Dissolution potentials

Similar considerations apply to oxidation. An anion which is considerably more stable than water will be unaffected in the neighbourhood of the anode. With a soluble anode, in principle, an anion only needs be more stable than the dissolution potential of the anode metal, but with an insoluble anode it must be stable at the potential for water oxidation (equation 12.4 or 12.5) plus any margin of polarisation. The metal salts, other than those of the metal being deposited, used for electroplating are chosen to combine solubility, cheapness and stability to anode oxidation and cathode reduction. The anions most widely used are SOj", Cl", F and complex fluorides BF4, SiFj , Br , CN and complex cyanides. The nitrate ion is usually avoided because it is too easily reduced at the cathode. Sulphite,... 

It was concluded from this and related works that suppression of the photodissolution of n-CdX anodes in aqueous systems by ions results primarily from specific adsorption of X at the electrode surface and concomitant shielding of the lattice ions from the solvent molecules, rather than from rapid annihilation of photogenerated holes. The prominent role of adsorbed species could be illustrated, by invoking thermodynamics, in the dramatic shift in CdX dissolution potentials for electrolytes containing sulfide ions. The standard potentials of the relevant reactions for CdS and CdSe, as well as of the sulfide oxidation, are compared as follows (vs. SCE) [68] ... 

A remarkable feature of the clusters generated by the present procedure was their unusual stability. In fact, it was found that Cu nanoclusters generated on Au(lll) surfaces presented the amazing property of remaining stable at potentials above the reversible dissolution potential for bulk Cu (Kolb et ah, 2002). 

Figure 9.24 Amounts of dissolved platinum, normalized to 1 cm and 1 mL of solution, plotted against the dissolution potentials for all the investigated surfaces. (Reproduced with permission from Komanicky et al. [2006].)...

An electroreductive Barbier-type allyla-tion of imines (434) with allyl bromide (429) also occurs inaTHF-PbBr2/Bu4NBr-(Al/Pt) system to give homoallyl amine (436) (Scheme 151) [533]. The combination of Pb(II)/Pb(0) redox and a sacrificial metal anode in the electrolysis system plays a role as a mediator for both cathodic and anodic electron-transfer processes. The metals used in the anode must have a less positive anodic dissolution potential than the oxidation potentials of the organic materials in order to be present or to be formed in situ. In addition, the metal ion plays the role of a Lewis acid to form the iminium ion (437) by associating with imine (435) (Scheme 151). 

All existing ocular endotamponades have a neglectable dissolution potential for approved drug substances. This situation could be significantly improved by... 

Therefore, the electrode will be dissolved when its dissolution potential will be lower than the potentials corresponding to any possible oxidation reaction of... 

In anodic dissolution of mercury in a solution of nitric acid, where both mercurous and mercuric salts are asumed to be completely dissociated, both the formed ions enter the solution in the ratio of their respective activities hKo+/ h1 ++ = 76. When alkali cyanide is used as electrolyte the bivalent ions formed on dissolution are predominantly consumed for the formation of the complex Hg(CN). As a result of the formation of this complex the concentration of free Hg++ jpns decreases considerably in accordance with the neghgible degree of dissociation of the above-mentioned complex, and consequently the dissolution potential of the system Hg/Hgt+ also decreases. For this reason, mercuric ions converted to mercuricyanide complex can be considered to be practically the sole product of the anodic process while the amount of univalent mercury ions is quite negligible. Contrary to this, on dissolving mercury in a solution of hydrochloric acid mercurous ions are predominantly formed due to the slight dissociation of mercurous chloride, the main product of the reaction. 

Dissolution and Deposition Potentials.—If a metal is placed in a solution of its ions a reversible electrode represented by M, is set up suppose its potential is E, Imagine now that an external source of potential is applied to this electrode so as to make it an anode of an electrolytic cell (p. 8) this will have the effect of increasing the potential, and since the electrode is reversible it will immediately commence to dissolve (cf. p. 184). It follows, therefore, that when a metallic electrode is made an anode, it wnll begin to dissolve as soon as its potential exceeds the reversible value E by an infinitesimal amount. In other words, the electrolytic dissolution potential of a metal when made an anode should be equal to its reversible (oxidation) potential (cf. p. 243) in the given electrolyte. The actual value depends, of course, on the concentration, or activity, in the solution of the ions with respect to which the metal is reversible. On the other hand, if the particular electrode under consideration is made a cathode, so that its potential is reduced below the reversible value, the reverse process, viz., deposition... 

In this section, we review approaches to predicting dissolution potentials in 6.1. Next we discuss the hydrogen evolution reaction and studies to understand it and predict new electrocatalysts in 6.2. Electrochemical oxidation reactions that are typical in fuel cells are discussed in 6.3. Finally, the studies on the increasingly important oxygen reduction reaction are reviewed in 6.4. 

Alloy stability is always of concern in heterogeneous catalysis, but in electrocatalysis there are new mechanisms for destabilizing alloys, namely electrochemical dissolution or corrosion. Greeley and Norskov developed an intuitive and simple thermodynamic framework for estimating the stability of alloy surfaces in electrochemical environments. " Their scheme is essentially an extension of an atomistic thermodynamic approach that uses chemical potentials to determine stability to one that uses electrochemical potentials to determine stability. They estimate the electrochemical potentials using total energies calculated within DFT and ideal solution behavior of the ions to consider concentration and pH effects. Within this formalism they are able to estimate the dissolution potential of metals in alloys. They further compared the trends in dissolution behavior to trends in segregation behavior and... 

Generally, in case of all tested minerals coupled with gold, it is seen that there is a certain different trend in the initial 15 minutes, then the corrosion rate decreased or became stable, relatively. This suggests that each oxide mineral has different effect on gold dissolution potentially due to the difference in conductivity and changes in the surface state. 

Secondary porosity development at greater burial depths is likely to be caused by mechanisms other than meteoric-water infl ux, which becomes exceedingly more difficult with depth and results in a decrease of the dissolution potential of such waters as they become saturated with the dissolved species along the way. In the Catalina Sandstone, secondary porosity is closely associated with sandstone-shale contacts (Fig. 21). Feldspar grains and shale clasts... 

All of the curves in Fig. 5.6 start in the active dissolution potential range and hence do not show the complete polarization curve for the iron extending to the equilibrium half-cell potential as was done in Fig. 5. 4. This extension was shown as dashed lines and the equilibrium potential was taken as -620 mV for Fe2+ = 10 6. Qualitatively, the basis for estimating how the active regions of the curves in Fig. 5.6 would be extrapolated to the equilibrium potential can be seen by reference to Fig. 4.16. There, the corrosion potential is represented as the intersection of the anodic Tafel curve and the cathodic polarization curve for hydrogen-ion reduction at several pH values. It is pointed out that careful measurements have shown that the anodic Tafel line shifts with pH (Ref 6), this shift being attributed to an effect of the hydrogen ion on the intermediate steps of the iron dissolution. 

As the potential is taken to more positive potentials, in the stripping step, initially no current will flow since the potential is still too negative to allow dissolution of the copper metal. When the dissolution potential is reached the copper metal will begin to dissolve and the current will rise exponentially. However there is only a finite limited quantity of copper metal available and as it is used up so the current must fall back to zero or in practice the base line. Thus the stripping signal takes the form of a peak as shown in Fig. 4.1a. 

However, formation of intermetallic compounds can cause problems. When metals such as copper and zinc are present in solution there is a tendency to form a Zn/Cu intermetallic compound when larger amounts are deposited at a mercury electrode. When an intermetallic compound is formed the stripping peaks for the constituent metals may be shifted, severely depressed, or even be absent altogether. When an alloy is formed at a solid electrode its dissolution potentials, in the stripping step, may be quite different to those of the constituent metals. 

The standard equihbrium dissolution potentials for nickel and iron are —0.25 and —0.44 V vs. SHE, respectively. The exchange current density for nickel dis-... 

Chapters 5 and 6 deal with systems where interaction between temperature gradient, concentration gradient and potential gradients without any barrier are involved. In these chapters, theoretical and experimental studies relating to thermal diffusion, Dufour effect, Soret effect, thermal diffusion potential, thermo-cells, precipitation and dissolution potential have been described. Physical implications of the experimental results have also been described. 

Systematic experimental and theoretical studies on generation of precipitation and dissolution potentials have been undertaken by Rastogi and co-workers [22, 25-27]. 

Figure 5.5. (a) Schematic diagram of cell for recording the dissolution potential of electrolytes A, B, platinum electrodes a, solid phase (electrolyte embedded on the electrode undergoing dissolution) 3, layer of solution in the immediate vicinity of the solid phase 8, solvent phase, (b) Schematic diagram of ceU for the measurement of precipitation potentials A, B, platinum electrodes a, precipitating solid electrolyte ss, super-saturated solution of electrolyte. 

The experimental set-up used for the purpose of recording precipitation and dissolution potentials are schematically recorded (Fig. 5.5a and b). 

It has been shown that precipitation or dissolution potential = observed potential -Nernst potential - diffusion potential - phase potential - thermo-emf. 

Each type of potential has been measured using elaborate experimental set-up. For uni-univalent electrolytes like KCI, KBr and KI, the estimated values of precipitation and dissolution potentials are found to be in the range of 20-100mV with a negative sign. Experimental values of some typical electrolytes are recorded in Table 5.1. 

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