Reduction potentials metallic couples

Figure 2. Schematic representation of some relevant ground and excited-state properties of Ru(bpy)j. MLCT and MLCT are the spin-allowed and spin-forbidden metal-to-ligand charge transfer excited states, responsible for the high intensity absorption band with = 450 nm and the luminescence band with = 615 nm, respectively. The other quantities shown are intersystem crossing efficiency energy (E°°) and lifetime (x) of the MLCT state luminescence quantum yield ( ) quantum yield for ligand detachment (O,). The reduction potentials of couples involving the ground and the MLCT excited states are also indicated.

A student was given a standard Cu(s) Cu2+(aq) half-cell and another half-cell containing an unknown metal M immersed in 1.00 M M(NO,)2(aq). When the copper was connected as the anode at 25°C, the cell emf was found to be —0.689 V. What is the reduction potential for the unknown M2+/M couple ... 

A quite different set of oxidations/reductions, but not fast, were the equilibria which governed the change of the environment, that is external oxidation/reduction potentials. They involve elements such as S, Se and metals but not all C or N couples. Their slow change in value was due to the slow release of oxygen by... 

Whereas the reduction potentials for the three metal ions range from +0.19V vs NHE(Cu) to +1.58V(Au), the potentials for oxidation of OH in their presence are -0.79V vs NHE(Cu), -0.30V(Ag), and -0.19V(Au). This is compatible with the proposition that oxidation occurs via the facilitated removal of an electron from OH and formation of an M-OH covalent bond. The only exception to the close agreement between gas-phase and redox-derived M-OH bond energies is the Cu-OH bond energy from aqueous redox data. This may be due to an inaccurate formal potential for the CuOH/Cu, OH couple (a value of 0.0V vs NHE rather than -0.36V would result in a more consistent bond-energy estimate). 

A number of rate constants for reactions of transients derived from the reduction of metal ions and metal complexes were determined by pulse radiolysis [58]. Because of the shortlived character of atoms and oligomers, the determination of their redox potential is possible only by kinetic methods using pulse radiolysis. In the couple Mj/M , the reducing properties of M as electron donor as well as oxidizing properties of as electron acceptor are deduced from the occurrence of an electron transfer reaction with a reference reactant of known potential. These reactions obviously occur in competition with the cascade of coalescence processes. The unknown potential °(M /M ) is derived by comparing the action of several reference systems of different potentials. 

In aqueous solution, thorium exists as Th(IV), and no definitive data have been presented for the presence of lower-valent thorium ions in this medium. The standard potential for the Th(IV)/Th(0) couple has not been determined from experimental electrochemical data. The values presented thus far for the standard reduction potential have been calculated from thermodynamic data or estimated from spectroscopic measurements. The standard potential for the four-electron reduction of Th(IV) ions has been estimated as —1.9 V in two separate references 12. The reduction of Th(OH)4 to Th metal was estimated at —2.48 V in the same two publications. Nugent et al. calculated the standard potential for the oxidation ofTh(III) to Th(IV) as +3.7 V versus SHE, while Miles provides a value of +2.4 V [13]. The standard potential measurements from studies in molten-salt media have been the subject of some controversy. The interested reader is encouraged to look at the summary from Martinot [10] and the original references for additional information [14]. 

While many metal centers can be reversibly cycled between two (or more) oxidation states, few organic moieties can match such reversibility especially in protic media. Nevertheless, the first supramolecular example of an electroswitch-able luminescent device involved the benzoquinone-hydroquinone couple. The luminescence of 55 " is switched off due to PET in the benzoquinone state of the redox couple. Electrochemical or chemical reduction of the benzoquinone under protic conditions to hydroquinone recovers the luminescence of the tris(2,2 -bipyridyl) Ru(II) unit. It is noted that the luminescence of tris(2,2 -bipyridyl) Ru(Il) itself is electroswitchable. Indeed tris(2,2 -bipyridyl) Ru(II) came to fame as a solar energy material from more humble beginnings as a luminescent redox indicator. However 55 achieves the same switching at a lower magnitude of reduction potential. Here lies the advantage of the supramolecular design. Like tris(2,2 -bipyridyl) Ru(II), many lumophores show electroswitchable luminescence. An... 

Although reduction potentials may be estimated for half-reactions, there are limits for their values that correspond to both members of a couple having stability in an aqueous system with respect to reaction with water. For example, the Na+/Na couple has a standard reduction potential of -2.71 V, but metallic sodium reduces water to dihydrogen. The reduced form of the couple (Na) is not stable in water. The standard reduction potential for the Co3 + / Co2 + couple is +1.92 V, but a solution of Co3+ slowly oxidizes water to dioxygen. In this case the oxidized form of the couple is not stable in water. The standard reduction potential for the Fe3T/Fe2+ couple is +0.771 V, and neither oxidized form or reduced form react chemically with water. They are subject to hydrolysis, but are otherwise both stable in the aqueous system. The limits for the stability of both oxidized and reduced forms of a couple are pH dependent,... 

The reduction potential for the nitrate(V)/nitrate(III) couple in acid solution of +0.94 V indicates from the limited data in Table 6.12 that nitrate(V) ion in acidic solution is a reasonably good oxidizing agent. However, nitric acid as an oxidant usually functions in a different manner, with the production of brown fumes with a metal (e.g. copper) or a metal sulfide (e.g. FeS2). The brown fumes consist of N204 (brown gas) and its monomer N02 (colourless gas). Concentrated nitric acid consists of about 70% of the acid in an aqueous solution. In such a solution there is some dissociation of the nitric acid molecules to give the nitronium ion, N02, which represents the primary oxidizing species ... 

The two positive oxidation states of P (+ 5 and + 3) are both more stable than their nitrogen equivalents, and phosphoric acid has no oxidant properties apart from those given by the hydrated protons produced from it in aqueous solution. A dilute solution of phosphoric acid will provide a sufficiently high concentration of hydrated protons to oxidize any metal to its most stable state, providing the reduction potentials for the metal ion/metal couple are negative. 

The metals of Group 11 all form + l states that vary in their stability with respect to the metallic state. The standard reduction potentials for the couples Cu+/Cu and Ag + /Ag are +0.52 V and +0.8 V, respectively. That for Au + /Au has an estimated value of + 1.62 V. The thermodynamic data for the calculation of the reduction potentials are given in Table 7.18, which also contains the calculated potentials for Cu and Ag. 

The electrochemical properties of (40)-(47) in the presence and absence of stoichiometric amounts of Na+ and K+ guest cations were investigated in acetonitile solution by cyclic voltammetry. Table VI shows that addition of alkali metal salt in 1 1 molar ratio produces anodic shifts (AE) in the original redox couple of 40-320 mV in the reduction potentials of the respective host s molybdenum redox center. Comparing (45)-(47) with the organic redox-active quinone systems described earlier (see Table I), in the case of Na+ guest cation these AE... 

Alkali metals have high oxidation-reduction potentials and low atomic masses. Thus they are attractive candidates for anodes in secondary batteries. In this context, it was shown in a couple of investigations that lithium and sodium can be electrodeposited from tetrachloroaluminate-based ionic liquids. 

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