Anal. Chem. 1990, 62, 903-909 (5) Prober, James M.; Trainor, (3eorge L.; Dam, Rudy J.; Hobbs, Frank W.; Robertson, Charles W.; Zagursky, Robert J.; Cocuzza. Anthony J.; Jensen, Mark A.; Baumeister, Kirk. Science 1987, 238, 336-341. (6) Smlth, Lloyd M. Am. Biotech. Lab. 1989, 7 (5), 10-25. (7) Jorgenson, James W.; Lukacs, Krynn D. Anal. Chem. 1981, 53, 1298-1302. (8) Hjerten, Stellan J. Chromatogr. 1983, 270, 1-6. (9) Cohen. A. S.: Najarian, D. R.; Paulus, A.; Guttman, A,; Smith, J. A,; Karger, B. L. R o c . Natl. Acad. Sci. U . S . A . 1988, 8 5 , 9660-9663. (10) Compton, S. W.; Brownlee, R. G. Biotechniques 1988, 6 (5), 432-439. (11) Ewing, Andrew G.; Wallingford, Ross A.; Olefirowicz. Teresa M. Anal. Chem. 1989, 61 (4), 292A-303A. (12) Radola, Bertold J. Nectrophoresis 1980, 7 , 43-56. (13) Bente, P. F.; Myerson, J. Hewiett-Packard Company, Eur. Pat. Appl. EP272925, June 29, 1988. US Appl. 946566, Dec 24, 1986.
903
Smith, Lloyd, M.; Kaiser, Robert J.; Sanders, Jane 2.; Hood, Leroy E. Meth. in Enz. 1987, 755. 260-301. Applied Biosystems Model 370A DNA Sequencing System I Taq Polymerase Technical Manual; Applied Biosystems: Foster City, CA, 1989. (16) Huang, Xiaohua; Gordon, Manuel J.; Zare, Richard N. Anal. Chem. 1988, 60, 375-377. (17) Cheng, Yung-Fong; Dovichi, Norman J. Science 1988, 242, 562-564. (18) Mead, D., unpublished results.
RECEIVED for review November 6,1989. Accepted February 8,1990. This work was supported in part by grants from the Whitaker Foundation and NIH Grant GM 42366.
Theory for Cyclic Staircase Voltammetry for First-Order Coupled Reactions Mary Margaret Murphy, John J. O’Dea, Dieter A m , and Janet G. Osteryoung* Department of Chemistry, SUNY University at Buffalo, Buffalo, New York 14214
A systematic computational study of three first-order kinetic systems has been performed for cyclic staircase voltammetry. The systems are electrochemically reversible but compikated by a preceding, following, or Catalytic chemical reaction. Theoretical working curves have been Calculated and the variation in peak currents and peak potentials with a change In rate parameter for the chemical step has been compiled in graphical form. The results resemble, but differ quantitatively from, those for square wave and linear scan (cyclic) voltammetry. I t is suggested that reverse current be measured with respect to the zero of current.
A considerable amount of research has already been done by others concerning the application of cyclic staircase voltammetry (SCV) to the study of first-order kinetic systems (1-13). Our purpose here is to explore as fully as possible by computation the usefulness of SCV for determining chemical rate parameters. In particular, we wanted to determine whether rate information for first-order kinetic systems could be gained by simply examining peak currents, peak potentials, peak current ratios, and peak separations and how they vary with the characteristic time of the experiment. Specialists in voltammetric techniques frequently use some type of curve-fitting procedure for analysis of voltammetric data. For the nonspecialist, especially for obtaining estimates of kinetic parameters, and for understanding the broad phenomenological properties of various mechanisms, working curves as presented here are a useful resource. The previous work cited above generally provides only partial information on how peak values can be expected to vary as the time of the experiment is changed. The results presented here are obtained from theoretical voltammograms calculated for the cases where a reversible electrochemical reaction is complicated by a preceding, following, or catalytic chemical reaction. A companion paper deals with slow heterogeneous charge transfer (14). These results are qualitatively or even semiquantitatively the same as results for the analog counterpart of SCV, i.e. linear scan voltammetry (LSV) (15-1 7). (For simplicity we refer to both the unidrectional and cyclic experiment as LSV.) Also, SCV can be viewed as square wave voltammetry with zero square wave amplitude. The relationship between the
net current for a square wave voltammetric experiment (SWV) (18)and the characteristics of the cathodic part of the cyclic staircase curves is easily seen. Although a detailed analysis of the forward and reverse currents in SWV has not been presented, by inspection there is an even closer analogy between the forward and reverse SW and SC currents (18). Cyclic voltammetry is useful for studying reactions with electrochemical or chemical complications because it provides information not only on the initial oxidation or reduction but also on the fate of that initial reduction or oxidation product. The staircase waveform is characterized by two parameters, the step height (AE),and the step width (7). The potential profile in SCV can be described as E = Ei - (j - 1)AE(for E 5 E J and E = E,, + (j - 1)AE (for E I ESP)where E,, is the potential a t which the scan is reversed, (j- 1 )5~t 5 j T and j is the step number (for further discussion see ref 14). The steplike waveform of staircase voltammetry provides the added advantage of discriminating against charging current. This in turn allows faster effective scan rates ( A E / 7 )at a fixed ratio of faradaic to charging current. The stepwise changes in potential are also readily controlled by digital computer. In principle, current can be sampled anywhere along the step (where potential is constant). Experimentally, the upper limit on A E / 7 , and the lower l i i i t on the sampling point along the step, is set by the cell time constant. For the theoretical curves presented below, the current is always sampled at the end of the step. Also, nAE, the normalized step height, is set at a constant value of 5 mV. Therefore the staircase period, 7, is the only variable. Experimentally, it is convenient to specify the time variable in terms of frequency, f = 1 / ~ . One of the criteria for reversibility in LSV experiments is that the peak current ratio (ipa/ipc)be equal to unity. However, determination of the peak current ratio is not always simple since the correct base line from which to measure the reverse (anodic) peak must be determined. First-order EC and CE kinetic systems are treated as the electrode kinetics system described previously (14). Three methods of determining peak current ratios are compared directly in terms of time and applicability. Included here are plots of iw/iw using the extension of the forward current as a base line for reverse peak measurement, a discussion of Nicholson’s semiempirical method for determining iw/iw (19),and a complete set of plots of peak current ratio for which the reverse (anodic) peak is
0003-2700/90/0362-0903$02.50/0 0 1990 American Chemical Society
904
ANALYTICAL CHEMISTRY, VOL. 62, NO. 9, MAY 1, 1990
Table I. First-Order Reaction Schemes
dimension 1ess
reaction type
mechanism
1. following chemical reaction (EC) 0
rate parameter
+ ne- + R
kbi
R S Y kb
2. preceding chemical reaction (CE)
y
S0 kb
O+ne-+R + ne- + R
3. catalytic chemical reaction (EC') 0
kbr
k,(T
R+Y&O+Z "The equilibrium constant is defined as K = k,/kb. *Thesum of + kb). 'k, = k,'[Y] is a pseudo-first-order rate constant.
the chemical rate constants is k (=k,
Table IV. Series Solutions for SCV" 1. EC Case
Table 11. Surface Concentrations" 1. EC Case
b, = {n(kr)l/'/c
Co(0,t) = CO* - (l/nFA(?rDo)'~')So'[i(u)/(t - u ) ' / ~du ]
CR(O,t) = (1/(1+ K)nFA(rDR)l,')So'[i(u)/(t- u)ll2] du
" As adapted from O'Dea (21) for square wave voltammetry.
+
(K/(1 + K)nFA(rDR)'/')Sot{eXp[-k(t - u)]i(u)/(t - u)'/'] du
(1
+ (K&'/(K + l))E1biR/ - 2(kT/1)'/'(1/ i=l
+ K ) + l/r)E1biSj']/{KRl'a1/2/(1 +K)+ i-1
2(kr/l)'/'(1/(1
+ K) + l/c)) 2. CE Case m-1
2. CE Caseb
b, = ( K ( r k ~ ) ' /-'
CO(0,t) =
i-1
C* - (1/(1 + K)nFA(*Do)1/2)Sotlexp[-k(t - u ) ] i ( u ) / (-t
u ) ' / ~du ) - (K/(1 + K)nFA(aDo)1'2)Sot[i(u)/(t - u)'/'] du
biRj' - 2(kr/~1)'/'(K
{R{ + 2(kr/r1)'/'(K
+ Kt + e))
m-1
+ Ke + t) i=l C biSj'}/
3. EC' Case m-1
b, = ( l / R 1 ' ) { ( ~ k ~ i ) ~ / ' /+( lC) - C biRj'] 1=1
CR(0,t) = (l/nFA(*DR)'/')Sot[i(u)/(t - u)'/'] du
" 1 = number of subintervals per staircase period, bi = the approximation to $ ( t ) at t = m k r / l , Si = jl/' - (j- l)'i2,j = m - i + 1, and R;' = erfCjkr/l)l/' - erf((i - l)kr/l)l/*.
3. EC' Casec Co(O,t) = Co* -
(l/nFA(rD)l/')Sot{exp[-k,(t - u)]i(u)/
( t - u)'/') du
CR(O,t)= (l/nFA(rD)1/2)Sot(exp[-k,(t - u ) ] i ( u ) / (-t u)"') du "All equations are adapted from Smith (20). bSince0 and Y are assumed in equilibrium prior to the experiment, Co = KCy and the bulk concentration is given by the total analytical concentration, C*, where C* = Co + Cy = Co(l + l/K). Also, Do = Dy. CItis assumed that Do = DR = D and C y >> CO.
referenced to the zero of current ((ip,Jo/iPc) (see also Figure 1 of ref 14 and discussions therein).
THEORY The integral equations used to define surface concentration for the cases of preceding, following, and catalytic reactions are those of Smith (20). A more thorough description of the method of solving the equations is provided by O'Dea (21). The reaction schemes and definition of constants are the same here as in the latter. For all three reaction systems, the dimensionless current, $ ( t ) , is defined by
i(t) = ~ ( t ) n F A ~ 0 1 / 2 C o / ( a r ) ' ~ 2
(1)
and the potential variable, t , is defined as e
= exp[(nF/RT)(E - ErlpJl
(2)
Table I describes the three reaction types and their respective dimensionless rate parameters. The expressions for surface concentrations from Smith (20) are included as Table 11. The equations for surface concentration, along with the definitions provided by eqs 1and 2, are then substituted into the Nernst equation to obtain the integral equations of Table 111. The last step, prior to calculation, is to convert all integrals of Table I11 to finite sums by the step function method (22). The resulting equations of Table IV were then coded into FORTRAN. All calculations were done either on a PDP/8 computer or on a Masscomp 5500 computer. Further computational details are provided in ref 14. For the three first-order systems described here, 20 subintervals/step were used. A potential window of 500/n mV was used unless otherwise noted, with the initial potential (Ei) a t 200/n mV and switching potential (EBp) 300/n mV negative of the reversible half-wave potential Dependence of reverse peak amplitude or position on switching potential was not examined comprehensively. The computation is fully normalized with respect to the number of electrons transferred. Since individual staircase voltammograms consist of discrete points, the "peak" heights and "peak" potentials are determined from the vertex of a parabola defined by the three highest points of the forward half of the curve and the three lowest points of the reverse part. For the case of a catalytic follow-up reaction,
ANALYTICAL CHEMISTRY, VOL. 62, NO. 9, MAY 1, 1990 905
i
0.5
0.
0 21
6 0
4
5
1 5
3 0
log
0 0
-1
5
-i
I
6.0
-3 0
I
0 -0.5
i
-? 3.0 l o g
4 . 5
tk;:k,,,
0.0
-1.5
Figure 1. Peak current ratio (ip/ipc) for the following chemical reaction as a function of log (k,+ k& for various values of log Kas indicated.
Cathodic peak potential for the following chemical reaction as a function of log (k, k b ) T for log K values as indicated.
Figure 3.
+
I
0 01
I
I
175-
13
0 0 6 0
-3.0
(kr+kb)7
r 5
3 0
1 5
I
25 6.0
0 0
log (kC*kb)T
Flgure 2. Peak current ratio ((i&/i ) for the foilowing chemical reaction as a function of log (k,+ k$ for various log K values as indicated.
when values of parameters result in a steady-state current, $pc = Each working curve is a compilation of values from 101 individual voltammograms (unless otherwise noted) of dimensionless peak currents, the peak current ratio, peak potentials, and the peak separation, as determined from the anodic and cathodic portions of the individual cyclic staircase voltammograms. Peak currents are presented as absolute values.
RESULTS AND DISCUSSION Following Reaction. Consider first a reversible electrochemical reaction followed by a chemical step, the kinetic effect of which varies with the characteristic time of the experiment (Table I, case 1). Figures 1-4 present easily measured characteristics of the resulting staircase voltammograms. The limiting cases and kinetic regions defined by Nicholson and S h a h (15)and Saveant and Vianello (16)for LSV are seen also for SCV. Consider first the dimensionless cathodic peak current. The chemical step has very little effect on the magnitude of the dimensionless cathodic current, which varies from 0.30 to 0.36 when the equilibrium constant is changing by 5 orders of magnitude; i.e., log K varies from -2 to 3. When the equilibrium constant K is less than unity, the variation in is negligible for all values of k r . Therefore, variation in $w is not useful for determining the rate parameter experimentally. The chemical step for an EC reaction prevents the reoxidation of R through conversion to electroinactive Y. The effect, which depends on the values of K and k r , is manifested mainly in the reverse (anodic) part of the cyclic staircase voltammogram. With this in mind, it seems that the peak current ratio may provide a good measure of chemical rate constants. There is an added advantage to using the peak
*I 3.0 log
4 . 5
(k 1.5 r + k b ) r 0.0
i
0 -0.5 -2 -1.5
- 3 4
Figure 4. Variation in anodic peak potential for the following chemical reaction with log (k, kb)7 for various values of log K as indicated.
+
current ratio for the EC case as will be seen below. Peak current ratio is plotted as a function of rate parameter for various values of equilibrium constant in Figures 1 and 2. Consider first the conventional ratio of Figure 1. For all log K values less than about -1, the peak current ratio is at or near the reversible value of unity. Within that range of equilibrium constant, no rate information can be obtained through measurement of either peak currents or peak potentials since the experiment is over before the conversion R Y has taken place to any significant degree. For larger equilibrium constant values (log K > -1) rate information can be gained if k r has an intermediate value (this range varies with K as shown in Figure 1). A potentially troublesome aspect of these peak current ratio curves is that for a given peak current ratio there are a t least two possible values of kT. This problem is not a major one in most cases. That is, in those portions of the working curve where the ratio is either increasing or decreasing significantly, Le., in the range of k r values from about 10 to 0.1, several experiments at different values of the characteristic time can be used to determine which portion of the working curve applies. The computations described here allowed for direct comparison of results obtained by using the conventional extended forward base line for measuring anodic peak current and using Nicholson’s method (19) of calculating peak current ratio. Nicholson’s method was originally devised for the case of a reversible electrochemical reaction followed by an irreversible chemical reaction (E,Ci) but has achieved wider use due to its simplicity. For the EC case, throughout the entire range of equilibrium and dimensionless rate constants studied, it was determined that agreement between the two methods was generally satisfactory. There was signifcant deviation between the two methods only for K > 10 when the current ratio is
-
906
ANALYTICAL CHEMISTRY, VOL. 62, NO. 9, MAY 1, 1990
-
Table V. Limiting Anodic and Cathodic Peak Potentials (kr m ) log K 3 2 1 0.5 0 -0.5 -1 -2
EC mechanism E,,, mV Epe,mV -n
-0
157 100 75.6 56.8 46.2 41.7 39.5
81.0 24.0 -1.0 -19.8 -30.6 -35.2 -37.4
log K 2 1 0.5 0 -0.5 -1 -2 -3
CE mechanism E,,, mV Epo mV 38.3 36.1 31.5 20.8 2.1 -22.6
-37.8 -40.1 -44.7 -55.5 -74.3 -99.3
-a
-0
-a
-0
50
No limiting value found in uotential range studied.
significantly different from unity. An alternative is to use the ratio (ipa),,/iPc,where (ipa)ois the anodic current measured from the zero of current. The quantity (ipa)o is easier to measure and to calculate than is ips. The ratios (ipa)o/ipcplotted in Figure 2 have the same general features and provide the same kinetic information as ip/!w of Figure 1. The ratio (i,Jo/iw is nearly zero in the same region where the Nicholson approximation (19) fails. Thus this troublesome region where kf >> kb and kf = 7-l should be avoided if possible by manipulating the chemical conditions to change K , or by changing 7. The choice of (ipa)o/ipcas the diagnostic parameter makes the difficulties of working in this region dramatically clear. Also, note that this region is the one avoided by separating the kinetic problem into two separate cases, one having a reversible, the other a totally irreversible chemical step (15). For large values of dimensionless rate parameter and large values of equilibrium constant, the peak current ratio is even larger than for an unperturbed system. This is due to enhanced anodic current from increased electrolysis time as both peaks move toward the potential for reversible reduction of 0 to Y (Table I) and the switching potential remains fixed. This effect would be eliminated if one chose to present the results using a potential scale normalized by a factor containing K (15). For uncomplicated reversible electron transfer, the ratio (ipa)o/ipcdepends on the separation of peak and switching potentials. The ratio has the values 0.5222,0.7068, 0.7699, 0.8356, 0.8653, 0.9010, and 0.9243 for values of -n(E,, - ErIjz) equal to 0.1, 0.3, 0.5, 1, 1.5, 1.8, 2.8, and 4.8 V, respectively. The peak potential data shown in Figures 3 and 4 also correlate closely with results for LSV (16,17) and SWV (18). For cyclic staircase voltammetry, the chemical step has a significant effect when log K 2 -0.5 and log k7 > -1.5. In this region, both cathodic (Figure 3) and anodic (Figure 4) peaks shift positive of Erljzdue to the thermodynamic effect (Y is more stable than R). In fact, for the case of log K = 3, the anodic peak shifts are so far positive that an anodic peak cannot be seen within the potential window for log k7 > -0.93. At the opposite end of the time scale, as log k7 m, limiting values of E,, and E, are found for most log K values (Table V, first three columns). Furthermore, as log K a,aEp/a log K = l / n f (from Table V). From the working curve for the cathodic peak potentials (Figure 3), for the largest K value shown, the straight line portion has a slope of 29.4 mV/decade. This value is identical with the one obtained by O’Dea et al. (18) for the net square wave peak and Nadjo and Saveant ( I 7) for linear sweep voltammetry. In the terminology of the latter article, this limiting case lies in the kinetic zone (KP) where there is pure kinetic control by the follow-up chemical reaction. For most values of log k7 the peak separation has its reversible value of about 76/n mV (Figure 5). But in the (KP) region, the peak separation can increase to more than 200/ n mV when K is large. As was the case for the peak current data,
-
-
1
6.0
3.0 l o g
4.5
(k 1 .c 5 + k b ) s 0.0
-1.5
i
-3.0
Figure 5. Peak separation for the following chemical reaction as a function of log (k, k b ) 7 for values of log K as indicated.
+
I -100 3
2
I
0
log
-1
-2
I
-3
(k,+k,)T
Figure 6. Anodic peak potential for the preceding chemical reaction function of log (k, kb)7 for log K values as indicated.
+
as a
7
I
1 -250-
-3
-300 -1
log
-2
-3
(kr*kb)7
Flgure 7. Cathodic peak potential for the preceding chemical reaction function of log ( k , kb)7 for values of log K as indicated.
as a
+
there are certain k7 values in the intermediate range (k7values which are centered around 1 with the range increasing as the value of the equilibrium constant increases), from which chemical rate constants can be obtained. Preceding Reaction. In the second case, a chemical reaction precedes a reversible electrochemical reaction (CE mechanism, Table I, case 2). Results for this case are presented in Figures 6-10. In general, both the forward and reverse parts of the staircase voltammogram are influenced significantly by the preceding chemical reaction, but the effect is larger for the forward (cathodic) part. The chemical effect immediately manifests itself in the forward (cathodic) part of the cyclic staircase voltammogram (except for the largest K values) because 0 and Y are in equilibrium prior to the experiment. By the time the reverse scan is taking place, more time has passed, making more R available for electrochemical
ANALYTICAL CHEMISTRY, VOL. 62, NO. 9, MAY 1, 1990
50 I
2
3
0
log
-2
-1
-3
(k,+k,)T
Figure 8. Peak separation for the preceding chemical reaction as a function of log (k, k,)r for values of log Kas indicated.
+
O
~
~
\
~
-
j
0 0
3
-I
log
-2
-3
(k,+k,)s
Figure 9, Variation in fiPc for the preceding chemlcal reaction as a function of log (k, k b ) r for values of log K as indicated.
+
2 8
I
I
I
1
I
0 8
3
2
0
1
log
-1
-2
-3
(kr+kb)T
Figure 10. Peak current ratio ((ip),,/iF) as a function of log (k, for the preceding chemical reaction for log
+ kb)r
K values as indicated.
oxidation and resulting in an anodic peak which is located nearer to its reversible position. As expected, when the equilibrium constant ( K ) for the chemical reaction is sufficiently large (log K 1 2) or the dimensionless chemical rate constant ( k r )is sufficiently small (log kr 5 -2) the preceding reaction has no influence. The peak potential curves for the CE case (Figures 6-8) deviate significantly from the reversible case only for log K I 0. At large values of k r , the anodic and cathodic peak potentials reach limiting values (Table V, last three columns). As log K decreases toward the smallest value used, -3, and k r increases to 0.1 or greater, the anodic and cathodic peaks both shift negative of their reversible values as the reaction comes increasingly under kinetic control. However, as shown in Figure 8, except for small values of log K ( 7-l. The intermediate values of kcr show mixed control by diffusion and kinetics. The shape changes for various values of k,r are seen in Figure 11,where cathodic and anodic branches of the cyclic staircase voltammogram coalesce into a single S-shaped curve when k,r is large. Since it is impossible to determine any “peak potential” in the region where steady-state current is obtained, and since both the anodic and cathodic parts of the cyclic voltammogram are affected equally, only the dimensionless peak (or maximum) current for the cathodic branch is presented in Figure 12. The value of log k,r at which the catalytic step becomes the controlling factor is about 1.3 (Figure 12). For larger values of log kCr,the magnitude of the current then increases without limit as the rate parameter increases. When steady state is achieved (e.g., Figure 11-4), the limiting current value is given by i, = nFACo(Dokc)1/2or IJ,~= (*k,r)’/*.
ANALYTICAL CHEMISTRY, VOL. 62, NO. 9, MAY 1, 1990
908
3 0. I
-0.I
0.0
Flgure 11. Theoretical cyclic staircase voitammograms for a catalytic follow-up chemical reaction: k , s = (1) 0.01, (2) 0.05, (3) 0.1, (4) 1.0. Vertical axis is dimensionless current, $, and horizontal axis is normalized potential, n(E - ,Er,,*).
I
l5
+:
/
t
101
/
/
51
log k E T
Flgure 12. Variation in
$pc
with log k,s for catalytic follow-up reaction.
These data show the range and the limitations of cyclic staircase voltammetry for investigating first-order kinetic systems. Significant kinetic information can be gained from cyclic staircase voltammetry with only time (frequency) as a parameter. This is equivalent to changing scan rate in LSV, but the current response in SCV has a much smaller component of charging current for comparable conditions. From the voltammetric response, the measurable features of peak heights and peak positions can be compared with the working curves presented here to determine chemical rate constants. The working curves can be used directly when the reaction scheme, the value of ErIl2and the equilibrium constant, K , are known. In the absence of this knowledge, only the catalytic case can be identified unambiguously by simple variation of frequency. The EC and CE cases can be differentiated by examining change in the peak current ratio with change in the characteristic time of the experiment. If the peak current ratio increases above the reversible value, a CE mechanism is suggested. Conversely, a decrease in the peak current ratio
below the reversible value (see Figures 1 and 2) suggests an EC mechanism. For each of the three schemes presented, there are well-defined regions where the effect of the chemical step is minimized and reversible behavior is seen; that is, peak potentials and dimensionless peak currents have their reversible values. It is not possible to obtain kinetic information in these regions. Also, there are regions on most of the curves involving current for which the value of the current or current ratio approaches zero. These regions may present experimental problems. This still leaves a 4 to 5 order of magnitude range of time in which to work. The additional work involved in using an extended forward base line to measure ,i for calculation of peak current ratio was found to be unnecessary. The semiempirical method developed by Nicholson for LSV (E,Ci mechanism) did have some utility for SCV within the limitations described above. Simply measuring the anodic peak current referenced to zero (and calculating (ipa)o/ipc)was found to be generally useful for both CE and EC mechanisms. We recommend this approach for experimental measurements. Further discussion of this point is found in ref 14. The results presented here apply quantitatively for normalized step height ( n u )of 5 mV. Both amplitude and shape of the response depend on step height, but results presented here are representative for other choices of n A E over the range normally used.
ACKNOWLEDGMENT The authors wish to thank M. Seralathan for helpful discussions and MASSCOMP for the donation of computer equipment. LITERATURE CITED (1) Ryan, M. D. J . Nectroanal. Chem. Interfacial Nectrochem. 1977, 7 9 , 105. (2) Miaw, L. H. L.; Boudreau, P. A.; Pichier. M. A,; Perone, S. P. Anal. Chem. 1978,50, 1988. (3) Christie, J. H.; Lingane, P. J. J . Nectroanal. Chem. 1965, 70, 176.
Anal. Chem. 1990, 62, 909-914 (4) Lam, K. W. L. Ph.D. Thesis, Wayne State University, 1974. (5) Ferrier, D. R.; Schroeder, R. R. J. Electroanal. Chem. Inferfacial Ebctrochem. 1973, 45,343. (6) Ferrier, D. R.; Chidester, D. H.; Schroeder, R. R. J. Electroanal. Chem. Interfacial Electrochem. 1973, 45,361, (7) Stefani, S.; Seeber, R. Anal. Chem. 1082, 5 4 , 2524. (8) Zipper, J. J.; Perone. S.P. Anal. Chem. 1973, 45,452. (9) Schachterle, S. D.; Perone, S. P. Anal. Chem. 1981, 53, 1672. (10) Surprenant, H. L.; Rigway, T. H.; Reilley. C. N. J . Elecfroanal. Chem. Inferfacial Electrochem. 1977. 75, 125. (11) Seralathan, M.; Osteryoung, R. A.; Osteryoung, J. G. J. Nectroanal. Chem. Interfacial Necfrochem. 1987, 222, 69. (12) Bilewicz, R.; Osteryoung, R. A.; Osteryoung, J. Anal. Chem. 1986, 58,2761. (13) Seralathan, M.; Osteryoung, R.; Osteryoung, J. J. Nectroanal. Chem. Inferfacial Electrochem, 1988, 2 74, 141. (14) Murphy, M. M.: O'Dea, J. J.; Arn, D.; Osteryoung, J. G. Anal. Chem. 1989, 67, 2249-2254. (15) Nicholson, R. S.; Shain, I.Anal. Chem. 1964, 36,706.
909
(16) Saveant, J. M.; Vianello, E. Electrochim. Acta 1963, 8 , 905. (17) Nadjo, L.; Saveant, J. M. J. Electroanal. Chem. Inferfacial Nectrochem. 1973, 48, 113. (18) O'Dea, J. J.; Osteryoung, J.; Osteryoung, R. A. Anal. Chem. 1981, 53,695. (19) Nicholson, R. S. Anal. Chem. 1986, 38, 1406. (20) Smith, D. E. Anal. Chem. 1963, 35,602. (21) O'Dea, J. J. Ph.D. Dissertation, Colorado State University, Fort Collins, CO, 1979. (22) Nicholson, R . S.;Olmstead, M. L. I n Necfrochemlstfy: Calculations, Simulation and Instrumentation; Mattson, J. S., Mark, H. B., MacDonald, H. C., Eds.; Marcel Dekker: New York, 1972; Vol. 2, Chapter 5. (23) Brumleve, T. R.; Osteryoung, J. J. Phys. Chem. 1982. 86, 1794.
RECEIVED for review September 18,1989. Accepted January 24, 1990. This work was supported in part by the National Science Foundation under Grant no. CHE8521200.
Conductive Polymeric Tetrakis(3-methoxy-4-hydroxyphenyl)porphyrin Film Electrode for Trace Determination of Nickel Tadeusz Malinski,* Aleksander Ciszewski,' and Judith R. Fish Department of Chemistry, Oakland University, Rochester, Michigan 48309-4401
Leszek Czuchajowski* Department of Chemistry, University of Idaho, Moscow, Idaho 83843
Stable polymer film electrodes were formed from tetrakis(3methoxy4hydroxyphenyi)porphyrinwlth NI( I I ) as the central metal. The nickel-porphyrin polymer films efficiently demetaiated In acidic media at pH 1. The resulting demetalated porphyrln polymer electrodes required no preconditioning treatment before exhlbitlng strong affinity for Ni( I I ) . Electrodes were placed in sample soiutlon to preconcentrate NI( I I ) and transferred to a blank electrolysis solution where the signal due to the Ni( I I)/NI( I I I ) oxidation In the film was observed by dlfferential pulse voltammetry. A detection limit of 8 X 10" M was obtained for a 60-s exposure to the sample solution. Accurate quantitatlon of Ni in certified standard reference material NBS 1643 6 (Trace Elements in Water) was achieved wlth the electrode developed. Electrodes wlth polymeric porphyrin, which can be used as amperometrlc sensors, were easlly and controllably formed and stable with both time and use. The demetalated film could be regenerated with exposure to acid and reused for chemical preconcentration. I n interference studies, a 10-fold excess of Co resulted In partial suppression of the Ni( I I ) signal, but no new signals were observed. Similar concentrations of cations of Zn, Cd, Pb, Cu and Fe did not appreciably influence the NI( I I ) response.
INTRODUCTION Application of chemically modified electrodes for trace analysis offers the advantage of selectivity coupled with the *To whom correspondence should be addressed. Permanent address: Department of Analytical Chemistry, Technical University of Poznan, 60-965 Poznan, Poland.
sensitivity enhancement of preconcentration. Electrode surfaces are designed and fabricated to preconcentrate a particular species by reaction and bonding with specific functional groups. The preconcentration, which has a purely chemical rather than electrochemical mechanism, can be effected by ion exchange ( 1 , 2 ) ,covalent linkage (3),or complex formation (4-8) reactions. Therefore, selectivity is determined by the chemical reactivity of the electrode modifying agents rather than the redox potential of the analyte. This allows construction and use of electrodes specifically optimized for an analyte of interest. These electrodes do not depend on electrochemical processes performed in or on the film and, therefore, make possible selective preconcentration of analytes that are either difficult or impossible to reductively deposit onto untreated electrodes. Quantitation of accumulated analyte is effected by the usual voltammetric methods. However, this process is facilitated because interfering species that would be codeposited a t the negative potentials necessary and/or exhibit overlapping stripping potentials are discriminated against during chemical preconcentration. Exchanging the sample solution with a clean medium prior to performing the actual quantitation step may effectively bypass a host of electroactive interfering substances encountered with stripping voltammetry in which electrolytic preconcentration is employed. Ni(I1) is an essential metal that occurs a t trace concentration levels in physiological and environmental systems. However, its quite negative reduction potential, -1.2 V vs SCE (9, l o ) ,and propensity to form intermetallic compounds with other metallic species codeposited result in complex, matrix-dependent stripping patterns not suited for quantitation (10). Therefore, development of electrodes with selective analyte collection properties is relevant for trace level analysis of this metal.
0003-2700/90/0362-0909$02.50/00 1990 American Chemical Society
