Working electrodes control

Controllcd-Currcnt Coulomctry The use of a mediator makes controlled-current coulometry a more versatile analytical method than controlled-potential coulome-try. For example, the direct oxidation or reduction of a protein at the working electrode in controlled-potential coulometry is difficult if the protein s active redox site lies deep within its structure. The controlled-current coulometric analysis of the protein is made possible, however, by coupling its oxidation or reduction to a mediator that is reduced or oxidized at the working electrode. Controlled-current coulometric methods have been developed for many of the same analytes that may be determined by conventional redox titrimetry. These methods, several of which are summarized in Table 11.9, also are called coulometric redox titrations. 

The methods can be classified by the controlled parameter (E or i) and by the quantities actually measured or the process carried out. Thus in controlled-potential techniques the potential of the working electrode is maintained constant with respect to a reference electrode. Since the potential of the working electrode controls the degree of completion of an electrolytic process in most cases, controlled-potential techniques are usually the most desirable for bulk electrolysis. However, these methods require potentiostats with large output current and voltage capabilities and they need stable reference electrodes, carefully placed to minimize uncompensated resistance effects. Placement of the auxiliary electrode to provide a fairly uniform current distribution across the surface of the working electrode is usually desirable, and the auxiliary electrode is often placed in a separate... 

Several designs for STM electrochemical cells have appeared in the literature [M]- hr addition to an airtight liquid cell and the tip insulation mentioned above, other desirable features include the incorporation of a reference electrode (e.g. Ag/AgCl in saturated KCl) and a bipotentiostat arrangement, which allows the independent control of the two working electrodes (i.e. tip and substrate) [ ] (figure BL19.11). 

Stripping voltammetry involves the pre-concentration of the analyte species at the electrode surface prior to the voltannnetric scan. The pre-concentration step is carried out under fixed potential control for a predetennined time, where the species of interest is accumulated at the surface of the working electrode at a rate dependent on the applied potential. The detemiination step leads to a current peak, the height and area of which is proportional to the concentration of the accumulated species and hence to the concentration in the bulk solution. The stripping step can involve a variety of potential wavefomis, from linear-potential scan to differential pulse or square-wave scan. Different types of stripping voltaimnetries exist, all of which coimnonly use mercury electrodes (dropping mercury electrodes (DMEs) or mercury film electrodes) [7, 17]. 

Figure C2.8.3. A tliree-electrode electrochemical set-up used for the measurement of polarization curves. A potentiostat is used to control the potential between the working electrode and a standard reference electrode. The current is measured and adjusted between an inert counter-electrode (typically Pt) and the working electrode.

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. 

Selecting a Constant Potential In controlled-potential coulometry, the potential is selected so that the desired oxidation or reduction reaction goes to completion without interference from redox reactions involving other components of the sample matrix. To see how an appropriate potential for the working electrode is selected, let s develop a constant-potential coulometric method for Cu + based on its reduction to copper metal at a Pt cathode working electrode. 

Instrumentation Controlled-current coulometry normally is carried out using a galvanostat and an electrochemical cell consisting of a working electrode and a counterelectrode. The working electrode, which often is constructed from Pt, is also... 

The ability to control selectivity by carefully selecting the working electrode s potential, makes controlled-potential coulometry particularly useful for the analysis of alloys. For example, the composition of an alloy containing Ag, Bi, Cd, and Sb... 

Another area where controlled-potential coulometry has found application is in nuclear chemistry, in which elements such as uranium and polonium can be determined at trace levels. Eor example, microgram quantities of uranium in a medium of H2SO4 can be determined by reducing U(VI) to U(IV) at a mercury working electrode. 

Anodic stripping voltammetry consists of two steps (Figure 11.37). The first is a controlled potential electrolysis in which the working electrode, usually a hanging mercury drop or mercury film, is held at a cathodic potential sufficient to deposit the metal ion on the electrode. For example, with Cu + the deposition reaction is... 

The electrical characteristics of the cell and electrode will comprise both capacitative and resistive components, but for simplicity the former may be neglected and the system can be represented by resistances in series (Fig. 19.36 > and c). The resistance simulates the effective series resistance of the auxiliary electrode A.E. and cell solution, whilst the potential developed across by the flow of current between the working electrode W.E. and A.E. simulates the controlled potential W.E. with respect to R.E. 

Accurate control of potential, stability, frequency response and uniform current distribution required the following low resistance of the cell and reference electrode small stray capacitances small working electrode area small solution resistance between specimen and point at which potential is measured and a symmetrical electrode arrangement. Their design appears to have eliminated the need for the usual Luggin capillary probe. 

In the common method of electro-gravimetric analysis, a potential slightly in excess of the decomposition potential of the electrolyte under investigation is applied, and the electrolysis allowed to proceed without further attention, except perhaps occasionally to increase the applied potential to keep the current at approximately the same value. This procedure, termed constant-current electrolysis, is (as explained in Section 12.4) of limited value for the separation of mixtures of metallic ions. The separation of the components of a mixture where the decomposition potentials are not widely separated may be effected by the application of controlled cathode potential electrolysis. An auxiliary standard electrode (which may be a saturated calomel electrode with the tip of the salt bridge very close to the cathode or working electrode) is inserted in the... 

Two distinctly different coulometric techniques are available (1) coulometric analysis with controlled potential of the working electrode, and (2) coulometric analysis with constant current. In the former method the substance being determined reacts with 100 per cent current efficiency at a working electrode, the potential of which is controlled. The completion of the reaction is indicated by the current decreasing to practically zero, and the quantity of the substance reacted is obtained from the reading of a coulometer in series with the cell or by means of a current-time integrating device. In method (2) a solution of the substance to be determined is electrolysed with constant current until the reaction is completed (as detected by a visual indicator in the solution or by amperometric, potentiometric, or spectrophotometric methods) and the circuit is then opened. The total quantity of electricity passed is derived from the product current (amperes) x time (seconds) the present practice is to include an electronic integrator in the circuit. 

Apparatus. The source of current is a potentiostat which is used in conjunction with a reference electrode (commonly a saturated calomel electrode) to control the potential of the working electrode. The circuit will be essentially that shown in Fig. 12.2(a) but with the addition of the integrator or of a coulometer. 

The electrode will of course be incorporated in a circuit similar to that previously described in which a potentiostat controls the potential of the working electrode and may also provide a counter-current facility to nullify the background current an electronic integrator will also be included. 

It should be pointed out that not all of the iR drop is removed by the potentiostatic control. Some fraction, called iRu (where Ru is the uncompensated solution resistance between the reference and working electrodes) will still be included in the measured potential. This component may be significantly large when resistive nonaqueous media are used, and thus may lead to severe distortion of the... 

The implications of Equation (4.30) for solid state electrochemistry and electrochemical promotion in particular can hardly be overemphasized It shows that solid electrolyte cells are both work function probes and work function controllers for their gas-exposed electrode surfaces. 

The development and the very widespread use of the polarographic technique to record i-E curves and the more recent designing of electronic devices known as potentiostats which automatically control the potential of the working electrode at a pre-set value has led to many examples in the literature of organic electrode reactions whose products depend on the potential. Some examples are cited below ... 

---