Half cells

A formulation consisting of catalyst particles and a binding agent is typically applied to a porous backing substrate such as carbon paper or carbon cloth. The backing provides not only a support to hold the catalyst mixture but also the electrical connection with other cell components. 

Such a half-cell fixture enables quick screening of an electrode of interest without complications from what happens on the counter electrode. The electrode of interest can be either a cathode or an anode. 

Electrolyte selection is very similar to what is described for the 3-electrode cell. Since the catalyst layer is separated from the aqueous electrolyte solution by a permselective ionomer membrane, only water and protons can transport through the membrane to reach the catalyst layer. This helps minimize or eliminate the adverse impact of anion adsorption (onto the catalyst surface) on the reaction kinetics. 

The membrane is likely to swell in the aqueous solution and this could cause warping of the electrode/membrane assembly. This will then change the distance between the membrane and the tip of the Luggin capillary, which alters the uncompensated solution resistance slightly. So it is better to position the tip of the reference electrode after warping is complete. If the warping causes delamination of the electrode from the membrane, the testing result will be adversely affected. 

One method to reduce the warping is to use a small membrane/electrode assembly. Another method is to add a stiff mesh on top of the membrane/electrode assembly. The mesh should be highly open and inert to the reaction environment. 

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A problem that has fascinated surface chemists is whether, through suitable measurements, one can determine absolute half-cell potentials. If some one standard half-cell potential can be determined on an absolute basis, then all others are known through the table of standard potentials. Thus, if we know E for... 

For a half-cell under standard conditions this becomes AG = - nFE ... 

Experimentally, the aqueous iron(II) is titrated with cerium(IV) in aqueous solution in a burette. The arrangement is shown in Figure 4.6, the platinum indicator electrode changes its potential (with reference to a calomel half-cell as standard) as the solution is titrated. Figure 4.7 shows the graph of the cell e.m.f. against added cerium(IV). At the equivalence point the amount of the added Ce (aq) is equal to the original amount of Fe (aq) hence the amounts of Ce (aq) and Fe (aq) are also equal. Under these conditions the potential of the electrode in the mixture is ( - - f)/2 this, the equivalence point, occurs at the point indicated. 

The two estimates for the first or a parameter of the parabolic fit are the intercepts on the voltage axis of Eig. 3-1, so both procedures arrive at a standard potential of the silver-silver chloride half-cell of 0.2225 V. The accepted modem value is 0.2223 V (Barrow, 1996). 

Also, by convention, potentiometric electrochemical cells are defined such that the indicator electrode is the cathode (right half-cell) and the reference electrode is the anode (left half-cell). 

The double vertical slash ( ) indicates the salt bridge, the contents of which are normally not indicated. Note that the double vertical slash implies that there is a potential difference between the salt bridge and each 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... 

Potentiometric electrochemical cells are constructed such that one of the half-cells provides a known reference potential, and the potential of the other half-cell indicates the analyte s concentration. By convention, the reference electrode is taken to be the anode thus, the shorthand notation for a potentiometric electrochemical cell is... 

Standard Hydrogen Electrode The standard hydrogen electrode (SHE) is rarely used for routine analytical work, but is important because it is the reference electrode used to establish standard-state potentials for other half-reactions. The SHE consists of a Pt electrode immersed in a solution in which the hydrogen ion activity is 1.00 and in which H2 gas is bubbled at a pressure of 1 atm (Figure 11.7). A conventional salt bridge connects the SHE to the indicator half-cell. The shorthand notation for the standard hydrogen electrode is... 

Redox Electrodes Electrodes of the first and second kind develop a potential as the result of a redox reaction in which the metallic electrode undergoes a change in its oxidation state. Metallic electrodes also can serve simply as a source of, or a sink for, electrons in other redox reactions. Such electrodes are called redox electrodes. The Pt cathode in Example 11.1 is an example of a redox electrode because its potential is determined by the concentrations of Ee + and Ee + in the indicator half-cell. Note that the potential of a redox electrode generally responds to the concentration of more than one ion, limiting their usefulness for direct potentiometry. 

In the second method to produce ADN, known as electrohydrodimerization, two moles of acrylonitrile [107-13-1] are combined and hydrogenated in an electrochemical cell where the two half-cells are separated by a membrane. 

Half-cells based on Ag-AgCl, Hg-HgO, Hg-HgSQj and other system.s can be used as reference electrodes (see Table 3-1) 14]. Electrodes have been developed to operate up to 100 bar and 250°C (see Fig. 21-8). 

This method involves very simple and inexpensive equipment that could be set up m any laboratory [9, 10] The equipment consists of a 250-mL beaker (used as an external half-cell), two platinum foil electrodes, a glass tube with asbestos fiber sealed m the bottom (used as an internal half-cell), a microburet, a stirrer, and a portable potentiometer The asbestos fiber may be substituted with a membrane This method has been used to determine the fluoride ion concentration in many binary and complex fluondes and has been applied to unbuffered solutions from Willard-Winter distillation, to lon-exchange eluant, and to pyrohydrolysis distil lates obtained from oxygen-flask or tube combustions The solution concentrations range from 0 1 to 5 X 10 M This method is based on complexing by fluonde ions of one of the oxidation states of the redox couple, and the potential difference measured is that between the two half-cells Initially, each cell contains the same ratio of cerium(IV) and cerium(tll) ions... 

As a result, the electromotive force (EMF) of the cell is zero In the presence of fluoride ions, cerium(IV) forms a complex with fluoride ions that lowers the cerium(IV)-cerium(IIl) redox potential The inner half-cell is smaller, and so only 5 mL of cerium(IV)-cenum (III) solution is added To the external half-cell, 50 mL of the solution is added, but the EMF of the cell is still zero When 10 mL of the unknown fluonde solution is added to the inner half-cell, 100 mL of distilled water IS added to the external half-cell The solution in the external half-cell is mixed thoroughly by turning on the stirrer, and 0 5 M sodium fluonde solution is added from the microburet until the null point is reached The quantity of known fluonde m the titrant will be 10 times the quantity of the unknown fluoride sample, and so the microburet readings must be corrected prior to actual calculations... 

Standard, reduction potentials are determined by measuring the voltages generated in reaction half-cells (Figure 21.2). A half-cell consists of a solution containing 1 M concentrations of both the oxidized and reduced forms of the substance whose reduction potential is being measured, and a simple electrode. 

Together, the oxidized and reduced forms of the substance are referred to as a redox couple.) Such a sample half-cell is connected to a reference half-cell... 

If electron flow between the electrodes is toward the sample half-cell, reduction occurs spontaneously in the sample half-cell, and the reduction potential is said to be positive. If electron flow between the electrodes is away from the sample half-cell and toward the reference cell, the reduction potential is said to be negative because electron loss (oxidation) is occurring in the sample halfcell. Strictly speaking, the standard reduction potential, is the electromotive force generated at 25°C and pH 7.0 by a sample half-cell (containing 1 M concentrations of the oxidized and reduced species) with respect to a reference half-cell. (Note that the reduction potential of the hydrogen half-cell is pH-dependent. The standard reduction potential, 0.0 V, assumes 1 MH. The hydrogen half-cell measured at pH 7.0 has an of —0.421 V.)... 

Figure 21.2a shows a sample/reference half-cell pair for measurement of the standard reduction potential of the acetaldehyde/ethanol couple. Because electrons flow toward the reference half-cell and away from the sample half-cell, the standard reduction potential is negative, specifically —0.197 V. In contrast, the fumarate/succinate couple and the Fe /Fe couple both cause electrons to flow from the reference half-cell to the sample half-cell that is, reduction occurs spontaneously in each system, and the reduction potentials of both are thus positive. The standard reduction potential for the Fe /Fe half-cell is much larger than that for the fumarate/ succinate half-cell, with values of + 0.771 V and +0.031 V, respectively. For each half-cell, a half-cell reaction describes the reaction taking place. For the fumarate/succinate half-cell coupled to a H Hg reference half-cell, the reaction occurring is indeed a reduction of fumarate. 

However, the reaction occurring in the acetaldehyde/ethanol half-cell is the oxidation of ethanol ... 

Some typical half-cell reactions and their respective standard reduction potentials are listed in Table 21.1. Whenever reactions of this type are tabulated, they are uniformly written as reduction reactions, regardless of what occurs in the given half-cell. The sign of the standard reduction potential indicates which reaction really occurs when the given half-cell is combined with the reference hydrogen half-cell. Redox couples that have large positive reduction potentials... 

This is the isocitrate dehydrogenase reaction of the TCA cycle. Writing the two half-cell reactions, we have... 

Halbelement, n. Elec.) half-cell, half-element, halben, v.t. halve. 

However, in the case of stress-corrosion cracking of mild steel in some solutions, the potential band within which cracking occurs can be very narrow and an accurately known reference potential is required. A reference half cell of the calomel or mercury/mercurous sulphate type is therefore used with a liquid/liquid junction to separate the half-cell support electrolyte from the process fluid. The connections from the plant equipment and reference electrode are made to an impedance converter which ensures that only tiny currents flow in the circuit, thus causing the minimum polarisation of the reference electrode. The signal is then amplifled and displayed on a digital voltmeter or recorder. 

Fig. 10.41 Silver/silver chloride half-cell (Admirally patiern). The elecirode is immersed in a chloride-containing solution which diffuses through the porous pot and thus comes into contact...

The measurement of current densities in the vicinity of a cathodically protected structure is a comparatively new principle used chiefly to monitor the effectiveness of offshore protection systems. These measurements are undertaken by twin half-cell devices either installed for stationary use or moved about the structure by diver or remote controlled vehicle. 

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. 

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