Polarography and the Dropping-Mercury Electrode DME

Mercury was used in large quantities in the chlor-alkali industry, for the production of chlorine and pure NaOH, However, its use in the industry was essentially stopped when its deadly impact on the environment was realized, and even in research its use has been limited as far as possible. In modem polarography, pressure is applied by an inert gas and the experiment is conducted using very small amounts of mercury. 

For the classical dropping mercury electrode, the diffusion-limited current is given by the llkovic equation 

The classical dropping-mercury electrode (DME) represents a rather complex system. As the volume of the drop grows, its surface moves towards the solution, while the diffusion layer is also growing. As a result, the thickness of the diffusion layer is less than that which would have been calculated for a stationary electrode. Therefore, the llkovic equation is an approximation, not an exact solution of the diffusion problem. The dependence of the diffusion-limited current on t results from the combined effects of the increasing surface area (at a rate that is proportional to t / ) and the decreasing diffusion-limited current (that is proportional to r / ). 

For a reversible process, the potential depends on the current and is expressed by 

The current-potential relationship for irreversible polarographic waves is more complex than that for the reversible situation. Instead of Eq. (4.20), one can use the approximate expression 

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