The Limitations of dc Polarography

The sensitivity of most instrumental techniques is limited by the signal to noise ratio. (See also 1.5.2). 

The electrolysis cell acts like a electronic component in the circuit with its own electrical resistance, capacitance etc. When a voltage is applied a certain current will flow. This current, and the conductivity it represents, is made up of several components. The most useful is that caused by simultaneous electron exchange (oxidation and reduction) involved in the electrochemical reaction at the two electrodes. Since this latter current obeys Faraday s Law of Electrolysis it is often refered to as the Faradaic current. 

With classical dc polarography the signal is the mean Faradaic current and the principal source of noise is the capacitive current for the growing mercury drop. 

For diffusion-controlled processes the faradaic or electrolysis current grows during the drop lifetime I = ktb (Fig. 1.5d). This growth is the resultant of two opposing processes. The first is the increase in the size of the growing mercury drop causing an increase in the current. The second is the depletion of the electroactive species from the solution and growth of a depletion zone around the electrode, due to electrolysis. The latter produces a decrease in the current Fig. 3.2a shows this decrease in current for an electrode of fixed size. 

The electrode/solution interface acts like a capacitor. The higher the charge density on the surface, the higher the potential. If the surface area increases more charge is required to maintain the density. That extra charge flowing represents a current. 

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