Question

Development of method for the determination of pka values

Answer 1

Abstract
The acid dissociation constant (pKa) is among the most frequently used physicochemical parameters, and its determination is of interest to a wide range of research fields. We present a brief introduction on the conceptual development of pKa as a physical parameter and its relationship to the concept of the pH of a solution. This is followed by a general summary of the historical development and current state of the techniques of pKa determination and an attempt to develop insight into future developments. Fourteen methods of determining the acid dissociation constant are placed in context and are critically evaluated to make a fair comparison and to determine their applications in modern chemistry. Additionally, we have studied these techniques in light of present trends in science and technology and attempt to determine how these trends might affect future developments in the field.

Keywords: review, history, dissociation constant, pKa, pH
Go to:
Introduction
History

The centennial of the concept and of the quantitative measurement of pH was celebrated not long ago.1–3 By now, accurate and precise determination of pH seems to have few secrets left. The related concept of the acid dissociation constant (pKa) as a substance property is recognized as being among the most commonly used parameters in modern-day chemistry. Both pH and pKa are essential for understanding the behavior of chemical substances in everyday life. This realization came gradually and under different names. The first notion of acids comes from ancient Greece, where people noticed that some substances tasted sour. This is also where the word “acid” comes from; it is derived from the Greek word “oxein” which in Latin is “acere,” meaning “to make sour.” It was also noted that acids and bases could color certain substances.

The first description of an equilibrium constant came from Guldberg and Waage in 1864 in their “law of mass action.” With the help of van’t Hoff’s work on osmotic pressures, Ostwald formulated his “dilution law” for solutions. Measurements of osmotic pressures and conductivity of solutions gave insight into the degree of dissociation.4

It was in 1907 that Henderson published a paper first describing the relation between the hydrogen ion [H+] and the composition of a buffer.5 In 1909, Sörenson6 proposed the more convenient pH and pKa terms as the negative logarithm of [H+] and the equilibrium constant, K, respectively. Although Henderson defined K in terms of a concentration ratio in 1908, it was not until 1916 that Hasselbalch7 proposed their now famous equation (1), which remains the most commonly used equation to calculate pKa values: the Henderson-Hasselbalch equation. It relates pH and pKa to the equilibrium concentrations of dissociated acid [A−] and non-dissociated acid [HA] respectively:

pH = pKa + log ([A-]/[HA]) (1)
In many experimental methods to determine pKa values, a certain parameter is measured as a function of pH. This results in a characteristic sigmoid curve (Fig. 1) from which the pKa may be determined by locating the inflection point. Generally speaking, for acidic components X ranges from a bulk property of a solution of only non-dissociated acid to the situation where only dissociated acid is present. A more specific example is represented if parameter X denotes the degree of dissociation α ranging between 0 and 1, the inflection point will be at α = 0.5, where pH equals pKa. The degree of dissociation α for acids is defined as:


Figure 1
A classic example of a sigmoidal curve created by plotting a measured quantity versus pH. The inflection point corresponds to pKa.
α = [A-]/([HA] + [A-])...

hope it will help