Polarography

Polarography is another important analytical technique that is based on electrochemistry and the interaction of electrical charges with ions in solution. Polarography was discovered in 1922 by Jaroslav Heyrovsky for which he received the Noble Prize in Chemistry in 1959. The polarograph is an interesting analytical tool that reached its peak of popularity in 1950’s and 1960’s but in due course a number of its established applications were replaced by alternative techniques. It still, however, has certain areas of application for which it is extremely useful and produces excellent results. Although invented in 1922, like chromatography, it took several years to become recognized and developed into a common instrumental technique.

The Processes Involved in Polarography

Polarography is another form of linear/sweep voltammetry where the electrode potential is programmed over a specific voltage range. As the current is determined by diffusion transport of the respective ions the usual sigmoid form of the current/potential curve is realized but in a somewhat modified form. A photograph of a commercial Polarograph is shown in figure 24. The unique nature of polarography is that it employs a dropping mercury electrode (the working electrode) and a stationary mercury anode (basically a stream of mercury drops (the cathode) that fall into a static pool of mercury. the anode). The surface of the electrode is constantly renewed under dropping conditions and, thus, the conditions under which reaction takes place are readily reproducible. As the currents are solely diffusion controlled they are significantly smaller than those where convection also plays a part. The trace relating current to potential shows a series of peaks (each peak produced by a falling drop) and the height of each peak is modified as the applied potential is changed. The peak maxima for each drop will trace out the usual sigmoid shape.

Figure 24.  A Princeton Applied Research Polarograph

The maximum current (basically the current resulting from diffusion of the ions to the mercury drop) is given by the following equation,

        where (D) is the diffusion coefficient of the analyte,     

                   (n) is the number of electrons transferred per mole,        

                   (m) is the mass flow rate of mercury through the capillary,

                   (t) is the drop life time in seconds.

           and  (C) is the concentration of the analyte in the solution.

Due to the ‘noise’ on the current measurement device resulting from the mercury dropping procedure and other effects the simple system has limited sensitivity. However by suitable electronic modification of the current signal and the introduction of carefully controlled potential pulse techniques, the sensitivity of the mercury dropping electrode system can be improved by as much as three orders of magnitude. After the initial application of a potential across the electrodes no current is passed until the deposition or reduction potential of one of the solution components is reached. A sharp increase in current then occurs and the solution interface becomes denuded of reducible material and further reaction is limited by the rate at which fresh materiel diffuses to the interface from the main bulk of the solution (a function of the ion concentration). Further increase in potential results in no further increase in current until the next deposition voltage is reached when the process is repeated with the next ion type. Thus, a stepped curve is produced (the polaraogram) where the step voltage identifies the ion and the step height identifies the concentration of the respective component.