Effect of pH on oxidation

Effect of pH on oxidation of phenol with Fenton s reagent. (From Eisenhauer, H.R., J. Water Pollut. Contr. Fed., 36, 1116-1128, 1964. With permission.)... 

Properties estimated by interpolation within the range of conditions over which a QSAR was calibrated should be reliable, but extrapolations beyond this range cannot be made with certainty. A similar restriction applies to experimental variables factored out of the training set data before deriving the QSAR (e.g., the effect of pH on oxidation of phenolic compounds, or the effect of surface area on reductions with Fe°). 

The effect of pH on oxidation-reduction potentials can be illustrated with the alcohol dehydrogenase system. Instead of pH 7.0, consider the reaction at pH 11.0. The acetaldehyde-ethanol couple will have an Eo of -0.200 - (4 X 0.06) = -0.440. The DPN-DPNH couple, involving only 1 hydrogen ion, changes from —0.320 to —0.440 volts. AE now is zero, and log IT, is zero. Ken, therefore, is 1. 

Stannate(II) ions are powerful reducing agents. Since, for tin, the stability of oxidation state -b4 is greater than that of oxidation state -b2, tin(II) always has reducing properties, but these are greater in alkaline conditions than in acid (an example of the effect of pH on the redox potential, p. 101). 

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 . 

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-... 

The combination of Na2S and amine was then introduced into the Gomo plant in 1954. The pH during flotation was found to be very important For flotation of calamine, the optimum pH was 10.5. In the presence of willemite-hemimorphite, the pH with RNH2 was about 11.5. The effect of pH on zinc oxide flotation in the presence of amine is illustrated in Figure 20.2. 

Figure 20.2 Effect of pH on mixed oxide zinc flotation using amine (Armac C).

It is appropriate now to return to the effect of pH on the [Co(phen)3] oxidation of PCu(I). If protonation at the remote site influences the reaction of PCu(II), then a similar effect might be expected for the reaction of PCu(I) with positively charged complexes. In the case of PCu(I) the kinetics are dominated by the inactivation resulting from the active site protonation. Whereas the pK for the [Fe(CN)g] oxidation is in good agreement with the HNMR independently measured value, the apparent pK obtained with [Co(phen)3] " is significantly higher, an effect which is clear from an inspection of Fig. 10. A two pK fit is possible in the case of [Co(phen)3], as has been illustrated [1,100],... 

The effect of pH on the periodate oxidation of seven anilines has been investigated. " The kinetics of periodate oxidation of aromatic amines have been studied. " - " Periodate oxidation of oxalic acid is catalysed by Mn(II). " The reaction of ethane-1,2-diol with periodate has been investigated under a variety of conditions and the results compared with those of earlier work and analogous studies on pinacol. " The 104 ion is the primary reactant, with H5IO6 as a secondary reactant the reverse is true for pinacol. The complex observed in previous work is shown not to be an intermediate, but rather to deactivate the reactants. 

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). 

Figure 1. Effect of pH on ir-A and AV-A isotherms of octadecyl-dimethylamine oxide (ClsDAO) at 25 C, O.OIM NaCl subsolutions (NaOH or HCl/NaCl). Substrate pH 1-10.9, 2-5.5, 3-2.2.

FIGURE 8.9 Schematic diagram of effect of pH on the rate constant k and on the concentration of dissolved StlV) and its total rate of oxidation represented by [S(IV)] for two cases (a) rate constant k decreases with pH (b) k increases with pH. 

Figure 11.1 Effect of pH on the oxidation-reduction potential of various systems (from...

Effects of Valinomycin on Oxidative Phosphorylation When the antibiotic valinomycin is added to actively respiring mitochondria, several things happen the yield of ATP decreases, the rate of 02 consumption increases, heat is released, and the pH gradient across the inner mitochondrial membrane increases. Does valinomycin act as an uncoupler or an inhibitor of oxidative phosphorylation Explain the experimental observations in terms of the antibiotic s ability to transfer K+ ions across the inner mitochondrial membrane. 

Understanding the behavior of acids and bases is essential to every branch of science having anything to do with chemistry. In analytical chemistry, we almost always need to account for the effect of pH on analytical reactions involving complex formation or oxidation-reduction. pH can affect molecular charge and shape—factors that help determine which molecules can be separated from others in chromatography and electrophoresis and which molecules will be detected in some types of mass spectrometry. 

The Ncrnst equation was given before (Eq. 10.115), and in this chapter the effect of pH on the reduction potential of the hydrogen ion has been mentioned, but the effect in general should be emphasized. There are several types of reactions in which concentrations of the reactants and products affect the stability of various oxidation states. This can be understood through application of the Nemst equation. The reduction potential of hydrogen will vary with the concentration of the hydrogen ion hence the commonly known fact that many reasonably active metals dissolve in acid but rot in base. 

Methionine can be oxidized to the sulfoxide in this system. In an investigation of the effect of pH on the photooxidation reaction, no... 

The fact that uncouplers are lipophilic weak acids (see above) explains their ability to collapse transmembrane pH gradients. Their lipophilic character allows uncouplers to diffuse relatively freely through the phospholipid bilayer. Because they are weak acids, uncouplers can release a proton to the solution on one side of the membrane and then diffuse across the membrane to fetch another proton. The chemiosmotic theory thus provides a simple explanation of the effects of uncouplers on oxidative phosphorylation. 

Beltran et al. (1998) reported that the oxidation kinetics of nitroaromatic hydrocarbons at different pH levels (between 2 and 12) was similar to those found in 03/H202 oxidation — for example, the positive effect of pH on removal rate between pH 2 and 7 and partial inhibition at pH 12. The ozone efficiency increased with pH, from 30 to 40% (pH = 4) to 95% (pH 9 or 11). At pH 4, about a 10% difference was observed between the ozone efficiencies obtained during UV / ozone radiation oxidation and ozonation alone, while no difference was observed at pH 9 or 11. Figure 8.3 shows this effect for the degradation of vanillic acid, as reported by Benitez et al. (1997). 

Effect of pH on chlorophenol oxidation rates at various pH values. (Data from Benitez, F.J. et at., Chemosphere, 41, 1271-1277,2000.)... 

J.- F. Thaburet, N. Merbouh, M. Ibert, F. Marsais, and G. Queguiner, TEMPO-mediated oxidation of maltodextrins and D-glucose effect of pH on the selectivity and sequestring ability of the resulting polycarboxylates, Carbohydr. Res., 330 (2001) 21-29. 

A similar effect of pH on dissolution rates of Mn(III/IV) oxides was observed by Stone (1987b) with substituted phenols. In this study, phenols with alkyl, alkoxy, or other electron, donating substituents were more slowly degraded. Stone (1987b) even found that p-nitrophenol, the most resistant phenol studied, reacted slowly with Mn(III/IV) oxides. 

In Figure 6, we show a theoretical calculation using Equations 3-6 of the effect of pH on the distribution coefficient for a case where the principal adsorbent is a hypothetical clay which contains a total of 0.8% oxides. In this case, we chose a mixture of three oxides as follows oxide 1, 0.1%, Kox =. 01, C =. 5 oxide 2, 0.2%, -. 005,... 

Figure 1. Effect of pH on the IR spectrum of 0.20 M trimethylamine oxide (CjAO) in water at 25 C.

In our next series of measurements we determined the effect of pH on the oxidation of H2S in buffered dilute solutions. The measurements were made at S5°C to speed up the acquisition of data. The results are shown in Figure 7. The results from pH = 2 to 8 are similar to the earlier measurements of Chen and Morris (41). Above a pH = 8 we find the rate to be independent of pH unlike the results of Chen and Morris (411 who find a complicated pH dependence. This could be related to trace metal impurities in the buffers used by Chen and Morris (41). Hoffmann and Lim (461 nave examined these trace metal effects and the base catalysis of the oxidation of H2S. 

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. 

---