Intro to Electroanalytical Chemistry

4/30/2012 Introduction to Electroanalytical Chemistry Lecture Date: April 27h, 2008 Reading Material ● Skoog, Holler and Crouch: Ch. 22 (An Introduction to ElectroanalyticalChemisty) ● See also Skoog et al. Chapters 23-25. ● Cazes: Chapters 16-19 ● For those using electroanalyticalchemistry in their work, the following reference is recommended: A. J. Bard and L. R. Faulkner, “ElectrochemicalMethods”, 2nd Ed., Wiley,2001. 1 4/30/2012 Advantages of Electroanalytical Methods Matched against a wide range of spectroscopic and chromatographic techniques, the techniques of electroanalytical chemistry find an important role for several reasons: – Electroanalyticalmethods are often specific for a particularoxidation state of an element – Electrochemical instrumentation is relatively inexpensive and can be miniaturized – Electroanalyticalmethods provide information about activities (rather than concentration) History of Electroanalytical Methods Michael Faraday: the law of electrolysis – “…the amount of a substance deposited from an electrolyte by the action of a current is proportional to the chemical equivalent weight of the substance.” Walter Nernst: the Nernst equation (Nobel Prize 1920) Jaroslav Heyrovsky: the invention of polarography: (Nobel Prize 1959) Michael Faraday (1791-1867) Walter Nernst (1864-1941) Jaroslav Heyrovsky (1890-1967) 2 4/30/2012 Main Branches of Electroanalytical Chemistry Interfacial methods Bulk methods Static methods (I = 0) Dynamic methods (I > 0) Conductometry (G = 1/R) Potentiometry (E) Voltammetry (I = f(E)) Controlled potential Amperometric titrations (I = f(E)) Constant current Electro-gravimetry (m) BasedonFigure 22-9in Skoog, Holler and Crouch,6th ed. Coulometric titrations (Q = It)  Key to measured quantity: I = current, E = potential,R = resistance, G = conductance,Q = quantity of charge, t = time, vol = volumeof a standardsolution, m = mass of an electrodispensed species Main Branches of Electroanalytical Chemistry  Potentiometry: measure the potential of electrochemical cells without drawing substantial current – Examples: pH measurements, ion-selectiveelectrodes, titrations (e.g. KF endpoint determination)  Coulometry: measures the electricity required to drive an electrolytic oxidation/reductionto completion – Examples: titrations (KF titrant generation), “chloridometers” (AgCl)  Voltammetry: measures current as a function of applied potentialunder conditions that keep a working electrode polarized – Examples: cyclic voltammetry, many biosensors 3 4/30/2012 Electrochemical Cells  Zinc (Zn) wants to ionize more than copper (Cu).  Wecan use this behavior to construct a cell: Voltmeter e- e- Salt bridge (KCl) Znelectrode 0.010MZnSO4 solution Zn  Zn2+ (aq) + 2e-a Zn 2+ = 0.010 Anode Cuelectrode 0.010MCuSO4 solution Cu2+ (aq) + 2e-  Cu(s) a Cu 2+ = 0.010 Cathode Electrochemical Cells and Analytical Methods Potentiometry: Measures equilibrium E Amperometry:Control E, measures I as functionof time Coulometry: Control E, measure total Q over a period of time control e- measurement e- working electrode indicator electrode detector electrode reference electrode counter electrode 4 4/30/2012 Electrochemical Cells Galvanic cell: a cell that produces electrical energy Electrolytic cell: a cell that consumes electrical energy Chemically-reversible cell: a cell in which reversing the direction of the current reverses the reactions at the two electrodes Conduction in an Electrochemical Cell Electrons serve as carriers (e.g. moving from Zn through the conductor to the Cu) In the solution, electricity involves the movement of cations and anions – In the salt bridge both chloride and potassium ions move At the electrode surface: an oxidation or a reduction occurs – Cathode: the electrode at which reduction occurs – Anode: the electrode at which oxidation occurs 5 ... - tailieumienphi.vn