present system. It differs from differential polarography discussed in the section immediately above in that the incremental derivative is produced on a point by point basis rather than a scan to scan basis. It should be noted that the recorded signal differs only in shape from a true derivative by a constant (9). Despite these shortcomings, this technique serves the same function as the true derivative in restoring the base line between successive polarographic waves and aids the analyst in detecting electroactive species in the presence of one another which are, potential-wise, poorly separated. Figure 14 shows the effect of the stair-step potential height on the detection of 1.15 to 1 molar ratios of indium and cadmium whose half-wave potentials are separated by only 50 mV in 0.1M KC1 supporting electrolyte. Although the detection improves as the potential step becomes smaller, even a IO-mV step is sufficient to show that two species are present. Complete resolution of these two elements in this supporting electrolyte is, of course, quite impossible (21). (21) E. P. Parry and R. A. Osteryoung, ANAL.CHEM.,37, 1634
(1965).
Table 111, the first three columns, are data stored on three consecutive incremental derivative scans of the same solution through the half-wave potential region. The same starting potential was used for each scan, the potential increasing from bottom of the column to the top. The total number of counts agree within a few hundredths of a per cent with the average of the three totals, although small variations are evident on a scan to scan basis. ACKNOWLEDGMENT
We thank Dr. R . M. Diamond, Dr. E. H. Huffman, and Dr. E. K. Hyde for their support in this undertaking. RECEIVED for review November 19,1970. Accepted February 19, 1971. This work was done under auspices of the U. S. Atomic Energy Commission, and was presented in part at the 13th Conference on Analytical Chemistry in Nuclear Technology, held at Gatlinburg, Tenn., September 30-October 2, 1969.
Rotating Ring-Disk Study of the Reduction of Oxidized Platinum by Mercurous Mercury and Its Adsorption on Reduced Platinum Stanley Bruckenstein and M. Z. Hassan Department of Chemistry, State University of New York at Buffalo, Buffalo, N. Y. 14214 A potential-time study at a rotating platinum disk electrode showed that Hg(l) in 1.OM HClO, reduces an oxidized platinum surface. Using a rotating platinum ring-disk electrode, the time-dependent ring collection curves demonstrated that the amount of Hg(ll) produced was less than the amount of Hg(l) consumed during the reduction of the oxidized platinum surface. The quantity of Hg(ll) produced agreed with the electrical charge required to reduce the oxidized platinum surface. The difference between the amount of Hg(l) consumed and Hg(ll) produced (357 f 47) pC/cmz corresponds to the amount of Hg(l) adsorbed on a reduced Pt surface. The quantity of adsorbed Hg(l) was independently established by determining the charge (1) required for reduction to Hg(0) on the disk; (2) required for oxidation to Hg(ll) on the disk before and after reducing the disk; and (3) corresponding to the amount of Hg(ll) produced in (2) using the ring electrode.
VARIOUSVOLTAMMETRIC STUDIES describing the reduction of Hg(1I) or Hg(1) to mercury on reduced Pt are reported in the literature (1-6). However, the oxidation of Hg(1) at an oxidized Pt electrode and the chemical reduction of Pt oxide (1) A. M. Hartley, A. G. Hiebert, and J. A. Cox, J. Electroanal. Chern.,17,81(1968). ( 2 ) F. L. Marsh, Ph.D. Thesis, University of Minnesota, Minneap-
olis, Minn., 1965. (3) J. A. Cox, Ph.D. Thesis, University of Illinois, Urbana, Ill.,
1967. (4) G. D. Robbins and C . G. Enke, J . Electroanal. Chem., 23, 343
(1969).
( 5 ) A. G. Hiebert, Ph.D. Thesis, University of Illinois, Urbana,
Ill., 1967. (6) R. L. Brubaker, Ph.D. Thesis, Princeton University, Princeton, N. J., 1966. 928
0
ANALYTICAL CHEMISTRY, VOL. 43, NO. 7, JUNE 1971
by Hg(1) have not been described, nor has evidence for the adsorption of Hg(1) and/or Hg(I1) on a Pt surface been given. The rotating ring-disk method is ideally suited to investigate such processes, and we report, in this paper, evidence concerning these processes. THEORETICAL
Thermodynamic considerations based on current-potential data for the reduction of an oxidized Pt surface and the standard potential of the Hg(II)/Hg(I) couple predict that the reaction Pt(axidized)
Hg(1)
=
Hg(I1)
+ Pt(reduoed)
(1)
should be spontaneous in 1.OM HCIOl solution containing Hg(1) but no Hg(I1). The occurrence of reaction 1 can be studied quantitatively at a rotating platinum ring-disk electrode (RPRDE) and the approach we have used is given below. First, the oxidized disk of the RPRDE is allowed to react with a Hg(1) solution while the ring electrode is held at a potential where the reaction
-
+
(i~,,
-iddt
Hg(1) Hg(I1) e (2) occurs. No current flows through the disk electrode in this or the next experiment since the disk is not in any electrical circuit. In this experiment, integration of the time-dependent ring electrode current yields the amount of Hg(1) consumed by reaction 1. The amount of Hg(1) consumed at the disk may be calculated from Equation 3, which is a charge balance between ring and disk, when the ring potential is oxidizing. QI,R =
1
= NIQI,D
(3)
In Equation 3, the subscripts R and D refer to the ring and disk electrodes and the subscripts t and m to the time-dependent and the steady-state ring current, iR. QI,Dis the charge associated with consumption of Hg(1) at the disk, and QI,Ris the amount of Hg(1) which fails to reach the ring as a result of the consumption of Hg(1) at the disk. NI is the RPRDE collection efficiency in this experiment. Second, the above experiment is repeated at a ring electrode potential where the overall ring electrode reactions are Hg(I1) + Hg
or [Hg(I)Iz
-
+ 2e
2Hg
+ 2e
(4)
(5)
In this experiment, integration of the time-dependent ring electrode current yields the amount of Hg(I1) produced by reaction 1. The amount of Hg(I1) produced at the disk may be calculated from Equation 6, which is a charge balance between ring and disk when the ring potential is reducing QII,R
=
Lrn
( i ~ ,t hz,m)dt= NIIQII,D
(6)
In Equation 6 , QII,Dcorresponds to the amount of Hg(I1) produced at the disk. NII corresponds to the geometric collection efficiency, N, while NI is about 85% of N because kinetic complications exist at oxidizing ring electrode potentials. If Hg(I1) is not adsorbed on an oxidized disk electrode, while Qc coulombs of Hg(1) are adsorbed on the reduced disk electrode, charge balance yields QI,D
= QII,D
+
(7)
Qc
or, substituting Equations 3 and 6 in 7, and solving for Qc
QC =
QI,D
-
QII,D
=
QI,R/NI- QII,R/NrI
(8)
Qc can be determined independently by linearly sweeping the disk electrode potential, w = 0 ( W = electrode rotation speed in revolutions per minute), from the rest potential attained after all the oxidized Pt surface has been reduced by reaction 1, to 0.0 V US. SCE, and reversing the scan to 1.35 V. During the cathodic sweep, a peak voltammogram will be observed because of the reaction [Hg(I)lads
+e
+
Hg
(9)
The charge corresponding to reaction 9 is Qc’ (= Qc). On the anodic sweep, another peak voltammogram will be observed with area Qa, given by Qa =
2Qc’
(10)
because of the reaction Hg
+
Hg(1I)
+ 2e
(11)
Alternatively, Qc can also be determined by sweeping the disk electrode potential of the chemically reduced electrode initially in an anodic direction. In this experiment, an anodic peak voltammogram is produced by the reaction [Hg(I)l.d.
-,Hg(I1) + e
and the area under the anodic peak ea’
= Qc’
Qat
(12)
is given by (13)
The occurrence of reactions 11 and 12 can again be quantitatively verified by simultaneously studying ring current-disk
potential curves with the ring potential set at a value where the ring electrode will detect and determine soluble Hg(I1) species. Integration of these ring current-time curves leads to another technique for determining Qat. EXPERIMENTAL
Cell and Chemicals. A 500-ml round bottomed borosilicate flask was used as the cell. The counter electrode, reference electrode, and other cell components are described elsewhere (7). The cell was cleaned at least once a week with a warm mixture of concentrated H&04 and ” 0 3 for 48 hours and then was rinsed thoroughly with triply-distilled water before being used. All experiments were performed at room temperature ( i 0 . 5 “C), which was (23 2) “C. Dissolved oxygen was removed from the 1.OM HC104 supporting electrolyte by passing purified and water saturated Linde’s nitrogen through the solution for about 20 minutes using a gas dispersion tube. Nitrogen was passed over the surface of the solution during experiments. All potentials were measured and are reported with respect to the saturated calomel electrode (SCE). HC1O4, l.OM, was prepared by diluting -7Ox “Baker Analyzed” perchloric acid. A 0.1M Hgz(C104)2stock solution was prepared and standardized by Pugh’s method (8). Hg(C104)z, 0.2M, needed to prepare a standardized 0.1M Hg2(C104)2stock solution by Pugh‘s method (8),was made by dissolving Mallinckrodt mercuric oxide in HC104. Triply distilled water was used ; the second distillation was performed with alkaline KMn04. Electrode and Rotator. The RPRDE used was made by the Pine Instrument Company, Grove City, Pa. It had a disk radius rl of (0.378 i 0.002) cm, an inner ring radius, rz, of (0.398 i 0.003) cm, and an outer ring radius, r 3 ,of (0.442 k 0.004) cm. The area of the disk electrode, A , was (0.449 0.005) cm2. The theoretically predicted parameters N, p 2 / 3 and S ( 9 ) for the electrode used, were, respectively, (0.178 f 0.016), (0.368 i 0.029), and (0.519 0.010). See reference 9 for the definition of these symbols. The above values of N, p2I3,and S were obtained from 27 independently determined values for each parameter and are based upon the extreme and average values of rl, r2, and r3. Experimentally determined average values of NII,(32/3,and S using the reduction limiting currents for Hg(1) and/or Hg(1I) solutions, using the method described in reference 7, were, respectively, (0.188 i 0.008), (0.377 0.015), and (0.485 0.005). The experimental values of NII,/3*13, and S were used for interpreting the data reported here, since they are indistinguishable from the theoretical values. NI was found to be 0.85 NIXby studying the steady-state current-potential curves (i measured at E = 1.15 V) for the oxidation of Hg(1) at the disk as well as at the ring. Both the shielded and unshielded steady-state ring currents were determined. The variable speed (400-1000 rpm) rotator used in the study was also made by the Pine Instrument Company, Grove City, Pa. Instrumental. A four electrode potentiostat built with solid-state operational amplifiers, was used in these studies. The analysis and the circuit diagram of a four electrode potentiostat have already been discussed by others (7, 9-11).
*
*
*
(7) D. C . Johnson, Ph.D. Thesis, University of Minnesota, Minneapolis, Minn., 1967. (8) W. Pugh,J. Chem. Soc., (London), 1937,1824. (9) D. T. Napp, D. C. Johnson, and S. Bruckenstein, ANAL. CHEM., 39,481 (1967). (10) B. Miller, J. Electrochem. Soc., 116,1117 (1969). (11) D. T. Napp, Ph.D. Thesis, University of Minnesota, Minneapolis, Minn., 1967. ANALYTICAL CHEMISTRY, VOL. 43, NO. 7, JUNE 1971
929
1.2
0.8
0.4
0
Ell Figure 1. Current-potential curve of Pt in 1.OM HC104
EDheld at 1.15 V, w = 2500, for 10 minutes before being scanned from 1.15 V to 0.00 V to 1.3 V at 100 mV/sec. -1st cathodic scan; 1st anodic scan
---
L
I
0
IO0
I
1
200
300
1
t,s e c s. Figure 3. Time dependence of ring current, in = 0
--
Disk opened at ED = 1.15 V; w = 2500; -Hg(I) disappearing, ER = 1.15 V; -Hg(II) appearing, ER = 0.25 V; iR - t curve without Hg(1) was a horizontal straight line
t
1.1 0
In the presence of Hg(1) or Hg(1I) in 1 .OM HC104, once Hg has been deposited on the disk electrode, the above pretreatment procedure fails to return the electrode exactly to the “reproducible” state. However, no serious errors are made if three successive experiments are performed before the electrode is repolished and pretreated as above. RESULTS
0
100
t,secs.
300
200
Figure 2. Time dependence of potential of oxidized Pt in presence of 2.22 X 10-5M Hg(1)
--
2500; --ED us. in 1.OM HCIOI; -EDus. t in 1.OM HClOa containing 2.22 X 10-5M Hg(1) w =
Experimental data were automatically recorded by an EA1 1131 X, Y, Y1Variplotter. All numerical disk electrode data are the average result of a minimum of seven replicate experiments conducted on several different days. Preparation of Reproducible RPRDE Surface. The RPRDE was initially polished using standard metallographic methods, and before each experiment was polished lightly with 0.3 p alumina. To prepare a “reproducible electrode,” both the ring and the disk were anodized at 1.15 V for ten minutes in 1 .OM HClOa. The RPRDE was rotated at the indicated w during anodization. Both the disk and the ring were then cathodized for ten minutes at 0 V, w = 0. Such a “reproducible” electrode gave the same i - E curves as those obtained in the fifth or later scan of an electrode that is cycled between 1.2 and 0.2 V in 1 .OM HClO4 (12). (12) D. C . Johnson, D. T. Napp, and S. Bruckenstein, Electrochim. Acta, 15,1493 (1970). 930
ANALYTICAL CHEMISTRY, VOL. 43, NO. 7, JUNE 1971
Current-Potential Curve of Pt in 1.OM HC104. A “reproducible” electrode was held at 1.15 V for ten minutes in 1.OM HC104. Its potential was then scanned from 1.15 V to 0.00 V to 1.35 V and the resultant i - E curve is shown in Figure 1. The oxidized Pt surface is reduced at an appreciable rate at potentials negative to 0.7 V. Cathodic currents at potentials more positive than 0.7 V are largely due to doublelayer charging. On the basis of seven replicate experiments the cathodic peak (0.7 to 0.2 V) has an area Qox = (943 5 31) pc/cm2, after correcting for double-layer charging. The roughness factor of the electrode, as determined by a hydrogen adsorption-desorption experiment in 1 .OM HC104, is 1.6. We make no use of the roughness factor in reporting any of our results, but we include it to characterize the platinum electrode surface. ED - t Curve. A freshly pretreated “reproducible electrode” was introduced into the cell containing 1.OM HC104. The disk electrode potential, En, was maintained at 1.15 V for ten minutes, 0 = 2500 rpm. At the end of ten minutes, the disk was open-circuited, in = 0, and EDus. time, t, was determined. Figure 2, Curve A , shows a typical ED- t plot in 1.OM HC104. The steady-state ED reached in five minutes, was -0.7 V. The electrode is in an oxidized state at this potential. The above sequence of experiments was repeated in 1.OM HC104 containing 2.22 X 10-5M Hg(1). Figure 2, Curve B, shows the change in EDwith time after opening the disk circuit in the presence of Hg(1). The steady-state rest potential in the presence of Hg(1) was reached in five minutes and corresponded to ED= 0.49 V. At this potential, the Pt electrode
32
I
I 1.2
1
0.4
008
I
0
ED Figure 4. Identification of [Hg(I)IZ,adsorbed
\
0, scan rate = 100 mv/sec; -.-.-.-ED scanned from rest potential to 0.0 V to 1.35 V in 1,OM HC104; -. EDscanned from rest potential to 1.35 V in 1.OM HCIOa; --ED scanned from rest potential to 0.0 V to 1.35 V in 1.0MHC104containing 2.22 X 10-5M Hg(1); ED scanned from rest potential to 1.35 V in 1.0 M HC104containing2.22 X 10-5MHg(I) w =
-
-
’ \ I
I
‘J
1.2 I
1.0
-----
0 I. 8
0.6 I
EO
Figure 5. Quantitative determination of the oxidation product of adsorbed Hg(1) or its reduction product
is in a reduced state. Hence, Hg(1) is capable of reducing an oxidized platinum electrode. iR - t Curves. Concurrently with the previous EDus. t experiment in the presence of 2.22 X 10-5M Hg(1) iR us. t curves were obtained for ER = 1.15 V, and in a duplicate experiment for ER = 0.25 V. Figure 3 shows a typical set of experimental data at the ring. The average value of QI,D according to Equation 3, was (571 f 48) pc, and that of QII,D,according to Equation 6, was (406 k 22) pc. Hence, ~ 0.192, Qc = (165 i from Equation 8, using N1jO.85 = N I = 35) pc. Current-Potential Curves for the Reduction and Oxidation of Adsorbed Hg(1). At the end of five minutes, after the ring currents and the disk potential in the previous experiments had reached steady-state values, the rotation of the electrode was stopped. EDwas then scanned at the rate of 100 mV/sec from its steady-state value of 0.49 V to 1.35 V, or to 0.0 V and then to 1.35 V. Figure 4 shows the residual i~ - EDcurve in 1.OM HC104, as well as the curves in presence of Hg(1). The values of Qc’, Qa, and Qa’ (See Equations 10 and 13) were, respectively, (165 i 22), (353 =t 13), and (166 f 18) pc. As predicted above, Qc’ = Qar and Qa = 2 Qcr, thus supporting the hypothesis that as the surface of the oxidized Pt electrode is reduced, Hg(1) is adsorbed on the reduced Pt surface. The possibility that some of the charge involved in Qa and Qa‘ may be due to some other process, e.g., Pt disk surface oxidation and not due to oxidation of adsorbed Hg(I), or Hg(O), formed by the reduction of Hg(1) during the cathodic scan, was tested by the following experiment. First, the chemically reduced Pt disk was scanned at 100 mV/sec from the steady-state value of 0.49 V to 1.35 V, w = 2500 rpm, and the ring collection curve was determined simultaneously at ER = 0.25 V. The curves for iR - ED (ER= 0.25 V) and i D - ED areshowninFigure5. Thearea
Scan rate 100 mV/sec -iR (or i D ) cs. ED ( E R = 0.25 V) curve, ED scanned from 0.49 V to 1.35 V, w = 2500; - -iR (or iD) os. ED ( E R = 0.254) curve, EDscanned from 0.49 V to 0.0 V to 0.49 V at w = 0 and then from 0.49 V to 1.35 V at w = 2500
-
~
Table I.
Comparison of Predicted and Experimental Peak Areas
(Charge, pc/cm2) QI~D
Q o ~
QII,D
Qc
Qc‘
ea’
Q.”
Qa
1270* 943.t 903.t 367i. 367k 370.t 320+ 785* 107 31 49 62 49 40 38 29
Q.”’ 743.t 120
under the peak in the iD - EDcurve ($0.49 V to 1.35 V) was calculated by dividing the area under the corresponding ring collection peak by NII. This quantity, QU”, was (143 + 17) pc, on the basis of four experiments. Second, the chemically reduced Pt disk was scanned at 100 mV/sec, w = 0, from the steady-state value of 0.49 V to 0.00 V and back to 0.49 V. Then the electrode was rotated at 2500 rpm and the ring collection curve, ER = 0.25 V, was followed as the EDwas swept from 0.49 V to 1.35 V. (See Figure 5.) Here again, the area under the peak in the i D - ED curve, e a r “ , was calculated as above from the iR - EDcurve. Quadruplicate experiments gave = (334 54) pc. and Qa’ and Qa”verifies The agreement between Qu and that the anodic process associated with the peak current results solely from production of a soluble Hg(I1) species, as postulated by Equations 11 and 12. Also, Equation 9 is again verified by the fact that Qu’ ’ = 2 Qu ’.
ea”’ ea”’
*
DISCUSSION
Values of QI.D,Qox,QII.D,Qc, Qc’, Qu, Q U I , Qa”,and Qa”” expressed in pc/cm2are given in Table I. ANALYTICAL CHEMISTRY, VOL. 43, NO. 7, JUNE 1971
931
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
