Oxidation-reduction stoichiometry

In this chapter we will examine oxidation-reduction stoichiometry, equilibria, and the graphical representation of simple and complex equilibria, and the rate of oxidation-reduction reactions. The applications of redox reactions to natural waters will be presented in the context of a discussion of iron chemistry the subject of corrosion will provide a vehicle for a discussion of the application of electrochemical processes a presentation of chlorine chemistry will include a discussion of the kinetics of redox reactions and the reactions of chlorine with organic matter finally, the application of redox reactions to various measurement methods will be discussed using electrochemical instruments as examples. 

Study, the students are taught the basic concepts of chemistry such as the kinetic theory of matter, atomic stmcture, chemical bonding, stoichiometry and chemical calculations, kinetics, energetics, oxidation-reduction, electrochemistry, as well as introductory inorgarric and organic chemistry. They also acquire basic laboratory skills as they carry out simple experiments on rates of reaction and heat of reaction, as well as volrrmetric analysis and qualitative analysis in their laboratory sessions. 

COVALENT COMPOUNDS, METAL IONS OXIDATION-REDUCTION tion (see Cr(Vr)-dithionate, p. 287). The stoichiometry is... 

The Incentive to modify our existing continuous-flow microunit to incorporate the square pulse capability was provided by our work on perovskite-type oxides as oxidation-reduction catalysts. In earlier work, it had been inferred that oxygen vacancies in the perovskite structure played an important role in catalytic activity (3). Pursuing this idea with perovskites of the type Lai-xSrxFeg 51 10 503, our experiments were hampered by hysteresis effects which we assumed to be due to the response of the catalyst s oxygen stoichiometry to the reaction conditions. 

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. 

The catalytic effect of metal ions such as Mg2+ and Zn2+ on the reduction of carbonyl compounds has extensively been studied in connection with the involvement of metal ions in the oxidation-reduction reactions of nicotinamide coenzymes [144-149]. Acceleration effects of Mg2+ on hydride transfer from NADH model compounds to carbonyl compounds have been shown to be ascribed to the catalysis on the initial electron transfer process, which is the rate-determining step of the overall hydride transfer reactions [16,87,149]. The Mg2+ ion has also been shown to accelerate electron transfer from cis-dialkylcobalt(III) complexes to p-ben-zoquinone derivatives [150,151]. In this context, a remarkable catalytic effect of Mg2+ was also found on photoinduced electron transfer reactions from various electron donors to flavin analogs in 1984 [152], The Mg2+ (or Zn2+) ion forms complexes with a flavin analog la and 5-deazaflavins 2a-c with a 1 1 stoichiometry in dry MeCN at 298 K [153] ... 

K. Tagawa and D. F. Arnon, Oxidation-reduction potentials and stoichiometry of electron transfer in ferredoxins, Biochim. Biophys. Acta, 153, 602-613 (1968). 

Often the first step in the electrochemical characterization of a compound is to ascertain its oxidation-reduction reversibility. In our opinion, cyclic voltammetry is the most convenient and reliable technique for this and related qualitative characterizations of a new system, although newer forms of pulse polarography may prove more suitable for quantitative determination of the electrochemical parameters. The discussion in Chapter 3 outlines the specific procedures and relationships. The next step in the characterization usually is the determination of the electron stoichiometry of the oxidation-reduction steps of the compound. Controlled-potential coulometry (discussed in Chapter 3) provides a rigorously quantitative means for such evaluations. 

Twenty years ago the main applications of electrochemistry were trace-metal analysis (polarography and anodic stripping voltammetry) and selective-ion assay (pH, pNa, pK via potentiometry). A secondary focus was the use of voltammetry to characterize transition-metal coordination complexes (metal-ligand stoichiometry, stability constants, and oxidation-reduction thermodynamics). With the commercial development of (1) low-cost, reliable poten-tiostats (2) pure, inert glassy-carbon electrodes and (3) ultrapure, dry aptotic solvents, molecular characterization via electrochemical methodologies has become accessible to nonspecialists (analogous to carbon-13 NMR and GC/MS). 

Reaction Stoichiometry in Solutions Oxidation-Reduction Titrations... 

Chemical reactions are frequently carried out in solution, and their description requires modifications to the rules of stoichiometry described in Chapter 2. We illustrate these modified rules by the important analytical techniques of titration in acid-base and oxidation-reduction reactions. 

Titrations are widely used in analytical chemistry to determine acids, bases, oxidants, reductants, metal ions, proteins, and many other species. Titrations are based on a reaction between the analyte and a standard reagent known as the titrant. The reaction is of known and reproducible stoichiometry. The volume, or the mass, of the titrant needed to react essentially completely with the analyte is determined and used to obtain the quantity of analyte. A volume-based titration is shown in this figure, in which the standard solution is added from a buret, and the reaction occurs in the Erlenmeyer flask. In some titrations, known as coulometric titrations, the quantity of charge needed to completely consume the analyte is obtained. In any titration, the point of chemical equivalence, experimentally called the end point, is signaled by an indicator color change or a change in an instrumental response. 

Tagawa, K., Arnon, D.I. Oxidation-Reduction Potentials and Stoichiometry of Electron Transfer in Ferredoxins, Biochim. Biophys. Acta 153, 602 (1968)... 

Chapter 4 Types of Chemical Reactions and Solution Stoichiometry," now gives a more qualitative and intuitive method for balancing oxidation-reduction equations, which is based... 

A general feature of these reactions, in which platinum reacts with monoelectronic reagents, is to undergo oxidation-reduction with formation of labile platinum(III) intermediates. These reactions can involve reduction of platinum(IV) complexes or oxidation of platinum(II) complexes, and the stoichiometry of these two sets of reactions is shown in equation (559). ... 

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