Introduction and Theoretical Basis

The current-versus-voltage curves obtained with the DME are very reproducible, since the surface of every new mercury drop is fresh, clean, and practically unaffected by electrolysis at earlier drops. The total amount of electrolysis is very small because of the small area of the electrode and the small currents involved for example, with 20 ml of a typical solution, 100 polarograms can be recorded without noticeable change in the curve. The small size of the DME permits the analysis of small volumes of solutions if necessary, less than 0.01 ml can be used. 

Mercury is chemically inert in most aqueous solutions, and hydrogen is evolved on it only at quite negative potentials consequently, the reduction of many chemical species can be studied at mercury electrodes, but not at electrodes made of most other materials. However, the anodic dissolution of mercury makes it impossible to study reactions at potentials more positive than about -1-0.4 V versus the saturated calomel electrode (SCE). 

When one polarographic wave is preceded by another, the limiting current is measured as the difference between the two plateaus, which are usually parallel to one another the wave heights are almost always additive. 

The Ilkovic Equation. The current that flows through the polarographic cell depends on the rate of the electrode reaction and on the rate of transport of the electroactive species to the electrode surface. At sufficiently negative potentials (that is, where the limiting current is observed) the rate of the electrode process is so fast that the rate of transport of the species to the surface becomes the limiting factor. 

In the absence of migration (eliminated by addition of supporting electrolyte) and convection (prevented by keeping the electrolyzed solution unstirred), diffusion is the only mode of transport involved. Therefore, the limiting current is proportional to the rate of diffusion, and 

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