Classical or DC Polarography

A very narrow capillary is connected to a mercury column, which has a pressure head that can be raised and lowered. A drop of mercury forms at the tip of the capillary, grows, and finally falls off when it becomes too large. Typically, the level of the mercury column above the tip of the capillary is about 60 cm and the natural interval between drops is 2-6 s. The choice of mercury for the electrode is important for several reasons  

Each fresh drop exposes a new Hg surface to the solution. The resulting behavior is more reproducible than that with a solid surface, because the liquid drop surface does not become contaminated in the way solid electrodes can be contaminated. Organic contaminants or adsorbants must undergo reequilibration with each new drop and are less likely to interfere. 

As mentioned earlier, there is a high overpotential for H+ ion reduction at mercury. This means that it is possible to analyze many of the metal ions whose standard reduction potentials are more negative than that of the H2/H ion couple. It is easier, too, to reduce most metals to their mercury amalgam than to a solid deposit. Conversely, however, mercury is easily oxidizable, which severely restricts the use of the DME for the study of oxidation processes. 

Solid electrodes have surface irregularities because of their crystalline nature. Liquid mercury provides a smooth, reproducible surface that does not depend on any pretreatment (polishing or etching) or on substrate inhomogeneity (epitaxy, grain boundaries, imperfections, etc.). 

In electroanalysis, diffusion currents are quite small ( 100 pA), which means that the aqueous solution IR drop between the reference electrode and the DME can be neglected in all but the most accurate work. Electrolytes prepared with organic solvents, however, may have fairly large resistances, and in some instances, IR corrections must be made. 

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