Oxidation-reduction potentials acid solutions

The oxidation-reduction potential of arsenious-arsenie acid solutions has been determined.10 A small quantity of iodide was added as catalyst,11 and it was found that true equilibrium values, varying normally with the concentration ratio, are obtained only if the solutions are acidified to an extent corresponding at least with X-HC1. For the cell... 

The oxidation-reduction potentials of metal ions differ in different solvents due chiefly to differences in the strength of coordination of the solvents to the metal ions. Thus, Schaap and coworkers,33 who measured reduction potentials polarographically in anhydrous ethylenediamine, found the order of half-wave potentials to be Cd2+ > Pb2+ > Cu2+ - Cu+ > Ti+, whereas, in aqueous solution, the order is Cd2+ > Ti+ > Pb2+ > Cu2+ -> Cu+. Oxidation—reduction potentials have been measured in a great variety of non-aqueous solvents, both protonic and non-protonic. Among the former are liquid ammonia and concentrated sulfuric acid.34 Among the latter are acetonitrile, cyanopropane, cyanobenzene, dimethyl sulfoxide, methylene chloride, acetone, tet-rahydrofuran, dimethylformamide and pyridine.34... 

For reductions in acid solution, oxidation potentials show that barium metal is a better reducing agent than calcium. The reverse is true in basic solution. Explain. 

Dinitrobiphenyl (57)44 88 is thus reducible in alkaline solution at a suitable potential to benzo[c]cinnoline N-oxide (58) in acid solution the reduction proceeds to 4,5-dihydrobenzo[c]cinnoline (59) which is very easily oxidized, e.g., by air, to benzo[c]cinnoline (60). 

The Aqua Ion. Electrolytic or zinc reduction of acidic solutions of Vv, Vlv, or V111 or dissolution of the metal in acid produces violet air-sensitive solutions containing the [V(H20)6]2+ ion. These are strongly reducing (Table 17-1) and are oxidized by water with evolution of hydrogen even though the standard potential V3+/V2+ would indicate otherwise. The oxidation of V2+ by air is complicated and appears to proceed in part by direct oxidation to V02+ and in part by way of an intermediate species of type VOV4+. 

Quinone-Hydroquinone Systems.—In the brief treatment of the quinone-hydroquinone system on page 270 no allowance was made for the possibility of the hydroquinone ionizing as an acid actually such ionization occurs in alkaline solutions and has an important effect on the oxidation-reduction potential of the system. Hydroquinone, or any of its substituted derivatives, can function as a dibasic acid. It ionizes in two stages, viz.,... 

In some cases the oxidation-reduction potential is close to the potential at which another process, e.g., hydrogen evolution, can occur in this event both reactions will take place simultaneously. For example, reduction of the titanic-titanous system in hydrochloric acid commences at about + 0.05 volt, while in the same solution the reversible hydrogen potential is approximately zt 0.0 volt. It follows, therefore, that if a platinized platinum cathode, which has almost zero hydrogen overvoltage, is employed, reduction of the titanic ions and evolution of hydrogen will take place at the same time. The reduction eflSciency will then be small. If a high overvoltage cathode is employed, however,... 

We should also note that the oxidative degradation of some dmgs in solution may be pH-dependent for example, the oxidation of prednisolone is base-catalysed. Similarly, the oxidation of morphine occurs more rapidly in alkaline or neutral solution than in acid solution. The reason for this may be the effect of pH on the oxidation-reduction potential, Eq, of the drug. 

Oxygen itself is a good oxidizing agent in acidic solution at pH 0 because it has a fairly high reduction potential ... 

To leach the more acid-resistant minerals containing tetravalent uranium, steam is fed to the second tank to bring the temperature to 49 to 60°C, and sodium chlorate NaQOj is added to bring the oxidation-reduction potential e, measured relative to the calomel electrode, to from —0.47 to —0.51 V. At —0.51 V, the equilibrium ratio of ferric iron to ferrous iron in the solution is 0.52. Ferric iron catalyzes the oxidation of insoluble tetravalent uranium to the soluble hexavalent uranyl form ... 

Table 9.7 Formal oxidation-reduction potentials for actinides and fission products in acid solutions ...

Pentaralent neptunium is the most stable state in solution. It hydrolyzes only in basic solutions, disproportionates only at high acidity, and forms no polynuclear complexes. As shown by the oxidation-reduction potentials of Table 9.6, hexavalent neptunium is much less stable in solution than is hexavalent plutonium in fact, hexavalent neptunium is a strong oxidizing agent and is easily reduced in the presence of oxidizable substances, such as those present in ion-exchange and solvent extraction separations [K2]. 

In field applications, evidence of enhanced reduction of Cr(VI) by a DC electric field was first reported in 1987 by Banarjee et al (1987) at a Superfund site in Corvallis, Oregon. Later, controlled laboratory experiments of kaolinite clay injected with Fe(II) showed that an externally applied electric field caused an additional cathodic current that drive forth the reduction of Cr(VI) in clay (Pamukcu, Weeks, and Wittle, 2004). These transformations were characterized as to have benefited the capacitive changes on the clay surfaces. The results in these experiments showed that the system oxidation-reduction potential (ORP) increased by a positive shift from the standard solution ORP in the presence of the clay medium and the induced electrical field. Rgure 2.13 shows the ORP measurements plotted against the reaction quotient of the Nemst relation, where the data is categorized by pH. Under anoxic conditions and acidic environments, Fe(II) can be the dominant reductant of Cr(VI), as given by the following redox reaction ... 

FIG. 5—Local cell polarization on grain faces and boundaries of corroding stainiess steel. In ferric sulfate-sulfuric acid solutions grain faces and grain boundaries are both polarized to the oxidation-reduction potential of the solution. The corrosion current at corroding boundaries is greater than that on grain faces. 

The standard reduction potential of O ig) to 02(g) is +2.07 V. Write a half-equation for this reduction in acid solution. How does ozone compare with chlorine gas, Cl2( ), as an oxidizing agent Of what practical, beneficial, environmental importance is the oxidiang abiUty of ozone Hint Refer to Chapter 11 if need be.)... 

The work of Gorichev on the dissolution of magnetite in N82EDTA closely paralleled his studies in mineral acids. The data were found to fit Equation (15) very well when A = 0.01. A measured activation energy of 39 kcal/mol was evidence for the lack of mass transport limitations on the rate. It should be noted that the pH dependence of the rate was found to be more complex than in the case of the acids. A maximum in the rate at 140-176°F (60-80°C) occurred at a pH (controlled by H2SO4) of 2.3 and at a = 0.6. The rate coefficient, k, was found to be independent of EDTA concentration (zero order). The rate of dissolution was found to depend both on (H ] and on the oxidation/reduction potential of the solution, just as in the case of the mineral acids. The reaction order with respect to H was assumed to be 0.5, leading to a rate expression similar to Equation (21) ... 

The major disadvantage of this technique is the interference of other oxidants or reductants. The pH glass bulb is very selective, whereas the metallic electrode is sensitive to all oxidation-reduction potentials in the sample. Solutions of high salt or acid content can also cause errors. 

Although all potentiometric measurements (except those involving membrane electrodes) ultimately are based on a redox couple, the method can be applied to oxidation-reduction processes, acid-base processes, precipitation processes, and metal-ion complexation processes. Measurements that involve a component of a redox couple require that either the oxidized or reduced conjugate of the species to be measured be maintained at a constant and known activity at the electrode. If the goal is to measure the activity of silver ion in a solution, then a silver wire coupled to the appropriate reference electrodes makes an ideal potentiometric system. Likewise, if the goal is to monitor iron(ni) concentrations with a platinum electrode, a known concentration of iron(II) must be present in the sample solution such that potential changes are only dependent on the iron(II) concentration. 

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