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ANALYTICAL CHEMISTRY, VOL. 51, N O . 2, FEBRUARY 1979
measurable change u p to a pulse repetition rate of 57 MHz, which is the limit of the pulse generator used. T h e output pulse of the comparator when the amplifier was driven by such pulses was about 11 ns wide. T h e maximum frequency a t which the counter would trigger reliably with a sinewave input was approximately 93 MHz, which is consistent with a pulse pair resolution of about 11 ns. T h e speed of several pulse counting systems incorporating this counter is examined experimentally in a companion paper (11). Stability. Some of the results of preliminary experiments using this pulse counter circuit indicated t h a t while the sensitivity was apparently sufficient to count all pulses when used in a photon counting system, the importance of good stability in the pulse counter electronics should not be neglected (11).An examination of the sources of instability in the two photon counting systems described in reference 11 revealed two major contributions. One was a low frequency variation in the light source output of some 170.By far the worst type of instability exhibited by the electronics of the pulse counting measurement system itself was the variation in effective threshold with temperature. Several factors contribute t o thermal drift of the effective threshold, but changes in the gain of the amplifier and in VAo(see Figure 4) were found to be most important. Direct measurement of the temperature coefficients of amplifier gain and offset voltage is extremely difficult because the changes are small and because so much low amplitude noise is introduced via the test probes t h a t measurements anywhere near the (nonperturbed) critical range are impossible. A more sensitive (and perhaps more meaningful) method of measuring the change in effective threshold is simply to monitor the count rate a t a "constant" light intensity as the temperature of the counter is varied. Such measurements were performed for several amplifier gain settings. The dependence of count rate on temperature was worst a t low amplifier gain (G = 60), exhibiting a 30% change from 30 to 50 "C. Much better results were obtained a t full amplifier gain (the normal operating conditions), where any count rate changes between 32 and 40 "C were so small as to be totally obscured by the
instability in the light source. T h e temperature controller used with the pulse counter maintains the temperature to within *0.5 "C indefinitely after an hour's equilibration time. The present data are insufficient to show whether such control is actually necessary for good stability. However, with this temperature control the instability of the counter is almost certainly negligible compared to light source instabilities, or to changes in photomultiplier cathode sensitivity with temperature (18).
ACKNOWLEDGMENT The authors are grateful to B. K. Hahn for suggesting the use of the pA733 and AM685 integrated circuits. LITERATURE CITED F. T. Arecchi. E. Gatti, and A. Sona, Rev. Sci. Instrum.. 37, 942 (1966). A. T. Young, Appl. Opt., 8, 2431 (1969). F. Robben, Appl. Opt., 10, 776 (1971). M. K. Muphy, S.A. Clybum, arid C. Veillon, Anal. Chem., 45, 1468 (1973). M. L. Franklin, G. Hotlick, and H. V. Malmstadt, Anal. Chem., 41, 2 (1969). K. C. Ash and E. H. Piepmeier, Anal. Chem., 43, 26 (1971). H. W. Wemer, H. A. M. m e f t e , and J. V. D. Berg, Int. J. Mss Spectnm. Ion Phys., 8, 459 (1972). J. D. Ingle, Jr., and S. R. Crouch, Anal. Chem., 44, 785 (1972). C. Smit and C. Th. J. Alkemade, Appl. Sci. Res., 108, 309 (1963-4). C. G. Enke, Anal. Chern., 43 ( l ) , 69A (1971). E. J. Darland, G. E. Leroi, and C. G. Enke, Anal. Chem., preceding paper in this Issue. E. J. Darlarid, Ph.D. Thesis, Michigan State University, East Lansing, Mich.,
1978. L. B. Robinson, Rev. Sci. Instrum., 3 2 , 1057 (1981). "Designlng with High-speed Comparators", AMD Application Note, Dec. 1975, Advanced Micro Devices, Sunnyvale, Calif., 94086. K. G. Harf, ComputerDes., 17, 130 (June, 1978). Additional, more detailed, construction information can be obtained by writing the authors. J. M. Hayes, D. E. Matthews, and D. A. Schoeller, Anal. Chern., 50, 25
(1978). Photomultiplier Manual, Technical Series PT-61, RCA Corp., Harrison, N.J.
RECEIVEDfor review July 14,1978. Accepted October 30,1978. Two of the authors (E.J.D. and G.E.L.) are pleased to acknowledge the support of the National Science Foundation (MPS 75-02525) a n d the Office of Naval Research (N00014-76-C-0434).
Micromolar Voltammetric Analysis by Ring Electrode Shielding at a Rotating Ring-Disk Electrode Stanley Bruckenstein" and P. R. Gifford Chemistry Department, State University of New York at Buffalo, Buffalo, New York
Ring electrode Shielding at a rotating rlng-disk electrode (RRDE) provides a sensitive solid electrode technique for the study of mlcromolar solutions of electroactive materials. At micromolar levels, shielded ring electrode currents are free of nonconvectlve dtffusion current complications which obscure faradaic rotating disk electrode currents. Ag( I), Bi( III), Cu( II), and Fe(II1) and mixtures of Ag(1) and Cu(I1) can be determined in the concentration range 0.1 to 10 X lo-' M.
In the determination of micromolar levels of electroactive species a t solid electrodes, the convective diffusion-controlled current for the electrode reaction of analytical interest can
14214
be obscured by charging currents and/or surface processes occurring a t the electrode. This problem can be minimized by use of the rotating ring-disk electrode (RRDE) in the ring shielding mode (1-3). T h e RRDE has been shown to be of great utility in the study of metal ions in solution since the ring electrode can monitor electroactive species generated or consumed a t the disk ( 4 ) . Moreover, if the ring electrode is held a t a fixed potential, complications due to charging and surface processes a t the disk do not affect the ring current, thus making the ring electrode shielding technique a sensitive measure of the current due to the disk electrode process. In this study, ring shielding a t a RRDE was applied to the determination of Cu(II), Bi(III), Ag(I), and Fe(II1). Simul-
0003-2700/79/0351-0250$01.00/00 1979 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 51, NO. 2, FEBRUARY 1979
taneous determination of Cu(I1) and Ag(1) was demonstrated. Quantitative results were obtained over the concentration range to lo4 M.
Ermp
251
1
In
THEORETICAL In order to describe our use of t h e RRDE shielding technique for the determination of micromolar levels of a species, the following reaction is considered:
Mn+ + ae
+ M(n-a)+
(1)
T h e expression relating the disk limiting current, iD,L, to concentration for this process was first derived by Levich ( 5 ) and is
iD,L=
(2)
where rl is the disk radius (cm), D is the diffusion coefficient (cm2/s), u is the kinematic viscosity (cm2/s), and w is the electrode rotation speed (rad/s). The analogous expression relating the limiting current to concentration a t a rotating ring electrode has been derived by Levich and Zaidel (6). Their result also applies to the ring electrode of a RRDE when the disk current is zero, and may be expressed as iR,L
=
P2I3iD,L
(3)
where is the disk limiting current given by Equation 2 and p is a geometric function of rl and the inner and outer ring radii, r2 and r3, respectively, Le.,
P = (r3/rJ3- (r2/rd3
(4)
If a n intermediate, M("-')+, is generated a t the disk from a solution containing only M"' according to Reaction 1 and detected at the ring by its reoxidation back to Mnf, then the ring current for a convective diffusion-controlled process is given by
iR = -NiD
(5)
where N is the collection efficiency (2, 7). For a convective diffusion-controlled process, the collection efficiency may be calculated from rl, r2, and r3 (8, 9). When M"' and M("*)+ are both present in solution and M"+ is reduced a t the disk electrode, the ring electrode current due to reoxidation of the intermediate is iR
=
i R , L -I-N i D
(6)
Thus, t h e ring current is the sum of two components-one arising from the current a t the ring electrode when there is no disk process and one arising from the amount of M(n-a)+ created at the disk. Analogously, if M("-")+ in solution is oxidized to M"+ at the disk, the ring current for its reoxidation will be decreased by NiD. I t is this ability to monitor the consumption of species a t the disk electrode by changes in the ring current that is the basis for the ring electrode shielding technique. Consider a solution of Mn+ having no M(n-a)+in which the ring electrode is potentiostated so that Reaction 1is convective diffusion-controlled. Initially, if no reaction occurs at the disk involving Mn+,the ring limiting current observed is given by Equation 3. As the disk electrode potential is scanned cathodically to produce the limiting current response for Reaction 1, the ring current decreases because of the consumption of M"' a t the disk. The ring current does not fall to zero since the disk cannot prevent all M"+ from reaching the ring. Rather, the ring current will fall to a value defined as t h e shielded ring current, iR,sh(1-3). Its value is
(7) According to Equations 2 and 7, the shielded ring electrode current is proportional to Cb. However, a t low solute con-
i@K
Figure 1. Circuit diagram for four-electrode potentiostat modified for ring current offset capability
centrations, the experimental measurement of the absolute value of iR,sh is difficult, while the measurement of the difference between the limiting (unshielded) and shielded ring currents, A i R , is much easier. The relationship between these currents is obtained by subtracting Equations 3 and 7 and substituting from Equation 2, Le.,
AiR = iR,L- iR,sh= NiD,L= 0 . 6 2 n F ~ r ~ ~ D ~ 1 ~ v - (8) '~G"~ Because iD,L in Equation 8 represents the faradaic current is measured a t constant associated with Equation 1, and - l i ~ ring electrode potential, AiR is free of currents associated with charging and surface processes a t both the disk and ring electrodes. Thus the measurement of AiR is a more sensitive analytical method than the direct measurement of i ~ , ~ .
EXPERIMENTAL Instrumental. The earlier circuit used for independent potentiostatic control of the ring and disk electrodes ( 3 ) was modified as shown in Figure 1 to allow an offset current to be summed into the ring current. This circuit still provided independent potentiostatic control of both electrodes and increased the sensitivity with which the small differences in ring currents could be measured. Current-potential and current-time curves were recorded on an EA1 X-Y-Y' recorder (Electronics Associates, Inc.). The analog voltage ramp for the potentiostat has been described elsewhere (10). Unless otherwise stated, a scan rate of 200 mV/s was employed for the ring shielding studies. The electrochemical cell was constructed from borosilicate glass, had an approximate volume of 500 mL, and had five ports. One port provided an inlet for the RRDE and a polyethylene cover was fitted around the electrode over this port to minimize the introduction of oxygen and other contaminants. The other ports provided entrances for the auxiliary electrode compartment, a gas dispersion tube, Luggin capillary, and for the introduction of reagents to the cell. The cell was periodically cleaned with a 50-50 volume mixture of concentrated sulfuric and nitric acids by overnight soaking, followed by extensive rinsing with reagent grade water. The RRDEs were rotated in a high-speed rotator (Pine Instruments Co., Grove City, Pa.). Precision pulleys allowed incremental steps of rotation speed from 400 to 10 000 rpm. The rotator, with electrode and cell in place, was enclosed in an aluminum case grounded to the instrument ground to provide shielding against external noise. Connections to the cell from the instrument were made through the shield via double shielded
252
ANALYTICAL CHEMISTRY, VOL. 51, NO. 2, FEBRUARY 1979
Table I. Rotating RingDisk Electrodes electrode no. type I Au ring; Au disk I1 Pt ring; Pt disk I11 Au ring; Pt disk
R , , cm
R , , cm
0.383 0.383 0.380
0.412 0.398 0.402
coaxial cable and shielded BNC connectors. Experiments were performed at a rotation speed of 2500 rpm unless otherwise stated. Electrodes. A coil of platinum or gold wire was used as an auxiliary electrode, depending on the metal composition of the ring-disk electrode being used. A commercial saturated calomel electrode (SCE) (Fisher Scientific Co.) was used for the reference electrode. All potentials are reported vs. the SCE. The RRDEs used in this study were constructed and polished by a previously described procedure (11). The various RRDEs used are described in Table I. Before each set of experiments the RRDE was polished using 0.05 pm alumina. Next the ring and disk electrodes were potential cycled in supporting electrolyte until reproducible i-E curves were obtained. Then the ring electrode was potentiostated at the working potential for approximately 15 min before performing the shielding experiments. For electrode reactions involving metal deposition, this time deposits an average thickness of several atom layers of the metal over the ring electrode. Reagents and Solutions. All reagents used were of AR grade and were used without further purification. Solutions were prepared with water obtained from a Milli-Q Reagent Grade Water System (Millipore Corp., Bedford, Mass.). Solutions were deoxygenated by bubbling with water-saturated nitrogen (boil-off from liquid nitrogen, Union Carbide, Linde Division). During experiments, nitrogen was passed over the surface of the cell solution. Known volumes of stock solution were added t o the cell with a Manostat micropipet. Procedure a n d D a t a Interpretation. Values of AiR were determined for a series of concentrations in the range of lo-' and lo4 M M"'. These values were obtained by taking the difference between the ring current at a disk potential where a ring limiting current was observed and the ring current at a disk potential where the ring was fully shielded. Values of AiR were plotted vs. concentration. Theoretically, one should obtain a straight line of slope rnkR with zero intercept. Experimentally we found that processes not of analytical interest (e.g., hydrogen and oxygen evolution a t the disk) and solution impurities produced small nonzero intercepts. Steady-state values of i D b were obtained over the concentration range of 10-j to lo-* M M"' and plotted vs. concentration to give linear plots of slope mrDL.An experimental value for the collection and efficiency N was calculated from the ratio of mAIR/mlDL compared to the theoretical N calculated from the electrode geometry.
RESULTS AND D I S C U S S I O N I n this study the validity and utility of the RRDE ring shielding approach for micromolar determinations was tested for solutions containing Cu(II), Bi(III), or Ag(I), and mixtures of Cu(I1) and Ag(1). Also, the Fe(III)/Fe(II) system was studied t o show the validity of the ring shielding technique for a system not involving underpotential metal deposition. Underpotential metal deposition is an important factor in the study of metal/metal ion couples a t solid electrodes, e.g., Ag (12)and Cu (13)on Pt and Bi (14) on Au. At the low metal ion concentrations and fast scan rates employed in these studies, the disk electrode reaction for these systems was underpotential metal deposition, not bulk metal deposition. Therefore, the ring electrode became shielded a t potentials more anodic than the formal reduction potential for the metal/metal ion couple. D e t e r m i n a t i o n of Cu(11). Since the behavior of Cu(I1) in 0.5 M KCl has been well characterized ( 3 ) ,this system was the first chosen to test the ring shielding technique. Electrode I was employed for these studies. Its ring was potentiostated
R , , cm 0.511
02/3
N 0.364 0.374 0.409
0.493 0.523
1.092 1.009 1.269
4
1OOpA
I -
I
I
+0.4
t06
to2
L
1
p
00 -0.2 ED (volts)
p
-04
L
A
-06
Figure 2. Current-potential curve for CcUc,,,= 5.0 X M-0.5 M KCI. Electrode I, potential scan rate 20 mV/s. Disk, upper curve; ring, lower curve
b-
z
LL CK
5 0 Y
a c" CK 0 0
z
CK
I -
I
106
,
t04
+02
RING
,
00
-02
I
-04
I
-06
El, (volts)
Figure 3. Current-potential curve for C,,,,,,, = 9 70 X IO-' M-0.5 M KCI. Electrode I, potential scan rate 200 mV7s. Arrows indicate direction of potential scan. E, = -0.50 V
a t 4 . 5 0 V, where the reduction of Cu(I1) to Cu(0) is convective diffusion-controlled. The steady-state i-E curve a t high concentrations of Cu(I1) shows two waves (Figure 2) due t o the reactions:
+e Cu(I1) + 2e Cu(I1)
-
Cu(1) Cu(0)
(9) (10)
At ED= -0.4 V, Reaction 10 is convective diffusion-controlled on the disk and the ring is fully shielded. At low concentrations and fast scan rates, very different behavior is observed (Figure 3). The ring current is constant in the region of +0.6 V I EDI+0.4 V. The ring then begins t o shield and a t ED = 0.0 V is completely shielded. Only a single wave is observed a t the ring with low Cu(I1) concentrations whereas two waves are observed with high concentrations. During the anodic scan, a single ring current peak occurs in the region +0.5 V IED I+ 0.2 V corresponding t o the oxidation of Cu(0) a t the disk. T h e sharp increase in iR at ED= +0.6 V is due to dissolution of gold a t the disk. Although a stripping peak is seen in the disk anodic scan, no reduction wave is detectable in the disk curve due to charging
ANALYTICAL CHEMISTRY, VOL. 51, NO. 2, FEBRUARY 1979
3
253
PI
01
I
1
I
I
+OB
+1.0
1
I
+0.4 t02 Eo(volts)
t06
1
I
00
-02
1
lo-'
Figure 5. Ring current-disk potential curve for C,,(,,,, = 4.86 X 00
2
4
Concentration x
M-0.1 M "0,. V
lo6 M
Electrode I, potential scan rate 200 rnV/s. E, = -0.25
-
Figure 4. Plot of concentration of M"' vs. [ A h (A4,)c=0]. (A) Fe(III), slope = 0.062 f 0.002 p A / p M . (0)Ag(I), slope = 0.214 f 0.002
pAIpM.
= 0.350
(0) Cu(II), slope = 0.247 f 0.004 pA/pM. (V)Bi(III), slope
* 0.005 p A / p M
and surface currents at the disk. No Cu(1) formation is observed until more than 500 pC of copper per cm2 of electrode surface has been underpotentially deposited ( 1 5 ) . Thus, a t low concentrations, the shielding observed a t the ring results from underpotential deposition of Cu(0) at the disk. T h e disk limiting current for Reaction 10 was determined for a series of 9.84 x M 5 Ccu(IIl5 9.63 X M solutions. A plot of ~ D , Lvs. concentration gave a straight line of slope 0.713 f 0.001 pA/pM. Shielding studies M ICcU(11)I6.82 were performed for a series of 2.19 X X lo4 M solutions. Values of AiR were plotted vs. concentration, giving a straight line of slope 0.247 f 0.004 pA/pM (Figure 4). Calculation of N gives a n experimental value of 0.346. The theoretical value of N is 0.364 for Electrode I. Determination of Bi(II1). The behavior of Bi(II1) on Au in 0.1 M HC104 was originally studied by Cadle and Bruckenstein using a RRDE ( 1 4 ) . They found that underpotential deposition of Bi(0) occurred in the region of 0.35 V IE D 2 -0.23 V and ring collection curves yielded theoretical collection efficiencies. In this study, Bi(II1) in 0.1 M HNO, was determined using Electrode I over the concentration range of lo-' to M Bi(II1). T h e ring was potentiostated a t -0.25 V, a potential a t which the reduction of Bi(II1) to Bi(0) is convective diffusion-controlled, A typical iR-EDcurve for Bi(II1) (CBi(III)= 4.86 X M) is shown in Figure 5. T h e ring does not shield until E D = +0.35 V and is fully shielded in the region E D 5 4 . 2 5 V. The anodic scan shows a ring current peak due to Bi(0) oxidation a t the disk in the region +0.2 V 5 E D 5 +0.5 V. The disk limiting current for the reduction of Bi(II1) to Bi(0) obtained in solutions 2.02 x M M 5 CBi(III)5 5.82 x gives a linear plot of slope 0.934 f 0.004 pA/pM. Linear plots of A ~ vs. R concentration (Figure 4) are obtained for 4.86 X M ICB~(III) I9.63 X lo+ M with a slope of 0.350 f 0.005 pA/pM, giving a n experimental value of N = 0.375. Results at lower concentrations gave slightly poorer agreement, Le., a value of 0.330 for N in the range 5 X M to 5 M Bi(II1). T h e theoretical N for Electrode I is 0.364. Determination of Ag(1). The underpotential deposition of Ag on Pt has been studied a t a Pt RRDE in 0.2 M H2S04 by Tindall and Bruckenstein (12) by potentiostating the ring a t E R = + O s l o V. In our investigation, Ag(1) was determined in 0.2 M H2S04 a t concentration levels of CAg(I)I 5X M using Electrode I1 and a ring potential of +0.10 V, a t which potential the
1
I
1
+I4
1
+12
+IO
I
108
+06 E o (volts)
1
+04
I
I
+02
00
Figure 6. Ring current-disk potential curve for Cns(,)= 1.55 X
M-0.2 M H,SO,. +0.10 v
Electrode 11, potential scan rate 200 rnV/s. E, =
reduction of Ag(1) to Ag(0) is convective diffusion-controlled a t the ring. A typical I'R-ED curve is shown in Figure 6 (CAg(I) = 1.55 X lo+ M). A constant ring current is observed until E D = +0.9 V, a t which potential ring shielding begins because of underpotential deposition of Ag(0) a t the disk. In the anodic scan, a peak a t +0.8 V IE D 5 +1.2 V occurs in the ring current due to the oxidation of Ag(0) a t the disk. Values of iD,Lobtained for the reduction of Ag(1) to Ag(0) in 9.57 X M 5 Ck,, I1.88 X lo4 M solutions gave a linear plot of slope 0.574 0.002 pA/pM. Shielding studies were performed for CAg(l)I 5x M. A plot of AiR vs. concentration was linear and had a slope of 0.211 0.008 pA/pM for 4.88 X lo-' M ICAg(l)I6.78 X 10.: M. This slope corresponds to a value of 0.368 for N . Experiments a t slightly higher concentrations (7.88 X lo-' M ICAg(I)5 6.90 x M) yielded a slope of 0.214 f 0.002 pA/pM, Le., N = 0.373 (Figure 4). The theoretical N for Electrode I1 is 0.374. Determination of Fe(II1). The systems described above all involve underpotential metal deposition as the reaction of analytical interest. However, it is not necessary that this be the case for the ring shielding technique to be useful. To demonstrate this, determinations of Fe(II1) were carried out for solutions of C,+(III)I 5 X lo-' M, in 0.2 M H2S04by the reduction of Fe(II1) to Fe(I1) a t Electrode 11. The ring was potentiostated a t E R = +0.05 V and a scan rate of 100 mV/s employed. RRDE studies a t 1 m M Fe(II1) in 0.2 M HzSO4 show a well-defined reduction wave for Fe(II1) to Fe(I1) starting near ED= +0.5 V and exhibit a limiting current a t E D = +0.05 V (Figure 7). With the ring potentiostated at +0.05 V, the ring reduction reaction of Fe(II1) to Fe(I1) is convective diffusion-controlled and the ring is shielded for E D < +0.50 V. At ring potentials more cathodic than -1-0.05 V, hydrogen evolution interferes.
*
*
254
ANALYTICAL CHEMISTRY, VOL. 51, NO. 2, FEBRUARY 1979
, +I2
I
+iO
+08
1
t06
1
to4
u +OZ
00
E, (volts1
Current-potential curve for 1.5 X lo3 M Fe(IIIt0.2 M H,S04. Electrode 11, potential scan rate 10 mV/s. Disk, upper curve; ring, lower curve. Arrows indicate direction of potential scan. E , = +0.05 V Figure 7.
+-
6
OI ir 3
1
1
I
I
+l,O
+0.8
+0.6
1
I
+0.4 +0.2 E D (Volts vs SCE)
, w
0.0
, j -0.2
Figure 8. Current-potential curve for 1.06 X M Fe(II1)-0.2 M H,S04. Electrode 11, potential scan rate 100 mV/s. Disk, upper curve: ring, lower curve. Arrows indicate direction of potential scan. E R = 4-0.05 V
Steady-state currents for Fe(II1) reduction to Fe(I1) were M IC F ~ ( ~ I~2.04 I) X M. A determined for 1.06 X plot of iD,L vs. concentration was linear with a slope of 0.167 f 0.003 bA/bM. At low concentrations of Fe(II1) (CFe(llI) < lo-' M), ~ D , Lis obscured by charging currents and surface processes occurring at the disk electrode. However, the i R - E D curve shows a definite shielding wave due to the reduction of Fe(II1) at the disk (Figure 8). A small stripping peak in the anodic iR-ED scan with an area of about 0.2 pC is observed at about 0.5 V. This peak is probably due to the impurities that contribute to the residual shielding of 25 nA. This residual current was fairly constant from day to day despite thorough cleaning of the cell and limited the determination of Fe(II1) to concentration levels of c ~ 2 ~ 5 (x 10-7 ~ ~M. ~ ) A plot of A i R vs. concentration of Fe(II1) is shown in Figure 4. For 5.30 X M ICw(III)I4.24 X lo4 M, a slope of 0.062 f 0.002 pA/pM was obtained, yielding an experimental value of N = 0.371. This value compares well with the theoretical value of 0.374 for Electrode 11. Simultaneous Determination of Ag(1) a n d Cu(I1). The shielding method is also applicable to the simultaneous determination of mixtures of metal ions, provided their half-wave
Figure 9. Ring current-disk potential curve for C,( = 7.62 X lo-' M, CndI) = 6.86 X lo-' M. Electrode 11, potential scan rate 200 mV/s. E R = -0.25 V
potentials are sufficiently separated. Tindall and Bruckenstein previously performed simultaneous determinations of Ag(1) and Cu(I1) by stripping voltammetry from the Pt disk of a RRDE and collection a t the ring electrode (16). Solutions M to M in Ag(1) and Cu(I1) in 0.2 M H2S04were determined by this method with 10-15% reproducibility. Therefore, this system seemed well suited for determination of Ag(1) and Cu(I1) by the ring shielding technique. Initially, studies were performed in 0.2 M H2SO4 using Electrode 11. Tindall and Bruckenstein (15) have studied Cu(I1) reduction in sulfuric acid a t a Pt RRDE. They used an ER = -0.22 V, and found poor agreement between the value of N as calculated from electrode geometry and that observed from experiment. They concluded that a potential of -0.22 V was not sufficient to attain the limiting convective diffusion-controlled current for Cu(I1) reduction. We, too, found this to be the case. Also, a t this potential pseudo-collection effects due to uncompensated ohmic interactions were a problem ( 1 7). A gold ring-platinum disk RRDE (Electrode 111) allowed using a more cathodic ring potential (ER= -0.25 V) than was possible at platinum. Even though this ER was not sufficient to reach the limiting convective diffusion-controlled current on gold, the results were superior to using a platinum ring a t a less cathodic ER. At more cathodic gold ring potentials, reproducible ring currents could not be obtained. Disk limiting currents for Ag(1) reduction were obtained as described above for the determination of Ag(1) in the absence of Cu(I1). Attempts to measure limiting currents for Cu(I1) reduction were unsuccessful a t both the Pt and Au RDEs. However, a well-defined wave for Cu(I1) reduction was found using a carbon RDE with a n area of 0.281 cm2. Steady-state currents for Cu(I1) reduction to Cu(0) were obtained at ED = -0.50 V and a plot of iD,Lvs. concentration gave a slope of 0.427 A 0.004 pA/pM for 2.02 X 10" M IC~"(II) 5 9.33 X 10-5 M. The calculated disk limiting current for Cu(I1) reduction to Cu(0) a t Electrode I11 is 0.691 pA/pM. Shielding studies were performed for series of solutions 1 x M Ag(1) and Cu(I1) in 0.2 M H2S04. M to 5 X Initially, the concentration of one ion was held constant while the other ion's concentration was changed to determine whether the observed ring shielding currents for Ag(1) and Cu(I1) were independent of one another. A typical iR-EDcurve for a mixture of Ag(1) and Cu(I1) is shown in Figure 9. The ring is initially shielded in the region +1.1 V 5 E D 2 +0.6 V because of underpotential deposition of Ag(0). The ring is then further shielded for E D I+0.5 V
ANALYTICAL CHEMISTRY, VOL. 51, NO. 2, FEBRUARY 1979
255
Table 11. Comparison of Analytical Sensitivities sensitivity in ng/mL NFAAS ICPAES (18) (18) 0.001 4
ring shielding' 4.3
element DPP(18) Ag 0.2 5.0 10.5 Bi CU 13.7 1 9.5 Fe 1000. 5 22. a Detection limit taken as amount required to double background response.
0
2
4
,
6 Concentration x lo7
I
8
I
io
Flgure 10. Plot of AiR vs. concentration of Ag(I) and Cu(I1) for simultaneous determinations. (0)Cu(II), slope = 0.257 f 0.015 pA/pM; intercept = 0.086 f 0.011 MA. (A)Ag(I), slope = 0.218 f 0.008 p A I p M ; intercept = -0.001 f 0.004 p A
because of underpotential deposition of Cu(0). Two ring current peaks are present in the anodic scan due to Cu and Ag oxidation a t the disk. If the total metal deposition exceeds one monolayer, an additional ring current peak is found (16). We observed this peak only a t higher concentrations in mixtures of Ag(1) and Cu(I1). Results for simultaneous determination of Ag(1) and Cu(I1) (approximately equal concentrations) gave good results for M ICAg(1)I6.86 X M. A plot of AiR VS. CAg,m 1.17 X yielded a straight line of slope 0.218 f 0.008 pA/pM (Figure 10). The experimental collection efficiency of 0.419 compares well with the theoretical value of N = 0.409 for Electrode 111. Plots of AiR vs. Ccu(II)for Ccu(II)2 5 x lo-' M were linear and had a slope of 0.257 & 0.015 pA/pM for 3.88 X M ICCu(=)I1.15 X lo4 M (Figure 10). This slope yields a value of N = 0.372. The rather poor agreement with the theoretical collection efficiency (N= 0.409) arises because an ER = 4 . 2 5 V on gold is not sufficiently cathodic to produce a convective diffusion-controlled ring current for Cu(I1) reduction. Satisfactory results a t Ccu(II)I5 x M could not be obtained. For C C ~ (1~5~x) 10* M, a plot of AiR vs. C C ~ (had ~ ~a) downward curvature because appreciable amounts of copper deposit a t the ring during the experiment. As was noted by Tindall and Bruckenstein (16),after more than about 10 atom layers of copper deposit on the ring, reduction of copper became very irreversible, and there is a decrease in the observed collection efficiency. This decrease is not encountered a t lower copper concentrations since the duration of the experiment is too short to deposit significant amounts of copper a t the ring electrode.
CONCLUSION T h e use of the ring shielding mode of a rotating ring-disk electrode affords a simple, fast method for the determination of micromolar and submicromolar levels of electroactive species in solution. The ability of the ring electrode to monitor species consumed at the disk provides a significant analytical advantage over direct electrode voltammetry a t low solution concentrations. The technique provides good accuracy for concentration levels of 1 5 X M. Table I1 presents a comparison of the sensitivity obtained by us for the ring shielding technique with that previously given ( 1 8 ) using differential pulse polarography ( D P P ) , nonflame atomic absorption spectroscopy (NFAAS) and inductively coupled plasma sources for atomic emission spectroscopy (ICPAES). As can be seen, the ring shielding technique is quite competitive. LITERATURE CITED S. Bruckenstein, Nektrokhlmiya, 2, 1085 (1966). W. J. Albery, S. Bruckenstein, and D. T. Napp, Trans. Faraday SOC., 62, 1932 (1966). D. T. Napp, D. C. Johnson, and S. Bruckenstein, Anal. Chem., 39, 481
(1967). S. Bruckenstein and B. Miller, Acc. Chem. Res., IO, 54 (1977). V. G. Levich, "PhysicochemicalHydrodynamics",Prentice-Hall, Englewood Cliffs, N.J. 1962, p 60. Ref. 5, p 107. A. N. Frumkin, L. N. Nekrasov, V. G. Levich, and Yu. B. Ivanov, J . Electroanal. Chem., 1, 84 (1959). W. J. Albery and S.Bruckenstein, Trans. Faraday SOC.,62, 1920 (1966). W. J. Albery and M. L. Hitchman, "Ring-Disc Electrodes", Oxford University Press, Ely House, London W. 1. 197 1. D. Untereker, W. Sherwood, G. Martinchek. T. Reidhammer, and S. Bruckenstein, Chem. Instrum., 6, 259 (1975). D. F. Untereker, Ph.D. Thesis, SUNY at Buffalo, Buffalo, N.Y., 1973. G. W. Tindall and S. Bruckenstein, Electrochem. Acta., 16, 245 (1971). S. H. Cadle and S. Bruckenstein. Anal. Chem., 43, 932 (1971). S. H. Cadk and S.Bwckenstein. J. Electrochem. Soc., 119, 1966 (1972). G. W. Tindall and S. Bruckenstein, Anal. Chem., 40, 1051 (1968). G. W. Tindall and S.Bruckenstein, J. Nectroanal. Chem.,22, 367 (1969). M. Shabrang and S. Bruckenstein, J. € k k x h ? m . Soc.,122, 1305 (1975). J. J. Dulka and T. H. Risbey, Anal. Chem., 48, 640A (1976).
RECEIVED for review September 18,1978. Accepted November 20, 1978. This work has been supported by the Air Force Office of Scientific Research under Grants AFOSR 783621 and 742572.
