Copper Oxidation-reduction potentials

Recently DFM, TFM and other methionine analogues were utilized to explore the contribution that methionine makes to the reduction-oxidation potential of the P. aeruginosa metalloprotein azurin [26, 45], Azurin is a copper metalloprotein that is involved in... 

Enache et al. (2000) developed 2 electronegativity QSARs. The test system for both QSARs is listed in Table 5.14. Both QSARs relied on Allred-Rochow electronegativity (X r), standard reduction-oxidation potential (AEg), and Pauling s electronegativity (X) (Table 5.11). The first QSAR is for 12 cations listed in Table 5.11 and the second is for 11 cations listed in Table 5.11. The statistical analysis for the 12 cations produced a reliable QSAR, but improved results were obtained when the copper ion was omitted from the analysis as explained in Section 5.2.3. 

Notice, however, that if the reaction at the Zn/Zn2+ interface is reversed and written as an electronation (reduction) rather than a deelectronation, then this electronation does not proceed spontaneously and its free-energy change is positive. This positive value of AG° = -nFE0 implies that E° must be negative. The standard reduction potentials for the zinc and copper systems are, therefore, -0.76 and +0.34 V, in contrast to the standard oxidation potentials, which are +0.76 and -0.34 V, respectively. 

In this process, ores containing copper(II) oxide and copper(II) sulfide are dissolved in sulfuric acid, and then hydrogen is bubbled through the solution. The reduction is thermodynamically favored, because the standard potential of the couple Cu2+/Cu is positive ( ° = +0.34 V). Metals with negative standard potentials, such as zinc ( ° = —0.76 V) and nickel ( ° = —0.23 V), cannot be extracted by reduction with hydrogen. 

J. Tafel found that while nitric acid is reduced only to hydroxylamine q.v.) by mercury or well-amalgamated electrodes, a copper cathode reduces it to ammonia and at the same time has no action on hydroxylamine. A. Brochet and J. Petit studied the electro-reduction of nitric acid by an alternating current. T. H. Jeffery described the electrolysis of nitric acid with a gold anode, and obtained from the anode liquor crystals of aurinitric acid, HAu(N03)4.3H20. R. Ihle s observations on the oxidation-potential of nitric acid have been discussed in connection with nitrous acid (q.v.). He found that if the cone, of the nitric acid be expressed by... 

Similarly at the cadmium electrode oxidation will take place at the oxidation potential s° = 0.402 V, when combining with the copper electrode, which due to its nobler nature acts in this system as a positive electrode with a reduction potential v ° — 0.345 V ... 

The electrochemical potential is a measure of the driving force (or free energy change) of the oxidation/reduction reactions that occur during metal dissolution. As mentioned, copper dissolution and redeposition may occur by the reduction-oxidation reaction ... 

Based on redox potentials and species concentrations, Goldstein et al. (80) suggested an alternative mechanism (reactions (41) and (42)) to explain the unique toxicity of O2 compared to other biological reductants like glutathione and vitamin C. In the alternative system, the copper is oxidized rather than reduced as in reaction (36). The active species in this mechanism is trivalent copper. 

Remember that the table only gives you the reduction potentials, yet the galvanic cell has both a reduction reaction and an oxidation reaction. For a galvanic cell, you want a positive voltage, so the cathode is the metal with the more positive reduction potential, in this case copper. Reduction occurs at the cathode ( red cat ), so the half reaction is written correctly. Oxidation,... 

From the values of the standard reduction potentials, we can see that copper is oxidized, and silver is reduced. We then use equation 13.2 to find the cell potential ... 

In aqueous solution, iron exists in two oxidation states Fe and Fe. In contrast to copper, the standard potential for the reduction of the less oxidized species, Fe, is the less noble of the two ... 

FIG. 8.4 The arrow through the ocean. Top-. The redox potential of different reduction-oxidation pairs corresponds to the order in which reduced and oxidized forms were encountered and used by life. Elements with lower redox potentials were oxidized earlier than elements with higher redox potentials. For example, ammonia/nitrogen, sulfide, and molybdenum (IV) are more easily oxidized than copper (I) and vanadium (III). Bottom Because iron (III) readily precipitates out of solution as iron oxide, over time iron concentrations decreased. Because copper (II) compounds are generally more soluble than copper (I), oxidation increased overall copper availability. This trend can be summarized by an arrow pointing away from iron and toward copper. 

The cell emf is the sum of the potentials for the reduction and oxidation halfreactions. For the cell we have been describing, the emf is the sum of (he reduction potential (electrode potential) for the copper half-cell and the oxidation potential (negative of the electrode potential) for the zinc half-cell. 

In both cases we have to decide on the likely oxidation and reduction processes. The low reduction potential of Cu (aq) makes this the likely reduction process in both cases. What about oxidation processes The possibilities are in (a) oxidation of the copper electrode (anode) (E° = 0.340 V), oxidation of sulfate anion (2.01 V), and oxidation of water (1.23 V). Thus, the most easily oxidized is the copper at the anode. In (b), the platinum electrode is inert and is not easily oxidized. Of the other two candidates, sulfate anion and water, water has the lower oxidation potential. 

A potential greater than 0.89 V is required to electrolyze water and deposit copper. Keep in mind that when calculating E°en as a difference between two E° values, the E° values are reduction potentials. Because -E° corresponds to the half-cell potential for the oxidation process, the difference between two reduction potentials is equivalent to the sum of a reduction potential and an oxidation potential. 

For example, if we have a silver anode, the standard oxidation potential is —0.800 V. Adding this value to the copper reduction half-reaction gives —0.458 V. Thus, at a minimum, a potential of 0.458 V must be apphed to get Ag. We can see that the higher up a half-reaction is in Table 9.1, the more it will tend to be in its reduced form. Conversely, the lower it is, the more readily it wiU be oxidized. Thus, Table 9.1 can be quickly scaimed to see what oxidation—reduction reactions will spontaneously occur and provide useful work and what reactions will require the input of work. 

Selecting a Constant Potential In controlled-potential coulometry, the potential is selected so that the desired oxidation or reduction reaction goes to completion without interference from redox reactions involving other components of the sample matrix. To see how an appropriate potential for the working electrode is selected, let s develop a constant-potential coulometric method for Cu + based on its reduction to copper metal at a Pt cathode working electrode. 

Pentafluorobenzene. Pentafluoroben2ene has been prepared by several routes multistage saturation—rearomati2ation process based on fluorination of ben2ene with cobalt trifluoride reductive dechlorination of chloropentafluoroben2ene with 10% pabadium-on-carbon in 82% yield (226,227) and oxidation of penta uorophenylbydra2ine in aqueous copper sulfate at 80°C in 77% yield (228). Its ioni2ation potential is 9.37 V. One measure of toxicity is LD q = 710 mg/kg (oral, mouse) (127). 

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