The Electrochemical Cell

The electrochemical cell consists of a Teflon block with three interconnected cylindrical wells (See Fig. 2.2). The counter electrode is a polycrystalline nickel disk (12.54 mm diameter) cast in Kel-F and then press fitted into an orifice bored on the Teflon block. The dynamic hydrogen reference electrode, is formed by inserting a platinum wire into 

Great care was taken in cleaning the TEF block and other TEF parts since they were heavily contaminated by grease and dust after fabrication. 11 the TEF parts were initially degreased in 6 M KOH for one day at room temperature after fabrication. After thorough washing with distilled water, all the Teflon items were immersed in a 1 1 mixture of HgSO and HNOj for several days. 

After rinsing with distilled water several times, the Teflon items were immersed for three days in distilled water which was changed five co ten times. Finally the Teflon pieces were steamed in water vapor. 

The reference electrode used in this experiment, as already mentioned, was a dynamic hydrogen reference electrode consisting 

In essence, all that is required for electrochemistry is two electrodes, the oxidizing electron-accepting anode and the reducing electron-donating cathode. 

It is possible to design a redox reaction such that the oxidation occurs at one location and the reduction occurs at another location. The device is called a galvanic or voltaic cell. The cathode (usually a metal bar or carbon rod) is the electrode where reduction takes place the anode (usually a metal bar or carbon rod) is the electrode where oxidation takes place. The salt bridge allows ions to slowly migrate from one beaker to the other to maintain electrical neutrality in each half-cell. The voltmeter measures the voltage (or potential), V, between the two electrodes. If the temperature is 298 K, and the solutions are 1 M, then the beakers with the electrodes are each considered to be a standard half-cell. 

When the switch is closed, the following is observed in the model  

What is the half-reaction occurring in the copper half-cell  

What is the overall (net) chemical reaction taking place in the galvanic cell (Note This reaction should not have any e in it.) 

The choice of solvent is determined by several factors, including conductance, solubility of electrolyte and electroactive substance, and reactivity with electrolytic products. The solvent can also have important properties such as decreasing usually unwanted effects (e.g., adsorption of the electroactive species at the electrode). Because of the importance of the solvent in electrochemical processes, it is sometimes desirable to consider the physical and chemical properties of the solvent in some detail. Solvent properties relevant to electrochemical experiments are listed in Table 2-1. The melting and boiling points define the useful temperature range for most solvents (with some variation due 

Low temperature studies can be very useful in electrochemical investigations. Reactive species can be stabilized and reversible electrochemistry obtained. Solvent properties can change drastically with temperature. For example, DMF dimerizes at below — 40°C and has a large potential window as well as a lower dielectric constant. 

In Fry and Britton s handy review of solvents and electrolytes, acetonitrile, ethanol, methanol, and methylene chloride are recommended as good oxidative (anodic) electrochemical solvents, while acetonitrile, DMF, and dimethyl sulfoxide (DMSO) are suggested for reductive (cathodic) electrochemistry. Acetonitrile is suggested as the best overall nonaqueous solvent on the basis of its electrochemical properties and its relative nontoxicity. The review of Fry and Britton also is a good place to start when looking for purification methods. 

to minimize evaporation of the cell solvent (and consequent changes in concentrations), the purge gas (N2, Ar) is passed through the same solvent used in the electrochemical cell. This step is particularly necessary for solvents with low vapor pressures. 

Mention should also be made of the increasing use of unconventional media in electrochemical experiments. Experiments in micellar solutions and microemulsions, for instance, can solubilize or concentrate reactants in micelles. Cyclic voltammetric results have been obtained below the freezing point of the solvent for instance, in frozen DMSO and in perchloric add.  

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The apparatus consists of a tip-position controller, an electrochemical cell with tip, substrate, counter and reference electrodes, a bipotentiostat and a data-acquisition system. The microelectrode tip is held on a piezoelectric pusher, which is mounted on an inchwomi-translator-driven x-y-z tliree-axis stage. This assembly enables the positioning of the tip electrode above the substrate by movement of the inchwomi translator or by application of a high voltage to the pusher via an amplifier. The substrate is attached to the bottom of the electrochemical cell, which is mounted on a vibration-free table [, and ]. A number... 

Two major sources of ultrasound are employed, namely ultrasonic baths and ultrasonic immersion hom probes [79, 71]- The fonuer consists of fixed-frequency transducers beneath the exterior of the bath unit filled with water in which the electrochemical cell is then fixed. Alternatively, the metal bath is coated and directly employed as electrochemical cell, but m both cases the results strongly depend on the position and design of the set-up. The ultrasonic horn transducer, on the other hand, is a transducer provided with an electrically conducting tip (often Ti6A14V), which is inuuersed in a three-electrode thenuostatted cell to a depth of 1-2 cm directly facing the electrode surface. 

From the theory of the electrochemical cell, the potential in volts of a silver-silver chloride-hydrogen cell is related to the molarity m of HCI by the equation... 

The diversity of interfacial electrochemical methods is evident from the partial family tree shown in Figure 11.1. At the first level, interfacial electrochemical methods are divided into static methods and dynamic methods. In static methods no current passes between the electrodes, and the concentrations of species in the electrochemical cell remain unchanged, or static. Potentiometry, in which the potential of an electrochemical cell is measured under static conditions, is one of the most important quantitative electrochemical methods, and is discussed in detail in Section IIB. 

The potential of the working electrode, which changes as the composition of the electrochemical cell changes, is monitored by including a reference electrode and a high-impedance potentiometer. 

Shorthand Notation for Electrochemical Cells Although Figure 11.5 provides a useful picture of an electrochemical cell, it does not provide a convenient representation. A more useful representation is a shorthand, or schematic, notation that uses symbols to indicate the different phases present in the electrochemical cell, as well as the composition of each phase. A vertical slash ( ) indicates a phase boundary where a potential develops, and a comma (,) separates species in the same phase, or two phases where no potential develops. Shorthand cell notations begin with the anode and continue to the cathode. The electrochemical cell in Figure 11.5, for example, is described in shorthand notation as... 

What are the anodic, cathodic, and overall reactions responsible for the potential in the electrochemical cell shown here Write the shorthand notation for the electrochemical cell. 

Substituting known values for the standard-state reduction potentials (see Appendix 3D) and the concentrations of Ag+ and gives a potential for the electrochemical cell in Figure 11.5 of... 

Making appropriate substitutions into the Nernst equation for the electrochemical cell (see Example 11.2)... 

Despite the apparent ease of determining an analyte s concentration using the Nernst equation, several problems make this approach impractical. One problem is that standard-state potentials are temperature-dependent, and most values listed in reference tables are for a temperature of 25 °C. This difficulty can be overcome by maintaining the electrochemical cell at a temperature of 25 °C or by measuring the standard-state potential at the desired temperature. 

In potentiometry, the potential of an electrochemical cell under static conditions is used to determine an analyte s concentration. As seen in the preceding section, potentiometry is an important and frequently used quantitative method of analysis. Dynamic electrochemical methods, such as coulometry, voltammetry, and amper-ometry, in which current passes through the electrochemical cell, also are important analytical techniques. In this section we consider coulometric methods of analysis. Voltammetry and amperometry are covered in Section 1 ID. 

Coulometric methods of analysis are based on an exhaustive electrolysis of the analyte. By exhaustive we mean that the analyte is quantitatively oxidized or reduced at the working electrode or reacts quantitatively with a reagent generated at the working electrode. There are two forms of coulometry controlled-potential coulometry, in which a constant potential is applied to the electrochemical cell, and controlled-current coulometry, in which a constant current is passed through the electrochemical cell. 

Reduction of Fe + to Fe + occurs at the working electrode, making it the cathode in the electrochemical cell. 

Scale of Operation Coulometric methods of analysis can be used to analyze small absolute amounts of analyte. In controlled-current coulometry, for example, the moles of analyte consumed during an exhaustive electrolysis is given by equation 11.32. An electrolysis carried out with a constant current of 100 pA for 100 s, therefore, consumes only 1 X 10 mol of analyte if = 1. For an analyte with a molecular weight of 100 g/mol, 1 X 10 mol corresponds to only 10 pg. The concentration of analyte in the electrochemical cell, however, must be sufficient to allow an accurate determination of the end point. When using visual end points, coulometric titrations require solution concentrations greater than 10 M and, as with conventional titrations, are limited to major and minor analytes. A coulometric titration to a preset potentiometric end point is feasible even with solution concentrations of 10 M, making possible the analysis of trace analytes. 

What is the purpose of waiting 10 s after mixing the contents of the electrochemical cell before recording the voltammogram ... 

Electrochemical methods covered in this chapter include poten-tiometry, coulometry, and voltammetry. Potentiometric methods are based on the measurement of an electrochemical cell s potential when only a negligible current is allowed to flow, fn principle the Nernst equation can be used to calculate the concentration of species in the electrochemical cell by measuring its potential and solving the Nernst equation the presence of liquid junction potentials, however, necessitates the use of an external standardization or the use of standard additions. 

Ethylene glycol can be produced by an electrohydrodimerization of formaldehyde (16). The process has a number of variables necessary for optimum current efficiency including pH, electrolyte, temperature, methanol concentration, electrode materials, and cell design. Other methods include production of valuable oxidized materials at the electrochemical cell s anode simultaneous with formation of glycol at the cathode (17). The compound formed at the anode maybe used for commercial value direcdy, or coupled as an oxidant in a separate process. 

These reactions can be carried out at room temperature. Hydrogen gas can also be produced on a laboratory scale by the electrolysis of an aqueous solution. Production of hydrogen through electrolysis is also used industrially. This involves the following reaction at the cathode of the electrochemical cell ... 

Product Recovery. Comparison of the electrochemical cell to a chemical reactor shows the electrochemical cell to have two general features that impact product recovery. CeU product is usuaUy Uquid, can be aqueous, and is likely to contain electrolyte. In addition, there is a second product from the counter electrode, even if this is only a gas. Electrolyte conservation and purity are usual requirements. Because product separation from the starting material may be difficult, use of reaction to completion is desirable ceUs would be mn batch or plug flow. The water balance over the whole flow sheet needs to be considered, especiaUy for divided ceUs where membranes transport a number of moles of water per Earaday. At the inception of a proposed electroorganic process, the product recovery and refining should be included in the evaluation to determine tme viabUity. Thus early ceU work needs to be carried out with the preferred electrolyte/solvent and conversion. The economic aspects of product recovery strategies have been discussed (89). Some process flow sheets are also available (61). 

A Perkin-Elmer 5000 AAS was used, with an electrically heated quartz tube atomizer. The electrolyte is continuously conveyed by peristaltic pump. The sample solution is introduced into the loop and transported to the electrochemical cell. A constant current is applied to the electrolytic cell. The gaseous reaction products, hydrides and hydrogen, fonued at the cathode, are flowed out of the cell with the carrier stream of argon and separated from the solution in a gas-liquid separator. The hydrides are transported to an electrically heated quartz tube with argon and determined under operating conditions for hydride fonuing elements by AAS. 

The electrical conductivity also increases with increasing metal oxide content, due to the high mobility of the metal ions. For example several glass compositions have been used as solid electrolytes in galvanic cells in which other metal ions apart from the alkaline and alkaline earth ions have been incorporated. The electrochemical cell... 

Now let s take a more detailed look into the electrochemical cell. Figure 12-5 shows a cross-section of a cell that uses the same chemical reaction as that depicted in Figure 12-1. The only difference is that the two solutions are connected differently. In Figure 12-1 a tube containing a solution of an electrolyte (such as KNOa) provides a conducting path. In Figure 12-5 the silver nitrate is placed in a porous porcelain cup. Since the silver nitrate and copper sulfate solutions can seep through the porous cup, they provide their own connection to each other. 

For the electrochemical cell reaction, the reaction free energy AG is the utilizable electrical energy. The reaction enthalpy AH is the theoretical available energy, which is increased or reduced by an amount TAS. The product of the temperature and the entropy describes the amount of heat consumed or released reversibly during the reaction. With tabulated values for the enthalpy and the entropy it is possible to obtain AG. ... 

The reversible reaction heat of the cell is defined as the reaction entropy multiplied by the temperature [Eq. (15)]. For an electrochemical cell it is also called the Peltier effect and can be described as the difference between the reaction enthalpy AH and the reaction free energy AG. If the difference between the reaction free energy AG and the reaction enthalpy AH is below zero, the cell becomes warmer. On the other hand, for a difference larger than zero, it cools down. The reversible heat W of the electrochemical cell is therefore ... 

An important experimentally available feature is the current-voltage characteristic, from which the terminal voltage ([/v ) supplied by the electrochemical cell at the corresponding discharge current may be determined. The product of current / and the accompanying terminal voltage is the electric power P delivered by the battery system at a given time. 

A variety of complexes exists in solid or liquid state at ambient temperature, in the range required for battery operation. Liquid polybromine phases are preferred since they enable storage of the active material externally to the electrochemical cell stack in a tank, hence enhancing the... 

The recent development of the convertible oxide materials at Fuji Photo Film Co. will surely cause much more attention to be given to alternative lithium alloy negative electrode materials in the near future from both scientific and technological standpoints. This work has shown that it may pay not only to consider different known materials, but also to think about various strategies that might be used to form attractive materials in situ inside the electrochemical cell. 

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