Oxidation-reduction reactions solution composition

In natural waters occur not one but several oxidation-reduction reactions. These reactions are associated with the presence of several elements, which are capable of changing their charge, and run in parallel. For this reason, total oxidation potential of the solution is defined by the nature and concentration of all redox-couples. Components which noticeably affect the solution s oxidation-reduction potential are called electroactive. Elements whose concentration and form of existence actually control solution s oxidation are culled potential-setting. In natural waters these are usually O, S, C, N and Fe. The medium whose oxidation potential value almost does not change with the addition of oxidizers or reducers is called redox-buffers. The redox-buffer may be associated with composition of the water itself, of its host rocks or with the effect of atmosphere. In the subsurface redox-buffers are associated, as a rule, with the content of iron, sulphur or manganese. Stably high Eh value in the surface and ground waters is caused by the inexhaustible source of in the atmosphere. 

The Composition of Solutions Dilution Acid-Base Titrations Oxidation States Method of Balancing Oxidation-Reduction Reactions... 

Table 23-2 lists ihe various types of ion-selective membrane electrodes that have been developed. These differ in the physical or chemical composition of the membrane. The general mechanism by which an ion-sclective potential develops in these devices depend.s on the nature of the membrane and is entirely different from the source of potential in metallic indicator electrodes. We have seen that the potential of a metallic electrode arises from the tendency of an oxidation-reduction reaction to occur at an electrode surface. In membrane electrodes, in contrast. Ihe observed potential is a kind of junction potential that develops across a membrane thal separates the anidyte solution from a reference solution. 

Coulometry measures the amount of cunent flowing dirough a solution in an electrochemical oxidation or reduction reaction and is capable of measuring at ppm or even ppb levels of reactive gases. Thus a sample of ambient air is drawn through an electrolyte in a cell and the required amount of reactant is generated at the electrode. This technique tends to be non-specific, but selectivity can be enhanced by adjustment of pH and electrolyte composition, and by incorporation of filters to remove interfering species. 

The standard reversible potential is that listed in the EMF series of Table 1 and represents a special case of the Nernst equation in which the second term is zero. The influence of the solution composition manifests itself through the logarithmic term. The ratio of activities of the products and reactants influences the potential above which the reaction is thermodynamically favorted toward oxidation (and conversely, below which reduction is favored). By convention, all solids are considered to be at unit activity. Activities of gases are equal to their fugacity (or less strictly, their partial pressure). 

Where might this be important As discussed above, biological activity can result in the simultaneous precipitation of mixtures of nanoscale sulfide minerals under certain conditions. Each mineral will exhibit a particular particle size distribution, dependent on the solution composition, bacterial activity, rate of crystal growth, and the nature of electrochemical interactions between the particles. These electrochemical reactions could lead to oxidation of one type of nanophase sulfide mineral of a certain size, and reduction of another type of nanophase sulfide particle or other species in the solution. In this way, a tremendous number of mineral-solution-mineral galvanic cells could develop, with potentially significant impact on dissolution kinetics, growth kinetics, and the mixture of phases observed. In addition to environmental relevance, these processes may shape the mineralogy of low-temperature ore deposits. 

In this chapter we will consider in detail acid-base reactions using relevant examples from water treatment processes and natural water chemistry to illustrate the various principles. Special attention will be given to techniques for calculating the composition of solutions of acids and bases. The chapters that follow and deal with precipitation, coordination chemistry, and oxidation-reduction chemistry will make use of many of the principles developed in this chapter. 

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