Anal. Chem. 1987, 59, 2853-2860 (19) Ruzlcke, J.; Hansen, E. H.; Ghose, A. K.; Mottola, H. A.; Anal. Chem. 1979, 57, 199. (20) Hou, J. P.; Poole, J. W. J . phefm. Scl. 1972, 67, 1594.
RECEIVED for review May 11,1987. Accepted August 17,1987. The work at Lawrence Livermore Laboratory was performed under the auspices of the US Department of Energy under
2853
Contract W-7405-Eng-48. The group at Lawrence Livermore Laboratory would like to express thanks to Gerald Goldstein of the Office of Health and Environmental Research for supporting their work under Contract No. RPIS-003906. The work at Tufts University was supported by the Environmental Protection Agency through the Tufts Center for Environmental Management.
Assessment of Conditions under Which the Oxidation of Ferrocene Can Be Used as a Standard Voltammetric Reference Process in Aqueous Media A. M. Bond,* E. A. McLennan,l R. S. Stojanovic, and F. G. Thomas2 Division of Chemical and Physical Sciences, Deakin University, Waurn Ponds 321 7, Victoria, Australia
The one-electron owldatlon process for ferrocene (Fc), Fc e-, has been studied extensively by cycllc, normal Fc+ pulse, and dlfferentlal pulse voltammetry and chronocoulometry to determine the condltlons under whlch this reactlon can be used as a voltammetrlc standard In aqueous medla. I n water, the oxldatbn of ferrocene Is not a shrple revenlMe one-electron process as Is the case In organlc solvents. Rather, weak reactant adsorptlon Is exhlblted, whlch Is electrode and electrolyte dependent. The oxldatlon of a saturated solutlon corresponds to conslderably less than a monolayer of coverage. The relatlve order of adsorption wlth respect to the electrode materlal follows the trend Hg > glassy carbon > Au > Pt, whlle wlth electrolyte lt Is NaCIO, > LI,SO, > NaF. Desplte the presence of weak reactant adsorptlon, essentially electrode, electrolyte, and technlque Independent voltammetric data can be obtalned at sufflclently slow scan rates or long pulse widths. The E,,, value calculated under these condltkns is 0.400 0.005 V vs NHE whkh agrees very well wlth the standard redox potential reported from potentkmetrlc measurements under genuine equlllbrlum condltlons. The data suggest that ferrocene can be used as a voltammetrlc standard under carefully chosen condltlons where the Influence of adsorptlon Is mlnlmal.
+
*
The standard or formal reduction potential (Eo or E3 is often the basic thermodynamic quantity used to characterize a redox system. In aqueous media, redox potentials are usually measured relative to reliable and universally accepted reference electrodes such as the standard hydrogen eledrode (SHE) or the saturated calomel electrode (SCE) (1,2). Unfortunately, no universally accepted reference electrode exists for work in the majority of nonaqueous solvents (3, 4): Frequently electrode potentials obtained in nonaqueous solvents have been reported versus aqueous reference electrodes such as the SCE, the silver-silver chloride electrode, or the SHE. The problem with using these reference electrodes in nonaqueous Present address: Division of Biological and Health Sciences, Deakin University. *On leave from James Cook University, Townsville 4811, Queensland, Australia.
solvents is that an unknown liquid junction potential is introduced into the measurements. In view of the solvent-dependent liquid junction potential problem, there has been considerable interest in finding solvent independent redox couples that can be used as reference redox systems in both organic and aqueous solvents (5-13). Unfortunately, the standard redox potential of a given redox system in solution invariably depends to some degree on the nature of the solvent in which it is measured (14). Some redox couples are very strongly solvent dependent (electrode potentials are a function of the coordinating ability of the solvent) and are clearly not suitable as reference redox systems (14-18). A systematic search for a solvent-independent reference redox system has been based on theory involving various extrathermodynamic assumptions. One conclusion reached from these assumptions is that the redox potentials of large ions, molecules, or complexes might be essentially independent of the nature of the solvent (6, 10, 11, 19). More recently, it has been proposed that large symmetrical ions in which the charge is deeply buried should have the same activity as an uncharged molecule of the same size and structure in all solvents. Such redox couples involving a one-electron transfer between the ionic and neutral forms will include only minor contributions from the solvent. Strehlow (6) used this concept to propose that the redox systems ferricenium ion-ferrocene (Fc+/Fc) (ferrocene is bis(q5-cyclopentadienyl)iron(II) and cobalticenium ion/cobaltocene (Cc+/Cc) (cobaltocene is bis(q5-cyclopentadienyl)cobalt(II)) should be solvent independent. Since then, several reviews have appeared in the literature (20-25) that suggest that the Fc+/Fc assumption (6)may not be perfect. In view of the lack of complete agreement of the use of ferrocene, other reference redox systems (7,10-13,26-28) have been suggested. These include bis(biphenyl)chromium(l) / bis(biphenyl)chromium(O) (Bcr+/Bcr) ( 1 0 , I I ) and redox systems based on polynuclear aromatic hydrocarbons and the respective radical ions (13). Despite some questions being raised with respect to the Fc+/Fc redox couple, extensive research into reference redox systems has led to the IUPAC Commission on Electrochemistry recommending that either of the redox couples ferrocene/ferricenium ion or bis(q-biphenyl)chromium(I)/bis(qbiphenyl)chromium(O)be used tw reference redox systems (29).
0003-2700/87/035Q-2853$01.50/0 0 1987 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 24, DECEMBER 15, 1987
Both systems fulfill the requirements for reference redox systems in many solvents. The difference between the respective redox potentials of these two reference systems has been found to be almost constant in a variety of solvents (14-18, 30, 31). While voltammetric studies on the Fc+/Fc couple using oxidation of ferrocene (Fc) have been extensively reported in nonaqueous solvents (3,6,14-17,30, 311, detailed studies on the oxidative voltammetry of Fc in aqueous media have been restricted to platinum electrodes and two different electrolytes (8, 32). Therefore, the exact usefulness of ferrocene as a voltammetric standard in one of the most important solvents is uncertain. Since it has been conclusively established that the Bcr+/Bcr couple cannot be used in water as a reference system (15,33,34),voltammetric studies on Fc in water have enhanced importance with respect to the IUPAC recommendations. Potentiometric studies (6, 20, 25, 35, 36) tend to suggest that the FcC/Fc redox couple can be used as a standard in nonaqueous solvents but that it may be less useful in water and aqueous mixtures due to specific interactions with water. Ferrocene is only sparingly soluble in water, with a reported solubility of the order of M in the presence of electrolyte (8,32). Consequently, use of this compound for voltammetric and polarographic studies requires working at or near the solubility limit to achieve sufficient sensitivity. It has been established in other instances that adsorption or precipitation phenomena are frequently associated with voltammetric studies undertaken near the solubility limit (37-39). Thus, specific electrode interactions may accompany the oxidation of ferrocene in water and ions such as perchlorate, which disrupt the water structure, may coadsorb with any adsorbed ferrocene. Additionally, solution effects such as ion pairing between anions and Fc+ may modify the voltammetric response. In the present report, detailed voltammetric and polarographic studies on the oxidation of ferrocene (Fc) have been undertaken in aqueous media a t dropping mercury (DME), glassy carbon (GC), gold (Au), and platinum (Pt)electrodes. The electrochemical techniques of cyclic voltammetry (CV), differential pulse voltammetry (DPV), normal pulse voltammetry (NPV), and chronocoulometry (CC) have been used to ascertain whether or not the electrode process F c + Fc+ + e-
(1)
is a simple one-electron charge-transfer process that can be used as a voltammetric reference redox system in aqueous media. Since it is possible that the supporting electrolyte can significantly affect redox properties and voltammetric responses (40-42), three supporting electrolytes Li2SO4,NaClO,, and NaF have been chosen to provide variation in the cation and anion as well as the charge on the anion. These extensive studies form the basis for our final recommendation that the Fc+/Fc redox couple only can be used as a voltammetric reference redox system in aqueous media with caution, under carefully controlled conditions where the influence of adsorption is negligible. EXPERIMENTAL SECTION Reagents and Solvents. Ferrocene (Merck) lithium sulfate
(BDH), sodium perchlorate (BDH), sodium fluoride (Hopkins and Williams), and tetraethylammonium perchlorate (Southwestern Analytical Chemicals) were of analytical or electrochemical grade purity and dried under vacuum prior to use. Triply distilled mercury (Engelhard Industries, Pty. Ltd.) was used in polarographic studies at the dropping mercury electrode. Water was obtained from a Millipore 25TS (twin stage) reverse-osmosis water purification system (Millipore, Bedford, MA) and had a specific conductance of 2 WScm-'. HPLC grade acetonitrile was used.
Electrodes. The working electrodes used were glassy carbon (area, 0.078 cm2),gold (area, 0.024 cm2),and platinum (area, 0.021 cm2)disks obtained from Bioanalytical Systems, Inc. The working electrodes were polished with 0.05-pm alumina between each measurement. The surface area of each solid electrode was deM Fc solution in acetonitrile termined from oxidation of a l X (0.1M Et4NC104)by using chronocoulometric measurements and the equation
Q = 2nFACD112t112/a'i2 (2) where Q is charge, n the number of electrons transferred, F the Faraday constant, A the electrode area, C the concentration, D the diffusion coefficient and t the time (seconds). In this medium the oxidation of ferrocene is known to be a reversible, one-electron cm2s-l at process (43) and the diffusion coefficient is 2.3 X 22 "C ( 4 4 ) . The dropping mercury electrode (DME) had an open-circuit flow rate of 0.8 mg s-l and an electrode area of 0.004 cm2 with a mechanically controlled drop time of 0.5 s. The reference electrodes used were Ag/AgCl (3 M NaCl), calomel (saturated KCl) and Ag/AgCl (saturated KCl) electrodes. The auxiliary electrode was a platinum wire. The diffusion coefficient of Fc in various aqueous supporting electrolytes (0.1 M Li2S04,0.1 M NaClO,, 0.1 M NaF) was determined under steady-state conditions (scan rate 50 mV s-l) by the use of a 5-pm radius carbon fiber microdisk electrode and the equation
iL = 4nFDCr
(3)
where iL is the limiting current, r the radius of the electrode, and the other terms have been previously defined. The diffusion coefficient of Fc was found to be (6.9 f 0.5) X lo4 cm2s - ~in 0.1 M Li2S04,(7.6 f 0.5) X lo4 cm2 s-l in 0.1 M NaClO,, and (7.1 f 0.5) X lo4 cm2s-l in 0.1 M NaF. These values are consistent with the literature data (32). Instrumentation. Voltammetric measurements were performed with either of the following instruments: (a) A BAS-100 electrochemical analyzer (Bioanalytical Systems) and a threeelectrode system containing a working electrode (GC, Au, Pt), a reference electrode (Ag/AgC1(3 M NaCl), calomel (saturated KCl), Ag/AgCl (saturated KCl)), and a platinum-wire auxiliary electrode. (b) A PAR Model 174 polarographic analyzer (Princeton Applied Research), equipped with a PAR Model 172 mechanical drop knocker. A conventional dropping mercury electrode was used as the working electrode in the three-electrode cell described in example a above. A Model 26 pH meter (Radiometer, Copenhagen)was used for pH measurements. Experimental Procedure. Voltammetric and polarographic measurements were carried out in the following manner: All glassware was placed in an acid bath containing 2 M nitric acid for approximately 24 h and rinsed thoroughly with distilled water. Saturated solutions of ferrocene were prepared by adding excess Fc to the supporting electrolyte solution and stirring for approximately 24 h at 25 f 0.1 "C to achieve a saturated solution. The saturated solutions of Fc were filtered directly into the thermostatically controlled jacketed cell controlled at 25.0 f 0.1 "(2. For voltammetric and polarographic measurements, the solutions were deoxygenated with nitrogen for a minimum of 3 min and a continuous stream of nitrogen was passed over the solutions during the measurements. The nitrogen was passed through a trap containing the electrolyte of interest, prior to being passed into the electrochemical cell. For pH measurements, solutions were filtered and deoxygenated with nitrogen for a minimum of 3 min. The pH of a saturated solution of Fc was found to be 6.96 f 0.05 in 0.1 M Li2S04,7.35 ?= 0.05 in 0.1 M NaClO,, and 7.44 f 0.05 in 0.1 M NaF at 25 OC. Determination of Ferrocene Concentration. The ferrocene concentration was determined by differential pulse voltammetry (DPV) at 25 f 0.1 "C in stock saturated solutions containing various aqueous supporting electrolytes (0.1 M Li2S04,0.1 M NaC104,0.1 M NaF). All measurements were performed by using a glassy carbon (GC) working electrode, a Ag/AgCl (3 M NaCl) reference electrode, and a platinum-wire auxiliary electrode. Standard solutions of ferrocene were prepared from a 50/50 mixture of water/acetonitrile containing the electrolyte of interest.
ANALYTICAL CHEMISTRY, VOL. 59, NO. 24, DECEMBER 15, 1987
for a Saturated Solution of Ferrocene in Various Aqueous
Table 1. Cyclic Voltammetric Data at a Scan Rate of 100 mV Supporting Electrolytes at 25.0 f 0.1 "C electrode
electrolyte, M
ET,O V
GC
0.1 Li2S04 0.1 NaC104 0.1 NaF 0.1 LizS04 0.1 NaC104 0.1 NaF 0.1 Li2S04 0.1 NaC104 0.1 NaF
0.210 0.205 0.218 0.218 0.205 0.215 0.204 0.203 0.212
Au
Pt
V
iT/itd
Elpa V
Elp,b V
3.26 2.50 4.76 1.92 1.26 2.83 2.60 1.66 1.44
0.188 0.180 0.192 0.185 0.175 0.186 0.178 0.177 0.184
0.164 d 0.161 0.156 d 0.158 0.160 d 0.159
0.164 0.155 0.165 0.151 0.145 0.156 0.153 0.150 0.156
"Volt vs Ag/AgCl (3 M NaCl). El/?= ( E T + E,'")/2. measurement precluded by K+C10, junction.
DISCUSSION
Cyclic Voltammetry. The electrochemical behavior of Fc in nonaqueous solvents has been studied in detail (3,6,14-17, 30,31). Initially, cyclic voltammograms for the oxidation of
1 mM Fc in acetonitrile (0.1 M Et4C104)were examined at a glassy carbon electrode at 25.0 f 0.1 "C. A well-defined response was observed with a peak separation (PEP)equal to the theoretical value of 56 mV predicted for an electrochemically reversible one-electron oxidation process. Additionally, the values of the oxidation peak current (ipos)and reduction peak current (ipd) are identical in magnitude, indicating that the system is also chemically reversible without any complicating processes from adsorption in this medium. This process can be described as
*
[Fe11(q5-C5H5)z] [Fe"'(s5-C5H5),]'
+ e-
(4)
and is suitable as an internal standard in acetonitrile, as stated in the literature. Cyclic voltammograms were also obtained for saturated solutions of Fc in various aqueous supporting electrolytes (LizS04,NaC104, NaF) by using a range of working electrodes (GC, Au, Pt) at 25.0 f 0.1 "C. The cyclic voltammetric data obtained at a scan rate ( u ) of 100 mV s-l are summarized in Table I, and Figure 1 shows typical examples at the GC electrode. The separation of the peak potentials (PEP)and values of ip/ipd were found to be dependent on the working electrodes and supporting electrolytes at 25.0 f 0.1 "C and not consistent with a simple reversible charge-transfer process as required for a voltammetric standard. The peak-to-peak separations at the GC electrode were less than Nernstian and the ratio ip"/ipd was considerably greater than unity. All of the data at the GC electrode indicate that weak reactant adsorption is present (2,38). The half-wave potential (El/z)obtained at the stationary GC electrode versus various reference electrodes was found to vary only slightly with change in supporting electrolytes with average values being 0.186 f 0.006 V vs Ag/AgC1(3 M NaCl), 0.163 f 0.002 V vs SCE, and 0.209 f 0.002 V vs Ag/AgCl (saturated KC1) at 25.0 f 0.1 "C. These E l j zvalues were obtained at a scan rate of 100 mV from the relationship Ellz = (Epm+ Epd)/2. Figure 2a shows a typical cyclic voltammogram at a Au electrode for a saturated solution of Fc in water (0.1 M Li2S04) at 25.0 0.1 OC. Although the peak-to-peak separation (Up) on the Au electrode at a scan rate of 100 mV s-l is close to that expected for electrochemically reversible behavior, the ratio ipoX/iTd is still not unity and corresponds to weaker reactant adsorption than that observed at the GC electrode.
*
Elp:
v
0.207 d 0.211 0.208 d 0.204 0.203 d 0.203
m,, mV 45 50 53 67 60 59 51 53 56
*Voltvs calomel (saturated KC1). cVoltvs Ag/AgCl (saturated KCl). dAccurate
The aqueous saturated solutions of Fc were mixed 111 with acetonitrile and the concentration of Fc in each aqueous electrolyte medium determined by the normal calibration method. The concentration of Fc determined in this manner was found to be M in 0.1 M Li2S04,(3.8 f 0.5) X M in (3.3 f 0.5) X M in NaF. NaC104,and (3.0 f 0.5) X
RESULTS AND
2855
0.
Y'
(b)
1500nA
(C)
0Volt .&*O
Ag/AgCl(3M NaCI)
VI
Figure 1. Cyclic voltammorgrams of a saturated solution of ferrocene in water containing (a) 0.1 M Li,SO,, (b) 0.1 M NaF, and (c) 0.1 M NaCIO, at 25.0 f 0.1 "C at a glassy carbon electrode.
Volt
VI
Ag/AuCI(OM NaCII
Flgure 2. Cyclic voltammograms at a scan rate of 100 mV s-' for oxidation of a saturated solution of ferrocene in water (0.1 M Li,SO,) at (a) gold and (b) platinum electrodes at 25.0 f 0.1 "C.
Unfortunately, the faradaic response was more difficult to calculate than at the GC electrode because of the relatively large contribution from the charging current. The half-wave
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 24, DECEMBER 15, 1987
Table 11. Effect of Scan Rate on Cyclic Voltammetric Data at the Glassy Carbon Electrode for Oxidation of a Saturated Solution of Ferrocene in Various Aqueous Supporting Electrolytes at 25.0 f 0.1 "C
electrolyte, scan rate, M
mV s-'
0.1 Li2S04
10 20 50 100 250 500 10 20 50 100 250 500 10 20 50 100 250 500
0.1 NaC10,
0.1 NaF
ipOl, pA ipo1/ipled E1,2?bV AEp; mV 0.183 0.245 0.539 1.231 2.640 3.840 0.241 0.350 0.601 0.862 2.155 4.100 0.170 0.267 0.427 0.674 1.500 2.487
1.10 1.58 2.38 3.26 3.84 4.35 1.26 1.51 1.96 2.50 4.00 4.34 1.75 2.70 3.03 4.76 5.69 7.00
0.177 0.179 0.184 0.188 0.202 0.207 0.175 0.176 0.180 0.180 0.189 0.202 0.178 0.185 0.187 0.192 0.204 0.211
"Volt vs Ag/AgCl (3 M NaCl). bE,,, = (Ep"'+ E,'*)/2. E,"" - Enred.
56 55 55 45 41 44 57 54 53 50 47 43 60 56 56 53 49 48 =
potential obtained at the stationary Au electrode versus various reference electrodes was found to be only slightly dependent of the supporting electrolyte with average values of 0.180 f 0.006 V vs Ag/AgC1(3 M NaCl), 0.157 f 0.001 V vs SCE, and 0.206 f 0.005 V vs Ag/AgCl (saturated KCl) at 25.0 f 0.1 "C and a scan rate of 100 mV s-l. The voltammetric data (Table I) obtained at the Pt electrode indicate that the adsorption of the reactant is even weaker than for Au electrodes with only slight departures from standard Nernstian behavior being observed. Unfortunately, the charging current again limited the accuracy of the cyclic voltammetric measurements. A typical cyclic voltammogram a t a platinum electrode in 0.1 M Li2S04in water at 25.0 f 0.1 "C is shown in Figure 2b. The Ell2obtained a t the stationary Pt electrode versus various reference electrodes was again found to be almost independent of the electrolyte and had average values of 0.180 f 0.004 V vs Ag/AgC1(3 M NaCl), 0.160 f 0.001 V vs SCE, and 0.203 f 0.001 V vs Ag/AgCl (saturated KCl) at 25.0 f 0.1 O C and a scan rate of 100 mV S-1.
The effect of the scan rate on the cyclic voltammograms obtained a t a GC electrode in a saturated solution of Fc in various supporting electrolytes is summarized in Table 11. Over the range of scan rates examined, the El12,AE,,and ipox/i l o 0 mV/s) indicate considerable departures from ideal behavior in water at 25.0 f 0.1 "C. The scan-rate dependence is also consistent with the reactant (Fc) being weakly adsorbed. The peak current (i,) in linear-sweep voltammetry for a reversible system is described by the Randles-Sevcik equation
i, = (2.69 X 106)n3/2AD1/2Cv1/2 where i, is peak current (A), n is the number of electrons, A is the electrode area (cm2),D is the diffusion coefficient (cm2 s-l), C is concentration (mol ~ m - ~and ) , u is the scan rate (V s-l). According to this equation, plots of ip vs u1I2 should be linear with intercepts at the origin. The scan rate dependence does not fit eq 5. The effect of commencing the scan
at a more negative potential caused ipoxto increase, while making the switching potential more positive caused iTdto increase. It is possible that weak product adsorption (Fc+) is also present, as the data is not completely consistent with only weak reactant adsorption. Recording cyclic voltammograms at different time intervals (30 s, 1min, 5 min, and 10 min) had no measurable effect on the voltammetric data which indicates that adsorption equilibrium is reached in less than 30 s. Cyclic voltammograms on the Au and Pt electrodes also revealed a scan rate dependence that is consistent with the presence of adsorption. However, importantly, at very low scan rates (10-20 mV/s) measured El12values at all working electrodes are essentially independent of electrode and electrolyte, and the value of ipoX/iFd approaches unity. These data suggest that such conditions may be appropriate for using ferrocene as a voltammetric reference redox couple in water at 25.0 f 0.1 "C since the process now phenomenologically behaves as a simple reversible redox couple. However, other measurements need to be made to confirm this possibility. Since weak reactant adsorption is considered to be present, the concentration dependence on voltammetric measurements can be important. The effect of concentration on cyclic voltammetry data at the GC electrode with CJ4, C,/2, and saturated (CJ solutions of Fc in vaious supporting electrolytes at 25.0 f 0.1 "C was examined. The half-wave potential (Ell2) was found to be independent of concentration within f l mV in all supporting electrolytes, although hE, and i?/ipd values were affected slightly, becoming closer to the values expected for a reversible process as the concentration was decreased. Normal Pulse Voltammetry. Pulse voltammetric techniques offer better sensitivity than dc voltammetric techniques because of the improved faradaic-to-charging current ratio (1, 2 ) . In the present study, normal pulse voltammetry (NPV) has been used as both a diagnostic tool for examining the effect of reactant adsorption on the shapes of waves and an indication of the electrochemical reversibility of the Fc+/Fc redox couple in water at 25.0 f 0.1 OC. This is particularly pertinent since a redox couple such as Fc+/Fc may behave differently in the normal pulse mode, compared with other techniques where the potential is swept (1, 2). The phenomenon of maxima in pulse voltammetry (polarography) has been studied in detail (45-52). Perturbations in the shapes of normal pulse voltammograms (polarograms) were fist reported by Barker and Bolzan (45), who interpreted the occurrence of peaked maxima as arising from reactant adsorption. Figure 3 shows normal pulse voltammograms obtained at the GC, Au, and Pt electrodes in 0.1 M Li2S04in water at 25.0 f 0.1 O C . The curves are characterized by maxima that are electrode, electrolyte, pulse width, and concentration dependent. Some of the voltammetric data obtained from NPV are summarized in Table 111. E3I4- E114 values were used as a diagnostic criterion for reversibility, and all but one of the values were found to be less than the theoretically predicted value of 56 mV for a reversible one-electron process. Unfortunately, the presence of a large charging current and the maximum (which causes the limiting current to be less than in the absence of adsorption) prevented highly accurate measurements from being made. The half-wave potential was found to be slightly dependent on the working electrode and supporting electrolyte under conditions of normal pulse voltammetry. Normal pulse voltammograms at the GC electrode showed pronounced maxima in 0.1 M Li2S04and 0.1 M NaC104, which is a direct consequence of reactant adsorption. The maximum was not as evident on the Au and Pt electrodes, which is consistent with cyclic voltammetry data when weaker adsorption was also indicated at these electrode
ANALYTICAL CHEMISTRY, VOL. 59, NO. 24, DECEMBER 15, 1987
2857
Table IV. Effect of Pulse Width on Normal Pulse Voltammetric Data" for Oxidation of a Saturated Solution of Ferrocene at the Glassy Carbon Electrode at 25.0 f 0.1 OC
A
0.5
0.0
electrolyte, M 0.1 LizSOl
(b)
,
W
O
1 1
0.1 NaC104
0.0
0.4
0.1 NaF
E314 -
pulse width, ms
E112,bV
iL, PA
Ell,, mV
25 40 60 100 25 40 60 100 25 40 60 100
0.202 0.180 0.188 0.170 0.206 0.172 0.176 0.168 0.202 0.188 0.182 0.178
15.515 4.221 3.998 1.537 24.024 4.791 4.255 1.931 5.871 2.870 2.037 0.942
30 32 44 44 32 33 43 44 46 48 51 56
" Scan rate = 2 mV s-l. Duration between pulses = 1 s. Ag/AgCl (3 M NaCl). I
0.0
0.5 Volt
VI
Ag/AgCI(SM NaCI)
Figwe 3. Normal pulse voltammograms at (a) glassy carbon electrode, (b) gold electrode, and (c) platlnum electrode at 25.0 f 0.1 OC for oxidation of a saturated solution of ferrocene in.water (0.1 M LI,SO,). Pulse width = 60 ms. Duration between pulses = 1 s. Scan rate = 2 mV s-I.
Table 111. Normal Pulse Voltammetric Data" for Oxidation of a Saturated Solution of Ferrocene in Various Aqueous Supporting Electrolytes at 25.0 k 0.1 "C electrode GC Au
Pt
electrolyte, M E i p b V Elp; V 0.1 Li2S04 0.1 NaC104 0.1 NaF 0.1 Li2S04 0.1 NaC104 0.1 NaF 0.1 Li2S04 0.1 NaClO, 0.1 NaF
V vs
0.188 0.176 0.182 0.172 0.157 0.160 0.164 0.166 0.168
0.162 e 0.158 0.150 e
0.138 0.138 e 0.126
Elj2,d
v
0.204 e
0.210 0.198 e 0.187 0.188 e
0.181
E3/4
mV
44 43 51 48 54 51 54 53 56
'Scan rate = 2 mV s-l. Duration between pulses = 1 s. Pulse width = 60 ms. bVolt vs Ag/AgCl (3 M NaCl). eVolt vs calomel (saturated KCl). dVolt vs Ag/AgCl (saturated KCl). eAccurate measurements precluded by K+C10cjunction. surfaces. It was also found that in NaF the maximum was not as pronounced as in the other supporting electrolytes, suggesting that Fc has a tendency to adsorb to a greater extent in Li2S04and NaC104 than it does in NaF. This is consistent with Anson's classification of adsorbing species, where fluoride is not likely to induce adsorption (53). Diagnostic criteria for reversibility under pulse voltammetric conditions are shown to be similar to dc polarography except that the time domain in NPV is governed essentially by the pulse duration (I) rather than the drop time. The equation for the i-E curve for the reversible case may therefore be written in an analogous fashion to the Heyrovsky-Ilkovic equation
Thus a graphic plot of E vs log [(iL - i ) / i ]should be linear with a slope of 2.303RT/nF for a reversible process in normal pulse voltammetry. For the oxidation of Fc in water, this plot yielded a slope less than the theoretical value of 59 mV at all working electrodes.
The effect of the pulse width on normal pulse voltammograms obtained at the GC electrode in a saturated solution containing various aqueous supporting electrolytes is summarized in Table IV. At short pulse widths, the E l j zvalues are shifted to more positive values as is the case with E," a t faster scan rates in cyclic voltammetry. At long pulse widths, the maximum decreases and Ellz becomes essentially independent of electrode, electrolyte, and pulse width and experimentally indistinguishable from the E l j zvalue obtained with very slow scan rate cyclic voltammetry. That is, the influence of adsorption becomes less with respect to departures from the ideal reversible process at long pulse width. The size of the maximum showed a scan rate dependence and increased at slow scan rates (
