Phase-Selective Fundamental and Second Harmonic Alternating Current Voltammetry of Bismuth(ll1) and Lead(l1) at a Continuously Mercury-Coated Rotating Platinum Disk Electrode Natsuko Kanzaki,' Yasushi Kanzaki,' and Stanley Bruckenstein" Chemistry Department, State University of New York at Buffalo, Buffalo, New York 14214
A continuously mercurycoated rotating platinum disk electrode was used wRh alternating current (ac) and direct current (dc) voltammetry to explore its effectiveness for low concentration analysis. Well-defined fundamental and second harmonic alternating current voitamograms were obtained in 2.5 pM B I ( I I 1 ) and Pb(I1) in the presence of 108-fold excess of Hg,''. The height of the fundamental and second harmonic alternating current voltammogram of Bi(II1) was proportional to its concentration in the range 10 to 0.5 pM. Double layer charging effects interfered at lower concentrations. Direct current voltammetry of B I ( I I 1 ) had a seneltivlty limit of 0.25 pM. Fundamental and second harmonic ac voltammetry of Pb(I1) had a sensltlvity limit of 0.1 pM, as compared to 0.5 pM using the dc technique. Second harmonic ac voltammetry has a slightly higher sensltlvity than fundamental ac voitammetry in the phase-selective mode.
Alternating current (ac) voltammetry has been widely used, particularly a t the hanging mercury drop electrode (1-4). With conventional a c voltammetry, the convenient range of conM centration determination is approximately 5 X 10-' t o ( I ) . Double-layer charging effects limit the accuracy a t lower concentration, and the use of phase-selective ac voltammetry has t h e ahility in principle t o remove this complication ( 5 ) . However, the existence of non-negligible cell resistances results in a phase angle shift which interferes with the complete elimination of the non-faradaic currents ( I , 6, 7 ) . Relatively few alternating current studies have been performed a t solid electrodes, and we thought it worth while t o investigate the potential of ac methods using mercury coated solid electrodes. One such study was conducted by Subramanian and Prabhakara Rao ( 3 ) , who obtained t h e conventional ac voltammogram of Cd(I1) at a mercury-coated electrode for a rather high concentration, 5 X lo-' M. T h e continuously mercury-coated rotating platinum disk electrode (CMCRPDE)(8) appeared t o us to be a likely candidate as a n indicator electrode in our ac studies considering the satisfactory dc voltammograms obtained previously by Hassan and Bruckenstein (8) for micromolar Bi(II1) solutions. We report below the comparison of the analytical sensitivities of conventional slow scanning d c voltammetry, and of phase-selective fundamental and second harmonic ac voltammetry using the CMCRPDE for Bi(II1) and Pb(I1) in micromolar and submicromolar concentration ranges.
EXPERIMENTAL Electrochemical Techniques. A platinum disk electrode of 0.47 cm2was used. The counter-electrode, a platinum wire of 1.6 cm', was in a compartment separated by a glass frit from the Present address, Department of Chemistry, Toklo Institute of Technology, Meguro-ku, Tokyo. Japan. Present address, Department of Chemistry, Aoyama Gakuin University Chitosedai, Setagaya-ku, Tokyo, Japan.
sample compartment. The three-electrode potentiostat, cell, and rotator were essentially those described elsewhere (8, 9). A Princeton Applied Research Co. Two Phase/Vector Lock-in Amplifier, Model No. 173, was used for the ac measurement. A 6-pole Butterworth low pass filter (3 db point a t 50 Hz) was used prior to the lock-in amplifier. The ac modulation amplitude (All) was 5 mV for the fundamental ac measurements and 20 mV for the second harmonic ac measurements. A frequency of 10 Hz was used to minimize the residual current ( 6 ) . The component of the ac current in phase with the alternating voltage signal was measured and taken to be the faradaic component of the alternating current response. All potentials are reported with respect to the saturated calomel electrode (SCE). Reagents. High purity water was obtained using the Milli-Q Reagent Grade Water System (Millipore Corporation). This water had a resistivity of 18 MR cm-'. Bi(NO& was prepared by dissolving a known amount of granular bismuth metal (Fisher Certified Reagent) in excess concentrated "03 (Baker Analyzed Acid Solution). Pb(NOJ2 (Fisher Certified Reagent) and NaN03 (Baker Analyzed Reagent) were used without further purification. Hg2(N0J2 was prepared from chemically purified Hg by the method described by Marsh (10). Supporting Electrolytes. The supporting electrolytes were 0.1 M H N 0 3 in the bismuth experiments, while 0.1 M NaN03 containing lo-' M H N 0 3 solution was used in the lead experiments. Nitrogen gas was passed through the solutions to remove the dissolved oxygen before the experiments and over the solutions during the experiments. Procedure. The platinum rotating disk electrode (RDE) was polished mechanically with 0.3 pm and 0.05 pm alumina, and then was electrochemically oxidized and reduced in the deoxygenated supporting electrolyte solution (8,9, 11:i. Sufficient stock Hg22+ solutions were then added to supporting electrolyte solution sufficient to make the concentration 2.5 X lo-' M. Mercury was deposited on the freshly pretreated RDE: by holding its potential at +170 mV for 15 min without electrlode rotation. After the coating by mercury, the electrode potential was kept at +I00 mV and metal ion solution was added to the cell contents. Next, dc and ac voltammograms were obtained by scanning the potential cyclicly between +200 mV and the potential of hydrogen evolution at selected scan rates in the range 100 to 400 mV/min. The electrode rotation speed was 1600 rpm. Direct current voltammograms were compensated for the IIg,'+ reduction currents ( 150 PA) electrically (8). Alternating current voltammograms needed no compensation for the Hg,'+ reduction currents.
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RESULTS AND DISCUSSIONS Bismuth. T h e hydrogen overpotential in 0.1 M HN03 supporting electrolyte containing 2.5 X HgZ2+on a CMCRPDE was -470 t o -490 mV a t a current of 1 FA, in agreement with previously reported results (8, 9). T h e cathodic d c voltammogram of 2.5 fiM Bi(III), as shown in Figure l a , had a half-wave potential. of -30 mV, and t h e limiting current for the reduction of Bi(II1) was proportional t o its bulk concentration as shown in Figure 2 (solid circles). M. T h e T h e limit of detection of Bi(1II) is about 2.5 X results we have obtained for this system are in agreement with those previously reported (8). The nonlinearity of the residual current curve is att,ributable to t h e potential dependence of the double-layer capacitance (12). Mechanical noise appears ANALYTICAL CHEMISTRY, VOL. 49, NO. 12, OCTOBER 1977
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Figure 1. Polarograms for 2.5 X M Bi(II1) in 0.1 M HN03 2.5 X M Hg,". (a) dc current-potential curve; (b) fundamental ac ( A E = 5 mV); (c) second harmonic ac ( A € = 20 mV); (d) second harmonic ac obtained by subtracting dotted line from solid line in c. Dotted lines show polarograms obtained from 0.1 M HN03 f 2.5 X M Hg,'" solution
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M Pb(I1) in 0.1 M NaN03 Figure 3. Polarograms for 2.5 X M HNO, C 2.5 X M Hg,". (a) dc current-potential curve; (b) dc current-potential curve obtained by subtracting dotted line in a; (c) fundamental ac (A€ = 5 mV); (d) second harmonic ac (A€ = 20 mV). Dotted lines show polarograms obtained from 0.1 M NaNO,, M HNO, 2.5 X M Hg," solution
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Figure 2. Current vs. Bi(II1) concentration. (a)limiting current for dc; (0) maximum current for fundamental ac; (0) current between maximum and minimum points for second harmonic ac
Figure 4. Current vs. Pb(I1) concentration. ( 0 )limiting current for dc; (0) maximum current for fundamental ac; (0) current between maximum and minimum points for second harmonic ac
to be t.he limiting factor in this experiment. T h e phase-selective fundamental and second harmonic ac voltammograms of Ri(TI1) are shown in Figures 1b and IC. Mechanical noise is, as expect,ed, less important in these measurements than in the dc case. The summit potential of the fundamental ac current-potential curve (E,) was -45 mV and had a half height width ( E ,p2) of 55 mV. The ac residual current-potential curve shape results from the nonlinear nature of the differential capacity. As can be seen from the d a t a in Figure 2, the fundamental ac current is a linear funct,ion of Ri(TI1) concentration (open circles). T h e second harmonic ac voltammogram is distorted by a double-layer charging component, as shown in Figure IC,and its true shape i s obtained by subtracting the residual current. T h e curve resulting from this subtrzction i s shown in Figure I d , and is unsymmetricai about the zero current potential of - I 5 mV (Eo).The shapes of the second harmonic ac (and also the fundamental) voltammoprams indicate that the electrode reaction of Bi(II1) is quasi-reversible ( 7 , 1 3 ) ,and were the same in the concentration range 10 to 0.5 pM. Plots of the current difference between the maximum and the minimum in the second harmonic voltammograms were proportional to concentration in the above mentioned concentration range and are shown in Figure 2 (open squares). Comparison of the various voltammetric calibration curves in Figure 2 indicates that the second harmonic technique is slightly more sensitive t h a n in the first harmonic technique, both having a low concentration limit for the detection of bismuth of approximately 5 X lo-' M. This limitation is attributable to the background double-layer charging current ( 5 - 7 ) . T h e CMCRDE dc method is approximately two times more
sensitive than the ac method, apparently because it is less affected by variations in the differential double-layer capacitance and by kinetic complications. The current of the fundamental ac wave for bismuth deposition decreases with successive potential scans. This effect was eliminated when the electrode surface was treated with concentrated HNO:, and fresh mercury was deposited on it. No such effect was apparent in the dc voltammogram, suggesting that kinetic effects are operative. These results can be rationalized by assuming that the electrode reaction for Bi(II1) reduction is retarded by the formation of some surface compound involving bismuth and mercury (and/or platinum). The existence of a surface compound of antimony at a mercury electrode has been reported (2). Lead. The hydrogen overpotential of the 10 M H N 0 3 and 0.1 M NaNORsupporting electrolyte containing 2.5 x M Hgz2+was -580 to -600 mV at i = 1 +A. Figure 3a shows the dc voltammogram obtained in this solution in the presence and absence of Pb(I1). No well defined limiting current is observed, and if an attempt is made to correct for hydrogen evolution, by subtracting the residual current curve from the one obtained in the presence of lead, the curve shown in Figure 3b results. The latter curve shows a peak current with a decreasing current at more cathodic potentials. Presumably the lead amalgam formed during the reduction of lead has a higher hydrogen overpotential than the mercury coating on the rotating platinum disk electrode (14, 1.5). T h e half-wave potential for this process was estimated to be -470 mV. Voltammograms for various concentrations of lead were obtained, corrected for the residual current, and the peak currents obtained are plotted vs. concentration in Figure
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ANALYTICAL CHEMISTRY, VOL. 49, NO. 12, OCTOBER 1977
4 (solid circles). A linear relationship is found in the concentration range 10 to 0.5 pM. It is difficult to measure lead concentration less than 0.5 pM because hydrogen evolution begins to mask the limiting current for this process. T h e fundamental and second harmonic ac voltammogram ( E , = -460 mV, E , = 80 mV, Eo = -460 mV) obtained are shown in Figures 3c and 3d. T h e second harmonic ac voltammogram is more symmetric than that found for Bi(III), as expected from kinetic data ( 7 , 16). T h e peak fundamental ac current (open circles) and the difference in the current between the peak of the second harmonic ac current (open squares) are plotted in Figure 4 vs. concentration of Pb(II), and straight lines are obtained in both cases. In contrast to the case of bismuth, the most sensitive analytical technique is the second harmonic ac method, followed by the fundamental ac method, vith the dc method being the least sensitive by a factor of 5 as compared to the second harmonic procedure. Distinct ac voltammograms could be obtained at lead concentrations as low as 1 X 10 M Pb(J1). Comparison. The dc method is capable of detecting lower concentrations of bismuth than lead because hydrogen evolution interferes in measuring the lead limiting current, but not the bismuth limiting current. Subtraction of the residual current from the lead limiting current is not satisfactory because the overpotential for hydrogen evolution is different on the mercury and the lead amalgam surfaces. Conversely, the ac techniques yield a lower detection limit for lead than for bismuth because double-layer charging effects are more important at the positive potential regions ir. which bismuth is reduced. Also, there is partial compensation of double-layer currents by the hvdrogen evolution current when lead is reduced. T h e differences in sensitivity among all the methods for either ion are about a factor of 5. T h e failure of the ac methods to show a more marked sensitivity advantage over the dc method at the RDE results from two factors. First, convective-diffusion produces a higher faradaic current density a t the RDE than does diffusion a t a dropping mercury electrode (DME), and second, a DME has a high charging current density produced by the growth of the mercury drop. T h e two ac techniques‘ rationale for use at the DME hinges on thc rejection of double-layer charging currents with a concomitant improvement in the ratio of faradaic current ~
density to charging current density. Thus, these ac methods ordinarily produce little improvement of the lower limit of detection of an electroactive species at the RDE where there is a lesser charging current problem. For similar reasons, the advantages of second harmonic ac techniques over fundamental ac techniques at the DME do not exist for practical purposes a t the RDE. Direct current voltammetry a t the CMCRPDE will ordinarily have a sensitivity advantage over ac techniques when irreversible electrochemical processes occur. However, ac techniques will have a clear advantage if (1)two reversible reactions of similar half-wave potential are involved, or ( 2 ) an interfering kinetically slow process occurs simultaneously with a kinetically fast process and the slow process response is to be suppressed. Comparing sensitivities a t the D M E and CMCRPDE, we conclude that (1)for reversible processes the dc voltammetric method a t the CMCRDE will usually be only slightly less sensitive than the ac techniques at the DME, and ( 2 ) the CMCRDE will be more sensitive for a n irreversible process.
LITERATURE CITED (1) B. Breyer and H. H. Bauer, “Alternating Current Polarography and Tensammetry”, Interscience, New York, N.Y., 1963. (2) G. E. Batley and T. M. Florence, €/ectro,sna/. Chem., 55, 23 (1974). (3) G. Subramanian and G. Prabhakara Rao, J . Nectroanal. Chem., 70, 133 (1976). (4) 5.Kalvoda and J. Ai-chua, J . Electroam/. Chem., 8, 378 (1964). (5) J. Heitbaum and W. Vielstich, Angew: CAem., 13, 683 (1974). (6) R. D. Jee, J . Nectroanal. Chern., 69, 109 (1976). (7) D. E. Smlth in “Electroanatykal Chemistry”. Vol. 1. A. J. Bard. Ed., Marcel Dekker, New York, N.Y., 1966, Chap. 1. (8) M. 2 . Hassan and Stanley Bruckenstein, A,?a/. Chem.. 46, 1827 (1974). (9) M. 2. Hassan, Ph.D. Thesis, University of hhnnesota, Minneapolis Minn., 1973. (10) F. L. Marsh, Ph.D. Thesis, University of Minnesota, Minneapolis Minn.. 1985. (1 1) Stanley Bruckenstein and M. 2.Hassan, Anal. Chem., 43, 928 (1971). (12) A. N. Frumkin, Z.Nektrochern., 59, 807 (1955). (13) A. A. Moussa and H. M. Sammour, J . Chem. Soc., 2151 (1960). (14) A. M. Hartley, A. G. Hiebert, and J. A. Cor, J . Nectroanal. Chem., 17 81 (1968). (15) J. A. Cox, P h D Thesis, University of Illinois, Urbana, Ill.. 1967. (16) G. C. Barker, R. L. Faircloth, and A. W. Ga,rdner. Nafure(London). 181, 247 (1958).
RECEI\~ED for review March 21, 1977. Accepted June 13,1977. This work was supported by the Air Force Office of Scientific Research through AFOSR Grant No. 742572.
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