Effects of pH on Reaction Rates

In general, the rates of solution reactions are proportional to the concentrations of specific species. When these are participants in protonic equilibria, the rates will be found to vary with H. One example is seen in the study of ligand exchanges such as 

The experimental rate is found to depend on the concentrations of the complex ion and on OH. One postulated mechanism includes as the rate-determining (slow) step this dissociation of the conjugate base of the complex ion, 

The concentration of this reactive species is related to the original complex ion through the K , 

Using the usual ap expression for a monoprotic case and H = K ,/OH, we obtain 

When is negligible, above a certain OH, the rate becomes independ- 

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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. 

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.

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]+,... 

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 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 ... 

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... 

FIGURE 2-32 Effect of pH on hydrolysis rate. The graph shows, in dashed lines, the rate of each hydrolysis process (acid-catalyzed, neutral, and base-catalyzed) as it varies with pH. Because reaction rates are proportional to [H + ] and [OH-] for the acid-catalyzed and base-catalyzed reactions, respectively, the slopes of the log (rate) versus pH lines for acid-catalyzed and base-catalyzed hydrolysis are — 1 and + 1, respectively. 

If one could conclusively show that there is an effect of plasticizers on reaction rate that is independent of moisture content or a , then this would demonstrate the significance of matrix properties for governing reaction rate. Labuza et al. (1977) studied the Maillard reaction and measured the effects of several parameters. These included temperature, reactant concentration, pH, buffers, and the addition of humectants. The formulation consisted of glucose and casein as reactants within a carrier matrix of microcrystalline cellulose and an inert lipid. Reaction rate was measured both as glucose loss, lysine loss, and pigment production (A42o). 

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. 

Figure 47. The effect of pH on the rate of < >0- reprotonation in an emitter—buffer system. lOOpJVf hydroxypyrene trisulfonate, 2mM imidazol buffer. The reaction was followed at 440 nm using CW dye laser. The continuous line is computed for the emitter-buffer system using the rate constants ki = 1.6 x 10u M sec-1 A5 = 3.5 x 1010 Af-1 sec-1 io = 2.1 x 109 AT-1 sec-1. Measurements in the absence of buffer ( ) or in the presence of 2mM imidazol ( ).

In effect this is a self-organized system that maintains a steady state, although not at chemical equilibrium, by a balance of reaction rates against reactant fluxes, a condition commonly seen in real-world systems. The titration experiments permit a rather direct measurement of the rate of nucleation and polymerization reactions that transform Ala monomers to Alb polymers. The titrations at pH 4.75 and 4.90 yielded only Ala and Alb. Although some Ale was formed in titrations at pH 5.00, the polymeric product was about 90% Alb. Hence, the data obtained in these experiments should be useful for determining the effect of pH on the rate of the polymerization process leading from Ala to Alb. At least a part of the information from titrations made at pH 5.20 should also be useful because during much of those experiments the principal product also was Alb. 

Effect of pH on the rate of an enzyme-catalyzed reaction. The enzyme functions most efficiently at pH 7. The rate of the reaction falls rapidly as the solution is made either more acidic or more basic. 

Figure 1. Effect of pH on the rate of reaction at 299 °C. (Data are taken from reference 5.)...

Figure 2. The effect of pH on the rate constants for the reactions of e ao with uracil and uridine. Also shown are the ionizable groups of uracil and uridine and their pKa values. The unionizable OH groups on the ribose molecules are not shown...

Figure 8. The effect of pH on the rate constant for the reactions of OH with uracil obtained using the CNS competition technique...

Either of these conditions can alter the effectiveness of an enzyme to catalyze a reaction by changing the concentration of free enzyme. Therefore, a general relationship between enzyme activity and pH is shown in Figure 6.3. Most enzymes function effectively over a pH range of about 1.0 unit. In Section 6.4 the effect of pH on the rate of enzyme catalyzed reactions will be examined in greater detail. 

K. J. Laidler and M. L. Barnard [Trans. Faraday Soc., 52, 497 (1956)] studied the effects of pH on the rate of the a-chymotrypsin-catalyzed hydrolysis of methyl hydrocin-namate. An initial rate approach based on potentiometric measurements led to the following data for a reaction at 25°C and pH 7.8. 

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