Effect of pH on Rate

Below pH 3, Elmer and Nordberg (153) found that the initial rate of dissolution reached a maximum at 0.8 N HNOj, corresponding to pH 0.1, and declined above this pH. However, Baumann.(154) has shown that from pH 3 to about 6, the rate of dissolution increased in proportion to the hydroxyl ion concentration. There may therefore be a small minimum between pH 0.1 and 3.0 that has gone unnoticed. 

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Whitaker, J. R. 1994. Effect of pH on rates of enzyme-catalyzed reactions. In Whitaker, J. R., Principles of enzymology for the food sciences (pp. 271-300). New York Marcel Dekker. 

Fig. 9. Effect of pH on rate of consumption of Af-bromosuccinimide (0.04 M) by leucine (0.01 M), From Schmir and Cohen (1961).

Fujimori BC, Nakajima H, Akutsu K, Mitani M. Chemiluminescence of Cypridina luciferin analogues part 1. Effect of pH on rates of spontaneous autoxidation of CLA in aqueous buffer solutions. J Chem Soc PCTkin Trans 2 1993 2405-9. 

Other Catalysts.—Copper(ii) alkoxides can be carbonylated, giving dialkyl carbonates, by insertion of carbon monoxide into the copper-alkoxide-oxygen bond. The first well-characterized copper(i)-carbonyl compounds are [CuCl(CO)(en)] and [(en)Cu(/i-CO)aCu(en)]Cl2. The effects of pH on rates of hydration of acetylene have been compared for the three catalysts CugSOi-CuSO, HgSOi, and PdS04-Fe2(S04)3. Alkyl and aryl halides can be carbonylated in the presence of ethanol, antimony pentachloride, and liquid sulphur dioxide ... 

FIG. 28-2 Effect of pH on the corrosion rate, a) Iron, (h) Amphoteric metals (aluminum, zinc), (c) Noble metals. 

I have carried out widespread studies on the application of a sensitive and selective preconcentration method for the determination of trace a mounts of nickel by atomic absorption spectrometry. The method is based on soi ption of Cu(II) ions on natural Analcime Zeolit column modified with a new Schiff base 5-((4-hexaoxyphenylazo)-N-(n-hexyl-aminophenyl)) Salicylaldimine and then eluted with O.IM EDTA and determination by EAAS. Various parameters such as the effect of pH, flow rate, type and minimum amount of stripping and the effects of various cationic interferences on the recovery of ions were studied in the present work. 

Figure 8.1 Effect of pH on corrosion of 1100-H14 alloy (aluminum) by various chemical solutions. Observe the minimal corrosion in the pH range of 4-9. The low corrosion rates in acetic acid, nitric acid, and ammonium hydroxide demonstrate that the nature of the individual ions in solution is more important than the degree of acidity or alkalinity. (Courtesy of Alcoa Laboratories from Aluminum Properties and Physical Metallurgy, ed. John E. Hatch, American Society for Metals, Metals Park, Ohio, 1984, Figure 19, page 295.)...

FIGURE 8.7 Effect of pH on retention of amino acids. Column and flow rate Same as Fig. 8.1. Mobile phase 10 mA1 potassium phosphate with SO mM HFIP pH as indicated (adjusted prior to the addition of HFIP). 

Fig. 2-17. The effect of pH on the retention, selectivity and resolution of coumachlor enantiomers on vancomycin CSP (250 X 4.6 mm). The mobile phase was acetonitrile 1 % tri-ethylammonium acetate (10/90 v/v). The flow rate was 1.0 mL min at ambient temperature (23 °C).

Fig. 4.42 Effect of pH on the rate of corrosion of zinc, A, rapid corrosion B, stable film — low corrosion rate C, rapid corrosion (after Roetheli, elal )...

Duncan and Frankenthal report on the effect of pH on the corrosion rate of gold in sulphate solutions in terms of the polarization curves. It was found that the rate of anodic dissolution is independent of pH in such solutions and that the rate controlling mechanism for anodic film formation and oxygen evolution are the same. For the open circuit behaviour of ferric oxide films on a gold substrate in sodium chloride solutions containing low iron concentration it is found that the film oxide is readily transformed to a lower oxidation state with a Fe /Fe ratio corresponding to that of magnetite . 

Significant differences are also apparent for the effect of pH on the fermentation rate. The narrow pH optimum characteristic of a free cell system is replaced by an extremely broad range upon immobilisation. This effect stems from the gradient pH that exists within the bead. 

Examination of the effect of pH on the rates of protodeboronation of the 2,6-dimethoxy compound at 90 °C in malonic acid-sodium malonate buffer solutions of ionic strength 0.14 gave the data in Table 199. A plot of these data revealed the curve shown in Fig. 3 (one of the points was misplotted on the original) and the linear portions of the plot were attributed to acid and base catalysis as shown on Fig. 3, and since the rates in the region of pH 4-5 are higher than would be... 

Figure 9.15. Effect of pH, on the rates of formation of N2 (a) and N20 (b) for fixed Pn0 and various fixed Uwr values. 6 Reprinted with permission from Academic Press.

Figure 3, Effect of pH on initial rate of nitrosation of glyphosate at 25 C ( 2), Initial concentrations of glyphosate and sodium nitrite, 10,5 and 7.5mM respectively.

In summary, the effect of pH on the dissolution rate of a drug from an oral dosage form depends on (a) the pH of the GI fluids, a patient variable (b) the acid or base strength of the drug, a pharmaceutical variable as well as (c) the physicochemical properties of the dosage form, another pharmaceutical variable. Furthermore, by intentionally designing the dosage form such that it buffers the diffusion layer, we can control a patient variable by a pharmaceutical variable. 

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),... 

The rate of oxidation of ethylene glycol was found268-269 to attain a broad maximum between pH 2.5 and 6. In a very complete study of the effect of pH on the periodate oxidation of carbohydrates, Neumiiller and Vasseur260 showed that the oxidation of maltose, melibiose, methyl a-D-... 

Several studies have been carried out to investigate the effect of pH on azo dye decolorization. In these assays, the decrease of absorbance at the wavelength corresponding to the maximum absorption for each dye is used as the method to evaluate the effectiveness of decolorization. Unfortunately, in most cases it is not clear if the isosbestic point of each dye was taken into account, and so it cannot be well understood if the different decolorization rate at different pH is due to a physical factor or to a differently influenced metabolic activity. 

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. 

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].)... 

Figure 3. The effect of pH on the average rate of methane formation from methanol. In 0.2 M Na2S04 at 60 °C and at constant over potential (see table 3).

In comparison to the approach of Ginski et al. [48], the Miyazaki s method appears to be more elaborate and complex and is thus coming closer to the in vivo situation. The device can simulate various effects of pH on dissolution and is, as an open system, closer to in vivo conditions compared to a closed one. However, it exhibits the drawback of not freely adjustable pH values acting on the drug. Low flow rate in the dissolution vessel may limit applications of complete dosage forms and allows predominantly only the use of granules, pellets, or grinded tablets. Furthermore, the application of compendial dissolution devices appears to be a more robust approach. 

Earlier suggestions that the two uncoordinated and invariant residues His35 (inaccessible to solvent and covered by polypeptide) and His83 (remote and 13 A from Cu) are, from effects of [H ] on rate constants (and related pKg values), sites for electron transfer may require some re-examination. Thus, it has been demonstrated in plastocyanin studies [50] that a surface protonation can influence the reduction potential at the active site, in which case its effect is transmitted to all reaction sites. In other words, an effect of protonation on rate constants need not necessarily imply that the reaction occurs at the site of protonation. His35 is thought to be involved in pH-dependent transitions between active and inactive forms of reduced azurin [53]. The proximity of... 

Given, C.J. andDierberg, F.E. Effect of pH on the rate of aldicarb hydrolysis. Bull Environ. Contam. Toxicol, 34(5) 627-633, 1985. 

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