working of dropping electrode
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Explanation:
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•Mercury drops form at the end of a capillary delivery tube, grow with time to a certain size, and then fall off. Typical drop lifetime is on the order of 5 seconds
•Electrochemistry is done on the mercury drop (e.g. amalgam formation) and the electrode is regenerated following the loss of the each droplet
•Increasing potential is applied as a linear function with respect to time between the DME and an auxiliary electrode
•Scan time is usually on the order of ten minutes for a typical voltage ramp ranging from 0 – 2000mV•Actual potential of the DME is measured using a reference electrode, typically a SCE.
•Mercury flows continuously through the capillary and adds to the growing drop at the end of the capillary that serves as the working electrode.
•As the drop grows, its surface area increases. There is an increase in current due to electrical double-layer formation (i.e.more and more counter-ions move in to surround the exposed mercury drop surface as it grows). The charging current follows the rate of growth of the drop surface (not it’s volume).
•An average current is measured, and for an ideally polarised electrode, this average current will remain constant until a formal redox reaction can occur at the electrode surface.
•When a potential is reached during the applied voltage ramp at which a redox reaction can initiate, an immediate increase in current is observed.
•The difference between the maximum current and the residual current is termed the Diffusion Limited Current, Id, and is related to the concentration of the analyte.
•The potential at which the current is equal to the sum of the residual current and half of the diffusion current is known as the Half-wave potential, E½, and is indicative of the species present in the sample.
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