CHEM 146C Experiment 8 Surface Electrochemistry Adsorption of Polyoxometalate on Graphite Electrodes Yat Li Department of Chemistry Biochemistry University of California Santa Cruz Objective In this laboratory experiment we will learn 1 The basic concept of electrochemistry and cyclic voltammetry 2 How to study the electrochemical behavior of a surface adsorbed redox species Electrochemistry Electrochemistry encompasses a group of qualitative and quantitative analytical methods based on the electrical properties of a solution of the analyte when it is made part of the electrochemical cell stiochiometry and rate of interfacial charge transfer the rate of mass transfer the extent of adsorption or chemisorptions the rates and equilibrium constants for chemical reaction Electrochemical cell 1 Three electrode configuration Working electrode usually graphite potential is varied linearly with time Reference electrode e g Ag AgCl potential remains constant throughout the experiment Counter electrode usually platinum coil simply conducts electricity from the signal source through the solution to the working electrode 2 Supporting electrolyte non reactive electrolyte conducts electricity 3 Analyte e g redox species Cyclic voltammetry excitation signal In voltammetry a variable potential excitation signal is impressed on a working electrode in an electrochemical cell Cyclic voltammetry potential will be cycled between two potentials Same scan rate and region Triangular waveform Cyclic voltammograms For example K3Fe CN 6 A B No current no reducible or oxidizable species B D Fe CN 63 e D F Diffusion layer is extended away from electrode surface Fe CN 64 F H I Reduction of Fe CN 63 stop current becomes zero again H I J Fe CN 64 Fe CN 63 e J K A Current decrease as the accumulated Fe CN 64 used up Procedure 1 Record cyclic voltammograms of electrolyte solution with a clean graphite working electrode as a function of scan rate Procedure 2 Record cyclic voltammograms of electrolyte solution with a graphite working electrode modified with phosphomolybdic acid as a function of scan rate Procedure 3 Record cyclic voltammograms of electrolyte solution with a graphite working electrode modified with phosphomolybdic acid as function of H2O2 concentration Cyclic voltammograms quantitative information 1 Number of charge Q The integrated area under each wave represents the charge Q associated with the reduction or oxidation of the adsorbed layer Q nFA n number of electrons F Faraday constant A the electrode surface area the surface coverage in moles of adsorbed molecules per surface area 2 Capacitance C The peak current is proportional to scan rate v I vC Icap current v scan rate Cd capacitance Cyclic voltammograms quantitative information 3 Number of electrons n For a reversible electrode reaction at 25 C the difference in peak potentials Ep is expected to be Ep Epa Epc 90 6 n 4 Surface coverage When the number of electrons is known the surface coverage can be calculated by the equation Ipeak n2F2vA 4RT