Rotating Disk Voltammetry Experiment Julie L. Town, Fiona MacLaren, and Howard D. Dewald' Ohio University, Athens, OH 45701 Voltammetry a t the rotating disk electrode (RDE) is a widely used method for the study of electrode processes at solid electrodes. The benefits of a RDE are: (1) a diffusion layer is developed with a thickness that does not change with time; (2) double-layer charging has a minimal effect on the measurement; and (3) the theoretical basis of the mass transfer process has been solved and equations are available torelate the experimental parameters to the mass transfer of reactants to the electrode surface. Thus the experiment described below is designed to acquaint the advanced undergraduate or entry graduate student with the theory and application of the RDE. The RDE as shown in Fieure 1is constructed from a disk ---.of electrode material (e.g., glassy carbon or platinum) imbedded in a rod of an insulating material (e.g., glass, Teflon, Kel-F). The electrode is attached t o a motor and rotated a t a . w is the angular velocity certain freouencv. ...f = ~ 1 2 swhere (S-I). Electrical contact is made to theelectode'hy means ofa hruvh contact: details of the construction of RDE's are given . by Adams ( I ) . When an electrode is rotated, mass transfer of reactants and products is by convective-diffusional mechanisms. The theory of convective flow a t the RDE is identical with the theory of flowing fluids, hydrodynamics (21, and thus the methods are termed hydrodynamic uoltammetry. At the RDE the hydrodynamic flow pattern results from centrifugal forces that move the liquid horizontally out and away from the center of the disk while fresh solution continually replaces i t by a flow normal to the electrode surface. The mass transfer equations for the RDE are given by Levich ( 3 , 4 ) . The hydrodynamic boundary layer, y, as described above, can he given as y = 3.6(u/w)'", where v is the kinematic viscosity of the fluid (cm2s-I), and represents the ~
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thickness of the layer of fluid dragged by the rotating disk. For water (u = 0.01 cm2 s-' at 20 'C) and a rotation rate of 1000 rpm (w = 100 s-I), y is 0.036 cm. If concentration gradients are included, the convective-diffusion equation can be determined. The equation for the limiting current that is derived by Levich for a reaction controlled by mass transfer is
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' Author to whom correspondence should be addressed
352
Journal of Chemical Education
Flgure 1. Rotatlng dlsk electrode. (A) Side view. (6)Bonon view: (a) disk; (b) insulator: (c)shaft, (d) brush contact.
where it is t h e limiting current (A), n is t h e number of electrons transferred (ea mol-I), A is t h e electrode area (cmz), CO is t h e solution concentration (mol cm-9, D is t h e diffusion coefficient (cm2 s-I), a n d F is t h e Faradav constant, 96,485 C eq-'. Thus, f o r t h e RDE, t h e diffusion layer For water, D cm2 thickness, 6 = 1.61 D1I3 w-'I2 s-1,6 0.05 y. More complete accounts of t h e Levich equation a r e available (5, 6). T h e limiting current is t h e current t h a t is attained when t h e potential of t h e R D E is adjusted t o he o n t h e plateau of t h e corresponding current-potential curve for t h e reactant. Following t h e application of a potential i n t h e plateau region, t h e limiting current a t t h e R D E quickly approaches t h e steady-state value, i.e., independent of time, a s predicted by t h e Levich equation. Information about a redox system that can be ohtained by rotating disk voltammetry includes redox potentials and reversibilitv. T h e aneulnr velocitv of rotation of the R D E is T h u s llectrode mechanisms t h a t a n adjustagle involve c o u ~ l e dhomoeeneous chemical reactions and cause deviations bf t h e experimental currents from t h e Levich equation are amenable t o study with t h e R D E (7,8).
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Experimental Reagents All chemicals are reagent grade and are used as such. A 100-mL stock solution of approximately 0.1 M K4Fe(CN)s3H20 is prepared in 0.1 M KNOa. From this each of approximately 1,2,4,6,8, and 10 mM solutions in 0.1 M KN01are made. A 100-mL salution of 1mM CuClz prepared in 0.1 M KC1 is also required. All solution concentrations should be accurately calculated. Apparatus The equipment consists of a polarographic analyzer, RDE, recorder, and an electrochemical cell. The equipment used is EG&G PARC Model 264A Polarographie AnalyzerIStripping Voltammeter, Model 616 RDE, and Model RE0089 X-Yrecorder. AlternativeIv. ., other commerciallv available instruments or homemade models can he used. The electrwh~mivalrell used is an EG&C PARC Model KO0661 KOOGO polarogrnphic rrll tophotrom. The rlertn,des are a glassy carbon disk working rl~rtroderHI)EOO8,4-mm diametprl, a hlvdel ~~~~
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KO077 saturated calomel electrode (SCE) and a P t wire auxiliary electrode. The cell top has five holes to accommodate the three electrodes, a tube far deoxygenating by bubbling with Nz, and one for solution addition. Procedure Pretreatment of the elassv .. . carbon electrode surface is reouired. Polishing with a 0.05-pm alumina slurry on a Microcloth felt disk (Duehler, Lake Rluff. ILI and rinsing w:th dtlute HNO. and deiunized, distilled water suffices. The cell is assembled and filled with 10 mM KnFe(CN)dHzOin 0.1 M KNOs just until the ends of the electrodes are immersed. The solution is deoxygenated by bubbling Na through the system for annroximatelv 10 min. After deoxveenation. N9 is allowed to flow over the solu&n to nrevent 0 9 froireenter& " the cell, While the deoxveenation . " ~ ~is occurrine. ...the scan narameters can be set. \\'ith thp working elemode i n standby rswitched off or disconnected, the initial purenrlal is set ar -0'10 V,and the final pot~ntial is set st 0.70 V. A scan rate of 100 mV s-' is used. An equilibration time of 15 s is set to allow for the current to attain a constant value when the working electrode is activated. Care should be taken that no huhbles remain on the electrodes after deoxygenation. The effect of the angular velocity on the voltammograms is observed hvrecordine voltammoerams at the followine rates: 1OO. 400. 900. 1600, 2500,3fi0ll. 4900, fii(l0, and Rl00 rpm. &I, the eff&t of concentration on the magnitude of the l~mitingcurrent is ~ b ~ e r w d . This is recn by obtaining potential scans fur 1, 2, I.R,H, and 10 m u K.Ve(CX,,4H?O using a rutation rare 01 1000 rpm. A wltammopram of an unknuan frrrocyamdr sohtim shvuld he r u n a, well. 'l'hecellisrinsedand refilled withO.l \I KC..'l'heinitialpot~nrial is set at 11.4 V, and thr final putrntial a set at -0.- V . Followine, the same pretreatment and de&ygenatian procedures, the pote&ial scan is initiated usine a scan rate of 100 mV s-' and a rotation rate of 1000 rpm, and a background volrnmmogram of the rupportmg electndyw is ohtained. Agmn, the rell is rinsed and rerilled wilh 1 mhl CuCI:, in 0.1 hl KC1 Fullowing the same procedurr as ahow, a voltammogram for the stepwise reduction of Cu(I1) to Cu(1) and Cu(I1) to Cu is obtained. ~~
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Resuns and Dlscusslon T h e effect of t h e aneular velocitv on t h e annearance of t h e voltc&nogram can b e s e e n i n ~ i & e 2A. equations for t h e R D E d o n o t apply a t very small or very large values of w. When w is small, t h e hydrodynamic boundary layer becomes laree. Also. the rate a t which t h e electrode uotential is scanned &st be small with respect t o w t o allow-the steady-
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Figw.92. (A)RMetingdiskvoItammograms of 10 mM K4Fe(CNk3H20in 0.1 M KNOs. Glassy carbon electrode. Scan rate: 100 mV s-'. Rotation rate: (a) 100:(b) 400: (c) 900: (d) 1600: (e) 2500 rpm. (Not shown: 3600,4900,6400,and 8100 rpm.) (0)Plot of limiting current versus rpm"' fromvoltammograms.
Volume 68
Number 4
April 1991
353
