Standard reduction potentials limitations

FIGURE3.7 The potential window for the redox chemistry of life. Redox chemistry in living cells is approximately limited by the standard potentials for reduction and oxidation of the solvent water at neutral pH. Approximate standard reduction potentials are also indicated for the commonly used oxidant ferricyanide and reductants NADH and dithionite. 

Figure 2.91 Schematic representation of the rotating disc electrode response Tor the reduction and oxidation of a reversible couple. / = F/RT, /, is the limiting current, / is the current, E is the potential of the electrode and ° is the standard reduction potential of the couple.

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,... 

Substituent or solvent effects may be similar for concerted and stepwise processes. It has been shown that provided the rates of reverse reactions are almost independent of changes in oxidation potential, plots of E°, the standard reduction potential for the half cell (8) against log kf for a series of acceptors, Ox +, reacting with a hydride donor must have a slope of 30 mV/ log unit whether the rate-limiting step is hydride transfer, or hydrogen-atom transfer, or electron transfer (Kurz and Kurz, 1978). 

These reactions are typically limited to the halogens. The procedure for predicting the outcome of these reactions is the same as for the metals. The halogens also appear on the table of standard reduction potentials, but for reasons we will discuss in Chapter 18, the halogens get more reactive as you go up the table of reduction potentials. An easy way to remember the reactivities of the halogens is they are less reactive going down the group (as atomic number increases). 

Fig. 1. Reduction potential E (referenced to the standard hydrogen electrode) versus pH for various species of vanadium. Boundary lines correspond to E, pH values where the species in adjacent regions are present in equal concentrations. The short dashed lines indicate uncertainty in the location of the boundary. The upper and lower long dashed lines correspond to the upper and lower limits of stability of water. Standard reduction potentials are given by the intersections of horizontal lines with the abscissa pH = 0. The half reactions are 02 + 4H+ + 4e = 2H20, E° = 1.23V V02+ + 2H+ + e = V02+ + H20, E° = 1.0V V02+ + 2H+ + e = V3+ + H,0, E° = 0.36V 2H+ + 2e = H2, E° = 0.0V and V3+ + e = V2+, E° = -0.25V. V2+ is therefore a strong reductant. Air oxidation of V02+ presumably proceeds by the reaction 4V02t + 02 + 2H20 = 4VOJ + 4H+, E° = 0.23V which is favored at higher pH. Not all known species are represented on this diagram. Reproduced with permission from Ref. 30...

The Tl -Tl relationship is therefore a dominant feature of thallium chemistry. The standard reduction potentials at 25 °C and unit activity of H+ are TIVtI = —0.336 V, T1 /T1 = +0.72 V, and Tl /Tli = +1.25V. Estimates have also been made for the couples T1 /T1 = +0.33 V and Tl /Tl = 2.22 V. The generally valid limitations concerning the use of standard electrode potentials to predict the redox chemistry of real systems are especially important in the case of thallium factors such as complex formation in the presence of coordinating anions or neutral ligands and pH dependence due to hydrolysis do affect the actual or formal redox potentials. For example, redox potentials have been measmed for TICI/TICI3 =+0.77 V in IM HCl and T10H/T1(0H)3 = —0.05 V in alkaline soluhon. These formal potentials differ from the standard value for Tiin/Tii = +1.25 V. The difference can be attributed to the substanhal difference between the complex forming abilities of Tl and Tl , which will be discussed in detail later. The... 

In practice there are several limitations to such measurement. Obviously it implies that both members of the half-reaction are sufficiently stable for a cell to be realized. This is a serious difficulty in organic chemistry owing to usual great reactivities of the species formed upon electron transfers. For the most frequent cases it is then impossible to rely on reversible thermodynamic transformations to determine experimental values of standard reduction potentials. However, these important figures, or at least very precisely approximated values, can be obtained from current intensity potential curves or transient electrochemical methods as is discussed in a later section. 

The evidence for H (aq) as a real entity is extremely limited, and estimates of the standard reduction potential of H atoms are highly uncertain the most recent estimate, however, implies that the hydrogen atom should be a fairly strong simple one-electron oxidant E(H/H ) 0.83 V.108 This implies that it may be rather difficult to... 

Though accelerating effect of redox mediators is proved, differences in electrochemical factors between mediator and azo dye is a limiting factor for this application. It was reported that redox mediator applied for biological azo dye reduction must have redox potential between the half reactions of the azo dye and the primary electron donor [37], The standard redox potentials for different azo dyes are screened generally between -430 and -180 mV [47],... 

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