Cell voltage silver hydrogen

The reaction continues and current passes until all the iodide is used up. At this point some means of endpoint detection is needed. Two methods are commonly adopted. The first uses an amperometric circuit with a small imposed voltage that is insufficient to electrolyze any of the solutes. When the mercury ion concentration suddenly increases, the current will rise because of the increase in the concentration of the conducting species. The second method involves using a suitable indicator electrode. An indicator electrode may be a metal electrode in contact with its own ions or an inert electrode in contact with a redox couple in solution. The signal recorded is potentiometric (a cell voltage vs. a stable reference electrode). For mercury or silver we may use the elemental electrodes, because they are at positive standard reduction potentials to the hydrogen/hydrogen ion couple. 

Figure 3-1 Voltage Measurements on a Silver-Silver Chloride, Hydrogen Cell at 298.15 K. The contribution of the Standard Hydrogen Electrode is taken as zero by convention.

The voltage we measure is characteristic of the metals we use. As an additional example, unit activity solutions of CuCE and AgCl with copper and silver electrodes, respectively, give a potential difference of about 0.45 V. We could continue with this type of measurement for aU the different anode-cathode combinations, but the number of galvanic cells needed would be very large. Fortunately, the half-reactions for most metals have been calculated relative to a standard reference electrode, which is arbitrarily selected as the reduction of hydrogen ... 

The impregnation of porous nickel discs with CoPc was difficult because of the limited solubility of the chelate in the usual solvents. CoPc cathodes with carbon as substrate were therefore prepared for use in H2/O2 fuel cells. A mixture of 72 mg CoPc and 48 mg acetylene black, with PTFE as binder, was pressed into a nickel mesh of area 5 cm2. Electrodes of this type were tested in an H2/O2 fuel cell with 35% KOH electrolyte in an asbestos matrix at 80° C. Figure 5 compares the current/voltage characteristics of CoPc cathodes (14 mg/cm2) with those of other catalysts, including platinum (9 mg/cm2), silver (40 mg/cm2), and pure acetylene black (20 mg/cm2). An hydrogen electrode (9 mg Pt/cm2) was used as the anode in all tests. To facilitate comparison of the activity of different cathodes, the pure ohmic internal resistance of the cells (of the order of 0.02 ohm) was eliminated. 

In Equation (18b), the activity quotient is separated into the terms relating to the silver electrode and the hydrogen electrode. We assume that both electrodes (Ag+/Ag and H+/H2) operate under the standard condition (i.e. the H+/H2 electrode of our cell happens to constitute the SHE). This means that the equilibrium voltage of the cell of Figure 3.1.6 is identical with the half-cell equilibrium potential E°(Ag+l Ag) = 0.80 V. Furthermore, we note that the activity of the element silver is per definition unity. As the stoichiometric number of electrons transferred is one, the Nemst equation for the Ag+/Ag electrode can be formulated in the following convenient and standard way ... 

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