Voltammetry. Bulk electrolysis: relatively large electrodes (on the order of cm 2 ) Voltammetry:

Transcription

1 Voltammetry varety of eletroanalytal methods rely on the applaton of a potental funton to an eletrode wth the measurement of the resultng urrent n the ell. In ontrast wth bul eletrolyss methods, the objetve s not to engage all of the analyte n reaton. Furthermore, voltammetr methods generally requre ahevement of polarzaton whereas t s ordnarly desrable to avod polarzaton n bul eletrolyss methods. Bul eletrolyss: relatvely large eletrodes (on the order of m 2 ) Voltammetry: relatvely small eletrodes (on the order of μm 2 -mm 2 ) In both ases, relatvely hgh onentratons of bul eletrolytes (that do not engage n eletrohemal reatons under the ondtons used) are used to avod mgraton.

2 Varous extaton waveforms are used for dfferent purposes.

3 Lnear sweep voltammetry: otental of worng eletrode hanged at a rate of mv/s Exess of nonreatve supportng eletrolyte to mnmze mgraton (moton of analyte ons due to external feld). Referene eletrode onneted va a hgh mpedane voltmeter. Current flows prmarly between worng eletrode and ounter eletrode. Typal worng eletrodes are a ds eletrode (left) and a merury eletrode (rght).

4 Voltage ranges assoated wth t, Hg, and C based worng eletrodes. Lmts establshed by oxdaton and reduton of water. (Hg gves lower negatve voltages due to large overvoltage of H 2 on merury.)

5 Current versus appled potental referred to as a voltammogram: By onventon, reduton (athod) urrents are postve and oxdaton (anod) urrents are negatve. + ne - -shaped urve s referred to as a voltammetr wave. ont Z lmtng urrent, l urrent lmted by polarzaton l = s a onstant, = analyte onentraton

6 The pont at whh = l /2, E 1/2, s referred to as the half-wave potental and t s approxmately equal to the standard potental for the half-reaton. Ths value an be used for dentfaton purposes. Hene lnear sweep voltammetry an be both a qualtatve and quanttatve tehnque. Two types: 1. Hgh onveton: hydrodynam voltammetry 2. Low onveton: polarography We wll dsuss only hydrodynam voltammetry Conveton suppled by ) vgorous strrng of the soluton n ontat wth the eletrode, ) rapd rotaton of the eletrode, or ) flowng analyte soluton over the eletrode. In all ases, an exess of supportng eletrolyte s used to mnmze mgraton of the analyte.

7 Near the eletrode:

8 + ne - E appl.59 = E log n E where E s the standard eletrode potental for the half-reaton, s the produt onentraton n the Nernst dffuson layer, δ, and s the analyte onentraton n the Nernst dffuson layer. δ s typally.1-.1 m, dependng upon strrng rate and solvent vsosty. Current at the eletrode s determned by the rate at whh arrves at the eletrode, whh s gven by: δ /δx where x s the dstane from the eletrode = nfd x where n s the number of e - transferred, F s Faraday s onstant, s the eletrode surfae area (m 2 ), D s the dffuson oeffent for (m 2 /s), and s onentraton (mol/m 3 ). ref

9 = x nfd ( ) ( ) nfd = = δ t the pont where, beomes l and l nfd = = δ pluggng l = nto ( ) = and rearrangng l = ( ) ( ) nfd = = δ nfd = = δ =

10 luggng E appl = l.59 and = E log n = E ref nto gves E appl.59 = E log log n n.59 l E ref t E 1/2, = l/2, and E 1 / 2.59 = E log n E solvng for E and pluggng nto the equaton for E appl yelds: ref E appl.59 = E1/ 2 log n l ths s the equaton for the -shaped urve f D and D are smlar, / 1 suh that E 1/2 E - E ref

11 Lnear sweep voltammetry an provde qualtatve (va E 1/2 ) and quanttatve (va l ) nformaton. rovded E 1/2 values are suffently dfferent (.e., they an be resolved) spees present n mxtures an be determned n a sngle expt.

12 Mxtures of the same spees n dfferent oxdaton states an also be determned Curve C: 1-4 M Fe 3+ Curve B:.5 x 1-4 M Fe 3+,.5 x 1-4 M Fe 2+ Curve : 1-4 M Fe 2+

13 Uses of lnear sweep hydrodynam voltammetry: 1.) For oxdzable or reduble spees n flowng streams, as n the outlet of a lqud hromatograph: e.g., LC/EC BS 2.) Sensors ells establshed to be seletve for partular spees (e.g., oxygen, gluose) usually a fxed V msmt (amperometry)

14 Cyl voltammetry: msmt of urrent as worng eletrode potental s swept frst n one dreton and then n the other forward san: san n dreton of more negatve potental reverse san: san n dreton of more postve potental swthng potentals: voltages at whh san reversal ours Uses: prmarly qualtatve study of redox reatons dentfaton of reaton ntermedates observaton of rapd sequental (follow-up) rxns

15

16 e.g.: Fe(CN) e - Fe(CN) mm n Fe(CN) 6 3-, 1 M n KNO 3 onts: : begnnng of san, small anod urrent due to oxdaton of water B: begnnng of reduton of Fe(CN) 6 3- to Fe(CN) 6 4- C: rapd nrease n urrent wth appled V D: maxmum athod urrent (see equaton) E: urrent deays due to extenson of dffuson layer (unstrred soluton) F: san reversal pont F-J: reoxdaton of Fe(CN) 6 4- to Fe(CN) 6 3- For a rapdly reversble redox reaton p = 2.686x1 5 D ½ υ ½ where p = pea urrent (ether athod or anod), = eletrode area, s onentraton, D = dffuson oeffent, υ = san rate (volts/s) E pa -E p = ΔE p =.59/n f ΔE p >.59/n, rate of eletron transfer s slow relatve to san rate, nets of eletron transfer an be determned va measurements taen as a funton of san rate.

17 Strppng methods: voltammetry experments nvolvng a bul eletrolyss preonentraton step followed by the removal of the analyte va a potental sweep whle measurng urrent Step 1: a fxed eletrolyss tme durng whh analyte s deposted onto an eletrode (allows for orders of magntude n preonentraton) Step 2: evoluton of the analyte va a voltage sweep of the eletrode (onentratons as low as nano-molar an be determned)