Direct current polarography DCP

The most original working electrode contains an active mercury micro-dropper. The voltammetric technique making use of this electrode is called polarography, invented by Heyrovsky in the 1920s. 

This electrode is constituted of a glass tube, maintained in a vertical position, of which the central passageway (10-70 pm in diameter), allows the transfer of mercury between a reservoir and the capillary tip, where very small droplets are formed. This dropping-mercury electrode is immersed in an unstirred solution containing the target analyte mixed with a support electrolyte. The surface of the droplet of mercury increases until it falls (around 4-5 s). The fall is most often provoked by a device producing a small impact on the electrode. Instantaneously, a new droplet, identical to the previous one, is formed presenting a fresh uncontaminated surface. 

This electrode, rather out of the ordinary, has some inconveniences. The mercury must be very pure (six-time distilled and conserved under nitrogen). Its vapours are poisonous, and after use it must be at least recovered and re-cycled. 

In the basic experiment - now rarely used - a linear changing time-dependant potential is applied to the mercury droplet, in the order of u= l-2mV/s from the initial potential selected. So, it can be written  

The mercury is quickly limited at positive potentials (-F0.25V with respect to the SCE) beyond which it is oxidized in its turn. Instead, for negative potentials it is usable up to —1.8 or —2.3 V depending upon whether the supporting electrolyte is acid or alkaline. This range offers many possibilities in analysis, particularly for the determination of heavy metals. 

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Each electroanalytical technique has certain characteristic potentials, which can be derived from the measured curves. These are the half-wave potential in direct current polarography (DCP), the peak potentials in cyclic voltammetry (CV), the mid-peak potential in cyclic voltammetry, and the peak potential in differential pulse voltammetry (DPV) and square-wave voltammetry. In the case of electrochemical reversibility (see Chap. 1.3) all these characteristic potentials are interrelated and it is important to know their relationship to the standard and formal potential of the redox system. Here follows a brief summary of the most important characteristic potentials. 

Investigation of chemical kinetics meet two limitations in direct current polarography (DCP) operating with dropping mercury electrodes the first is the narrow kinetic window given by the drop time 0.1 s < < 5 s... 

DCP Acronym for direct current polarography. See - po-larography, and subentry DC polarography. 

Direct current polarography is a basic method, from which improved analytical techniques of polarography and voltammetry have been developed. With DCP, inorganic and organic analytes can be analyzed with a sensitivity of about 10" mol/L. For determining several analytes simultaneously, their half-wave potentials must be at least 100 mV apart. 

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