Fundamental Aspects of Polarography

Here E° is the standard reversible potential for the electrode process, F is the Faraday, and R and T have their usual significance. For simplicity, the concentrations (Cr) and (Cox)s in the electrode surface region are used in place of surface activities. 

For other potentials more negative than Er, a net cathodic current passes corresponding to the current of the polarographic wave and Er, of course, depends on the activities of oxidized and reduced forms in a reversible process. The net current density is given by Eq. (71) with /o given by Eq. (72) and ho by (73). 

In a typical polarographic study, the concentration of the electroactive species may vary from 10 to perhaps 10 molliter and if the rate of electron transfer is fast, then with increasing potential the rate of reaction (and therefore the current) becomes eventually limited by the rate of diffusion of the reducible species to the surface of the electrode. It should be noted that the current may also be limited by adsorption, or by the rate of a chemical 

In a polarographic wave for a reversible process, the ratio of activities of R and Ox species at the electrode surface progressively increases from zero at the foot of the wave, through unity at the half-wave potential to infinity as the limiting current is approached. The potentials throughout the wave are determined by the Nernst equation in (RT/F) logaR/aox and the current densities by Eq. (71) with both forward and backward component terms comparable. With increasing potential, the rate of the reaction and therefore the current increases and eventually becomes limited by the maximum rate of diffusion determined by the bulk solution concentration, the solution viscosity, and the diffusion constant. 

In an irreversible process, the potential is no longer given by a Nernst relation throughout the wave and only the forward current-potential term in Eq. (71) applies, especially at elevated potentials. Many electro-organic processes behave in the latter manner. 

---

In a later section (Section 8), some of the fundamental aspects of polarography will be briefly considered with particular reference to the question of reversibility of various electro-organic reactions studied by means of polarography. Finally, an attempt will be made to discuss those techniques complementary to the basic polarographic method, but essential for any deductions regarding the nature of intermediates and products in an organic electrode reaction, and therefore regarding its overall reaction mechanism. 

---