Selection of the pH scale

In some studies of the pH dependence of the reaction rate (or the first-order rate coefficient, ft) on the hydrogen or hydroxide ion concentrations, kinetic experiments are done with different concentrations of strong acid or strong base in the solution. This is practical if measurable rates are obtained with acid or base concentrations of 10-3 N or higher. For kinetic experiments in the pH range between 3 and 11, constant hydrogen and hydroxide ion concentrations are maintained with the acid or buffer solutions whose pH values must be known. This leads to the problem of the choice of a pH scale suitable for the treatment of kinetic data. 

It is an established procedure to define second-order rate coefficients in terms of concentrations rather than activities. Even if the activities of substrate and catalyst are known the rate coefficient includes the activity coefficient of the activated complex (see Vol. 2, pp. 311-312). Therefore, it is reasonable to continue to follow the same procedure no matter whether the data are obtained for solutions of strong acids or bases or for buffer solutions. (However, it is recommended to use activities rather than 

In comparisons of rate coefficients measured at different pH values, it is necessary to keep a constant ionic strength by addition of a neutral salt such as NaCl, KC1, NaNOs, or NaC104, as the pH of a buffer system may be altered by a change of ionic strength (secondary salt effect [1]). Furthermore, the catalytic rate coefficient of a hydrogen ion or hydroxide ion catalyzed reaction is dependent on the ionic strength also (primary salt effect [1], see also Vol. 2, p. 337). 

Some kinetic data found in literature are referred to the conventional (Sorensen) pH scale [7] or, in a few cases, to the paH scale [7] (paH = —log aH). Application of the conventional pH scale is certainly useful in kinetic experiments which are done for practical purposes (such as stability studies of drugs in solution). Matters are different, however, if it is intended to determine well defined values of the catalytic coefficients fcH or ft oh As mentioned above, the pcH scale is most recommendable for an evaluation of the rate equation from experimental data — particularly if the dependence of the rate on [H+] is complicated. Only if the pcH scale is used, ftH and fcoH values (referred to concentrations) determined with dilute solutions of strong acids or strong bases will be identical with those measured in buffer solutions at the same ionic strength.  

For buffer solutions, pcH can be calculated from the equation pcH = pifc + log ([A ] /[HA]), provided pKc of the buffer acid HA is known for the particular ionic strength and neutral salt. Otherwise, pcH values must be measured with a glass electrode (and a silver- silver chloride electrode) in a cell without liquid junction [7]. 

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