Electrolysis potentiostatic control

Controlled potential electrolysis (potentiostatic control) requires a three-electrode cell, so as not to polarize the reference electrode. Controlled potential methods enable one to be very selective in depositing one metal from a mixture of metals. If two components have electrochemical potentials that differ by no more than several hundred millivolts, it may still be possible to shift these potentials by complexing one of the species. One disadvantage of exhaustive electrolysis is the time required for analysis, and faster methods of electrochemical analysis are described. 

Figure 1. Typical H-cell for electrolysis using two-electrode setup (reference electrode can be inserted into working compartment for potentiostatic control).

Generally, except for the simpler apparatus involved, controlled-current electrolysis offers no advantages over controlled-potential methods. With the commercial availability of suitable potentiostats, controlled-current methods are being used less frequently in analysis and lab-scale preparative electrolysis. For large-scale electrosynthesis or separations involving very high currents, especially in flow systems where the reactants are... 

The experiments in Section 1.2 cannot actually be done as described— no electrochemical measurement can be made with an isolated single electrode. In the simplest actual arrangement a potential is applied between the electrolysis or working electrode (WE) and a reference half-cell (REF), which completes the circuit for electron flow. This circuit works perfectly well for very low current applications, but a three-electrode system with potentiostatic control is most commonly used. 

If a commercial polarograph which includes a potentiostat is employed, then the three-electrode procedure (Sections 16.7 and 16.8) is conveniently used with the controlled potential supplied by the potentiostat applied between the dropping electrode and the calomel reference electrode, while the electrolysis current flows between the working (mercury) electrode and the auxiliary... 

For a potential controlled electrolysis, one needs a potentiostat [177] and a reference electrode. The Ag/AgCl electrode can he used as a reference electrode with a fixed potential. It is prepared from a silver wire, which is covered with AgCl, when employed as the anode in 2N HCl. 

In coulometry, the analyte is quantitatively electrolyzed and, from the quantity of electricity (in coulombs) consumed in the electrolysis, the amount of analyte is calculated using Faraday s law, where the Faraday constant is 9.6485309 xlO4 C mol-1. Coulometry is classified into controlled-potential (or potentiostatic) coulometry and controlled-current (or galvanostatic) coulometry, based on the methods of electrolysis [19, 20]. 

The choice of the electrolysis procedure either working at controlled potential (potentiostatic or controlled-potential electrolysis, cpe) or at a fixed current density (galvanostatic)... 

In general, controlled-current electrolyses need less expensive equipment. Only a controlled-current source in combination with a coulomb integrator is necessary. Therefore, in industry, electroorganic reactions are always performed at a fixed current density. In the laboratory, it is advisable to start with controlled-potential electrolyses using a potentiostat and a three-electrode electrolysis cell (Fig. 22.8). In this way, the reaction can be controlled at the redox potential of the substrate determined analytically, and the selectivity of the process can be studied at different potentials. After determination of the selectivity controlling factors, it is usually possible to change over to current control by proper selection of the current density and the concentration of the substrate. Using a continuous process, the concentration can be fixed at the desired value. Thus, selectivity can also be obtained under these conditions. 

The voltage or current is applied across the electrolysis cell using the Potentiostat and Power Supply . This instrumentation is controlled by the Computer , which also collects and stores experimental data. 

One of the most important, yet latent, applications of controlled-potential electrolysis is electrochemical synthesis. Although electrolysis has been used for more than a century to synthesize various metals from their salts, application to other types of chemical synthesis has been extremely limited. Before the advent of controlled-potential methods, the selectivity possible by classical electrolysis precluded fine control of the products. The only control was provided by appropriate selection of electrode material, solution acidity, and supporting electrolyte. By these means the effective electrode potential could be limited to minimize the electrolysis of the supporting electrolyte or the solvent. Today potentiostats and related controlled-potential-electrolysis instrumentation are commercially available that provide effective control of the potential of the working electrode to 1 mV, and a driving force of up to 100 V for currents of up to several amperes. Through such instrumentation electrochemical syn-... 

Electrode geometry in controlled-potential electrolysis. When fast response and accuracy of potential control are desired, considerable attention must be paid to the design of the cell-potentiostat system, and several papers have discussed the critical parameters and made recommendations for optimum cell design.8"11 In general, to achieve stability and an optimum potentiostat rise time for a fast potential change, the total cell impedance should be as small as possible, and the uncompensated resistance should be adjusted to an optimum (nonzero) value that depends on the characteristics of the cell and potentiostat.9,12 The electrode geometry also should provide for a low-resistance reference electrode and a uniform current distribution over the surface of the... 

Lingane was a leader in the field of - electro analytical chemistry and wrote, with Kolthoff, the definitive, two volume monograph, Polarography [i] that remains a useful reference work. He also helped develop other electroanalytical techniques, like controlled potential electrolysis, -> coulometry, -> coulometric titrations, and developed an early electromechanical (Lingane-Jones) potentiostat, He wrote the widely-used monograph in this field, Electroanalytical Chemistry (1st edn., 1953 2nd edn., 1958). Lingane received a number of awards, including the Analytical Chemistry (Fisher) Award of the American Chemical Society in 1958. Many of his Ph.D. students, e.g., -> Meites, Fred Anson, Allen Bard, Dennis Peters, and Dennis Evans, went on to academic careers in electrochemistry. 

Electrogravimetry, which is the oldest electroanalytical technique, involves the plating of a metal on to one electrode of an electrolysis cell and weighing the deposit. Conditions are controlled so as to produce a uniformly smooth and adherent deposit in as short a time as possible. In practice, solutions are usually stirred and heated and the metal is often complexed to improve the quality of the deposit. The simplest and mqst Vapid procedures are those in which a fixed applied potential or a cqqp nt cell current is employed, but in both cases selectivity is poor and they are generally used when there are only one or two metals present. Selective deposition of metals from multi-component mixtures can be achieved by controlling the cathode potential automatically with a potentiostat. This device automatically monitors and maintains the cathode potential at a pre-determined value by means of a reference electrode and servo-driven potential-divider. The value chosen for the cathode potential is such that only the metal of interest is deposited and there are no gaseous products formed. 

Note that the calculated value of n might be a fraction if the current efficiency is less than 100 % or the mechanism followed is complex. Preparative electrolysis can be performed under potentiostatic or galvanostatic conditions. In the former case, the use of a potentiostat ensues that the potential of the working electrode relative to a reference electrode is held at a constant value during the whole electrolysis process independently of the current supplied. Preferentially, the potential should be selected so that the substrate can be electrolyzed without the products or the medium being reduced or oxidized at the same time. It is possible by proper potential adjustment to control the fraction of electrolysis X which is defined according to Eq. 98 as the fraction of the substrate transformed during electrolysis. 

Theoretical considerations and actual measurements of the working electrode potential under working conditions show that a uniform potential distribution is possible only with concentric electrodes of the same area [7, 8]. There is thus a limitation to how exactly the potential indicated by the potentiostat reflects the working potential at different parts of the electrode. In actual practice, a controlled potential electrolysis (CPE) is carried out in a small potential range around some average value. In the author s experience, a selective electrolysis requires a separation of 0.15-0.2 V in Ei/o between the two reactions. 

Very often it is of great interest to determine the n value of an electrode reaction or the current efficiency of an electrosynthesis. Both involve a measurement of the electricity consumption, that is, an integration of the electric current over the time of electrolysis. In constant-current experiments this is, of course, an easy task. For controlled potential electrolysis, an integrating device is included in the circuit. Electronic integrators can usually be obtained from the companies that supply potentiostats. 

---