A Glassy-Carbon Electrode for Voltammetry .
E. ZITTEL
and
F. J. MILLER
Analytical Chemistry Division, Oak Ridge National Laboratory, Oak Ridge, Tenn. A new form of carbon known as "glassy" carbon has been used as an indicator electrode in voltammetry. Glassy carbon is a gas-impermeable, electrically conductive material highly resistant to chemical attack. It is suitable for use over the potential range from about +1.2 to —0.8 volts vs. S.C.E. in acid medium. Voltammetric studies have been made of solutions of Ce(lll), Ce(IV), Cr(VI), Fe(ll), Hg(l), Fe(CN)6-4, Ag(l), Cu(ll), and U02(ll). For most of these ions a peak type wave that approaches ideality The Ep and ip values was obtained. for these ions are for the most part reproducible under the conditions stated and appear to be analytically usable. The electrode was fabricated by molding glassy carbon into an epoxy rod with a central tube to permit an Hg contact. The assembly is rugged and the electrode requires no pretreatment or treatment between uses. In ease of fabrication and maintenance the glassy carbon electrode (G.C.E.) compares very favorably with the pyrolytic graphite electrode (P.G.E.) and other solid electrodes.
Although
of carbon for the use electrodes in voltammetry is not
new, recently introduced forms of this material are of particular interest. Primary among these are pyrolytic, impregnated, and glassy carbon. Adams (1) discusses the behavior of graphite, as well as other solid electrodes, in voltammetry. Pyrolytic graphite is being studied as an electrode for voltammetry of both aqueous and nonaqueous systems (7, 9). “Glassy carbon" is a proprietary preparation of the Tokai Electrode Manufacturing Co. (11). It is gas-impermeable, highly resistant to chemical attack, electrically conductive, and obtainable in a relatively pure state. Although its properties are discussed by Yamada and Sato (12), many analytical chemists apparently have been unaware of its existence. Use of glassy carbon as an electrode in analytical chemistry has not been reported. It has many with pyrolytic properties in common graphite; however, it is isotropic rather than anisotropic. An electrode of pyrolytic graphite must be properly oriented, and the edges of the planes of the graphite must be sealed. These limitations should not exist for an electrode made of glassy carbon.
200
·
ANALYTICAL CHEMISTRY
Figure 1. Usable potential range for G.C.E. in various mineral acids acid concentration, 0.1N rate, 0.1 volt*min.-1
Test conditions:
Voltage
scan
EXPERIMENTAL
Apparatus. Voltammeter. ORNL controlled-potential voltammeter as
described previously (5). Recorder. X-Y recorder, Model 2 D2A, F. L. Moseley Co., 409 Fair Oaks Ave., Pasadena, Calif. Electrode System. The electrode system consisted of a glassy-carbon indicator electrode (G.C.E.), a pyrolytic graphite counter electrode (P.G.E.) (9) and a saturated calomel reference electrode (S.C.E.). All potentials were measured vs. S.C.E. by way of a saturated KNOs-agar bridge. The G.C.E. was prepared by cutting a piece from a small “boat” made of the material and sealing the piece into the end of a cylinder of epoxy resin (Shell Co. Epon 820). The resin cylinder was formed by pouring the molten resin between two sections of glass tubing; the outer section of glass tubing was removed, and the inner section was left in place. A small amount of mercury was poured into the central glass tube, and electrical contact was made by means of a copper wire inserted into the mercury. The hardness and brittleness of glassy carbon require that it be shaped by grinding
cavitation. In this case, cavitation used. The d.c. electrical resistance through the electrode was 6 ohms; 0.11 cm.2 of electrode surface area was exposed to the test solution. Reagents. All standard solutions were prepared from reagent-grade chemicals and were standardized, when necessary, by conventional methods. Dilutions were made with appropriate supporting media. or
was
Cylinder grade argon was used without purification to deaerate the
test solutions.
RESULTS
A major difficulty in voltammetry with nearly all stationary solid electrodes is high background current. This current may be due to impurities in the electrolyte or may arise from the double-layer charging process. Therefore, the capacitance of the double layer at the G.C.E. was studied. From the relationship (2) io
=
AC¿)
(1)
where ic is the capacitance current in µ&., A is the electrode area in sq. cm., Cd is the differential capacitance of the electrode in µ ., and v is the voltage scan rate in µ . sec.-1, the differential capacitance of this electrode in 1M H2SO4 was about 200 µ cm.-2 at a scan rate of 1
volt min.-1 This relatively high figure "indicates that the electrode should exhibit a fairly large background current, as is indeed the case. This large current does not, however, in itself prohibit the use of the electrode in voltam-
metry. To determine the usable potential of the G.C.E. in acid systems, which is limited by the decomposition potentials of H20, the effect of different mineral acids was studied. Figure 1 gives the results. Of the common mineral acids, HNO3 exhibits the most limited potential range in the cathodic region, whereas HC1 is
the most limited in the anodic region. For both these acids, the anion is probably reacting at the electrode. The widest usable potential span—i.e., about + 1.3 to —0.8 volts vs. S.C.E.—is obtained in phosphoric acid solution, whereas the usable potential ranges for H2SO4 and HCIO4 are intermediate. The response of the G.C.E. to change in pH was studied. The data of Table I show that the G.C.E. is relatively insensitive to changes in pH. To compare the voltammetric behavior of the G.C.E. with that of the P.G.E. (9), current-voltage curves were recorded for ions similar to those studied with the P.G.E. Representative curves for cathodic and anodic electrode processes are shown in Figures 2 and 3, respectively. In general, the w'ave form is that expected for linear-sweep voltammetry with a solid planar electrode, for some ions although the curves approach ideality much more closely than do those for others. Extreme examples of nonideal behavior are the cathodic waves for Cr(VI) and Hg(I), respectively. Copper(II) exhibits a double wave at the G.C.E., whereas only one wave is detectable with the P.G.E. The voltammetric characteristics for several ions over a range of concentrations are given in Table II. For all these ions except Cr(VI), the waves are well defined and reproducible. The wave for Cr(VI) in dilute solutions is adequate for analytical purposes; for Cr(VI) in concentrated solutions, the wave becomes very irregular. The data of Table II show only those values obtained at a voltage scan rate of 1 volt min.-1. The accuracy and precision of values obtained at other scan rates are comparable. To ascertain the effective working area of the electrode, the oxidation of Fe(CN)6-4, as K4Fe(CN)6 in \M KC1, was studied. The
Table I. Response of the G.C.E. and Glass Electrode in pH Changes Measuring instrument, Beckman research pH meter" Glass electrode, Beckman glass electrode (standard 1199-30). Reference electrode, Beckman calomel reference electrode (standard 1199-31). Electrode response Test solution Glass electrode G.C.E.
Identity
pH
pH
mv.
HC1, 1M
1
data of Table III indicate that the measured current is diffusion controlled since the it112 product is fairly constant. If the average if1'2 value from Table III is substituted in the equation (3) it1/2irl/2 (2)
nFCD,/!
tained
=
i
=
t
=
=
F
=
C
=
ip
D
=
voltage Curves:
1.
rate, 0.1 volt-min.Ci^Ot-2
2. 3.
Ag(l)
4. 5.
Cu(ll) U02(ll)
scan
Hg(l)
=
A
=
C°
=
D
=
time, sec. equivalents mole-1 faraday, coulombs concentration, moles cm.-3 diffusion coefficient, cm. sec.-1
V
=
·
the area calculated is 0.121 cm.2 This calculated value is in good agreement with the measured value 0.11 cm.2 Thus, it is evident that the electrode is active over its entire surface. The anodic reaction of K4Fe(CN)6 in IM KC1 was used to verify the linear relationship between the peak current and the square root of the rate of polarization. A linear dependency was ob-
taken with the G.C.E.
(2.72 X 10°)ni/2AD^2C°V1/2
=
electrode area, cm.2 current, amp.
·
1.2
+1.0
rate of 0.1 to 1.0
where ip
·
scan
·
where
A
the
over
volt min.-1 for a concentration range from 0.5 to 11 X 10-3 in accordance with the equation (4).
peak current, amp. electrode area, cm.2 concentration, mole cm.-3 diffusion coefficient, cm.2 sec.-1 voltage scan rate, volt sec.
Similarly,
a
linear relation
was
·
obtained
between ip and C° for the same range of ferrocyanide concentration as predicted by equation 3. The Ce(III)-Ce(IV) couple was essentially reversible at the P.G.E. (10). To compare the G.C.E. and P.G.E, and to test the performance of the G.C.E. at the extreme anodic potential range, this same couple was studied briefly. The reduction of Ce(IV) and the oxidation of Ce(III) are not reversible at the G.C.E. Figure 4 shows the currentfor the reduction of voltage curve
+0.8 E,.(electrode) "
Test conditions:
5.385 6.746 6.850 6.968 8.181
Buffer 4.00 Buffer 6.86 +6.5 -176.1 Buffer 10.00 -344.1 -71.1 NaOH, 1M Meter standardized with pH-6.86 buffer and glass electrode.
+
curves
pH
mv.
+93.4 + 13.4 +7.0 +0.6
0.082 4.011 6.860 9.945 12.808
+407.6 + 174.7
+ '
volts
0.6
+0.4
0.2
vs. S.C.E.
Figure 3. Composite of typical voltammetric anodic taken with the G.C.E. Test conditions: voltage scan rate, 1 volt Curves: 1. Ce(lll) 2. Fe(ll) 3. Fe(CN)6-4
curves
min.-1
VOL. 37, NO. 2, FEBRUARY 1965
#
201
Table
II.
Voltammetric Data for Various Ions Taken with the G.C.E. Conditions: Electrode area (measured), 0.11 cm.2 Voltage scan rate, 1 volt-min.-1
t,/C,
Ion Concn.,
X 10s 10.1 1.01 0.50 41.5 4.15
Identity Ce(III)» Ce(IV)
1.0 0.5 0.1 63.0
Cr(VI)
Fe(II)"
0.63 10.1 1.0
Hg(I)
0.2 11.4 8.55 5.07 2.85 1.14 0.51 10.0 1.0 1.0
Fe(CN)6-4»
Ag(I)
Cu(II)
Supporting medium H2SO4, 1M
Ep/2,
) µ l#
41.4 4.27 2.12
H2SO4, 1M H2SO4,
138.0 13.3 22.3 12.1 2.9 388.0 38.8
1M
HCIO4, 1M
3.9
HNOs, 2M
116.5 11.5
2.4 84.6 64.4 38.5 22.3 8.5 3.9 86.4 8.72 3.1
KOI, 1M
HNO,, 1M HC1, 2M
21.0 H2SO4 0.5M 2.10 Anodic electrode reaction.
168.0 17.0
UO2GD “
Ce(IV) and subsequent oxidation of Ce(III). Curves of this type were obtained regardless of the initial oxidation state of the cerium. The Ce(IV)
Ce(III) cathodic wave is broad and is not nearly so well defined as the
-*·
The Ce(IV) anodic wave. of the two waves differ from each other by about 0.3 to 0.4 volt. This difference further substantiates the irreversibility of this reaction.
Ce(III)
-*
peak potentials
DISCUSSION
The manufacturer of glassy carbon it into four grades according to the “proof-temperature.” Grade GC 30 S is the purest and is recommended for use as electrode material. Although the cutting, shaping, and polishing of electrodes from glassy carbon requires special techniques, the material is available in sizes and shapes that require little reworking. To make an electrical connection to glassy carbon is no problem since either a mercury or metal-pressure contact can be made. Because glassy carbon is isotropic, it does not require definite orientation, as is the case with pyrolytic graphite. Most stationary solid electrodes are susceptible to poisoning or film formation during voltammetric use. Thus, it is necessary to pretreat such an electrode classifies
202
e
ANALYTICAL CHEMISTRY
vs.
volts
S.C.E.
+ 1.18 + 1.17 + 1.15 + 1.14 + 1.17 +0.6 +0.5 +0.5 +0.46 +0.46 +0.44 +0.35 +0.33 +0.33 +0.22 +0.22 +0.22 +0.22 +0.22 +0.21 +0.26 +0.27 +0.26 -0.44 -0.13 -0.12
A
µ ./mmol liter/cm. 37.3 38.4 38.5 30.2 29.2 20.3 22.0 26.4 56.0 56.0 57.0 105.9 104.5 109.0 67.4 68.5 68.0 71.0 67.8 70.0 78.6 79.4
72.7 73.5
to treat it between successive uses. The G.C.E. showed no evidence of this difficulty with the solutions studied. Neither was it necessary to treat the electrode between uses. During repeated use of the electrode and subsequent to an electrode reaction, it is necessary only to return the applied potential to its original setting and to allow sufficient time for any deposited material to be removed from the surface or for the double layer to be dispersed. At the beginning of the work, the surface of the electrode was shiny and glassy. No microscopically detectable change in the surface properties occurred during the work. The G.C.E. is remarkably free of maintenance, and its electrical characteristics are constant as far as was ascertained. The G.C.E. compared very favorably with the P.G.E. in all phases of this work. All current values given in this work are the corrected peak currents, ip, and all potential values are stated as peak potentials, Ep, or half-peak potentials, or
Change of / with Time at Constant Potential Test Conditions: [Fe(CN)6-4], 2.5 X 10 ~3M Supporting medium, 1M KC1 Applied potential, +0.04 volt vs. S.C.E. Electrode area (measured), 0.11 cm.1
Table III.
t, sec.
µ&.
5
19.3 12.8 10.4 9.0 8.1 7.5 7.0 6.6 6.3 6.1
10 15
20 25 30 35 40 45 50
¿íVz,
.
µ . sec.1/1
43.2 40.5 40.2 40.2 40.5 41.0 41.3 41.6 42.2 43.0 41.4
trade (D.M.E.) and the platinum electrode. In basic media, the cathodic limit of the G.C.E. may be extended to 1.6 volts vs. S.C.E. This very wide potential range makes the electrode very useful when other solid electrodes and the D.M.E. are limited. The current-voltage curves of Figures 2 and 3 illustrate both ideal and nonideal reactions. Without exception, the anodic waves have the ideal peak shape that should result from linear diffusion to a planar electrode in still solution. The cathodic waves shown for Ag(I), Hg(I), and U02(II) are also of this nature. However, the Cr(VI) wave deviates from ideality, as does the Ce(IV) wave in Figure 4. This deviation may result from the fact that these electrode reactions are kinetically rather than diffusion controlled.
Ep/2·
The curves in Figure 1 show the wide range of potential of this electrode. If the correct acid is chosen as supporting electrolyte, it is possible to use a range of from ~+1.3 to 0.8 volt vs. S.C.E. Thus, as is the case with the P.G.E., this electrode overlaps the usable ranges of the dropping-mercury elec-
form of Change in wave reduction and subsequent reversal of direction of voltage scan (reoxidation) Figure 4. Ce(IV) on
Test
conditions!
supporting
electrolyte.
H2SO4
[Ce(IV|], 4.15 X 1CT3M Voltage scan rate, 2 volts· min.
1
1M
Some of the wave forms recorded with the G.C.E. differ from those taken with the P.G.E. The wave for the voltammetric reduction of U(VI) at the G.C.E. is exceptionally well defined as compared with that obtained with the P.G.E. and even with the D.M.E. The current-voltage curve for the reduction of Cu(II) in 2M HC1 at the G.C.E. has two distinct peaks in contrast with that for the reduction at the P.G.E. which gives only one discernible peak. Both waves are easily measured and increase in direct ratio to an increase in Cu(II) concentration. Presumably, these peaks represent the reductions Cu(II) — Cu(I) and Cu(I) -+ Cu°. The cathodic wave of Fe(III) in 1M HCIO4 is well formed; in HC1 it is much less ideal, being extremely broad at the peak. This nonideal behavior is undoubtedly due to the complexing nature of HC1, which changes the mechanism of reaction at the electrode. The Cr(VI)-H2S04 system presents an anomaly. At millimolar or lower concentrations of Cr(VI), the current-voltage curves are fairly well defined and measurable; at higher Cr(VI) concentrations, they are poorly defined and have a high noise level. The reduction
of Cr(VI) at the platinum electrode pro-
that interferes with the reduction of Cr(VI) duces a coating on the electrode
(6). Possibly, such a coating could also form on the G.C.E. In general, for each of the ions studied the E„ values at the G.C.E. and P.G.E. are close. A more direct comparison is not possible, since the ions were not studied under the same conditions. The precision attainable with the G.C.E., as indicated by the data of Table II, compares very favorably with that attainable with both the P.G.E. and D.M.E. The relative insensitivity of the G.C.Ei to changes in pH is in decided contrast with the behavior of the P.G.E. (8), which responds more quickly to a pH change. This difference in behavior is difficult to explain. It may result from the difference in structure, glassy carbon being completely isotropic, whereas pyrolytic graphite has a well-ordered structure. The G.C.E. has several possible advantages for use in voltammetry. It appears to be inert to strong acids and oxidizing agents. For some currentvoltage curves, especially those for Fe(II) and U02(II), it provides better definition than does the P.G.E.
LITERATURE CITED
(1) Adams, R. N., “Progress in Polarography,” P. Zuman and I. M. Kolthoff, eds., Chap. 23, Interscience, New York, 1962.
(2) Delahay, P., “New Instrumental Methods in Electrochemistry,” p. 131, Interscience, New York, 1954. (3) Ibid,., p. 51. (4) Ibid., p. 119. (5) Fisher, D. J., Maddox, W. L., Kelley, . T., Stelzner, R. W., “ORNL Analytical Chemistry Division Annual Progress Report for the Period Ending November 15, 1963,” ORNL-3537, p. 16.
(6) Kolthoff, I. M., Shams el Din, A. M., J. Phys. Chem. 60, 1564 (1956). (7) Laitinen, . A., Rhodes, D. R., J.
Electrochem. Soc. 109, 413 (1962). (8) Miller, F. J., Anal. Chem. 35, 929 (1963). (9) Miller, F. J., Zittel, . E., Ibid., 35, 1866 (1963). (10) Miller, F. J., Zittel, . E., J. Electroanal. Chem. 7, 116 (1964). (11) Tokai Electrode Mfg. Co. Ltd., 20 Ahasaka. Tameike-Cho. Minato-Ku. Tokyo, Japan. (12) Yamada, S., Sato, H., Nature 193, 261 (1962).
Received for review September 14, 1964. Accepted November 12, 1964. Research sponsored by the U. S. Atomic Energy Commission under contract with Union Carbide Corp.
Ligand-Catalyzed Polarographic Reduction of Indium(lll) for Determination of Halides and Certain Organic Sulfur and Nitrogen Compounds A. JAMES ENGEL,1 JOAN LAWSON, and DAVID A. AIKENS
Department of Chemistry, Rensselaer Polytechnic Institute, Troy, N. Y. Traces of a number of inorganic and organic substances catalyze the polarographic reduction of ln(H2G)e+3 and can be determined from the resulting catalytic current. Halides, thiocyanate, and a number of organic sulfur and nitrogen compounds can be determined in this manner. Substances need not be electroactive in the usual sense to be detected and the catalytic nature of the reaction ensures high sensitivity. The key factor is the ability of catalytic substances to coordinate with ln(H20)6+s and to labilize the hydration sheath and thus facilitate the electrodeposition reaction. Determination of iodide is given as an example and optimum results obtained with iodide concentraare tions from 1 X 1 0-5 to 2 X 1 O-4 M.
ligands catalyze polarographic of certain metal ions and greatly enhance the limiting current. The resulting catalytic current
reductions Many
provides a sensitive and selective means for determination of such ligands, even though the ligands may not be electroactive in the usual sense. Analytical application of this effect has, however, received little attention. It has long been known that ligands such as the halides accelerate the normally slow polarographic reduction of many aquated metal ions such as In(H20)6+3 and that addition of high concentrations of these ligands shifts the entire reduction wave to more positive potentials (7, 8, 16, 19). Catalytic waves were not observed by early workers, however, because at the high ligand concentrations used, the current was limited by the rate of diffusion of metal and not by the rate of metal complex formation. Catalytic waves of metal complexes were first observed by Mark and Reilley (18) for reduction of Ni(II) in the presence of low concentrations of certain amines and the waves were used for determina-
tion of catalytically active amines
The mechanism was (10, 12, 14). studied by Mark (11), who concluded that the critical step is complexation of Ni(II) by ligand adsorbed on the electrode surface. The significance of complexation was pointed out by Nelson and Iwamoto (17), who proposed that complexation increases the ease of desolvation and facilitates deposition. This mechanism is supported by the work of Connick and Coppel (2), which shows that introduction of a foreign ligand such as chloride into the coordination sphere of an aquo ion increases the rate of displacement of the Hence remaining water molecules. labilization of the hydration sphere probably plays a key role in all ligandcatalyzed reductions of metal ions. The ligand-catalyzed reduction of In (III) resembles the ligand-catalyzed 1 Present address, Columbia College, Takoma Park, Md.
VOL 37, NO. 2,
FEBRUARY
1965
Union
·
203
