Cathode half-cell

In potentiometry, the concentration of analyte in the cathodic half-cell is generally unknown, and the measured cell potential is used to determine its concentration. Thus, if the potential for the cell in Figure 11.5 is measured at -1-1.50 V, and the concentration of Zn + remains at 0.0167 M, then the concentration of Ag+ is determined by making appropriate substitutions to equation 11.3... 

The explicit aims of boiler and feed-water treatment are to minimise corrosion, deposit formation, and carryover of boiler water solutes in steam. Corrosion control is sought primarily by adjustment of the pH and dissolved oxygen concentrations. Thus, the cathodic half-cell reactions of the two common corrosion processes are hindered. The pH is brought to a compromise value, usually just above 9 (at 25°C), so that the tendency for metal dissolution is at a practical minimum for both steel and copper alloys. Similarly, by the removal of dissolved oxygen, by a combination of mechanical and chemical means, the scope for the reduction of oxygen to hydroxyl is severely constrained. 

Overcharge tests were carried out in LiCo02 cathode half-cells that contained these additives, and a redox shuttle effect was observed between 4.20 and 4.30 V, close to the redox potentials of these additives. The same shuttling effect was observed even after 2 months of storage for these cells, indicating the stability and redox reversibility of these additives. A closer examination of the capacity retention revealed that 4-bromo-l,2-dimethoxybenzene seemed to have the best shuttle-voltage performance for the 4.0 V lithium cell used." The stability of these additives against reductive decomposition was also tested by the authors on metallic lithium as well as on carbonaceous anodes, and no deterioration was detected. 

Figure 76. Cathodic stability on graphitic anode Differential capacity vs voltage plots for anode and cathode half-cells containing different concentrations of (a) TMP, (b) TEP, and (c) HMPN. Only the first cycles are shown. Concentrations are indicated in the graph with the Cou-lombic efficiency for each cycle in the parentheses. (Reproduced with permission from ref 526 (Eigure 2). Copyright 2002 The Electrochemical Society.)...

Table 2. Cathodic half-cell voltage for a cobalt phthalocyanine electrode 1-a)...

Discharge curves for typical cathodic half-cells are shown in Fig. 2.23. In curve (a) reactants and products are both in liquid or solid solution, while curve (b) represents the case where they form two distinct solid phases. In Fig. 2.24, a two-stage discharge takes place, where all three electroactive components form separate solid phases and the electrolytic phase remains virtually invariant, e.g. 

Fig. 2.23 Discharge curves for typical cathodic half-cells, (a) Reactants and products in solution phase, e,g, Fe3 (aq) and Fe2+(aq) (b) reactants and products both form solid phases, e,g. Ag20(s) and Ag(s)...

Aqueous Corrosion. Several studies have demonstrated that ion implantation may be used to modify either the local or generalized aqueous corrosion behavior of metals and alloys (119,121). In these early studies metallic systems have been doped with suitable elements in order to systematically modify the nature and rate of the anodic and/or cathodic half-cell reactions which control the rate of corrosion. 

The shorthand for the cathode half-cell includes both reactant (Fe3+) and product (Fe2+) as well as the electrode (Pt). The two ions Fe3+(aq) and Fe2+(aq) are separated by a comma rather than a vertical line because they are in the same phase. 

The cell consists of a platinum wire anode dipping into an Sn2+ solution—say, Sn(N03)2(fl(j)—and a silver cathode dipping into an Ag+ solution—say, AgNC>3(aq). As usual, the anode and cathode half-cells must be connected by a wire and a salt bridge containing inert ions. 

Since the anode and cathode half-cell potentials must sum to give the overall cell potential, the E° value for oxidation of Zn to Zn2+ must be 0.76 V and the standard reduction potential for the Zn2+/Zn half-cell is therefore -0.76 V. 

Benzophenone hydrazone (5.88 g, 20 mM) was dissolved in methylene chloride (20 ml) and over-layered with 1 M sodium hydroxide (40 ml) containing, as phase transfer catalyst, tetrabutylammonium sulfate (0.68 g) and sodium iodide (300 mg). The cathode half cell contained 1 M sodium hydroxide (60ml). The whole cell was cooled to 0°C, the anode compartment stirred and electrolysed at a current of 50 mA. Formation of DDM was followed using the DDM absorption peak at 525 nm. The chart obtained was as shown in Figure 2. 

To maintain electrical neutrality in both compartments, positive ions (Zn2+ and Na+) migrate through the salt bridge from the anode half-cell to the cathode half-cell and negative ions (N03 ) migrate in the opposite direction. 

D. To permit positive ions to flow from the cathode half-cell to the anode half-cell... 

D The anode receives electrons from the oxidation half-reaction (choice A) and the circuit conducts electron flow (choice C) to the cathode which supplies electrons for the reduction half-reaction. This flow of electrons from the anode to the cathode is relieved by a flow of ions through the salt bridge from the cathode to the anode (answer D). The salt bridge relieves the buildup of positive charge in the cathode half-cell (choice B is incorrect). 

Finally, there is the issue of ion movement. The cations in the salt bridge will move toward the cathode half cell while the anions in the salt bridge... 

B Cations in the salt bridge will migrate to the cathode half cell of the voltaic cell. 

Under open circuit conditions, the PEVD system is in equilibrium after an initial charging process. The equilibrium potential profiles inside the solid electrolyte (E) and product (D) are schematically shown in Eigure 4. Because neither ionic nor electronic current flows in any part of the PEVD system, the electrochemical potential of the ionic species (A ) must be constant across both the solid electrolyte (E) and deposit (D). It is equal in both solid phases, according to Eqn. 11, at location (II). The chemical potential of solid-state transported species (A) is fixed at (I) by the equilibrium of the anodic half cell reaction Eqn. 6 and at (III) by the cathodic half cell reaction Eqn. 8. Since (D) is a mixed conductor with non-negligible electroific conductivity, the electrochemical potential of an electron (which is related to the Eermi level, Ep) should be constant in (D) at the equilibrium condition. The transport of reactant... 

Each electrode reaction, anode and cathode, or half-cell reaction has an associated energy level or electrical potential (volts) associated with it. Values of the standard equilibrium electrode reduction potentials E° at unit activity and 25°C may be obtained from the literature (de Bethune and Swendeman Loud, Encyclopedia of Electrochemistry, Van Nostrand Reinhold, 1964). The overall electrochemical cell equilibrium potential either can be obtained from AG values or is equal to the cathode half-cell potential minus the anode half-cell potential, as shown above. 

Ox refers to the oxidized species and Red to the reduced species x and y are their coefficients, respectively, in the balanced equation. The Nernst equation for any cathode half-cell reduction half-reaction) is... 

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