The Effect of pH on Reaction Rates

The influence of pH on reaction rates may be looked upon as just another concentration effect, which can be dealt with in terms of the reaction orders just discussed. It merits special attention, however, for two reasons first, because it allows us to change the concentration of the reactant H3O or OH ) over many orders of magnitude, without encountering solubility limitations at the high concentration end and mass transport limitations at the low concentration end (as long as the solution is buffered). The second reason is that, in aqueous solutions, the solvent itself can be the reactant or the product in the reaction being studied. 

A few words of caution might be appropriate in regard to the use of this concept, particularly for readers who did not major in chemistry. The pH of an aqueous solution is formally defined as 

When the temperature is changed, the equilibrium constant for the dissociation of water is also changed. As a result, pH 7.0 represents the point of neutrality only at 25 °C. At higher temperatures the point of neutrality moves to lower pH values. Similarly, in mixed solvents the point of neutrality is not necessarily at pH 7.0. 

Let us now return to the effect of pH on electrode kinetics, using concentrations instead of activities. Consider the hydrogen-evolution reaction, and assume that it proceeds in the following two steps, vnth the second step being ratedetermining. 

This is identical to Eq. (6.16), derived for the first step in chlorine evolution, except for the change in the sign of the exponent, which is necessary because we are now dealing here with a cathodic reaction. 

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Many solution reactions are catalyzed by hydrogen or hydroxyl ions and consequently may undergo accelerated decomposition upon the addition of acids or bases. The catalysis of a reaction by hydrogen or hydroxyl ions is known as acid-base specific catalysis. In many cases, in addition to the effect of pH on reaction rate, there may be catalysis by one or more species of the buffer system. This type of catalysis is known as the acid-base general catalysis. Solutions of vitamin were found to be... 

Ionic equilibrium constants for the free enzyme (E) and the enzyme-substrate complex (ES) have to be determined to quantify the effect of pH on reaction rate (Eq. 3.96). Experimental design is simple since it consists on a matrix in which initial rate data are collected at varying s and pH as shown in Table 3.7. 

Figure 10.8 Atypical plot of the effect of pH on reaction rate.

FIGURE 9.2 The effect of pH on reaction rate of a typical enzyme catalyzed reaction. (From Mathewson, P.R. Enzymes, Eagan Press, St. Paul, 2004. With permission.)... 

The Effect of pH on Reaction Rate, Hydrothermal experiments in gold bag liners, as well as stainless steel and titanium vessels, have shown that the rate of decarboxylation of the acidic form of acetic acid is faster than the rate for the anion form (Kharaka et al. 1983 Palmer and Drummond 1986 Bell 1991 Bell et al. 1993). In addition, decarboxylation was found to be first order with respect to either acetic acid or acetate (Bamford and Dewar... 

The example of uranyl reduction shows the utility of this approach. The concentrations of the two surface complexes vary strongly with pH, and this variation explains the observed effect of pH on reaction rate, using a single value for the rate constant k+. If we had chosen to let the catalytic rate vary with surface area, according to 17.12, we could not reproduce the pH effect, even using H+ and OH-as promoting and inhibiting species (since the concentration of a surface species depends not only on fluid composition, but the number of surface sites available). We would in this case need to set a separate value for the rate constant at each pH considered, which would be inconvenient. 

The influence of pH on reaction rates may be looked upon as just another concentration effect, which can be dealt with in terms of the reaction orders just discussed. It merits special attention, however, for two reasons first, because it allows us to change the concentration... 

While there are abundant rate data available on ester hydrolysis in homogeneous aqueous solution (e.g., Mabey and Mill, 1978), quantitative data on the effect of surfaces on reaction rates are rather scarce. Hoffmann (Chapter 3, this volume) and Stone (1989) have investigated the catalytic effect of oxide surfaces on the hydrolysis of a few carboxylic acid esters, and have found a rate enhancement for compounds for which base catalysis is important at neutral pH. 

Effects of pH on reaction rates in prototypical rhodium-catalyzed hydrogenation in water have been examined carefully. The complex [Rh(DPPBTS)(NBD)][03SCF3] (DPPBTS = tetrasulfonated l,4-bis(diphenylphosphino)butane) upon reaction with hydrogen gives different complexes [Rh(DPPBTS)(H20)3(H)] +, [Rh(DPPBTS)(H20)2]+,... 

The salt (buffer) type and concentration may also influence reaction rate. While buffers vary in their effect on the Maillard reaction, it is generally accepted that phosphate is the best catalyst [27], The effect of phosphate on reaction rate is pH dependent with it having the greatest catalytic effect at pHs between 5-7. Potman and van Wijk [27] found the Maillard reaction rate in a phosphate buffered model system increased from 10- to 15-fold compared to a phosphate free reaction system. 

The Effect of pH on Reactivity, In spite of the different charges on oxidants Co(phen)2 + and Fe(CN) 3- and evidence that they use different reaction sites on PCu(I) (see below), remarkably similar pH profiles of rate constants are observed, Figure 4. Dependence on [H+] are described by (9),... 

This finding is the consequence of the distribution of various ruthenium(II) hydrides in aqueous solutions as a function of pH [RuHCl(mtppms)3] is stable in acidic solutions, while under basic conditions the dominant species is [RuH2(mtppms)4] [10, 11]. A similar distribution of the Ru(II) hydrido-species as a function of the pH was observed with complexes of the related p-monosulfo-nated triphenylphosphine, ptpprns, too [116]. Nevertheless, the picture is even more complicated, since the unsaturated alcohol saturated aldehyde ratio depends also on the hydrogen pressure, and selective formation of the allylic alcohol product can be observed in acidic solutions (e.g., at pH 3) at elevated pressures of H2 (10-40 bar [117, 120]). (The effects of pH on the reaction rate of C = 0 hydrogenation were also studied in detail with the [IrCp (H20)3]2+ and [RuCpH(pta)2] catalyst precursors [118, 128].)... 

The reasons for the effect of pH on the catalytic properties of enzymes are numerous and will not be discussed here. For most enzymes, however, there is a pH at which they are optimally effective changing the pH to lower (more acidic) levels or to higher (more basic) levels will decrease the overall rate at which the associated chemical reaction occurs. In the region of the optimum pH, the reaction rate vs. pH response surface can usually be approximated reasonably well by a second-order, parabolic relationship. 

Effect of pH on Lignin Peroxidase Catalysis. The oxidation of organic substrates by lignin peroxidase (Vmax) has a pH optimum equal to or possibly below 2. Detailed studies have been performed on the pH dependency of many of the individual reactions involved in catalysis. The effect of pH on the reaction rates between the isolated ferric enzyme, compounds I or II and their respective substrates has been studied. Rapid kinetic data indicate that compound I formation from ferric enzyme and H2O2 is not pH dependent from pH 2.5-7.5 (75,16). Similar results are obtained with Mn-dependent peroxidase (14). This is in contrast to other peroxidases where the pKa values for the reaction of ferric enzyme with H2O2 are usudly in the range of 3 to 6 (72). 

The effect of pH on the formation of dioxygen adducts of 6 (25c), as well as the establishment of volume profiles for the reaction (25b) have been reported. In aqueous solution, dioxygen adducts of 6 form at pH >7, with the kinetics of the reaction remaining approximately constant with those measured at pH < 7. However, at high pH values, the rate drops off by a factor of about five, corresponding to the point at which complex 6 is deprotonated (pK = 11.68). When the rate... 

The effect of pH on the hydroxyl radical reaction rate constants was studied in buffered solution at pH = 3.5, 7.0, and 11.0. As the pH increases above 9, the presence of alternate scavenger HOy will react with hydroxyl radicals, and the rate constant is nearly 300 times that of H202. The data for haloben-zenes showed that the reactivity of hydroxyl radical with these derivatives does not vary much among the compounds studied, and an average rate constant of 5.0 x 109 M 1 s 1 could be used for these compounds. 

The effect of pH on the rate of oxidation of H2S with H202 was determined from pH = 2 to 13 at 5, 25 and 45°C. These results are shown in Figure 18. Our results at 25°C from pH = 5 to 8 are in good agreement with the results of Hoffmann (46) (See Figure 19). At lower values of pH, his results are faster than ours. This may be due to problems with the emf technique he used. For the slower reactions of H2S with 02 or H202, the emf technique may yield unreliable results due to problems with the electrode response. 

Question What is so useful about studying the effect of pH on the rates of enzymatic reactions ... 

In addition to the effect pH has on the overall reaction rate, it is also important to note the effect of pH on rearrangement reactions of (+)-catechin. At alkaline pH, these secondary rearrangement reactions dominate. Epimerization of (+)-catechin to (-b)-epicatechin is a prominent reaction at pH 9.0. This is not serious in terms of adhesive formulation because it does not alter the reactivity of the aromatic nucleus toward condensation with benzyl alcohols. However, in reactions at pH 10.0 or 11.0, the intramolecular rearrangement to catechinic acid dominates and results in loss of the phloroglucinol functionality. 

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