Equilibrium oxidation-reduction reactions

Several types of reactions are commonly used in analytical procedures, either in preparing samples for analysis or during the analysis itself. The most important of these are precipitation reactions, acid-base reactions, complexation reactions, and oxidation-reduction reactions. In this section we review these reactions and their equilibrium constant expressions. 

It is now possible to calculate the equilibrium constants of oxidation-reduction reactions, and thus to determine whether such reactions can find application in quantitative analysis. Consider first the simple reaction ... 

Table 9.3 can be used to calculate the point of equilibrium in an oxidation-reduction reaction. 

A production process has evolved from this original work, and is presently used for extracting americium from kilogram amounts of plutonium metal. This process is based upon equilibrium partitioning (by oxidation-reduction reactions) of americium and plutonium between the molten chloride salt and the molten plutonium phase. The chemistry of this process is indicated by the following reactions ... 

Analytical methods based upon oxidation/reduction reactions include oxidation/reduction titrimetry, potentiometry, coulometry, electrogravimetry and voltammetry. Faradaic oxidation/reduction equilibria are conveniently studied by measuring the potentials of electrochemical cells in which the two half-reactions making up the equilibrium are participants. Electrochemical cells, which are galvanic or electrolytic, reversible or irreversible, consist of two conductors called electrodes, each of which is immersed in an electrolyte solution. In most of the cells, the two electrodes are different and must be separated (by a salt bridge) to avoid direct reaction between the reactants. 

Oxidation-reduction reactions may affect the mobility of metal ions by changing the oxidation state. The environmental factors of pH and Eh (oxidation-reduction potential) strongly affect all the processes discussed above. For example, the type and number of molecular and ionic species of metals change with a change in pH (see Figures 20.5-20.7). A number of metals and nonmetals (As, Be, Cr, Cu, Fe, Ni, Se, V, Zn) are more mobile under anaerobic conditions than aerobic conditions, all other factors being equal.104 Additionally, the high salinity of deep-well injection zones increases the complexity of the equilibrium chemistry of heavy metals.106... 

Material balances can be written for moieties which are conserved during the reaction, such as the atoms of a particular element or the total charge, or for reactant or product species if the stoichiometry is unambiguous. Oxidation-reduction reactions may be particularly troublesome. In the following situation, for example, one cannot write a material balance relating protons to water molecules. Consider the oxidation of O2 to H2O and the equilibrium dissociation of I O. 

There existed oxidation-reduction reactions with the same reaction speed on the sulphide mineral surface in water. One is the self-corrosion of sulphide mineral. Another is the reduction of oxygen. If the equilibrium potential for the anodic reaction and the cathodic reaction are, respectively, E and, and the mineral electrode potential is E, the relationship among them is as follows ... 

Thus, if we start with our reactants and products under standard conditions and allow the reaction to proceed to equilibrium, an amount of energy AG° becomes available for external work. In the context of doing external electrical work, an oxidation—reduction reaction can generate a standard electromotive force AE° given by... 

How does the anionic alkyl of the original trialkylaluminum or of the dialkylaiuminum chloride, which has sufficient anionic character to undergo anionic hydride exchange or CH3OT reaction, form a catalyst which becomes cationic under certain polymerization conditions No studies of this have been reported. One possibility is an internal oxidation-reduction reaction that converts an anionic alkyltitanium trichloride to a cationic alkyltitanium trichloride (Equation 10). Basic and electrophilic catalyst components would determine the relative contributions of the anionic and cationic forms. This type of equilibrium or resonance structures could also explain the color in transition metal compounds such as methyltitanium trichloride (73). 

Because of the bulk of comparable material available, it has been possible to use half-wave potentials for some types of linear free energy relationships that have not been used in connection with rate and equilibrium constants. For example, it has been shown (7, 777) that the effects of substituents on quinone rings on their reactivity towards oxidation-reduction reactions, can be approximately expressed by Hammett substituent constants a. The susceptibility of the reactivity of a cyclic system to substitution in various positions can be expressed quantitatively (7). The numbers on formulae XIII—XV give the reaction constants Qn, r for the given position (values in brackets only very approximate) ... 

Standard half-cell potentials can be used to compute standard cell potentials, standard Gibbs free energy changes, and equilibrium constants for oxidation-reduction reactions. 

This chapter mainly focuses on the reactivity of 02 and its partially reduced forms. Over the past 5 years, oxygen isotope fractionation has been applied to a number of mechanistic problems. The experimental and computational methods developed to examine the relevant oxidation/reduction reactions are initially discussed. The use of oxygen equilibrium isotope effects as structural probes of transition metal 02 adducts will then be presented followed by a discussion of density function theory (DFT) calculations, which have been vital to their interpretation. Following this, studies of kinetic isotope effects upon defined outer-sphere and inner-sphere reactions will be described in the context of an electron transfer theory framework. The final sections will concentrate on implications for the reaction mechanisms of metalloenzymes that react with 02, 02 -, and H202 in order to illustrate the generality of the competitive isotope fractionation method. 

When a biochemical half-reaction involves the production or consumption of hydrogen ions, the electrode potential depends on the pH. When reactants are weak acids or bases, the pH dependence may be complicated, but this dependence can be calculated if the pKs of both the oxidized and reduced reactants are known. Standard apparent reduction potentials E ° have been determined for a number of oxidation-reduction reactions of biochemical interest at various pH values, but the E ° values for many more biochemical reactions can be calculated from ArG ° values of reactants from the measured apparent equilibrium constants K. Some biochemical redox reactions can be studied potentiometrically, but often reversibility cannot be obtained. Therefore a great deal of the information on reduction potentials in this chapter has come from measurements of apparent equilibrium constants. 

Carbon isotopes are fractionated in biological cycles and in inorganic oxidation-reduction reactions. In equilibrium systems of various oxidation states, 13C enrichment up to 500 °K occurs in the following sequence ... 

Natural waters are often in a highly dynamic state with regard to oxidation-reduction rather than in or near equilibrium. Most oxidation-reduction reactions have a tendency to be much slower than acid-base reactions, especially in the absence of suitable biochemical catalysis. Nonetheless, equilibrium diagrams can greatly aid attempts to understand the possible redox patterns in natural waters and in water technological systems. 

The work described in the foregoing sections is of a preliminary nature. Nevertheless, it offers hope that experimental scales of free hydrogen ion concentration (pcn or pmn) in seawater may be feasible. One need not know pmn or pan to derive meaningful equilibrium data, such as acid-base ratios and solubilities, provided that suitable apparent equilibrium constants are chosen (7). In these cases, the unit selected for the acidity scale disappears by cancellation. Nevertheless, the acidity of seawater is a parameter of broader impact. It plays a role, for example, in the kinetics of organic oxidation-reduction reactions and in a variety of quasi-equilibrium processes of a biological nature. The concentration of free hydrogen ions is clearly understood, and its role in these complex interactions is more clearly defined than that of a quantity whose unit purports to involve the concept of a single-ion activity. 

As you know, oxidation-reduction reactions can involve molecules, ions, free atoms, or combinations of all three. To make it easier to discuss redox reactions without constantly specilying the kind of particle involved, chemists use the term species. In chemistry, a species is any kind of chemical unit involved in a process. For example, a solution of sugar in water contains two major species. In the equilibrium equation NH3 + H2O NH/ + OH , there are four species the two molecules NH3 and H2O and the two ions NH/ and OH. ... 

---