Gas electrodes

If the circuit is broken after the e.m.f. has been applied, it will be observed that the reading on the voltmeter is at first fairly steady, and then decreases, more or less rapidly, to zero. The cell is now clearly behaving as a source of current, and is said to exert a back or counter or polarisation e.m.f., since the latter acts in a direction opposite to that of the applied e.m.f. This back e.m.f. arises from the accumulation of oxygen and hydrogen at the anode and cathode respectively two gas electrodes are consequently formed, and the potential difference between them opposes the applied e.m.f. When the primary current from the battery is shut off, the cell produces a moderately steady current until the gases at the electrodes are either used up or have diffused away the voltage then falls to zero. This back e.m.f. is present even when the current from the battery passes through the cell and accounts for the shape of the curve in Fig. 12.1. 

Indicator electrodes for anions may take the form of a gas electrode (e.g. oxygen electrode for OH- chlorine electrode for Cl-), but in many instances consist of an appropriate electrode of the second kind thus as shown in Section 15.1, the potential of a silver-silver chloride electrode is governed by the chloride-ion activity of the solution. Selective-ion electrodes are also available for many anions. 

Data for the pc-Au/DMF + LiC104 interface have been collected by Borkowska and Jarzabek.109 The value of ffa0was found to be 0.27 V (SCE in H20) and the roughness factor / = 1.3 (Table 8). Unlike Hg, Bi, In(Ga), and Tl(Ga) electrodes and similarly to the Ga/DMF interface, the inner-layer capacity for pc-Au in DMF depends weakly on a, and thus the effect of solvent dipole reorientation at pc-Au is less pronounced than at In(Ga), Bi, and other interfaces. 

Electrodes at which gases are evolved or consumed [e.g., by reaction (1.27)] can be called gas electrodes. In the conventional formulation of the electrode system, the reacting gas is indicated in parentheses. 

The underlying gas electrode for ammonia, responds to NH3 formed in the last reaction, which is proportional to the concentration of NTA in the sample. 

The concept of a biocatalytic membrane electrode has been extended to the use of a tissue slice as the catalytic layer. An example of this approach is an electrode for AMP which consists of a slice of rabbit muscle adjacent to an ammonia gas electrode. NHj is produced by enzymatic action of rabbit muscle constituents on AMP The electrode exhibits a linear range of 1.4 x 10 to 1.0 x 10 M with a response time varying from 2.5 to 8.5 min, depending on the concentration. Electrode lifetime is about 28 days when stored between use in buffer with sodium azide to prevent bacterial growth. Excellent selectivity enables AMP to be determined in serum. 

Oxidation- reduction An inert metal dips into a solution containing ions in two different oxidation states. An example consists of a platinum wire dipping into a solution containing ferrous and ferric ions. Such a cell is described by Pt Fe2 (c,). Fe3 (c2). The comma is used to separate the two chemical species which are in the same solution. These electrodes are similar to the gas electrodes, except that the two species involved in the electrode reaction are ions. The electrode reaction in the example is Fe3 + e Fe2, and there is the possibility of the electrode either donating or accepting electrons. 

Figure 6.10 (A) A gas electrode in general and a hydrogen electrode in particular ...

The decrease in free energy (—AG) which provides the driving force in a cell may ensue either from a chemical reaction or from a physical change. In particular, one often studies cells in which the driving force is a change in concentration (almost always a dilution process). These cells are called concentration cells. The alteration in concentration can take place either in the electrolyte or in the electrodes. As examples of alterations in concentration in electrodes, mention may be made of amalgams or alloy electrodes with different concentrations of the solute metal and in gas electrodes with different pressures of the gas. 

Electrodes of the first kind. These include cationic electrodes (metal, amalgam and, of the gas electrodes, the hydrogen electrode), at which equilibrium is established between atoms or molecules of the substance and the corresponding cations in solution (see Eqs 3.1.21 and 3.1.65), and anionic electrodes, at which equilibrium is established between molecules and anions. 

Electrodes of the first kind can be divided into anionic and cationic. The system of a gas electrode includes a gas interacting with a suitable metal or semiconductor surface in the cell reaction. However, gas electrodes can also... 

Theoretically, the most important cationic electrode of the first kind is the hydrogen electrode. This is a gas electrode at which equilibrium is... 

The capaciatance C depends on the gas-electrode-electrolyte interline "area" but not on the total electrode surface area S. If the porosity of all the electrode catalysts used is the same, which is a reasonable assumption since they were all prepared by the same calcination procedure, it follows that the interline "area" is proportional to the flat electrolyte surface area A, i.e. the constant X equals X A, where X is another constant which does not depend on any macroscopic dimension. 

Gas electrodes are similar to ISEs and usually work on the same basic principles. They look much like a standard ISE except that they have a gas-permeable, water-impermeable membrane at the tip. Gas present in the environment passes through the membrane, reacts with reagents in the interior, and produces a voltage. This voltage is related to the partial pressure of the gas being measured. 

Gas electrodes may also be regarded as electronic-and-ionic electrodes covered with a gaseous film. The electron transfer occurs between the electrode and the gaseous film (2 + 2 eoii H2( >) and the ion transfer occurs between the... 

To reduce and overcome this concentration polarization, several techniques are available. For reactants soluble in the electrolyte, high bulk concentrations are used and also the electrolyte is circulated by pumping which uses a fraction of the electricity produced by the fuel cell and hence reduces the available power. For gaseous reactants, porous gas electrodes are used to achieve larger contact of the three phases, namely the gas, the liquid and the solid phases. There are different types of such electrode and two examples are shown in Figures 3a et 3b. 

The electrodes and electrode protective coating of the oxygen sensor play a crucial role in determining the performance characteristics and durability (2). The electrodes used are the inner or air-reference electrode and the outer or exhaust gas electrode. The protective coating goes over the outer or exhaust electrode. While a complete discussion of the requirements and properties of the electrodes and electrode protective coating is beyond the scope of this paper, a brief description will be given. 

The edl structure of a Cd-Ga (0.3 atom % Cd) liquid electrode in aqueous [20, 21], methanol, and propylene carbonate (PC) [21, 22] solutions with inactive electrolyte (LiCl04, liBF4) was investigated. The double layer at the liquid Cd-Ga alloy differs from that of a Ga electrode, and was virtually identical with edl at the Cd electrode. 

Table 6.6 Performance characteristics of Severinghaus-type gas electrodes...

Ammonia Gas Sensor. The determination of ammonia is important in process analysis. An ammonia gas electrode is usually used for this purpose. However, volatile compounds such as amines often interfere with the determination of ammonia. Therefore, a sensor based on amperometry is desirable for the determination of ammonia. 

Other potentiometric electrode systems are ion-selective electrodes such as fluoride, calcium, magnesium, sodium, potassium and chloride, selective gas electrodes based on membranes such as 02, C02, CO, NO, N02 and S02, and enzyme electrodes. These electrodes fall beyond the scope of this book and are not discussed further. 

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