Voltammetry Stirred - Advanced Analytical Chemistry - Lecture Slides, Slides of Analytical Chemistry

Download Voltammetry Stirred - Advanced Analytical Chemistry - Lecture Slides and more Slides Analytical Chemistry in PDF only on Docsity! Voltammetry (stirred) • Stationary electrode • Stirred = mass transport by convection • Vary potential linearly with time • Measure current vs time Theory assume Ox + n e-
 Red - both Ox and Red are soluble - reversible reaction (electrochemically) - potential varies docsity.com Define - Limiting Current as steady state current when [Ox] = 0 at electrode surface i.e., applied potential is sufficiently cathodic such that all Ox is reduced at electrode E I transport-limited current limiting current nFADCbulkI = -----------------δ Nernst controlled current RT [Red]E = Eo - ----- ln --------nF [Ox] Gives quantitative information docsity.com Linear Scan Voltammetry (stirred) I E Half wave potential (E½) is E when I = IL/2 IL IC E½ Ox + e-
Red docsity.com Linear Scan Voltammetry (stirred) I E I is proportional to [Red], IL represents the situation where Red is maximum and Ox is zero. IL IC E½ Ox + e-
Red docsity.com Linear Scan Voltammetry (stirred) I E When I = IL/2, then [Red] = [Ox] and E = E½ IL IC E½ RT (IL – I) E = E½ - ------ ln ---------- nF I Ox + e-
Red docsity.com Linear Scan Voltammetry (stirred) • Normally use Pt or C (graphite) electrodes • Better to use rotating electrode than stir bar • LSV can be used for quantitative analysis • Can measure many metal ions & organics • Fairly sensitive due to convective mass transport, i.e., IF is large • The output signal in the form of a wave is considered a drawback – can be difficult to perform data analysis – multiple components gives stacked waves docsity.com Linear Scan Voltammetry (stirred) I E For a two component system it is difficult to measure the second species in the presence of a large concentration of the first species IL IL The second IL becomes small since the range is set by the first species This problem is inherent for techniques that produce waves docsity.com Voltammetry (unstirred) • Stationary electrode • Unstirred solution = mass transfer by diffusion • Vary potential linearly with time • Measure current vs time Theory assume Ox + n e-
 Red - both Ox and Red are soluble - reversible reaction (electrochemically) - potential varies docsity.com Linear Scan Voltammetry (unstirred) I E More than one component Can be difficult to determine baseline for second peak IP ? docsity.com Applied Signal for Linear Scan Voltammetry E time Slope = scan rate in mV/sec docsity.com Convention for plotting current & potential Current due to reduction Current due to oxidation reducing potentials oxidizing potentials Ecathodic Ianodic Icathodic Eanodic Ox + e-
Red Red - e-
Ox docsity.com Convention for plotting current & potential Ecathodic Ianodic Icathodic Eanodic Ox + e-
Red Red - e-
Ox A B C D E F H GI J Going from A to B to C to D is the same as for LSV in an unstirred solution scanning in one direction docsity.com Convention for plotting current & potential Ecathodic Ianodic Icathodic Eanodic Ox + e-
Red Red - e-
Ox A B C D E F H GI J Going from D to E there is a sharp decrease due to the change in sign of the capacitive current docsity.com Convention for plotting current & potential Ecathodic Ianodic Icathodic Eanodic Ox + e-
Red Red - e-
Ox A B C D E F H GI J Going from E to F the current decreases slowly due to capacitive current building (opposite in sign) docsity.com ∆EP/2 increase with ν < 1 Not constant with ν Irreversible Charge Transfer Constant with ν1 Constant with ν Reversible Charge Transfer ∆EP/2Ianodic/IcathodicIP/ν½ CV Diagnostics Ecathodic Eanodic Icathodic Ianodic ∆EP docsity.com Ox + e-
Red Red - e-
Ox ? Cyclic Voltammetry of Complex Systems Red 
A A - e-
B docsity.com Cyclic Voltammetry • Powerful technique for elucidating mechanisms of oxidation & reduction • Good for studying electrode kinetics • http://www- biol.paisley.ac.uk/marco/Enzyme_Electrod e/Chapter1/Cyclic_Voltammetry1.htm docsity.com