One has to be sure that there is no ionic or other interaction between the analyte and these other components. Many of the techniques mentioned in the subsequent section measure solutions in which not only the analyte is present, but also various other components for buffering. If these details are omitted, it cannot be assumed they were measured in water, at room temperature, and extrapolated to infinite dilution. Reporting pK a values in literature also require that exact conditions of temperature, ionic strength, and solvent be stated. Before use, pH meters should be calibrated under the same conditions of temperature, ionic strength, and solvent. It is often overlooked that this is also the case with pH measurements.

It must be stressed that when performing pK a measurements, all parameters mentioned will have to be kept constant in order to produce a meaningful result. The dependency of pK a of different Benzoic-acids on the solvent composition, methanol in water at 25 вЊ and I = 0.02 M. It relates pH and pK a to the equilibrium concentrations of dissociated acid and non-dissociated acid respectively: Although Henderson defined K in terms of a concentration ratio in 1908, it was not until 1916 that Hasselbalch 7 proposed their now famous equation (1), which remains the most commonly used equation to calculate pK a values: the Henderson-Hasselbalch equation. 5 In 1909, SГ¶renson 6 proposed the more convenient pH and pK a terms as the negative logarithm of and the equilibrium constant, K, respectively. It was in 1907 that Henderson published a paper first describing the relation between the hydrogen ion and the composition of a buffer. Measurements of osmotic pressures and conductivity of solutions gave insight into the degree of dissociation.

The first description of an equilibrium constant came from Guldberg and Waage in 1864 in their вЂњlaw of mass action.вЂќ With the help of vanвЂ™t HoffвЂ™s work on osmotic pressures, Ostwald formulated his вЂњdilution lawвЂќ for solutions. This is also where the word вЂњacidвЂќ comes from it is derived from the Greek word вЂњoxeinвЂќ which in Latin is вЂњacere,вЂќ meaning вЂњto make sour.вЂќ It was also noted that acids and bases could color certain substances. The first notion of acids comes from ancient Greece, where people noticed that some substances tasted sour. This realization came gradually and under different names.

Both pH and pK a are essential for understanding the behavior of chemical substances in everyday life. The related concept of the acid dissociation constant (pK a) as a substance property is recognized as being among the most commonly used parameters in modern-day chemistry. 1вЂ“ 3 By now, accurate and precise determination of pH seems to have few secrets left. The centennial of the concept and of the quantitative measurement of pH was celebrated not long ago.

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