Oxidation pH dependence

Table XI-1 (from Ref. 166) lists the potential-determining ion and its concentration giving zero charge on the mineral. There is a large family of minerals for which hydrogen (or hydroxide) ion is potential determining—oxides, silicates, phosphates, carbonates, and so on. For these, adsorption of surfactant ions is highly pH-dependent. An example is shown in Fig. XI-14. This type of behavior has important applications in flotation and is discussed further in Section XIII-4.

The intermediate HCIO2 is rapidly oxidized to chloric acid. Some chlorine dioxide may also be formed. Kinetic studies have shown that decomposition to O2 and chloric acid increase with concentration, temperature (88), and exposure to light (89—92), and are pH dependent (93). Decomposition to O2 is also accelerated by catalysts, and decomposition to chlorate is favored by the presence of other electrolytes, eg, sodium chloride (94—96). 

The iodometric analysis method for CIO2 and its coproducts is based on the pH-dependant oxidation of potassium iodide to selectively distinguish the various oxychlorine species from each other (42,89). The reactions of the oxidizer species with iodide at various pH buffered conditions ate... 

L/(mol-s) (39,40). QDI is also attacked by hydroxide ion (eq. 4) to produce a quinone monoimine (QMI), itself an oxidized developer derived from /)-aminopheno1. Such compounds can further react with coupler, albeit at a slower rate than QDI, to form a dye and were cited in the seminal patent as color developers (32). However, the dyes derived from this deaminated developer have different hues from the QDI dyes, and these hues are pH-dependent as a consequence of the phenoHc group contributed by the developer. Although the deamination reaction to produce QMI is fast, the rate constant is 10 to 10 L/(mol-s) (40—42), its effect is somewhat offset by the redox reaction of the QMI with the reduced developer, present in large excess, to regenerate the desired QDI. The primary net effect of the deamination reaction is to enlarge the resulting dye cloud (43). 

The thermodynamic data pertinent to the corrosion of metals in aqueous media have been systematically assembled in a form that has become known as Pourbaix diagrams (11). The data include the potential and pH dependence of metal, metal oxide, and metal hydroxide reactions and, in some cases, complex ions. The potential and pH dependence of the hydrogen and oxygen reactions are also suppHed because these are the common corrosion cathodic reactions. The Pourbaix diagram for the iron—water system is given as Figure 1. 

Alkynic esters react with nitrile oxides in a pH dependent reaction to product isoxazolin-5-ones (Scheme 145) (71JCS(C)86). Alkynic ethers also react with benzonitrile oxide to produce an isoxazole-ether which on treatment with HCl or HBr gave an isoxazolinone (Scheme 145) (63CB1088,58MI41600). The reaction of benzonitrile oxide with dimethoxyketene yielded a dimethyl acetal which was split with acid into the isoxazolinone (Scheme 145) (59G15H). 

Anode reactions can be problematic. The anode may dissolve or be oxidized. Or, depending on the pH and the chloride concentration ... 

If electron flow between the electrodes is toward the sample half-cell, reduction occurs spontaneously in the sample half-cell, and the reduction potential is said to be positive. If electron flow between the electrodes is away from the sample half-cell and toward the reference cell, the reduction potential is said to be negative because electron loss (oxidation) is occurring in the sample halfcell. Strictly speaking, the standard reduction potential, is the electromotive force generated at 25°C and pH 7.0 by a sample half-cell (containing 1 M concentrations of the oxidized and reduced species) with respect to a reference half-cell. (Note that the reduction potential of the hydrogen half-cell is pH-dependent. The standard reduction potential, 0.0 V, assumes 1 MH. The hydrogen half-cell measured at pH 7.0 has an of —0.421 V.)... 

The possibility of oxidation to Fe is a crucial theme in the chemistry of Fe and most of its salts are unstable with respect to aerial oxidation, though double sulfates are much less so (e.g. Mohr s salt above). However, the susceptibility of Fe to oxidation is dependent on the nature of the ligands attached to it and, in aqueous solution, on the pH. Thus the solid hydroxide and alkaline solutions are very readily oxidized whereas acid solutions are much more stable (see Panel opposite). 

The overall pattern of behaviour of titanium in aqueous environments is perhaps best understood by consideration of the electrochemical characteristics of the metal/oxide and oxide-electrolyte system. The thermodynamic stability of oxides is dependent upon the electrical potential between the metal and the solution and the pH (see Section 1.4). The Ti/HjO system has been considered by Pourbaix". The thermodynamic stability of an... 

Oxidation-reduction (redox) Inert metal (normally Pt but certain other metals can act in a similar manner) in a solution containing two species that give rise to a redox system. E depends on of the system and the relative activities of the oxidised and reduced forms. Quinone-hydroquinone QH4O2 -1- 2H+ -1- 2e-CjH4(OH)2, which is thus pH dependent Fe - -/Fe + Mn04-/Mn +... 

Metals in practice are usually coated with an oxide film that affects the potential, and metals such as Sb, Bi, As, W and Te behave as reversible A//A/,Oy/OH electrodes whose potentials are pH dependent electrodes of this type may be used to determine the solution s pH in the same way as the reversible hydrogen electrode. According to Ives and Janz these electrodes may be regarded as a particular case of electrodes of the second kind, since the oxygen in the metal oxide participates in the self-ionisation of water. 

The best known of the M/Mfiy electrodes that are used for the determination of pH are the Sb/SbjOj electrodes, but metals such as bismuth and arsenic filmed with their respective oxide act in a similar manner. Copper in alkaline solutions appears to behave as a pH-dependent Cu/CujO,... 

The pH dependence of the rate suggests that nucleophilic attack by the sulphur atom of the sulphoxide on the undissociated hydroperoxide is unimportant while nucleophilic attack by the hydroperoxide anion on the sulphoxide is the preferred reaction under these conditions. Changing the alkyl substituent of the sulphoxide had little effect on the rate and thus the reaction probably proceeds in a similar manner as for oxidation by peracids... 

This corresponds with MacDiarmid s observations which show that the second redox step is strongly pH-dependent. MacDiarmid further differentiated his redox model to take account of the fact that pure leucoemaraldine with its amine-N is already protonated at pH values 2, and that the totally oxidized pemigraniline with its less basic imine-N can also be protonated. This gives the following (simplified) reaction scheme ... 

Several demonstrations of this concept have recently been published The first one is based on the pH dependence of redox transitions in oxide semiconductors that are connected with conductivity changes. If the bridging polymer layer in Fig. 6 is WO3 sputtered onto the electrode array or electrochemically deposited Ni(OH)j the transistor amplification is a function of the pH of the... 

Fig. 18. pH dependence of the oxidized Rieske fragment from bovine heart mitochondria (ISF). (a) Redox potential determined by cyclic voltammetry. The line was fitted to the data points, giving = 7.6 and pi a, x2 = 9-2. (b) CD intensity of the oxidized... 

Removing two electrons from each P cluster renders each MoFe protein molecule oxidized by four electrons. Further oxidation leads to removal of electrons from the FeMoco centers. The potential of this oxidation is both species and pH dependent. At pH 7.9 the for Kpl is 180 mV, whereas for Cpl it is 0 mV and for Avl -95 mV (46). 

The one-electron reduction of the Ni-C form results in the diamagnetic species Ni-R. From the redox titration studies of Lindahl s group, a plausible catalytic cycle can be postulated where the enzyme in the Ni-Sl state binds H2 (77) and becomes the two-electron more reduced Ni-R state. Sequential one-electron oxidations from Ni-R to Ni-C and then to Ni-Sl will close the cycle (Fig. 6). The various redox states differ not only in the extent of their reduction, but also in their protonation, as shown by the pH dependence of their redox potentials (87). It is remarkable that both EPR (which monitors the magnetic... 

The redox potential of the [3Fe-4S] redox couple shows a wide variability (400 mV). These redox potentials are pH dependent (103-106), as are those linking lower oxidation levels, and so can be modulated by the medium. D. africanus Fdlll is one of these examples where the conversion 0/-2 states is suggested to be a 2e-3H process. 

Gerischer H (1989) Neglected problems in the pH dependence of the flatband potential of semiconducting oxides and semiconductors covered with oxide layers. Electrochim Acta 34 1005-1009... 

The oxidation of potassium thiocyanate by AuBrJ is first-order in oxidant and the pH dependence indicates that it is also first-order in thiocyanate ion, which is oxidised much faster than HCNS . The activation parameters are E = 6.4+0.4 kcal.mole and A5 = 26 + 2 eu. 

The rate of oxidation of sulphite ion by tetrabromoaurate is pH-dependent and analysis of the kinetic data leads to the rate law... 

As far as we know there is only one reference to induced change caused by neutralization. Raschig observed a strong air oxidation during the conversion of bisulphite-sulphite. We are of the opinion that, here, not the air oxidation of sulphite induced by neutralization but the pH dependence of oxidation is involved. 

---