V

S0, the solubility of the undissociated species (base), is determined at high pH, and S is determined at several different lower pH values. A plot of log[S0/(S - S0)] versus pH will have the pKa as the intercept on the pH axis.

Alternatively, S may be plotted against [H+] as in Fig. 5.4(b). Equation (5.14) can be written in the form

Plotting data as in Fig. 5.4(b) yields S0 when the line crosses the x-axis as [H+] = 0 (and S = S0). The intercept on the y-axis gives Ka and the slope of the line is Ka/S0.

where A U is the molar energy and AH is the molar heat of vaporisation of the solvent. A H is determined by calorimetry at temperatures below the boiling point at constant volume. V is the molar volume of the solvent. The solubility parameter is thus a measure of the intermolecular forces within the solvent and gives us information on the ability of the liquid to act as a solvent. Table 5.11 gives the solubility parameters of some common solvents calculated using equation (5.26).

A U/V is the liquid's cohesive energy density, a measure of the attraction of a molecule from its own liquid, which is the energy required to remove it from the liquid and is equal to the energy of vaporisation per unit volume. Because cavities have to be formed in a solvent, by separating other solvent molecules, to accommodate solute molecules (as discussed earlier) the solubility parameter

Table 5.10 Water solubilities and pKa values of aminoalkylphenothiazines and related compounds0

Table 5.10 Water solubilities and pKa values of aminoalkylphenothiazines and related compounds0

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