Voltammetric Applications of Rotating and Stationary Ca rbon-Epoxy Electrodes HAROLD S. SWOFFORD, Jr., and ROY L. CARMAN 111 Department of Chemistry, University of Minnesota, Minneapolis, Minn.
b Carbon electrodes consisting of carbon black suspended in an epoxy resin matrix have been constructed and used successfully for voltammetric investigations. The carbon-epoxy material was fabricated into both a rotating carbon disk electrode for current-voltage investigations, and a linear diffusion cell for chronopotentiometric work. Their residual behavior is examined and their applicability has been tested on the “reversible” Fe(cN)~,--Fe(cN)6-~ couple; speculation as to the nature of carbon surface is made.
55455
Contact
&
Teflon
Hs
Carbon-woxv disk
HILE AN INCRI:A5ING AMOWKT Of
work is being done with carbon electrodes, there is a conspicuous lack of published work on rotating solid carbondisk electrodes. The rotating carbon paste electrode contributed by Adams (1) comes closest in design to the latter. Therefore, an electrode combining many of the desirable features of rotating electrodes with those of carbon is of interest; such an electrode has been constructed. The carbon-epoxy resin suspension used has low resistivity, a high degree of chemical inertness, and a polishable surface. Its applicability is demonstrated in the following work. EXPERIMENTAL
Electrodes. The type of carbon used was Graphon (2) obtained from the Cabot Corp., Boston Mass. I t \vas ground finer, with the aid of a mortar and pestle, to reduce particle size. Two types of epoxy resin were employed. Early electrodes were constructed using Resiweld KO.1004 (Fuller Adhesives, St. Paul, llinn.), while later ones utilized a similar epoxy having a n amine curing agent with a lower viscosity. The type of eposy used in the fabrication did not appear to affect either the observed voltammetric behavior or the analytical results obtained with the electrodes. A nongrainy product can still be maintained up to a maximum of 25% carbon (by weight) added to the epoxy resin. Because the epoxy resin-carbon mixtures contain much trapped air prior to curing, centrifugation was used to effect its removal. This procedure also served to concentiate the carbon near the bottom of the centrifugation tube, thus helping to lower the resistance in 966
ANALYTICAL CHEMISTRY
Figure 1. Schematic of rotating carbon-epoxy electrode
the final product. The mixture was allowed to cure for 36 to 48 hours in the tube in which it mas prepared. The resulting machinable rod shrank slightly upon curing and was removed from the glass tube using the glass saw. A section from the bottom of t h e carbon-epoxy rod was fitted into a piece of Teflon (Du Pont) and the surface was refinished on a lathe and polished with emery paper; contact mas made with the carbon disk using mercury. Figure 1 is a schematic representation of the details of electrode fabrication. Geometric areas were determined with the aid of a travelling microscope. Equipment. For the voltammetric studies the electrode was rotated using a synchronous rotator with speeds variable from 2.51 t o 8.94 radians . A Sargent xv POlarograph (E. H. Sargent CO.) was used to-record current-voltage curves, and all potentials are reported with respect t o an S.C.E. reference electrode. The chronopotentiometric work employed a previously-described constantcurrent source (8). Chronopotentiograms were first displayed on the Tektronix oscilloscope (Type 532) and then, following adjustment of allpropriate experimental parameters, recorded on a hleterite recording oscillograph (Model PR-301). Reagents. Chemicals were Reagent Grade and were used without further purification. *ill solutions were prepared with conductivity water.
(a)
Procedure. A 250 - ml., threenecked flask served as the electrolytic cell in the current-voltage studies, while a 150-ml. beaker with rubber stopper bored for introduction of necessary supporting equipment-i.e., electrodes, gas purging tubes, etc,was found to be appropriate in the chronopotentiometric work. Deaeration was accomplished by purging ivith nitrogen in the usual manner. Residual current-voltage curves were recorded separately in deaerated 0.1M H?S04and 0.lJf KCl supporting electrolytes. Current-voltage studies were carried out using the Fe(CX)6-4 anion; the effects of rotation speed and concentration were observed and recorded. Chronopotentiometric investigations were done using the Fe(CS)6-3 anion and the effects of concentration and current density were studied. RESULTS
The resistance of an experimental cell employing a rotating carbon-epoxy electrode (R.C.E.E.) as the indicating electrode coupled with an S.C.E. reference electrode was found to be low. For a typical cylindrical electrode -0.5 cm. in radius and length, immersed in either 0 . 1 X KCl or H2SO4as a supporting electrolyte, the total resistance was on the order of 90-100 ohms. The residual behavior of a polished R.C.E.E. was investigated in deaerated aqueous solutions of KCl and H2SO4, and a residual current-voltage curve typical of both is reproduced in Figure 2. A current of only 2 pa. is observed a t a potential of +0.80 volt us. the S.C.E on an electrode of area equal to 0.772 cm.2 (geometric). If the potential was never allowed to become more positive than 1-0.80 volt, the electrode could be cycled between f0.80 and -0.60 volt (the cathodic limit due to hydrogen evolution) reproducing the initial residual current-voltage curve almost indefinitely (see Figure 2a). hpplication of potentials more positive than +0.80 volt resulted in marked changes in subsequent residual current-voltage curves. If, following the application of a potential more positive than +0.80, the electrode is cycled cathodically, a cathodic “hump” is observed in the current-voltage trace with a “halfhump” potential a t about +0.45 volt (see Figure 2b). This effect has been
Table 1. Limiting Currents Observed at the R.C.E.E. for Solutions of Fe(CN)6-4
0.1M KC1 supporting electrolyte; w = 63 rad./sec. Concn. ( x 10-4 mole/liter) i (pa.) 46
1.00 ... 1.50 2.00 2.50 3.00
is consistent with previously published work (3). The limiting current is found to be proportional to concentration (Table I) and the square root of rotation speed according to the Levich prediction (4) (Figure 3). Figure 4 is a typical plot of E us. log (i, - i)/i giving a straight line with a slope of 0.060 which is in reasonable agreement with the theoretically expected value of 0.059. The half-wave potential as evaluated from the log plot is f0.216 volt and in good agreement with previously reported results for the reduction of Fe(CN)6-3 on platinum ( 3 ) . Chronopotentiometric studies on the reduction of Fe(CS)8-3 using a stationary carbon-epoxy electrode (S.C.E.E.) also indicate reversible behavior over the current range used in the present investigation (4.65 to 75.2 Fa.) on an electrode of area equal to 0.772 cm.2 Initial studies were carried out on an unshielded electrode, while later work involved an electrode built into a linear diffusion cell fabricated from Teflon. Because of the size of the indicating electrode used in each case (0.772 em.*), there were no obvious differences in the
Residual current-voltage curve for R.C.E.E. in 0.1 M HzS04 Before poising a t potentials greater than $0.80 volt Cathodic cycling after poising electrode greater than +O.SO Anodic cyding after (b)
reported by others when carrying out voltammetric investigations using pyrolytic graphite indicating electrodes (6). If, following the cathodic cycle, the electrode is now scanned anodically, an anodic “hump” appears which has a “half hump” potential a t about f0.52 volt (Figure Zc). This would appear to be the reoxidation of a product reduced previously in the region of the cathodic hump. After poising the indicating electrode a t f l . O volt for 2 minutes, repeated cycling now reproduces both the anodic and cathodic humps and neither grows appreciably with further anodization of the electrode. Neither can the phenomenon be removed by the cathodic evolution of hydrogen from the surface of the electrode for extended periods of time. It would seem that whatever is formed on the surface of the electrode in the initial oxidation (potential more positive than f0.80 volt) remains attached and can be alternately reduced and oxidized depending on the direction of scan. The original residual current-voltage curve can only be reproduced by repolishing the electrode surface and not subjecting it to the more positive potentials. The number of coulombs associated with both the cathodic and anodic humps are nearly equal and approximate 11 X 10-4 coulombs. It is of interest to note that the calculated quantity of electricity necessary to put a monolayer of oxygen on the electrode surface (0.772 ems2),assuming the entire surface to be closely packed carbon atoms, is on the order of 7 X 10-4 coulombs. However, it must be remembered that the epoxy matrix used in the construction of the R.C.E.E. contains only 25 wt. yo carbon, and
volt
that the approximate agreement of the two values may be fortuitous. Because this phenomenon occurs using electrodes constructed from two different types of epoxy, and further because similar occurrences are observed using pyrolytic graphite (vide supra) it is felt that this is not an artifact of the epoxy resins. That the two humps constitute a surface process unique to carbon in the supporting electrolytes used seems undeniable. Voltammetric studies show the electrode to behave in a manner reversible to the oxidation of Fe(CN)6-4 which
50
-
40
-
1 30
-
PO
-
’O
1
-m a
70 92 115 131
!
1
2
3
4 wl/z
Figure 3.
ili,
5
6
7
8
9
(rad/rec.)l/z
(pa.) vs. w ’ / ~ (rad./sec.)1/2 for R.C.E.E. in
1.0 X 10-4M Fe(CN)e-4 VOL. 38, NO. 8, JULY 1966
a
967
io 9
8 7
6
5
4 3
9.
3
P
7.
2 6
.T
U
3
& 1
b 9
-I
8
7 6 5
4 1
P
3
4
5
6
7
8
9
10
(Trans. Timo)‘/r
3
Figure 5. Plot of concentration of Fe(CN)6-3vs. o ’ / ~at a stationary carbon-epoxy disk electrode (area = 0.772 cmS2)
P i = 22.8 pa. (b) i = 17.0 pa.
(a)
I 0.1 P
0.15
0.18
0.24
0.P1 E (volb)
Figure 4. Plot of E,, Fe(CNI6-‘ at R.C.E.E.
(volts) vs. log w1/2
(ili,
Table II. Constancy of i+ in the Chronopotentiometric Study of Fe(CN)6-3 on a Stationary CarbonEpoxy Electrode of Area Equal to 0.772 cm.2
Concn. (x10-4
5.0
2.0
968
i (pa.)
37.9 26.6 22.8 11.2 22.8 17.0 11.2 9.42
T
0.30
- i ) / i for
1 .O )( lo-4M
take part in the heterogeneous charge transfer react’ion. Using the Sand equation,
= 7.93 (rad. sed’/(
data resulting from either. Table I1 demonstrates the constancy of i W with concentration, and Figure 5 indius. cates the linearity of a plot of concentration. Cathodic chronopotentiograms followed by current reversal gave a ratio for r f : 7 b of 3 :1 as expected for a diffusion controlled pro-
mole/ liter)
0.97
(sec.)
3.69 7.30 9.40 43.4 1.10 2.50 6.20 6.80
ANALYTICAL CHEMISTRY
cess (Table 111). i71/z/C is also observed to be reasonably constant for differing values of the various experimental parameters. A study of the “effective electrode area” available for electrochemical reaction was carried out as a function of the extent of polishing of the carbonepoxy surface. The model of the surface envisioned essentially “islands” of conducting carbon, in electrical contact, dispersed in a “sea” of the electrically inert resin. By “effective electrode area” is meant that fraction of the total projected geometric area exposed to the solution which is composed of the conducting carbon, as opposed to the inert matrix material, and available to
where the symbols have their usual electrochemical significance (6), and accepting a value for the diffusion coefficient of Fe(Cx)6-3from the published (7), the “effecliterature (8.9 X tive electrode area” may be calculated from the chronopotentiometric E us. time data; this was done as a function of the degree of polish with a fine grade of emery paper (No. 600). The areas so calculated varied from a value of about 70y0 of the geometric area (rough polish) to values which approach 50% of the geometric area (smooth polish). Because the indicating electrode contains only 25 wt. yo carbon, it was expected the calculated
i+’2
73.0 71.9 70.0 73.6 24.0 27.0 27.9 24.6
Table 111. ChronopotentiometricStudy of Fe(CN)6-3 with Current Reversal on a Stationary Carbon-Epoxy Electrode of Area Equal to 0.772 cm.2
Concn. (xi?-4
mole/liter) 10.0 5.0 5.0 2.0
i (pa.)
71 (sec.)
75.2 22.8 17.0 11.2
3.80 9.40 18.4 6.20
Tb
(sec.)
1.15 3.10 6.80 2.05
T//Th
3.1 3.0 2.7 3.0
areas would be less than the geometric area. Although it might be supposed that the value of the effective electrode area approached would be 25% of the geometric area, there is no reason to expect that the carbon should be arranged a t the interface in a manner providing an electrochemical area equal to the weight fraction of carbon used in electrode construction. If a roughness factor of 2 (not a totally unrealistic value) was assumed, a value of 25% of the geometric area would be approached. However, it might \Tell be that the value of 5ooj, of the geometric area obtained with a smooth polish is fortuitous, and characteristic of the particular carbon-epoxy mixture. It seems obvious that the centrifugation procedure used in the fabrication of the electrode would serve to concentrate the carbon a t the bottom end of the tube. Hence, the fraction of carbon in the disk, which was taken from the lower end of the centrifuge tube, would undoubtedly be greater than that present in the initial 25 wt. % mixture. I n practical application, the electrode should be calibrated. As is usually the case when working
with most solid indicating electrodes reproducing the character of the electrode surface from run to run can be a problem. The general shapes of both the voltammograms and chronopotentiograms can be distorted by subjecting the electrode to extremes of potential. However, the reversible appearance of the waves could usually be restored by lightly polishing the surface; a t times it was necessary to remove the electrode from solution, lathe off a few millimeters of surface, and then repolish.
greater than +0.80 volt, the scan direction and magnitude are important considerations. The reader should be cautioned about using this electrode in highly oxidizing or reducing media; it is only reasonable to assume that under appropriate conditions the bonds formed in the polymerization of the resin could be attacked and thus introduce anomalous electrode behavior.
CONCLUSIONS
(3) Delahay, P., J. Am. Chem. SOC.76,
Under the conditions of the present work, the carbon-epoxy indicating electrode performed in a manner consistent with what might reasonably be expected of any solid electrode. From all available data it appears that epoxy resin does serve satisfactorily as an inert binder for the conducting carbon, and in the practical application of the electrode does not produce anomalous behavior. Because of the previously discussed surface complication due to poising the electrode a t potentials
LITERATURE CITED
(1) Adams, R. N., Prater, K. B., ANAL. CHEM.38. 153 11966). (2) Beilby, ’A. L.; Mather, B. R., Ibid., 37, 766 (1965).
874 (1954). (4) Levich, V., “Physicochemical Hydrodvnamics.” Prentice-Hall. New York. 1562.
’
(5) Lingane, J. J., “Electroanalytical Chemistry,” Interscience, New York, 1958. (6) Miller, F., Zittle, H., Mamantov, G., Freeman, D., J . Electroanal. Chem. 9, 305 (1965). (7) Parsons, R., “Handbook of Electrochemical Constants,]’ New York Academic Press, 1959. (8) Reilley, C., Scribner, W., ANAL. CHEM.27, 1210 (1955). for review January 27, 1966. RECEIVED ACCEPTEDApril 15, 1966.
Acidity of Aromatic Sulfonic Acids and Their Use as Titrants in Nonaqueous Solvents DONALD J. PIETRZYK and JON BELISLE Deparfment o f Chemistry, University of lowo, Iowa City, Iowa Twelve aromatic sulfonic acids and several inorganic acids were titrated potentiometrically with basic titrants in nonaqueous media. The data were used to show the acid strength of the acids in methyl isobutyl ketone and glacial acetic acid. Titration characteristics in other solvents are also reported. A straight-line correlation between half-neutralization potential and the sigma value for p- and m-substituted benzenesulfonic acids was 2,4 Dinitrobenzenesulfonic found. acid, which is a solid, readily obtained, readily purified, and shown to be close in acid strength to HC104, was evaluated as a titrant for bases. Data for the titration of a wide variety of bases in glacial acetic acid, acetonitrile, and chloroform ore reported.
-
A
SUCCESSFUL nonaqueous
titration of bases will depend, in addition to selection of a suitable solvent, on the strength of the acidic titrant. Perchloric acid, one of the strongest acids
known, has been the one most frequently used. Although other common inorganic acids can be used as titrants, their acid strengths are less by varying degrees. An indication of their strength in comparison to HClOd in methyl isobutyl ketone is reported by Bruss and Wyld ( 1 ) . Of the acidic organic compounds, the sulfonic acids are strong acids in water. Most of the work with this type of acid in nonaqueous solvents has been with benzene, naphthalene, p-toluene, and simple aliphatic sulfonic acids. Caso and Cefola (3) studied the stoichiometry and general characteristics of the potentiometric titration of potassium acid phthalate with the above acids in glacial acetic acid. A variety of alkaloids in dichloromethane and chlorobenzene were determined by visual titration with p-toluenesulfonic acid ( 7 ) . Numerous other applications have been reported (6, 7, IS, 14). Acidity function, Ho, values for several other sulfonic acids were determined by Smith and Elliott ( l a ) . Other sulfonic acids studied as titrants were fluorosulfonic
acid in acetic acid (IO,11) and alcohols (9) and trifluoromethane sulfonic acid in acetic acid (8). This work was begun to evaluate a wide variety of substituted aromatic sulfonic acids. One of these, 2,4-dinitrobenzenesulfonic acid, which appeared to be close in acid strength to HC104, was studied as a strong acid titrant for the titration of bases. In the course of these potentiometric titration measurements, the acid strength of 12 aromatic sulfonic acids was determined and compared to several inorganic acids. To the author’s knowledge, measurements of this type of strong acids of similar structure have not been reported before. EXPERIMENTAL
The inorganic reagents and organic solvents were obtained in good grades from readily available sources. All solvents were used as received except glacial acetic acid, distilled from acetyl borate, and acetonitrile, distilled from PzOs.Methyl isobutyl ketone, Eastman Chemical Reagents.
VOL. 38, NO. 8, JULY 1966
969
