From Table I, it is seen that: (1) The quantity of charge to reduce completely the oxidized Pt surface by electrochemical means, Qox, agrees with the quantity of Hg(II), QII,D,produced during the chemical reduction of the oxidized surface by Hg(1). Hence, the oxidized Pt surface, on reduction by Hg(1) produces an equivalent amount of Hg(I1). (2) The difference between the amount of Hg(1) consumed at the oxidized disk electrode during its reduction, QI,D, and QII,D, should represent the amount of Hg(1) adsorbed at 0.49 V on a reduced Pt electrode, Qc. Qc agrees numerically with Qc', the charge necessary to reduce the adsorbed species present at 0.49 V, and with QUI,the charge necessary to oxidize the adsorbed species. These data support the view that the adsorbed species is Hg(1). (3) If the adsorbed species present at 0.49 V is reduced, and then oxidized, the oxidation charge Qa, is twice the Q a t . This experiment is most simply explained by the reduction of adsorbed Hg(1) on the t'F surface, followed by its oxidation to Hg(II).
(4) Finally, the quantitative use of the ring electrode showed that a soluble species, having the properties of Hg(II), is produced on the oxidation of the adsorbed species, or its reduction product. Also, the amount of Hg(I1) reaching the ring electrode, as determined from ring currents, agrees with the amount of charge consumed at the disk electrode, thus ruling out any interpretations based on a surface process involving oxidation or reduction of the Pt electrode. CONCLUSION
Hg(1) quantitatively reduces an oxidized Pt electrode in 1.OM HC104. Hg(1) is adsorbed ( ~ 3 6 0pc/cm2), on a reduced Pt surface, while Hg(I1) is not detectably adsorbed on an oxidized Pt surface. RECEIVED for review November 16, 1970. Accepted March 25, 1971. The support of the U. S. Air Force Office of Scientific Research under Grant No. AFSOR 70-1832 is gratefully acknowledged.
NOTES
Rotating Ring-Disk Study of the Underpotential Deposition of Copper at Platinum in 0.5M Hydrochloric Acid S. H. Cadle and Stanley Bruckenstein Chemistry Department, State University of New York at Buffalo, Buffalo, N . Y . 14214
DEPOSITION OF COPPER at underpotential on platinum ekctrodes has been studied in various aqueous acid media. Tindall and Bruckenstein ( I , 2) reported that the deposition of two monolayers of Cu(0) at underpotential is necessary before Nernstian behavior for copper reduction is observed in sulfuric acid solution. The second monolayer is deposited just before bulk deposition occurs. J. w. Schultz (3) has concluded that a copper species is adsorbed at underpotential in sulfuric acid media which obeys the Temkin isotherm. He also reports y = 0.83, where 2 (1 - y) represents the net charge on the adsorbed copper species. Breiter (4) has reported the underpotential deposition of copper from perchloric acid. In hydrochloric acid solution, the reduction of Cu(I1) occurs in a stepwise manner (5). At a copper concentration of 10-6M +0.20 V
E
< -0.25
V
E < -0.25 V
> E > -0.25
V
+e Cu(I) + e or Cu(I1) + 2e Cu(I1)
+
Cu(1)
-+
Cu(0)
-+
Cu(0)
(1)G. W. Tindall and S. Bruckenstein, ANAL.CHEM.,40, 1051 (1968). (2) G. W. Tindall and S . Bruckenstein, Electrochim. Acta, in press. (3) J. W. Schultz, Ber. Bunsenges. Phys. Chem., 74, 7 (1970). (4) M.Breiter, J. Electrochem. SOC.,114,1125 (1967). (5) D.T.Napp, D. C. Johnson, and S. Bruckenstein, ANAL.CHEM., 39, 481 (1967). 932
ANALYTICAL CHEMISTRY, VOL. 43, NO. 7, JUNE 1971
B. J. Bowles (6, 7) has used a radioactive isotope and an electrochemical technique to study Cu(I1) in hydrochloric acid solution and he concluded that Cu(0) is deposited at underpotential, one monolayer existing at 0.0 V. Napp and Bruckenstein (8) also studied the behavior of Cu(1I) in hydrochloric acid solution. They interpreted their ring-disk data to indicate that Cu(1) was adsorbed. In attempting to repeat the latter experiments, we experienced considerable difficulty in obtaining reproducible results, for reasons which are discussed below. Therefore we have used a different ring-disk technique to study the reduction of copper(I1) in hydrochloric acid solution with the aim of determining whether Cu(1) is adsorbed on platinum, or Cu(0) is deposited at underpotential. The principle on which this method depends requires that deposition be convective-diffusion controlled. Consider a Cu(I1) solution in hydrochloric acid. Assume that the disk electrode potential is jumped from a potential at which no current flows through the disk to one at which the reaction Cu(I1)
+ ne
+ Cu(II-n).ds,
n=O,l,or2 O T is iJim = n (11 The consumption of Cu(I1) at the disk can be verified using the ring electrode, while the oxidation state of the adsorbed species can be found from ( I ) .
EXPERIMENTAL The platinum ring-disk electrode was polished between each experiment with 0.05-p alumina. The disk area was 0.454 cm2, The electrode parameters, N and , B 2 I a , were 0.250 and 0.567, respectively. The four-electrode potentiostat and the cell have been previously described (9). All potentials reported are us. the SCE. Solutions were prepared from reagent grade chemicals and triply distilled water, and were deoxygenated with nitrogen. The supporting electrolyte was 0.5M hydrochloric acid. RESULTS AND DISCUSSION Potential jump experiments were conducted in a 2 x lO-5M Cu(I1) solution. The electrode was oxidized at $1.0 V for two minutes before stepping the electrode to 0.0 V (Figure 1, Curve A), and the disk current was recorded us. time. The current peak observed on jumping the disk electrode potential arises from double layer charging, platinum oxide reduction, and nonsteady state convective-diffusion reduction of Cu(I1). At longer times, the disk current is caused by steady-state reduction of Cu(I1). Twenty seconds after the potential jump, the disk current decreases rapidly until it reaches a limiting current one half its previous value. Ring current US. time curves were recorded during the previous experiment at ER = 0.00 V and 0.40 V (Figure 1, Curves B and C). At ER = 0.00 V, the ring electrode detects Cu(I1) by reduction to Cu(J), while at ER = 0.40 V, the ring electrode detects Cu(1) by oxidation to Cu(I1). When the disk electrode is stepped to 0.0 V, the amount of Cu(I1) reaching the ring decreases to a value that corresponds to complete shielding (5)--i.e., the surface concentration of Cu(I1) at the disk is zero. Throughout the remainder of the experiment the ring remains shielded, with the exception of a small decrease in shielding when t is in the vicinity of T . Curve C, Figure 1 shows that no Cu(1) escapes from the disk until t = T . Since Cu(I1) is consumed at the disk, while no Cu(1) escapes from the disk during the interval t < T , either Cu(1) is adsorbed, or Cu(0) is deposited at underpotential. The ratio, n, in Equation 1 is two. Thus Cu(0) is plated at underpotential. A plot of a*'*us. iD, t < T , was linear. Using n = 2, the slope of this plot gives a value of 6.9 X 10-6 cm2/sec for Dc"(II),which is in good agreement with the value reported by Napp and Bruckenstein (8). Thus it can be concluded that the underpotential plating of Cu(0) is diffusion controlled, except for times at which t 'v T . Napp and Bruckenstein's experiment, which is in disagreement with the above result was repeated. Their experiment involves plating at -0.10 V from a Cu(I1) solution in 0.5M hydrochloric acid, and then scanning the disk potential in an anodic direction. The ring electrode potential is held at -0.10 V, thus permitting the ring to detect Cu(I1) produced at:the disk. If Cu(1) is adsorbed at the disk, then on oxidizing the disk the ratio of the ring current to the disk current would be 0.25, the collection efficiency (8). However, if Cu(0) is (9) D. C. Johnson, Ph.D. Thesis, University of Minnesota, Minneapolis, Minn., 1968.
0
20
40
T I M E , SEC
Figure 1. Current-time curve for RPRDE. CcucrI,= 2 1OW5MandCEcl = OSM. Rotation speed = 2500 rpm
x
A . Disk current cs. time curve. Disk potential stepped from 1.0 V to 0.0 Vat t = 0 B. Ring shielding curve. Ring potential = 0.00 V at all I C. Ring collectioncurve. Ring potential = $0.40 V at all f
Table I. Quantity of Cu (0) Deposited at Underpotential as a Function of Disk Potential EDisk
$0.50 $0.40 +O. 30
$0.20 $0.10 0.00 -0.10
pcoullcm2 00
48 279 559 612 648 654
being oxidized from the disk, the ring current to disk current ratio would be 0.125, one half the value of N (8). In these new experiments we found 0.125, not 0.25 as reported earlier. In these new experiments, both the ring and the disk were oxidized at f1.0 V before each plating experiment. If this procedure was not followed, higher collection efficiencies were obtained. The previous erroneous value of 0.25 was obtained because the ring and the disk were not oxidized at +1.0 V at the start of each experiment. The quantity of the underpotential Cu(0) plated was determined as a function of potential by integration of the ring collection curves obtained in the previous experiment. These results are shown in Table I. At a platinum electrode with a roughness factor of 1.3 a monolayer of copper corresponds to 540 pcoul/cm2 ( I ) . Thus, according to Table I, monolayer coverage of the electrode occurs at +0.20 V, the potential at which Cu(1I) begins to be reduced to Cu(1). Maximum coverage is obtained at -0.0 V, Le., on the limiting current for the convective diffusion controlled production of Cu(1). The quantity of Cu(0) plated at underpotential was found to be independent of the Cu(1I) concentration in the range from 2 x 10-6M to 2 X lO-4M. RECEIVED for review November 23,1970. Accepted February 26, 1971. Work supported by the U. S. Air Force Office of Scientific Research under Grant No. AFSOR 70-1832. ANALYTICAL CHEMISTRY, VOL. 43, NO. 7, JUNE 1971
933
