Mixed potential principle

In the case of mineral-mineral interactions, a mineral with higher potential acts as a cathode, while a mineral with lower potential acts as an anode. For a multiple mineral/grinding media(steel)system. The galvanic interactions become more complex than the two-electrode systems. The galvanic reactions among multielectrode systems are also governed by the mixed potential principle as shown in an example of polarization curves involving pyrite, pyrrhotite and mild steel in Fig. 1.9 (Pozzo and Iwasaki, 1987). 

Although there have been a lot of investigations on the interactions of sulphide minerals with thio-collectors in terms of the mixed potential principle, there are still much controversy about the products formed on a sulphide mineral in the presence of a collector in different conditions. In the following sections, the effects of potential on the flotation and formation of surface products of many kinds of sulphide minerals will be introduced based on the results of flotation, electrochemical measurement, surface analyses and thermodynamic calculations. 

Multiple Reactions With multiple reactions, the total current is the sum of the currents from the individual reactions with anodic currents positive and cathodic currents negative. This is called the mixed potential principle. For more details see Bard and Faulkner, Electrochemical Methods Fundamentals and Applications, 2d ed., Wiley, 2001. 

Ever since the mixed potential model has been proposed, the interaction mechanism between thio-collector and sulphide minerals has been usually explained on the basis of this model. The principle of the mixed potential model can be schematically shown in Fig. 4.1. Here, E respectively... 

Electrochemists will recognize that the ECP is a mixed potential, the value of which is determined by the balance of the oxidizing and reducing species in the environment and the kinetics of dissolution (corrosion) of the substrate. In order to calculate the ECP, it is important, in principle, that the concentrations of all of the radiolytic species be determined, since all of these species are electroactive. However, theory shows that the contribution that any given species makes to the ECP is determined primarily by its concentration, so that only the most prevalent electroactive species in the system determine the ECP. This is a fortunate finding, because the various radiolysis models that are available for calculating the species concentrations do not determine the concentrations of the minor species accurately nor are there electrochemical kinetic data available for these species. 

The successful development of electrochemical processes for some of the cited reactions requires an understanding not only of surface interactions but also of the changes in activity and selectivity that may incur from cell design, mixing, potential or current distribution, and transport processes. Here, we shall examine some simple reaction engineering principles that would be helpful for elucidating these effects. 

This section analyzes electrochemical sensors for CO2 detection. The two main categories of electrochemical sensing principles are reviewed (amper-ometric and potentiometric) and among potentiometric sensors, special attention is given to mixed potential sensors, because carbon monoxide sensors belong to this group. The influence of the materials and fabrication processes in the sensor performance is also evaluated. 

Cathodic protection (CP) is defined as the reduction or elimination of corrosion by making the metal a cathode by means of impressed current or sacrificial anode (usually magnesimn, aluminum, or zinc) [11]. This method uses cathodic polarization to control electrode kinetics occurring on the metal-electrolyte interface. The principle of cathodic protection can be explained by the Wagner-Traud mixed potential theory [12]. 

Figure 15.4 illustrates the origin of the corrosion potential and also the principles of cathodic and anodic protection for a single oxidation reaction (M M ) and a single reduction reaction (H H2) occurring at the metal surface (the dashed lines represent the current-potential behavior of the reverse reactions and are not important to the present discussion). Because charge balance must be maintained, the potential is pinned at a value, Ecom where the cathodic current and the anodic current are equal (i.e., where the two curves intersect). This corrosion potential (Ecorr) is called a mixed potential, as it is determined by a mixture of two (sometimes more) electrochemical reactions. The anodic current (also the cathodic current, as they are equal) at this potential is the corrosion current (torr)- It is important to note that E orr and icorr are influenced by both the thermodynamics of the two reactions, manifested by the equilibrium potentials E(h+/h2) and E(m/m+)> and by the kinetics of the two reactions, manifested by the exchange current densities io(H+/h2) and o(m/m+)> and by the slopes of the two linear curves (the Tafel slopes). 

Models with simplified geometries embodying descriptions of corrosion phenomena based on first principles, as well as existing measured and calculated data for corrosion parameters, are commercially available. Some codes incorporate mixed potential models that are used for the prediction of corrosion potential and current density. Fundamental concepts are used but calibration with experimental data is frequently required in order to estimate values for poorly known model parameters. 

Fig. 2 Basic principles of solid electrolyte cells Left oxygen sensor (Nemst sensor), Right combustible sensor (mixed potential sensor) [2]...

This chapter is coniined to analyze the complex aqueous corrosion phenomaion using the principles of mixed-potential, which in turn is related to the mixed electrode electrochemical corrosion process. This theory has been introduced in Chapter 3 and 4 as oxidation and reduction electrochemical reactions. Basically, this Chapter is an extension of the principles of electrochemistry, in which partial reactions were introduced as half-cell reactions, and their related kinetics were related to activation and concentration polarization processes. The principles and concepts introduced in this chapter represent a unique and yet, simplified approach for understanding the electrochemical behavior of corrosion (oxidation) and reduction reactions in simple electrochemical systems. 

In order to construct mixed-potential diagrams to model a corrosion situation, one must first gather (1) the information concerning the activation overpotential for each process that is potentially involved and (2) any additional information for processes that could be affected by concentration overpotential. The following examples of increasing complexity will illustrate the principles underlying the construction of mixed-potential diagrams. 

The following sections go through the development of detailed equations and present some examples to illustrate how mixed-potential models can be developed from first principles. 

The electrochemical behavior of heterometallic clusters has been reviewed clsewbcre."" The interest in examining clusters stems from their potential to act as "electron sinks " in principle, an aggregate of several metal atoms may be capable of multiple redox state changes. The incorporation of heterometals provides the opportunity to tune the electrochemical response, effects which should be maximized in very mixed"-metal clusters. Few very mixed -metal clusters have been subjected to detailed electrochemical studies the majority of reports deal with cyclic voltammetry only. Table XII contains a summary of electrochemical investigations of "very mixed"-metal clusters. 

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