Voltammetry. Voltammetry and Polarograph. Chapter 23. Polarographic curves -- Voltammograms

Transcription

1 Chapter 23 Voltammetry Voltammetry and Polarograph Electrochemistry techniques based on current (i) measurement as function of voltage (E appl ) Voltammetry Usually when the working electrode is solid, e.g., Pt, Au, GC. Polarograph A special term used for the voltammetry carried out with a (liquid) MURCURY electrode. Voltammogram The plot of the electrode current as a function of potential. Polarographic curves -- Voltammograms Tl + Typical polarographic curves (dependence of current I on the voltage E applied to the electrodes; lower curve - the supporting solution of ammonium chloride and hydroxide containing small amounts of cadmium, zinc and manganese, upper curve - the same after addition of small amount of thallium. 1

2 Mercury Droplet Electrode Electrochemical Cell Supporting electrolyte: alkali metal salt does not react with electrodes but can reduce the effect of migration and lower the resistance of the solution. Working electrode: place where redox occurs, surface area few mm 2 to limit current flow. Reference electrode: constant potential reference Counter (Auxiliary) electrode: inert material, plays no part in redox but completes circuit 2-Electrode vs. 3-Electrode Cell 2-electrode cell is OK in potentiometry-- very small i Now in voltammetry, measuring (big) i vs. applied E, but (1) Potential drops when current is taken from electrode due to solution resistance (ir drop): The actual E WE is smaller than E Appl (vs E Ref. E ) (2) Large i passes the ref. electrode instability of the reference potential (not constant) WE i E Appl R REF. E E E ir E Appl WE Ref.E E E E ir Appl Ref.E WE or E (vs E ) E ir Appl Ref.E WE E E (vs E ) ir WE Appl Ref.E 2

3 Advantages of 3- over 2-electrode Cell System Remember: In 3-electrode cell system, electrochemical cell current passes between WE and Counter electrode 3-electrode system 1. Provides great flexibility in location of the reference and the working electrodes and minimizes the effect of solution ir drop. 2. Virtually has no current passes through the reference electrode. Potentiostat Voltage source that drives the cell Supplies whatever voltage needed between working and counter electrodes to maintain specific voltage between working and reference electrode Very high impedance (so that current passes though the reference electrode is minimized) ~NOTE~ Almost all current carried between working and counter electrodes Voltage measured between working and reference electrodes Analyte dissolved in cell not at electrode surface! 3

4 Potential Excitations vs Voltammogram/Polarograms Staircase Voltammetry Square Wave Voltammetry Electrodes Mercury electrodes (liquid) dropping mercury electrode, hanging mercury drop electrode, static mercury drop electrode. Solid electrodes: mm in diameters, Pt, Au, GC. Micro(Ultramicro) electrodes: m in diameter: Pt, Au, Carbon fiber. Solid/liquid electrode: Mercury film electrodes, carbon paste electrode. Chemically modified electrodes ITO electrode (Transparent glass coated with In- SnO 2 4

5 Dropping Mercury Electrode (DME) Hanging Mercury Drop Electrode (HMDE) Static Mercury Drop Electrode Electrode material vs. Potential Window Potential window varies with material/solution due to overpotentials 5

6 Advantages of Mercury Electrodes A more reproducible surface No polish needed easy to prepare More negative potentials can be attained in aqueous systems due to overpotentials Amalgamation with heavy metals (e.g., lead and cadmium) preconcentration of metal ions very high sensitivity in stripping voltammetry n+ n+ M E E + ne M(Hg) M [Red on Hg] [OX] (+Hg) (Preconcentration Stripping) Disadvantages of Mercury Electrode Hg easily oxidized, limited use as anode (E < +0.4 V) 2Hg + 2Cl - Hg 2 Cl 2 + 2e non-faradaic residual currents (Impurities and charging current limit detection to >10-5 M cumbersome to use (toxic mercury) sometimes produce current maxima for unclear reasons maxima suppressor is needed to add In the rest of the chapter, all discussions are based on solid electrodes. X = 0 (Ox + ne Red) (reversible) 6

7 (A - ) (A) A is reduced to A - at potential E 2 i: Current F: Faraday constant A: area of electrode D: Diffusion Coefficient : diffusion layer thickness = ( Dt) 1/2 Flux Cottrell Equation i co co Do( ) x 0 Do( ) x 0, δ Dt nfa x δ * * * co 0 co 0 Doco D o( ) D o( ) δ Dt i nfa Dc nfad c Dt t * 1/2 * o o o o 1/2 1/2 7

8 Diffusion Layer Thickness D = 1x10-5 cm 2 /s δ Dt Diffusion layer m) Time, t (s) Sampled Current Voltammetry (Normal Pulse Voltammetry) 8

9 Voltammogram for a Reversible (Nernstian) Reaction Half-wave Potential (Qualitative) Limiting Current (Quantitative) i l or i d i l /2 (E-E o )/V RT i i( ) RT D E E E E 1/2 l o' R 1/2 ln 1/2 ln 1/2 nf i( ) nf Do Hydrodynamic Voltammetry Voltammetry in which analyte solution is kept in continuous motion. Two ways: Stirring the solution, and rotating the electrode. Rotating-disk electrode Electrode Rotator Flow patterns and regions of interest near the working electrode in hydrodynamic voltammetry 9

10 Linear Scan (Sweep) Voltammetry Peak-shaped i~e profile Cyclic Voltammetry Cyclic Voltammograms for a Reversible Reaction i / i 1.0 pc pa o ( Epa Epc) 57 mv/n at 25 C (independent of the scan rate) 0' E = ( Epa Epc) / 2 3/2 1/2 * 1/2 ip = constant n ADo cov o 5 (at 25 C, constant = ) 10

11 Diffusion Controlled Reversible Process For electrode adsorption process i p v Stripping Voltammetry Electrode: Hg film Detection: DPV-- Differential Pulse Voltammetry. Cd (Ultra)microelectrodes 11

12 UME Radial Diffusion Macro-disk Electrode Planar Diffusion 12

13 UMEs Applications Measure electrochemical behavior without added electrolyte (a small i small ir 0) Bioanalytical Applications. Scanning Electrochemical Microscopy imaging & chemical information, fast electron transfer rate measurement. 13