Definitions - Activity versus Concentration Thermodynamic Scales

Definitions - Activity versus Concentration Thermodynamic Scales 

The ionization reactions for acids, bases and ampholytes (diprotic) may be represented by the general forms 

Equations (3a)-(3d) indicate that when the concentration of the free acid, HA (or conjugate acid, BH ), equals that of the conjugate base, A (or free base, B), the pH has the special name, pKf. 

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]. 

The effect of temperature on acid or base pKa values cannot be reliably predicted [2, 17, 23]. For many nitrogenous bases, the pKa decreases by 0.1-0.3 for every 10 °C rise in temperature. For some carboxylic acids (e.g. acetic, benzoic, salicylic acids), the pKj remains essentially unchanged between 25 and 37°C. 

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