Lecture 12: Electroanalytical Chemistry (I)

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1 Lecture 12: Electroanalytical Chemistry (I) 1

2 Electrochemistry Electrochemical processes are oxidation-reduction reactions in which: Chemical energy of a spontaneous reaction is converted to electricity or Electrical energy is used to drive a nonspontaneous reaction 2

3 Electrochemistry is everywhere Energy conversion and storage E-chem sensors Electrolysis Corrosion Materials research Paul A Garris, Nature Methods,

4 An electrochemistry cell Electrodes Working, reference, counter Two electrodes? Solution Solvent supporting electrolyte Redox species Three electrode cell: Working (WE), reference (RE), counter (CE) WE: reaction of interest studied on WE RE: provides a stable reference point for the potential on WE CE: provides a path for most of the current passes through the circuit 4

5 Electrochemical reactions e e Reactions are on surfaces Electrons as special reagent working electrode K + Cl - Fe(CN) 6 3- reference electrode Electrostatic potential can be continuously varied Reaction kinetics can be easily monitored Electrodes may participate in the reaction 5

6 Electrode potential and electrochemical reactions Electrostatic potential on the electrode generates & changes the reactions at the electrode/solution interface Co 3+ + e Co Au + + e Au Ce 4+ + e Ce AgCl + e Ag + Cl Sn e Sn

7 Electrode/solution interface 7

8 Electrochemical reaction rate Current as an expression of the reaction rate: Faraday s Law: Q = nfn i dq dt nf dn i nf dt dn i dt nf dn dt -0.5 voltage 1.2 V rate( mol s 1 cm 2 ) i nfa 8

9 Faradaic & nonfaradaic processes, double layer Faradaic process involves electron transfer at the electrode/solution interface ET Ox Red Metal IHP OHP f 1 f Double layer charging is a nonfaradaic process which only involves movement of ions in and out of the double layer ET Ox Red Metal f M f S f 2 s i s d s S = s i + s d = -s M x

10 The thickness of electric double layer is dependent on salt concentration Electric double layer f Metal 3k 1 k 1 Debye length metal f(x) f solution solution At room temperature, in H 2 O For a 1:1 electrolyte, e.g., KCl: 3k -1 ~ 10/C 1/2 Å. C molar concentration. C / M 1 3k 1 1 nm nm nm mm mm Normal region for Echem investigations 10

11 Double layer charging current Faradaic Process E i Double layer charging t E 11

12 Pathway of a general electrode process Electrode surface region Bulk solution Electrode ET O surf O bulk R surf R bulk 12

13 Mass transport Modes of mass transfer: Diffusion, Migration, Convection Nernst-Planck equation: J Diffusion coefficient, D Stokes Einstein Relation i Ci ( x) zif f( x) ( x) Di DC i i Civ( x) x RT x D kt 6 r Ions: Cl -, Ru(NH 3 ) 6 3+, nm molecule, M? 10 nm protein, P? 100 nm nanoparticle? 13

14 Standard hydrogen electrode The convention is to select a particular electrode and assign its standard reduction potential the value of V. This electrode is the Standard Hydrogen Electrode. 2H + (aq) + 2e H 2 (g) H 2 Pt The standard aspect to this cell is that the activity of H 2 (g) and that of H + (aq) are both 1. This means that the pressure of H 2 is 1 atm and the concentration of H + is 1M, given that these are our standard reference states. H + 14

15 Standard potential tables All of the equilibrium electrochemical data is cast in Standard Reduction Potential tables. F 2 + 2e 2F Co 3+ + e Co Au + + e Au Ce 4+ + e Ce Br 2 + 2e 2Br Ag + + e Ag Cu e Cu AgCl + e Ag + Cl Sn e Sn H + + 2e H Pb e Pb Sn e Sn In e In Fe e Fe Zn e Zn V e V Cs + + e Cs Li + + e Li

16 Lecture 13: Electroanalytical Chemistry II 16

17 Electrochemical reaction rate How can one use current as an expression of the reaction rate? Faraday s Law: Q = nfn dq dt i nf dn nf dt dn dt dn dt An electrochemical reaction is a heterogeneous process, so we should consider the area of the electrode i nf rate( mol s 1 cm 2 ) i nfa 17

18 i / ma Cyclic voltammetry Peak current: pa, na, µa, ma Peak potential: V or mv vs REF Peak-peak separation: (60/n) mv for a reversible reaction Scan rate: mv/s 20 E p,a 10 i p,a E E HL 0 i p,c E initial t E final -10 potential waveform -20 E p,c E E o / V 18

19 Concentration of redox at the electrode surface and the current response The Nernst equation: C t 1 t 2 5 t 6 E Red Ox ne Re d E o Red RT nf i a ln( a Ox Red ) t 3 t 5 t6 Ox Red t 3 t 4 x t 2 t 1 E 19

20 Peak current is proportional to ν 1/2! E p i E 1/2 E p/2 Diffusion controlled process! i p i p 3/ 2 1/ 2 * 1/ 2 ( ) n AD O C O E E p RT E1 / / 2 mv nf ( E 28.5/ n ( ) i p 20

21 Charging current in cyclic voltammetry Charging current is proportional to the ν! ic C DL 21

22 Ultramicroelectrodes (UMEs) r < 25 mm, ultramicroelectrode r < 100 nm, nanoelectrode 25 mm r 50 nm 6-nm Pt disk nanoelectrode 22

23 UME in neurochemistry -2e - -2H + Wightman group, UNC Oxidation of dopamine Carbon-fiber microelectrodes Venton lab, U Virginia Sombers lab, NCSU 23

24 Preparation of UMEs a) Sealing a microwire in glass b) Pulling a Pt microwire with glass Pt microwire 24

25 UME and exocytosis Soma Dendrite Axon Neurotransmitters Secretory vesicles Synapse G.D. Fischbach, Sci. Amer. 1992, 267, Neuronal Communication Ca 2+ Plasma membrane Extracellular Fluid Fusion Pore Exocytosis is a Vesicle Fusion Process 25

26 UME and exocytosis Carbon Microelectrode Amperometry: Constant applied potential Measures current as a function of time High temporal resolution Quantification of released molecules Cell Single-cell recording -2e - -2H + Oxidation of dopamine N Q nf Faraday s Law 26

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

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