A Cyclic Voltammetry Experiment Using a Mercury Electrode Enrique Brillas, J& A. Garrido, and Rosa M. Rodr@uez Universitat de Barcelona, Avda. Diagonal, 647. 08028-Barcelona, Spain Javier D o d n e c h Universitat Autbnoma d e Barcelona. Bellaterra (Barcelona). Spain where Dredis the diffusion coefficient for metal in mercury, R Cyclic voltammetry (CV) is a modern electrochemical technique that possesses extensive capabilities for the kinetis the gas constant, T is the temperature, F is the Faraday ic and mechanistic analvsis of electrode nrocesses (1.2). For constant, and ECis the cathodic peak potential. Thus, the II;l/lep ratio is inaependent of the metallic cation concentrathis reason, CV has been widely used in the study of'n&erous oreanic. inoreanic. and hioloeical redox svstems. The tion, whereas i t increases as more time is given for rising basic principles and theories of chis technique have been amalgam concentration, such as when either scan rate deresented recentlv in a format suitable for students (3.4). creases or switchine is shifted to more neeative - ootential . " Some experiments introducing students to CV and its applivalues. In addition, the difference between anodic and cacations have also been reported (5, 6). However, these exthodic neak ootentials (EB.- E )is sliehtlv hieher than 57/n periments utilize only a few electrochemical reactions, all a t mV expected for a sim&, re&rsibley diffusron-controlled, solid electrodes, and thus miss the opportunity to investielectron transfer reaction without amalgam formation (1-4). gate the preconcentration phenomenon that provides the The reversihle half-wave potential (Eli2) for the metallic basis for anodic stripping voltammetry and is observed only cation reduction can he determined ( 7 ) by means of the a t a mercurv electrode. In this oauer. expression . . .we will reoort an easv and inexpensive experiment that we have designed speriiiodlv to illustrate CV nuulication in the studs of a simole reversible reaction such' a s Cd2+ 2e- (Hg) = ~ d ( ~ g f i n aqueous solution. The Cd2+/Cd(He)svstem has heen chosen which allows the formal reduction of metallic cation (EO') to a s an example of a redox which the oxidized be calculated by species is present in the solution, whereas the reduced one forms an amalgam. The experiment demonstrates determiEd' = (RTInF) In ( D ~ J D ~ ~ ~ ) ' ~ (4) nation of the following magnitudes: the diffusion coeffiThe formal reduction potential, as well as the reversible cients for Cd2+(D,,) and for Cd in mercury (Dred),the formal half-wave one.. denend on the ionic streneth of the solution . reduction potential of Cd2+ (Eo'), and also the effects of used. varying concentration, scan rate, and switching potential.
+
+
Theory For a reversible, diffusion-controlled, electron transfer reaction between a metallic cation in solution and its corresponding metal in mercury a t a stationary electrode, cyclic voltammograms contain distinctly peaked cathodic and anodic waves for the electrolyzed species since no convective mass transfer is employed and the time interval between forward and reverse potential sweeps is relatively short. When the scan is initiated in the cathodic direction and a hanging mercury drop electrode (HMDE) is used as stationary electrode, the observed cathodic peak current (le,) in amperes a t 298 K is given by
where n is the number of transferred electrons per metallic cation reduced, A is the electrode area, u is the scan rate, Dm is the diffusion coefficient for metallic cation. and c* is the bulk concentration in moles cm-3 ( I , 2, 7). In kq 1spherical corrections to the ~ e a current k are ienored (2). On reversine the scan a t a given potential (switccing potential), the an; dic peak current (I;) does not verify eq 1due to the concentration effect of amalgam formed in the HMDE. In such a case, the ratio of anodic to cathodic peak currents (lI$/I;) is greater than unity and depends on scan rate, switchmg potential (EA),electrode radius (ro), and diffusion coefficients, according to the theoretically derived relation by Tokuda et al. (7)for any reversible system
Reagents All chemicals are reagent grade and are used as such. A 50-mL stock solution of approximately 10 mM CdS04 is prepared. From this, 25 mL of approximately 0.2, 0.5, and 1.0 mM solutions in 0.05 M LiCI04are made. This salt is employed as supporting electrolyte hecause C10; does not form ionic pairs with Cdz+. All solution concentrations must he accurately known. Apparatus The CV equipment consists of a potentiostat with potential scan capability, a recorder, and a three-electrode cell. In this work, the potentiostat used is a Bioanalytical Systems, Inc., model CV-1B. The current versus potential curves are directly displayed on a Philips model 8043 X-Yrecorder. The cell is designed to accommodate the three electrodes and a tube for deoxygenating by bubbling with a stream of Nz.The working electrode is a Metrohm model E410 HMDE. The area of the mercury drop produced is 0.0222 em2. The reference electrode ig a saturated calomel electrode (SCE) and the auxiliary electrode is a Pt wire. Procedure Solutions are introduced in the cell and are deoxygenated by purging with Nz for approximately 15 mi". Following this, Nz is allowed to flowover solutions to prevent Ozfrom reentering the cell during the experiment. The initial potential of the working eleetrode is set at -0.400 V and different cyclic voltammagrams are recorded using switching potentials of -0.700, -0.750, -0.800, -0.850, -0.900, and -0.950 V, and scan rates of 10, 20, 40, 50, 60, and 80 mV s-I. Before each scan, the mercury drop of the HMDE is renewed. The temperature of solutions is always kept at 298 K. Special care must be taken when storing and handling Kg and solutions of Cd2+due to their high toxicity and environmental perVolume 64
Number 2
February 1987
189
-
I
- 0.4
I
- 0.6
I
-0.8
Figure 2. Plot of I1$1: versus [(ZFIRTKC EX)]^.''^ for Cd2+ in 0.05 M LiClO,. Scan rate = (A) l o . (A) 20. (.)40, ( 0 ) 50. (m) 60. and (0) 80 mV s?. Temoerature = 298 K.
-1
POTENT~AL/ V vs SCE Figure 1. Cyclic voltammogramsof 0.5 mM Cd2+ in 0.05 M LiCIOlat a hanging rnercuy drop electrode. Scan rate = 10 mV s-', and temperature = 298 K. Starting potential = -0.400 V, and switching potential = (a) -0.700, (b) -0.750, (c) -0.800. (d) -0.850. (e) -0.900, and (f) -0.950 V.
sistence. All solutions and residual water containing Cd2+should be treated with NanS solutions and filtered before disposal. Results and Discussion All cyclic voltammograms show a redox couple corresponding to the Cd2-/Cd(Hg) system. Values of I;,,q, E:, and Ea for each redox couple can be easily determmed as descriged in references 3 and 4. These data give a considerahlr inforniatiun uhout the system. The anodic imd rarhndic peak potentials are found to be independent of the different exp&mental parameters employed, their separation being close to 30 mV. Such results are characteristic of a reversible two-electron transfer reaction controlled by diffusion. This is also confirmed from the observed effects of Cd2+ concentration (c*) and scan rate upon cathodic peak current. As describkdby eq 1, IC increases either as "112 or as c*. A d o t of I: versus c*) yieldi a straight line, the slope of which allgws t o determine the drffusion coeffirient for Cd?+(D.. in cm's '1. Evidence indicating amalgam formation is found in the variation of anodic peak current with the different experimental parameters. Figure 1shows the effect of the switching potential on the cyclic voltammograms of 0.5 mM Cd2+ solution at a scan rate of 10 mV s-'. A gradual increase in 8, with rising E Acan be observed. In fact, the Iq!l$ ratio is independent of Cd2+ concentration, whereas it Increases when either E xrises or u decreases. This behavior can be seen in Figure 2, where the plots of lB,//I;versus [(2FIRT)(E; EA)]O-~I* obtained for different scan rates are presented. As described by eq 2, these plots fall on straight lines with an intercept of unity on the ordinate. The values of the slope for the straight lines in Figure 2 depend linearly on (RTI
Figure 3. Plot of the values of the slope obtained in Figure 2 against (RTI ZFV)~~~'.
Z F L J ) ~ . " ~as , can be observed in Figure 3. Thus, the slope of the straight line in Figure 3 can be used to determine the diffusion coefficient for Cd in mercury (Dredin cm2 s-I). From the resulting values of D, and DEd for the Cd2+/ Cd(Hg) system, the formal reduction potential of Cd2+ can be determined bv use of eas 3 and 4. All these results can be checked against. data repo;ted in the literature (7-91. Values cd I).,,and F ' for Cd" could additionalls he confirmed hv polnroyraphir measurements of the same solutions. The wriation of anodic r)enk current with the different experimental parameters aiso demonstrates the preconcentration principle underlyinc anodic stripping .. - voltammetw. In fact, this el&tn,analytic~ltechnique is hased o n a preroncentratim pnwesu in which a rnetnllir cation (such as Cd2-) in solution is reduced by a controlled potential more nega-
tive than the reduction potential of the species, the reduced metal forming an amalgam in the HMDE (10).Under such conditions, the concentration of metal in the amalgam is found to he proportional to the metallic cation concentration and to the preconcentration time. These dependencies are from the CV results taking into account that for definite Eh and u values (i.e., for the same preconcentration time), I; varies linearly with c*, whereas for definite c* and u values, I; increases gradually with rising preconcentration time, such as when EAbecomes more negative, as Figure 1 shows.
Llterature Clted 1. G ~ I U S z., ~undomentdao l ~ ~ ~ r t r o r h ~ m~i c a~i dH Q ~W O~~chicheater, : i ~ ;,976. 2. ~ a r d A. . J.; ~ ~ ~L. R.l Els~rmcherni~al k ~ ~ ~hiethods: , ~ ~ ~ m d Ad . D ~ I~~ C ~ -~ tiom: Wiloy: New York, 1980. 3. ~ s b b o t tG. . A. J. them. ~ d u rI.S ~ S . ~ O , ~ W . 4. ~ i r ~ i n g eP.T.: r. ~ ~W. R.J. c ih s m E ~ U~C1983.60. . ~702. ~ ~ ~ S VmBenachote~.J.J.;Lewir,J.Y.;Hcinernsn, W.R.: Raaton,D.A.;Kissinger,P.T.J. Chem Educ. 1983.60.772. 6. ~ d d ~R. i~ ~ . , : ~ ~ ~K .i : ~ o h~ ~R. ~s Ko~~J.. dc h e~m .~~ d u~ e1984,61.820. .. 7. Tnkuds, K.: Enornoto, N.: Matsuda, H.; Koizurni, N. J. Elartroanmi. Chem. 1985,159, 23. 8. G ~ I UZ. ~ , R. c C ~ i RPU. r AWL c ~ P ~ . I s ~ ~ . ~ , ~ s s . 9. nriaas, E.; ~ a r r i d oJ. , A,; ~ o d ~ i ~R.u M.; e ~ ~, ~ ~J. chsrnico e ~ scripto ~ ~1 ~ 8 5h , ~ s. . 349. LO. Ewing, G.W. J. ChemEduc. 1913,50,A131.
c
Volume 64
Number 2
Februaw 1987
191
~
~
,
