Irreversible systems polarographic waves

Many handbooks like the CRC Handbook of Chemistry and Physics provide, on behalf of electrochemistry investigation, values of standard reduction potentials, listed either in alphabetical order and/or in potential order. These must be considered as potentials of completely reversible redox systems. In current analytical practice one is interested in half-wave potentials of voltammetric, mostly polarographic analysis in various specific media, also in the case of irreversible systems. Apart from data such as those recently provided by Rach and Seiler (Spurenanalyse mit Polarographischen und Voltammetrischen Methoden, Hiithig, Heidelberg, 1984), these half-wave potentials are given in the following table (Application Note N-l, EG G Princeton Applied Research, Princeton, NJ, 1980). 

Hence, for a reversible system, the well-known linear relation is obtained between the potential E and log (/iim -///). Other equations have been derived for those reversible systems that involve semiquinone formation, dimerization, or the formation of complex compounds with mercury. Logarithmic analysis of the polarographic wave is often the only proof of reversibility which is considered but recently several authors, in particular Zuman and Delahay, " have pointed out that it is inadequate to assume that an electrode process is reversible on this evidence alone. For a reversible reaction, plots of E vs. In (/lim - ///) give the electron number z from the slope of the plot, RT/zF, A clearer indication of irreversibility is the evaluation of slopes of log i-E curves for higher concentrations (for i < /lim). Irreversible processes will give Tafel behavior. 

The Metrohm 646 VA Processor is another microprocessor based instrument. Used in tandem with the 647 VA Electrode Stand and 675 VA Sample Changer, this system is capable of performing automated data acquisition, including the use of the standard additions method. Data analysis features include smoothing and differentiation, and a peak shape analysis routine that performs independently of the base current. Pulse polarographic techniques that can be performed include dp, which can be optimized for reversible and irreversible systems staircase, with current measurement during the final 20 ms of each current step and Barker square wave, which employs a waveform composed of five square wave oscillations superimposed upon a staircase, with currents measured for 2 ms at the end of each half cycle of the second, third and fourth oscillations. The 1988 price of this instrument is 14,000. 

Other examples are irreversible systems, in which the electrode process is accompanied by a proton-transfer reaction. Such systems, e.g. benzaldehyde, also show two waves on polarographic curves in unbuffered solutions. The more positive wave corresponds to the reduction at lower pH-values. When the rate of supply of hydrogen ions is not sufficient, the reduction occurs at... 

Mishra and Gode developed a direct current polarographic method for the quantitative determination of niclosamide in tablets using individually three different buffer systems, namely Mcllraine s buffers (pH 2.20 8.00), borate buffers (pH 7.80—10.00), and Britton Robinson s buffers (pH 2.00—12.00) as well as 0.2 M sodium hydroxide. The drug was extracted from the sample with methanol, appropriate buffer was added to an aliquot and the solution then polarographed at the dropping-mercury electrode versus saturated calomel electrode at 25°C [36], The resultant two-step reduction waves observed were irreversible and diffusion-controlled. For the quantitative determination, the method of standard addition was used. Niclosamide can be determined up to a level of 5—10 pg/mL. 

Initial dissociation of a NCS ligand is also thought to lead to the electrochemically active [Rh(SCN)s]2- ion. One irreversible reduction wave ( l/2 = —0.36V vs. SCE) was observed in a polarographic study of Rh111 in aqueous NCS solution, but the system was complicated by poisoning of the DME due to catalytic decomposition of SCN- and sulfate formation.1168... 

The shapes and positions of irreversible waves can furnish only kinetic information. One may be able to determine such parameters as kf, 1, or a, but thermodynamic results, such as and free energies, are not available (28, 33, 34). As a rule of thumb, a system with > 2 X 10 cm/s appears reversible on the classical polarographic time scale of a few seconds when D is on the order of 10 cm /s. A heterogeneous charge transfer with < 3 X 10 cm/s will behave in a totally irreversible manner under the same conditions, and one can evaluate the rate parameters as described above. Systems with between these limits are quasireversible, and some kinetic information can be obtained from them through the treatment prescribed by Randles (33, 34). Naturally, the precision of the kinetic information deteriorates as the reversible limit is approached. See Section 5.5.4 for much more information about the interpretation of irreversible waves. 

---