Voltametric techniques Chapter 2 Prof. Rezvani

Redox behavior of a species Electron transfer Redox behavior of a species Thermodynamic aspects Kinetic aspects

Cyclic voltammetry 1

What happen? Overview of the redox aptitude Varying applied potential to an electrode with time in a solution of the species Recording the relevant current-potential curves

What reveal? Potential of redox processes Shape of the response as a function of the potential scan rate (coupled chemical complications) Generated current by the faradic processes (proportional to concentration of the electro active species) What reveal?

The most popular voltammetric techniques in the inorganic chemistry Cyclic voltammetry The most popular voltammetric techniques in the inorganic chemistry (always coupled with complementary techniques) bases on a linear potential sweep chronoamperometric technique

What does chronoamperometry mean? Potential step Voltammetry (Chronoamperometry) Electroanalysis by measuring at a working electrode the rate of change of current versus time; the potential is controlled. time i i ∝ 1 / √t

1-Cyclic voltammetry Fig 1 The meaning of cyclic Carrying out under stationary conditions(dominant diffusive mass transport) Varying applied potential from Ei to Ef at a constant rate(single sweep voltammetry - a in fig1) Reversing scan rate at Ef in same scan rate, back to initial value(cyclic voltametry – b in fig1) Fig 1 V = 0.02 – 100 Vs-1

Linear Sweep Voltammetry Concept similar to Chronocoulometry * but a voltage sweep applied instead of a pulse. time V2 V1 * involves measurement of the charge vs. time response to an applied potential step waveform. The shape of the resulting chronocoulogram can be understood by considering the concentration gradients in the solution adjacent to the electrode surface. It is useful for measuring electrode surface areas, diffusion coefficients, the time window of an electrochemical cell, adsorption of electroactive species, and the mechanisms and rate constants for chemical reactions coupled to electron transfer reactions

1-1- reversible (Nernstian) processes A simple reversible reduction Fig2 - Trace ABC

Maximum value of increasing Initial concentration gradient fig3 Rising concentration gradient Maximum value of increasing decreasing concentration gradient

When the direction of scan is reversed Fig2 - Trace CDE Same phenomenon for the newly generated species Red

discussion Quantitative By solving fick's law: Boundary conditions: Initial (assumption: Dox=Dred=D) Semi-infinite Electrode surface If reversible Jox(0,t)= -Jred(0,t) Then:

Potential – time relationship Initial potential In which the potential scan is reversed Potential waveform: E A New Waveform Ei 0  t

Laplace’s transform and fick’s second law: 25oC Randles – sevcik equation: Forward peak current (experimental)

1-1-1- Diagnostic criteria to identify a reversible process ipr/ipf The parameter which allows one to judge the chemical reversibility of an electrode reaction Calculation Computerized instrumentation Graphical method(base line) or empirical equation ipr/ipf =1 then Red is stable reversible

What is the baseline? Graphical method (ipr)0 : (if)0 : Fig 4b Reverse peak current not corrected for the baseline (if)0 : Forward peak current at Ef Fig 4b Graphical method

Properties of the potential Reversible process Properties of the potential Epf is independent of scan rate ΔEp(250C)=59/n mV ; constant with scan rate and dependent on T ΔEp=2.3RT/nF Properties of the current ipf/v1/2 is independent of scan rate ipr/ipf=1 and constant

1-1-2-The chemical meaning of electrochemically reversible process Activation barrier of a redox process The extent of geometrical reorganization(accompanied with e.t.) geometrical reorganization reversibility

Ferrocene/ferrocenium oxidation process Fig 5 Electrochemical reversibility Maintenance of the original molecular geometry( fc fc+) Ferrocene/ferrocenium oxidation process

1-1-1-1-Cyclic voltammetry at spherical electrodes Fig 6 Radial diffusion Sigmoidal profile amount of Ox at the electrode surface in the unit time for unit area with respect to a planar electrode Micron size radius of electrode Long time or slow scan rate Domination over the linear diffusion