Exp5(FeCN6CycVolt) - 59 EXPERIMENT 5 CYCLIC VOLTAMMETRY...

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59 EXPERIMENT 5: CYCLIC VOLTAMMETRY ANALYSIS OF IRON Introduction Cyclic voltammetry (CV) is a very versatile electroanalytical technique, used for the study and analysis of electroactive species. A redox reaction is often represented in the generic form of reaction (1), but can be of many different types (examples are given in reaction (2)): Ox + ne - R (1) Fe 3+ (aq) + e - Fe 2+ (aq) (2a) Cd 2+ (aq) + 2e - Cd(s) (2b) Dopamine + 2e - + 2H + H 2 Dopamine (2c) AgCl(s) + e - Ag(s) + Cl - (aq) (2d) During a CV experiment, the current which flows between two electrodes (the working and counter electrodes) is plotted on the y-axis, and the potential of the electrode of interest, vs a reference electrode potential, is plotted on the x-axis. The resulting plot is called a cyclic voltammogram (also referred to as a CV), which is sometimes described as an electrochemical spectrum. This is because the current which flows, particularly at the current peaks, is often directly proportional to the concentration of the analyte in solution, similar to an absorbance peak in spectroscopy, while the potential of the peaks, or the potential half-way between peaks, can be related to the identity of the analyte, through its E o value, analogous to the wavelength. The CV technique can be used not only to determine the concentration of species in solution, but also to determine the diffusion coefficient of the analyte, if desired, and, under certain circumstances, the rate constants for the electron transfer reaction can be determined. CV can be used in a wide range of analyses, from trace metals and inorganic complexes, to vitamins and neurotransmitters. CV is usually carried out in a small glass cell, holding ca. 20 ml of solution, but it can also be carried out in vivo , in the brain, for example. Typical Experimental Set-up
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60 In the CV technique, three electrodes are employed. The main electrode at which the desired redox reaction of the analyte is made to occur is called the working electrode (WE), usually consisting of a noble metal (Pt or Au), various forms of carbon, or Hg. The WE can take the shape of a wire, a plate, or a disc, and can range in surface area from ca. 1 cm 2 to as small as 1 m m 2 (the special domain of ‘microelectrodes’). As current flows at the WE during oxidation (electron loss from the analyte to the WE) or reduction (electron is transferred to the analyte from the WE), electrons flow to or from the counter electrode (CE), usually a high area inert Pt gauze electrode. Therefore, when the WE is the anode, the CE is the cathode, and vice versa. The reason that current flows at the WE is because its potential is changed vs. a reference electrode (RE) by the controlling instrumentation, the potentiostat. The reference electrode can be a saturated calomel electrode, SCE (E o = 0.245 V vs SHE), an Ag/AgCl electrode (E o = 0.222 V vs SHE) or others. If the potential of the WE, sometimes called the excitation signal, is made positive of E o , a net oxidation reaction occurs. If the WE potential is made negative of the equilibrium potential, reduction of the analyte occurs. In CV, the potential of the WE vs the RE is scanned linearly in time
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