Determination of the Constants of Acidity in Aqueous Environment

Among the physico-chemical properties of molecules, the acidity constant (K _a or pK _a ) is of particular importance. Indeed, very many chemical species possess an acid-base character. This article is devoted to its determination in aqueous media.

The acid dissociation constant K _a , or its alter ego the quantity pK _a , is a thermodynamic constant. It is therefore defined in terms of activities. This has a number of consequences. For this reason, we devote the first section to a brief review of solution thermodynamics. In the second part, we define it in an aqueous medium. It then becomes possible to identify the importance of determining it. This we do in the third section. In the fourth section, we present a general methodology for determining equilibrium constants (among other constants). This involves simulating experimental results as accurately as possible, using a least-squares procedure.

We then look at the determination of pK _a by different methods. It should be pointed out that there are often several variants of each of these methods. We deliberately choose to mention only a few of them. It is based essentially on scientific relevance or simplicity of use. In particular, we focus on the potentiometric and UV-visible spectrophotometric methods, which are the easiest and least expensive to implement. In fact, they are the most widely used. We will insist on certain methodological aspects concerning them. In addition to the spectrophotometric method, we describe the determination of extreme pK _a values using Hammett functions, which are based on spectrophotometry. This is the subject of the fifth and sixth parts. In part seven, we also consider determination by conductometry, which historically has rendered great service in this field.

The eighth section is devoted to a number of methods that are less widely used than the previous ones, namely those based on solubility measurements and those involving the use of more expensive equipment such as NMR, capillary electrophoresis, HPLC, polarography and voltammetry. Although these latter methods are undoubtedly the way of the future, given their more expensive and less common use, we will confine ourselves to outlining their principles and advantages in this field.

Determining the pK _a values of microscopic constants poses a very interesting problem. Indeed, according to the very "corpus" of thermodynamics, they cannot be determined rigorously. They...