Metal/ion electrode

Reference electrodes are used in the measurement of potential [see the explanation to Eq. (2-1)]. A reference electrode is usually a metal/metal ion electrode. The electrolyte surrounding it is in electrolytically conducting contact via a diaphragm with the medium in which the object to be measured is situated. In most cases concentrated or saturated salt solutions are present in reference electrodes so that ions diffuse through the diaphragm into the medium. As a consequence, a diffusion potential arises at the diaphragm that is not taken into account in Eq. (2-1) and represents an error in the potential measurement. It is important that diffusion potentials be as small as possible or the same in the comparison of potential values. Table 3-1 provides information on reference electrodes. 

The standard equUibrium potential of a metal/metal ion electrode is shifted to cathodic potentials with decreasing particle size. Taking a compact metal electrode as reference, the change in potential for a microelectrode of radius r is... 

A metal/metal-ion electrode consists of a metal immersed in a solution containing ions of the metal. The electrode potential of this electrode depends on the concentration (more exactly, the activity) of the metal ions in solution. An example is Cu immersed in a CUSO4 solution, Cu/Cu... 

In the general case of a metal/metal-ion electrode, a metal M is in an equilibrium with... 

RedOx electrode potentials are the result of an exchange of electrons between metal and electrolyte. In Section 5.4 we have shown that the metal/metal-ion electrode potentials are the result of an exchange of metal ions between metal and electrolyte. In the RedOx system the electrode must be made of an inert metal, usually platinum, for which there is no exchange of metal ions between metal and electrolyte. The electrode acts as a source or sink for electrons. The electrolyte in the RedOx system contains two substances electron donors (electron-donating species) and electron acceptors (electron-accepting species). One example of a RedOx system is shown in Figure 5.4. In this case the electron donor is Fe ", the electron acceptor is Fe , the electrode is Pt, and the electrode process is... 

According to Nernst s equation, there should be a linear relationship between the equilibrium potential of the metal/metal-ion electrode (M/M ) and the logarithm of the concentration of ions [Eq. (5.13)]. This linear relationship was observed experimentally for a low concentration of the solute MA (e.g., 0.01 mol/L and lower). For higher concentrations, a deviation from linearity was observed (see, e.g.. Fig. 5.12). The deviation from linearity is due to ion-ion interactions. In the example in Figure 5.12, the ion-ion interactions include interaction of the hydrated Ag ions with one... 

Processes at metal/metal-ion electrodes include crystallization partial reactions. These are processes by which atoms are either incorporated into or removed from the crystal lattice. Hindrance of these processes results in crystallization overpotential The slowest partial reaction is rate determining for the total overall reaction. However, several partial reactions can have low reaction rates and can be rate determining. 

Figures 6.5 and 6.6 illustrate the RedOx and the metal/metal-ion electrodes in a nonequilibrium state, respectively. When current flows through an electrode, its potential A(/>(/) deviates from the equilibrium value A(/>eq by the amount ry, which was defined by Eq. (6.1a). Thus, the nonequilibrium potential difference A(p in Eqs. (6.33) and (6.34) can be substituted by the nonequilibrium value A(/)(/) ...

Figure 6.6. Metal/metal-ion electrode deviates from the equilibrium potential two currents flowing in opposite directions.

It should be remembered that in the case of a metal/metal ion electrode, the current can only be mass-transport limited in the cathodic direction. For net anodic currents, an accumulation of metal ions occurs at the electrode surface. The anodic current can greatly exceed the limiting cathodic current (Fig. 2.17), and under these circumstances equations can be simplified to give... 

The other group comprises silver-silver halide electrodes, mercury pools, metal-metal-ion electrodes, and others normally prepared In the solvent used for the compound being studied (and often, indeed, employed as internal "reference" electrodes). For such an electrode, the abbreviation alone signifies that the solvent was the same throughout the cell, while the symbol "(w)" for ("water") following the abbreviation signifies that the reference electrode was prepared with water and used as an external reference electrode. 

Equality of i and i on an atomic scale means that a constant exchange of charge carriers (electrons or ions) takes place across the metal-solution interphase. Figure 6.3 illustrates a RedOx electrode at equilibrium. Figure 6.4 illustrates a metal/ metal-ion electrode at equilibrium. 

Metal-metal ion electrode Here, a metal is in equilibrium with its ion in solution. Metals such as Cu or Zn have an appreciable tendency to dissolve and form ions in water, but little tendency to react with water. 

The number of reversible metal-metal ion electrodes is limited so that the accurate direct potentiometric measurement of the activity of a metal ion with an electrode of the same metal usually is not feasible, except perhaps with the Ag/Ag,(OH2)4 system. However, a number of metal ion-metal half-reactions are sufficiently reversible to give a satisfactory potentiometric titration with a precipitation ion or complexing agent. These couples include Cuu(OH2>6+/Cu, Pbn(OH2>4+/Pb, Cdu(OH2)l+/Cd, and Znn(OH2)i+/Zn. However, all these metals can be determined by EDTA titration and the mercury electrode that is described in the preceding section. 

In the example of the Zn/Cu cell we have been using, the electrode reaction involves a metal and its hydrated cation we call such electrodes metal-metal ion electrodes. There are a number of other kinds of electrodes which are widely encountered in electrochemistry and analytical chemistry. 

The very fast metal-metal ion electrode processes, for which the exchange current density is very high. At steady state the overall rates of those electrode processes are controlled by the rates of mass transfer of the electroactive components to and from the electrode-melt interface. 

A metal (Me) acts as an electrode in the electrolyte containing its ions. The metal is said to be electroactive according to Me <-> Me"+ + n c. An example is the Ag/ Ag+ electrode represented in Figure 3.1.3 (i.e. a silver metal electrode immersed in an electrolyte containing silver ions). The Zn/Zn++ electrode is another metal/ metal ion electrode. It is widely used in batteries. 

Corrosion — Corrosion current density — Figure. Polarization curves of a metal/metal ion electrode and the H2/H+ electrode including the anodic and cathodic partial current curves, the Nernst equilibrium electrode potentials E(Me/Mez+) and (H2/H+), their exchange current densities / o,M> o,redox and related overpotentials Me) and 77(H), the rest potential r, the polarization n and the corrosion current density ic at open circuit conditions (E = Er) [i]... 

Passivation — Metals usually dissolve in acidic electrolytes when their electrode potential becomes more positive than the value of the related -> Nernst equilibrium potential of the metal/metal ion electrode. The dissolution current density increases exponentially with... 

Me/Me + is the Nemst equilibrium potential of the Me/Me metal ion electrode, denotes the standard potential of the Me/Me metal ion electrode, and is the activity of Me jy ions in the electrolyte. For a pure 3D Me bulk phase, flMe is equal to unity. 

Figure 2 illustrates the metal/metal-ion electrode in nonequilibrium state. From Eqs (16-18), we get the Butler-Volmer... 

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