Electrolysis circuit, controlled potential

Torsi et al. [395] have carried out a systematic investigation to establish the potential value of such an apparatus. The apparatus is basically an electrothermal device in which the furnace (or the rod) is replaced by a small crucible made of glassy carbon. Figure 5.10 provides an overall view of the apparatus. Figure 5.11 shows a block diagram of the electrolysis circuit the crucible (6) acts a cathode, while the anode is a platinum foil dipped into either the sample solution reservoir (1) or the washing solution reservoir (2). Pre-elecrolysis was performed at constant current with a 500 V dc variable power supply (5). Under these conditions, the cathode potential is not controlled, so that other metals can be codeposited with lead. 

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. 

It may need an explanation why we consider a current flow during the measurement of the cell potential even in the case of a galvanic cell or an electrolysis cell at so-called open-circuit conditions. K we close any electric circuit and there is a potential difference between the two terminals, current should flow. If we want to measure the potential we have to make a circuit otherwise no measurement can be made. In this sense the open-circuit potential only means that no current is applied in an electrolysis cell. While the classical compensation method can still be used, nowadays almost exclusively, high impedance voltmeters are applied. In an electrolysis cell the potential of the working electrode can be controlled by using a potentiostat. 

Fig. 5.32 The circuits for (a) con-trolled-potential electrolysis and (b) controlled-current electrolysis (for the circuits based on operational amplifiers, see Figs 5.43 and 5.44).

The important feature of this technique is that the working electrode is a microelectrode, which restricts the current to a few microamperes and allows limiting currents to be reached. Conditions are maintained so that the current is diffusion controlled i,e., the only way the electroactive test substance (one that can be reduced or oxidized at the available potentials) can reach the electrode is by diffusion. When the applied potential reaches the decomposition potential of the test substance a current flows, and it increases linearly as the potential is increased, in accordance with Ohm s law (since the resistance of the circuit remains constant). But with a microelectrode and under diffusion-controlled conditions, the number of ions or molecules of test substance that can diffuse to the electrode and maintain the electrolysis current is limited. Thus, with a dilute solution (10 to 10" M and less) a limiting current is ultimately reached and an S-shaped plot of ciurent versus applied potential results. The limiting current is proportional to the concentration of test substance. The potential at which the current is one-half of the limiting current (called the diffusion current, id) is the half-wave potential (E1/2) and is independent of concentration. The half-wave potential is characteristic of the particular test substance being electrolyzed. 

Fig. 3 Scheme of potentiostatic operation for a preparative electrolysis, using in principle a simplified cyclovoltammetry equipment. The potential of the working electrode is measured by a Luggin capillary, coupled with a reference electrode (RE, see Sect. 2.5.1.6). The control circuit in the potentiostat adjusts the cell current until the potential of the working electrode is equal to the voltage at the control input. 

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. 

Figure 2.64. Electronic control circuits with operational amplifiers. Left stabilization of a current flowing through a load (e.g. a LED). Right potentiostat for control of electrolysis potential...

An extended and improved potentiostat circuit is shown in Fig. 2.65. This circuit, the so-caUed inverting type, is widely used in practice as it is very reliable. One of the three OAs in Fig. 2.65, OA3, is connected to act as current follower. It keeps the working electrode input WORK at zero potential and outputs a voltage signal proportional to the electrolysis current. OA2 ensures that the reference electrode is kept free from current flow. OAl is the controlling amplifier. In this special case, it makes the potential at WORK equal to the inverted reference voltage I7ref. 

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