Electrochemistry under standard conditions

The combination of hydrogen gas, H3 O ions, and a platinum electrode is referred to as a hydrogen electrode. This electrode appears in the right-hand portion of Figure 19-8. When a hydrogen electrode operates under standard conditions, PH2 — 1-00 bar and H3 O ] — 1.00 M, it is a standard hydrogen electrode (SHE). The standard hydrogen electrode is particularly important in electrochemistry, as we describe in Section 19-1. 

Perhaps the most common example of overvoltage encountered in electrochemistry is that needed to reduce H" " ions at a mercury electrode. On a catalytic Pt surface (platinized Pt, which is a large surface-area, black Pt deposit on Pt metal) the H2/H ion couple is said to behave reversibly. This means that one can oxidize hydrogen gas, or reduce H ions, at the standard reduction potential, 0.00 V, under standard conditions. At a mercury... 

We have seen how to calculate the emf of a cell when the reactants and products are under standard conditions. As a voltaic cell is discharged, however, reactants are consumed and products are generated, so concentrations change. The emf progressively drops until = 0, at which point we say the cell is dead. In this section we examine how the emf generated under nonstandard conditions can be calculated by using an equation first derived by Walther Nernst (1864—1941), a German chemist who established many of the theoretical foundations of electrochemistry. 

For the study of electrochemistry and corrosion one must be able to compare the equilibrium potentials of different electrode reactions. To these ends, by convention, a scale of standard electrode potentials is defined by arbitrarily assigning the value of zero to the equilibrium potential of the electrode (2.42), under standard conditions (Pfj2 = 1 bar =1.013 atm, T = 298 K, au+ = 1) ... 

Other Coordination Complexes. Because carbonate and bicarbonate are commonly found under environmental conditions in water, and because carbonate complexes Pu readily in most oxidation states, Pu carbonato complexes have been studied extensively. The reduction potentials vs the standard hydrogen electrode of Pu(VI)/(V) shifts from 0.916 to 0.33 V and the Pu(IV)/(III) potential shifts from 1.48 to -0.50 V in 1 Tf carbonate. These shifts indicate strong carbonate complexation. Electrochemistry, reaction kinetics, and spectroscopy of plutonium carbonates in solution have been reviewed (113). The solubiUty of Pu(IV) in aqueous carbonate solutions has been measured, and the stabiUty constants of hydroxycarbonato complexes have been calculated (Fig. 6b) (90). 

If a solution forms part of an electrochemical cell, the potential of the cell, the current flowing through it and its resistance are all determined by the chemical composition of the solution. Quantitative and qualitative information can thus be obtained by measuring one or more of these electrical properties under controlled conditions. Direct measurements can be made in which sample solutions are compared with standards alternatively, the changes in an electrical property during the course of a titration can be followed to enable the equivalence point to be detected. Before considering the individual electrochemical techniques, some fundamental aspects of electrochemistry will be summarized in this section. 

The standard picture of slip predicts an increase of frequency upon bubble formation. This has repeatedly been observed. " Tsionski et al. went through a particularly careful study, which yielded a negative result these authors did not see a frequency shift under conditions where they expected the presence of nanobubbles on the basis of their knowledge of the electrochemistry going on in the cell.55... 

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