Reduction potentials metal oxidants

Desorption of water often converts Bronsted to Lewis acids, and readsorption of water can restore Bronsted acidity. Probe molecules, such as ammonia, pyridine, etc., are used to evaluate Bronsted and Lewis acidity. These compounds may contain water as an impurity, however. Water produced by reduction of metal oxides can also be readsorbed on acid sites. Probe molecules can in some cases react on surface acid sites, giving misleading information on the nature of the original site. Acidity, and accessibility, of hydroxyl groups or adsorbed water on zeolites and acidic oxides can vary widely. Study of adsorbed nitrogen bases is very useful in characterization of surface acid sites, but potential problems in the use of these probes should be kept in mind. 

Reduction potentials for the reversible reduction of metal ions to the corresponding metal in aqueous solution and in liquid NH3 are listed in Table 8.5. Note that the values follow the same general trend, but that the oxidizing abihty of each metal ion is solvent-dependent. Reduction potentials for oxidizing systems cannot be obtained in liquid NH3 owing to the ease with which the solvent is oxidized. 

Otsuka, K., Murakoshi, S., Morikawa, A. (1983). Potential metal oxides for the production of hydrogen from water by a reduction-oxidation cycle using carbon as a reductant. Fuel Process Technology, 7, 203—211. 

Effective oxidizing agents, such as Cu + and F2, have positive E9 values. Half-reactions with negative reduction potentials prefer oxidation over reduction. Effective reducing agents, such as Li and Zn, have negative values, indicating that the metal or other electrode is more... 

The blends are aged in 5-6 hi oak barrels (pipas) for several years in oxidative conditions that maintain an elevated oxidation-reduction potential. Metal ions, in particular copper and iron, play an essential role in polyphenol oxidation. Even in the bottle, these wines conserve a high oxidation-reduction potential. The iron remains in its ferric state, as if all of the reducing components had been destroyed by oxygen. Their prolonged oxidation and intense esterification give these wines a rich and complex bouquet. 

Table A. 13 in the appendix gives values for standard reduction potentials for a number of half-cells. Longer versions of such tables are available in handbooks. Unfortunately, some older works use values that are the negative of the standard reduction potentials (they are called oxidation potentials). If you are not certain whether an old table gives reduction potentials or oxidation potentials, look for an active metal electrode such as sodium or potassium. If the table contains reduction potentials, the half-cell potential of such a metal electrode will be negative.

Thennodynamic stability is generally provided for noble metals in most media as tlieir oxidation potential is more anodic tlian tire reduction potential of species commonly occurring in tire surrounding phase. However, for many materials of technological and industrial importance tliis is not tire case. 

For example, for iron in aqueous electrolytes, tlie tliennodynamic warning of tlie likelihood of corrosion is given by comparing tlie standard electrode potential of tlie metal oxidation, witli tlie potential of possible reduction reactions. 

Highly protective layers can also fonn in gaseous environments at ambient temperatures by a redox reaction similar to that in an aqueous electrolyte, i.e. by oxygen reduction combined with metal oxidation. The thickness of spontaneously fonned oxide films is typically in the range of 1-3 nm, i.e., of similar thickness to electrochemical passive films. Substantially thicker anodic films can be fonned on so-called valve metals (Ti, Ta, Zr,. ..), which allow the application of anodizing potentials (high electric fields) without dielectric breakdown. 

Two methods are used to measure pH electrometric and chemical indicator (1 7). The most common is electrometric and uses the commercial pH meter with a glass electrode. This procedure is based on the measurement of the difference between the pH of an unknown or test solution and that of a standard solution. The instmment measures the emf developed between the glass electrode and a reference electrode of constant potential. The difference in emf when the electrodes are removed from the standard solution and placed in the test solution is converted to a difference in pH. Electrodes based on metal—metal oxides, eg, antimony—antimony oxide (see Antimony AND ANTIMONY ALLOYS Antimony COMPOUNDS), have also found use as pH sensors (8), especially for industrial appHcations where superior mechanical stabiUty is needed (see Sensors). However, because of the presence of the metallic element, these electrodes suffer from interferences by oxidation—reduction systems in the test solution. 

Although it is only slowly oxidized in moist air at ambient temperature, cadmium forms a fume of brown-colored cadmium oxide [1306-19-0] CdO, when heated in air. Other elements which react readily with cadmium metal upon heating include the halogens, phosphoms, selenium, sulfur, and tellurium. The standard reduction potential for the reaction... 

Various other observations of Krapcho and Bothner-By are accommodated by the radical-anion reduction mechanism. Thus, the position of the initial equilibrium [Eq. (3g)] would be expected to be determined by the reduction potential of the metal and the oxidation potential of the aromatic compound. In spite of small differences in their reduction potentials, lithium, sodium, potassium and calcium afford sufficiently high concentrations of the radical-anion so that all four metals can effect Birch reductions. The few compounds for which comparative data are available are reduced in nearly identical yields by the four metals. However, lithium ion can coordinate strongly with the radical-anion, unlike sodium and potassium ions, and consequently equilibrium (3g) for lithium is shifted considerably... 

The oxidation-reduction potentials of metal complex ions. D. D. Perrin, Rev. Pure Appl. Chem., 1959, 9, 257-285 (111). 

In damp air, materials with standard reduction potentials less than 0.88 V oxidize spontaneously. Atmospheric O2 easily oxidizes iron and aluminum, the most important structural metals ... 

One of the most common chemical reducing agents for metallurgy is coke, a form of carbon made by heating coal at high temperature until all of the volatile impurities have been removed. Metals whose cations have moderately negative reduction potentials—Co, Ni, Fe, and Zn—are reduced by coke. For example, direct reaction with coke in a furnace frees nickel from its oxide NiO(.j) + C( ) Ni(/) -F CO(g)... 

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