Potential reference systems, voltammetry

In fact, the potentiometric or voltammetric measurement is carried out using a conventional reference electrode (e.g. Ag+/Ag electrode).3 After measurement in the test solution, Fc or BCr+ (BPhJ salt) is added to the solution and the half-wave potential of the reference system is measured by polarography or voltammetry. Here, the half-wave potential for the reference system is almost equal to its formal potential. Thus, the potential for the test system is converted to the value versus the formal potential of the reference system. The example in Fig. 6.2 is for a situation where both the test and the reference systems are measured by cyclic voltammetry, where E1/2=(Epc+Epi)/2. Curve 1 was obtained before the addition of Fc and curve 2 was obtained after the addition of Fc. It is essential that the half-wave potential of the test system is not affected by the addition of the reference system. 

When voltammetry measurements are made in nonaqueous solvents, the problems of an adequate reference electrode are compounded. Until the 1960s the most common reference electrode was the mercury pool, because of its convenience rather than because of its reliability. With the advent of sophisticated electronic voltammetric instrumentation, more reliable reference electrodes have been possible, especially if a three-electrode system is used. Thus, variation of the potential of the counter electrode is not a problem if a second non-current-canying reference electrode is used to monitor the potential of the sensing electrode. If three-eleetrode instrumentation is used, any of the conventional reference electrodes common to potentiometry may be used satisfactorily. Our own preference is a silver chloride electrode connected to the sample solution by an appropriate noninterfering salt bridge. The one problem with this system is that it introduces a junction potential between the two solvent systems that may be quite large. However, such a reference system is reproducible and should ensure that two groups of workers can obtain the same results. 

All potentials refer to SCE, except for Pd(5-H ) for which the potential refers to Fc" /Fc. The systems are reversible except in few cases indicated by qrev (quasi-reversible) or irr (irreversible). The potentials have been determined by cyclic voltammetry on both Pt and Hg electrodes, leading to identical values, unless otherwise noted. 

Instruments suitable for voltammetry and am-perometry consist of three basic components a wave-form generator, some form of potential control, and an electrochemical cell. Modem electro-analytical systems employ a three-electrode arrangement for the electrochemical cell. A device called a poleiuiosiat is used to maintain a programmed or fixed potential difference between the two current-carrying electrodes (the working electrode and the auxiliary electrode) relative to a third electrode (reference electrode), the function of which is to provide a fixed potential reference in the cell [64], [65]. 

The Fc /Fc couple has been recommended by Gritzner and Kuta as a reference redox system for potentials in nonaqueous solutions (see Chap. 2 or [190]). Because the reference electrode of the couple can be used only under limited conditions, the potential of the couple should be measured, in general, as a half-wave potential in cyclic voltammetry. 

Cyclic voltammetry investigation on a DMSO solution of the supercomplex, abbreviated as [ Ni (Lpy))2Pt Cl2], disclosed just one reversible wave, with Ei/2 = 0.023 V vs Fc /Fc. This potential is very close to that observed for the oxidation of the reference system [Ni (7)](C104)2, under the same conditions 0.032 V vs Fc" /Fc. Moreover, the coulometry experiment on a solution of [ Ni (Lpy) 2Pt kI l2] at a potential 200 mV more positive than Ey2 showed the consumption of 2 electrons. This indicates that the [ Ni (Lpy) 2Pt Cl2] supercomplex undergoes a two-electron oxidation process according to the following two one-electron reversible steps ... 

FIGURE 1.23. Variations of the transfer coefficient with the electrode potential derived from convolutive cyclic voltammetry of the following systems with double layer correction, t-nitrobutane in acetonitrile ( ), r-nitrobutane in DMF ( ), nitrodurene in acetonitrile + 2%H20 (a), nitrodurene in acetonitrile ( ), nitromesitylene in acetonitrile (y). Data from reference 64 and references therein. 

Cyclic voltammetry was conducted using a Powerlab ADI Potentiostat interfaced to a computer. A typical three electrode system was used for the analysis Ag/AgCl electrode (2.0 mm) as reference electrode Pt disc (2.0 mm) as working electrode and Pt rod (2.0 mm) as auxiliary electrode. The supporting electrolyte used was a TBAHP/acetonitrile electrolyte-solvent system. The instrument was preset using a Metrohm 693 VA Processor. Potential sweep rate was 200 mV/s using a scan range of-1,800 to 1,800 mV. 

Occasionally, when the voltammetry solution is non-aqueous, it is difficult to find a reference electrode of known potential. If this is the case, it is useful to add a tiny amount of ferrocene, Fe(cp)2, to the voltammetry solution. The FefcplJ, Fe(cp)2 couple is wholly reversible in almost every solvent system except water, so the CV will contain all of the peaks of the analyte of interest, plus a small pair of peaks due to the ferrocene couple. The potentials of the peaks of interest can then be cited with respect to the for the ferrocene couple in the solvent system in question (cf. adding tetramethylsilane (TMS) to an NMR sample). 

The chemical stability and electrochemical reversibility of PVF films makes them potentially useful in a variety of applications. These include electrocatalysis of organic reductions [20] and oxidations [21], sensors [22], secondary batteries [23], electrochemical diodes [24] and non-aqueous reference electrodes [25]. These same characteristics also make PVF attractive as a model system for mechanistic studies. Classical electrochemical methods, such as voltammetry [26-28] chronoamperometry [26], chronopotentiometry [27], and electrochemical impedance [29], and in situ methods, such as spectroelectrochemistry [30], the SECM [26] and the EQCM [31-38] have been employed to this end. Of particular relevance here are the insights they have provided on anion exchange [31, 32], permselectivity [32, 33] and the kinetics of ion and solvent transfer [34-... 

---