Polarography diffusion current

Ilkovic equation The relation between diffusion current, ij, and the concentration c in polarography which in its simplest form is... 

Polarography Electrochemical reaction Exchange of charge Diffusion current Current, voltage i=m Polarogram ... 

Diffusion Currents. Half-wave Potentials. Characteristics of the DME. Quantitative Analysis. Modes of Operation Used in Polarography. The Dissolved Oxygen Electrode and Biochemical Enzyme Sensors. Amperometric Titrations. Applications of Polarography and Amperometric Titrations. 

Polarography is the classical name for LSV with a DME. With DME as the working electrode, the surface area increases until the drop falls off. This process produces an oscillating current synchronized with the growth of the Hg-drop. A typical polarogram is shown in Fig. 18b. 10a. The plateau current (limiting diffusion current as discussed earlier) is given by the Ilkovic equation... 

To learn that in polarography, the magnitude of the diffusion current /j is proportional to analyte concentration according to the llkovic equation. 

Diffusion current, /d The maximum current in polarography (see Section 6.3.2). 

Figure 19.3—Polarographic cell and diffusion current. Dissolved oxygen, which leads to an interfering double wave, has to be removed from the sample solution by degassing. On the right features of the diffusion current are shown. These increase with time for every drop of mercury in a static (unstirred) solution. Direct polarography is a slow method of analysis. More than 100 droplets are needed to record the voltammogram.

Voltammetry is a collection of methods in which the dependence of current on the applied potential of the working electrode is observed. Polarography is voltammetry with a dropping-mercury working electrode. This electrode gives reproducible results because fresh surface is always exposed. Hg is useful for reductions because the high overpotential for H+ reduction on Hg prevents interference by H+ reduction. Oxidations are usually studied with other electrodes because Hg is readily oxidized. For quantitative analysis, the diffusion current is proportional to analyte concentration if there is a sufficient concentration of supporting electrolyte. The half-wave potential is characteristic of a particular analyte in a particular medium. 

As Equation 3.28 shows, the diffusion current id is directly proportional to the concentration of the electroactive species. Consequently, polarography has been used extensively for the analysis of solutions containing electroactive materials. The optimum concentration range for determinations by conventional polarography as described here is lO -lO-4 M. Reproducibility is generally 3%. 

Polarography is valuable not only for studies of reactions which take place in the bulk of the solution, but also for the determination of both equilibrium and rate constants of fast reactions that occur in the vicinity of the electrode. Nevertheless, the study of kinetics is practically restricted to the study of reversible reactions, whereas in bulk reactions irreversible processes can also be followed. The study of fast reactions is in principle a perturbation method the system is displaced from equilibrium by electrolysis and the re-establishment of equilibrium is followed. Methodologically, the approach is also different for rapidly established equilibria the shift of the half-wave potential is followed to obtain approximate information on the value of the equilibrium constant. The rate constants of reactions in the vicinity of the electrode surface can be determined for such reactions in which the re-establishment of the equilibria is fast and comparable with the drop-time (3 s) but not for extremely fast reactions. For the calculation, it is important to measure the value of the limiting current ( ) under conditions when the reestablishment of the equilibrium is not extremely fast, and to measure the diffusion current (id) under conditions when the chemical reaction is extremely fast finally, it is important to have access to a value of the equilibrium constant measured by an independent method. 

Diffusion currents. Half-wave potentials. Characteristics of the DME. Quantitative analysis. Modes of operation used in polarography. The dissolved oxygen electrode and biochemical enzyme sensors. Amperometric titrations. Applications of polarography and ampero-metric titrations. 

Polarography (discovered by Jaroslav Heyrovsky in 1922) is a technique in which the potential between a dropping mercury electrode and a reference electrode is slowly increased at a rate of about 50 200 mV min while the resultant current (carried through an auxihary electrode) is monitored the reduction of metal ions at the mercury cathode gives a diffusion current proportional to the concentration of the metal ions. The method is especially valuable for the determination of transition metals such as Cr, Mn, Fe, Co, Ni, Cu, Zn, Ti, Mo, W, V, and Pt, and less than 1 cm of analyte solution may be used. The detection hmit is usually about 5 X 10 M, but with certain modifications in the basic technique, such as pulse polarography, differential pulse polarography, and square-wave voltammetry, lower limits down to 10 M can be achieved. 

Single-sweep i-E curves are step-like in shape for kinetic currents whereas, for diffusion currents, they show peaks. Similarly the incisions on the dEjdt = f E) curves recorded in oscillographic polarography are L-shaped for kinetic currents, whereas they are V-shaped for diffusion currents. 

Diffusion current is the limiting current observed in polarography when the current is limited only by the rate of diffusion to the dropping mercury electrode surface. 

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