The Influence of pH

As iodhydric acid is a strong acid, the pH value is of no influence on the couple hlV. 

Most redox systems exhibit an apparent standard potential that decreases when the pH increases (for example, this is the case of the couples Mn04 /Mn + and Cr207 /Cr see Chap. 20). By comparison with these couples, iodine becomes a strong oxidant in slightly acidic and weakly alkaline media. We must recall that in alkaline media (about pH = 9), iodine disproportionates into hypoiodous acid (or into hypoiodites) and into iodide ions, these species being in false equilibrium. Moreover, the second time, iodine disproportionates into iodate and iodide ions that are in true equilibrium. Hence, the potential of the couple I2/I is independent of pH in the approximate range 0 pH 9. 

6 Existence of the Fundamental Reaction of Direct and Indirect lodometries 

The existence of the fundamental reaction of direct and indirect lodometries permits us to determine an excess of iodine or iodine liberated from iodide ions. This reaction 

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The charge on a droplet surface produces a repulsive barrier to coalescence into the London-van der Waals primary attractive minimum (see Section VI-4). If the droplet size is appropriate, a secondary minimum exists outside the repulsive barrier as illustrated by DLVO calculations shown in Fig. XIV-6 (see also Refs. 36-38). Here the influence of pH on the repulsive barrier between n-hexadecane drops is shown in Fig. XIV-6a, while the secondary minimum is enlarged in Fig. XIV-6b [39]. The inset to the figures contains t,. the coalescence time. Emulsion particles may flocculate into the secondary minimum without further coalescence. 

Consistent with this, even KI3 is rapidly decolorized in alkaline solution. The example is a salu-tory reminder of the influence of pH, solubility, and complex formation on the standard reduction potentials of many elements. 

The influence of pH on the residual metal concentration [M] was studied at a constant polymer concentration of 10 mg/1 and copper sulphate concentration of 10 gm/1, results are shown in Table 4. It is clear that [M] decreases with increasing pH value. Results are given in Table 4, which shows that the concentration of Cu decreases with an increase in the pH value for each polymer used. This is attributed to the effect of the pH value on the active groups, which are distributed along the polymer chains. At a low pH value the amide groups... 

Figure 4.34 illustrates, by means of potential/anodic current density curves, the influence of pH and Cl ions on the pitting of nickel The tendency to pit is associated with the potential at which a sudden increase in anodic current density is observed within the normally passive range ( b on Curve 1 in Fig. 4.34). It can be seen that in neutral 0-05 M Na2S04 containing 0-02m Cl" (Curve 1) has a value of approximately 0-4 V h- When pitting develops, the solution in the pits becomes acidic owing to hydrolysis of the corrosion product (see Section 1.6) and when this occurs the anodic current density increases by at least two orders of magnitude and tends to follow the curve obtained in 0 05 m H2SO4-t-0-02 m NaCl (Curve 2). Comparison of Curves 2 and 3 illustrates the influence of Cl" ions on the pitting process.

Figure 4.36 shows the influence of pH on the breakdown potential of nickel in alkaline solutions containing Cl ions, and it is apparent that the breakdown potential becomes more positive as the pH increases, i.e. breakdown is unlikely unless the solution has a very high redox potential. 

Fig. 10.3.1 Bioluminescence spectrum of the centipede Orphaneus brevilabiatus (left panel), and the influence of pH on the luminescence of the exudate of the same centipede in 0.1 M potassium citrate/phosphate buffers (right panel). From Anderson, 1980, with permission from the American Society for Photobiology.

The influence of pH on the affinity of Hb for oxygen known as the Bohr-effect indicates that protons retain the allosteric regulation of oxygen transport. It is also an indirect confirmation of the ability of Hb and Im Hb for transporting carbon dioxide. The values of the Bohr-effect d log P50/d pH for Hb and Im Hb are close to each other in the pH range 7.1-7.4. It is possible that the effect of the micro-environment of carboxylic CP on immobilized Hb and its polyfunctional interaction represents the interaction between Hb and the structural elements inside the red cell [99]. 

The influence of pH, ionic strength, and protein concentration on the extraction of a-lactalbumin and 3-lactoglobulin from an aqueous solution with water/AOT/isooctane microemulsions and their separation has been reported [168],... 

Other diagrams could be devised indicating the influence of pH, electrolyte concentration, etc., on the appearance of one or two phases. Three-dimensional diagrams might also be of great aid. 

The influence of pH, Emulsifier, and Accelerated Ageing upon Preservative Requirements of O/W Emulsions, by Gene Jacobs, M.S. at all Soc. Cosmet. Chem., 26,105-117 (February, 1975)... 

Benzoic acid derivatives also altered the electrical potential across the cell membrane in neurons of the marine mollusk Navanax lnermls (46). Salicylic acid (1-30 mM) caused a depolarization very rapidly (1-2 min) and decreased the ionic resistance across the membrane. As pH was decreased, more salicylic acid was required to reverse the effect of pH on the membrane potential (47). This result is contradictory to the influence of pH on the amount of salicylic acid required to affect mineral absorption in roots (32). The ability of a series of salicylic and benzoic acid derivatives to increase PD correlated with their octanol/water partition coefficients and pKa values (48). The authors proposed that the organic acid anions bound directly to membranes to produce the observed results. 

Also of great interest is the so-called FIA scanning as a method for investigating, for instance, the influence of pH on the solvent extraction of metal dithizonates95 by controlled-potential continuous alteration of the pH of the carrier stream. Many other investigations can thus be made, such as the catalytic activity of enzymes and the influence of pH on ion-selective electrodes. 

Babich, H. and Stotzky, G., Sensitivity of various bacteria, including actinomycetes, and fungi to cadmium and the influence of pH on sensitivity, Appl Environ Microbiol, 33 (3), 681-695, 1977. 

In 1940, Jacobs [4] described the theory of nonionic membrane permeation of organic compounds in quantitative terms. The influence of pH and pKa on drug... 

We close this section with a note on the influence of pH on reduction potentials. Many redox reactions are pH-dependent, which can be understood with reference to the simple model in Figure 13.4, in which a redox compound in its oxidized state has a pK,t for proton dissociation that is different from (i.e., lower than) the corresponding value for its reduced state the positive charge of Xox is higher than that of Xred, so it is more difficult for Xox to accept a proton (i.e., its pKa is lower). The °(pH) is now... 

Shi SR, Imam SA, Young L, et al. Antigen retrieval immunohistochemistry under the influence of pH using monoclonal antibodies. J. Histochem. Cytochem. 1995 43 193-201. 

There are a number of various commercial products designed for DNA/ RNA extraction, but their use may lead to variable results. Based on the evidence that formalin-induced modification of protein is similar to that of nucleic acid modification by formalin (Fig. 3.1),15-19 our research group at the University of Southern California has conducted a serial study of AR based heating protocols for DNA/RNA extraction from FFPE tissues following our experience of the AR principle as applied to IHC on tissue sections heating under the influence of pH.19-21... 

Shi S-R, Cote RJ, Wu L, et al. DNA extraction from archival formalin-fixed, paraffin-embedded tissue sections based on the antigen retrieval principle heating under the influence of pH. /. Histochem. Cytochem. 2002 50 1005-1011. 

Therefore, a systematic examination of the influences of pH and electrolyte concentration on the state of flocculation of both silica types seemed promising. For a quantitative examination of the state of flocculation a Laserphoto Sedimentometer was used to follow the turbidity and the sedimentation of the suspension as a function of time. This method was described in detail in our earlier work (1,10). 

Since acidity (Lewis or Brpnsted) impacts adversely on the yield of epoxides, Clerici and Ingallina (204) added basic compounds in low concentrations to TS-1 catalysts during epoxidation of alkenes to inhibit the oxirane ring opening and enhanced the epoxide yields. A comprehensive investigation of the influence of pH on product selectivity in epoxidation of allylalcohol, allylchloride, and styrene catalyzed by various titanosilicates was reported recently by Shetti et al (205). 

The influence of pH on epoxidation of styrene with aqueous H202 catalyzed by TS-1 was also investigated. Conversion of styrene decreased, and styrene oxide selectivity increased marginally at high pH values (Table XXI). 

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