A Nonaqueous Carbon Paste Electrode Lynn S. Marcoux, Keith B. Prater, Barbar'a G. Prater, and Ralph N. Adams, Department of Chemistry, University of Kansas, Lawrence, Kan.
development of the carbon S paste electrode in this laboratory ( I ) , a continuing search for a similar INCE THE
electrode material suitable for voltammetry in nonaqueous solvent systems has been in progress. The Nujol carbon paste electrode (CE-NjP) (4) disintegrates in those nonaqueous solvents commonly used in voltammetry-e.g., W,N-dimethylformamide (DMF) , acetonitrile, etc. (2). This disintegration was first thought to be due to the mutual miscibility of the solvent with the pasting liquid (Xujol); however, in some cases Nujol and the solvent are immiscible. More recently the preferential wetting of the graphite by the solvent has been found to be an important factor in electrode decomposition. The present paper reports the development of a carbon paste electrode in which the problem of preferential wetting has been circumvented by the addition of a surface active agent to the electrode material. EXPERIMENTAL
The electrode material used in this study was prepared with 3.3 grams of graphite, 1.4 grams of sodium lauryl sulfate, and 2.5 ml. of Nujol. The sodium lauryl sulfate was Matheson, Coleman & Bell U.S.P. powder, and the graphite was Acheson grade 38. The aromatic amines were prepared in this laboratory or were recrystallized from commercially available compounds. The
I 41.1
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supporting electrolyte, tetraethylammonium perchlorate (TEAP), was prepared from tetraethylammonium bromide and lithium perchlorate, and was 0.1M in all experiments. Acetonitrile was purified by two distillations from phosphorus pentoxide and a final distillation from calcium hydride. Nitromethane was passed over an alumina column and then over a column of Linde 4A molecular sieves. The propylene carbonate, obtained from Jefferson Chemical Co., Inc., was purified by vacuum distillation and subsequent treatment with molecular sieves. The geometric area of the carbon paste electrode was 0.17 sq. cm. All carbon paste electrodes and electrochemical techniques were as previously reported (3,4). The voltage scan rate employed was 2 volts/minute and all potentials are reported us. an aqueous saturated calomel electrode. RESULTS AND DISCUSSION
The properties of this electrode material were studied in acetonitrile, nitromethane, and propylene carbonate. The studies in acetonitrile were the most extensive as this solvent is especially useful for anodic voltammetry. Background currents of less than 3 pa. were obtained from +1.1 to -0.7 volts in this solvent. This is compared in Figure 1 with the background current obtained in the same solution with a platinum electrode (geometric area 0.22 sq. cm.). The
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Figure 1.
Background cyclic voltammograms
(Evs. S.C.E.)
a. b.
Acetonitrile at carbon paste Acetonitrile at platinum c. Nitromethone a t carbon paste d. Propylene carbonate at carbon paste Supporting electrolyte 0.1M TEAP in all cases. All potentials ated calomel electrode.
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ANALYTICAL CHEMISTRY
peak current (i,) for the first oxidation wave of o-dianisidine varied less than 1% when five different electrode faces were used. The variation was less than 2% when this experiment was repeated using five different batches of paste, all of the same formulation. This i, did not show appreciable variation when the electrode was allowed to remain in the solution for a period of 30 minutes. The effect of the variation of the sodium lauryl sulfate concentration upon the peak current for this compound is shown in Table I. As expected, i, varied linearly with the concentration of the electroactive species. The ratio i,/C for five different concentrations of trianisylamine was found to be 29.8 f 6.1 pa./mmole. The cyclic voltammograms of several compounds ( p aminophenol, dianisylamine, W-methyldianisylamine, and hydroquinone) obtained using this electrode in acetonitrile indicated no irregularities. The cyclic voltammogram of 1.0 X 10-3M solution of trianisylamine is shown in Figure 2. A background scan in nitromethane is shown in Figure 1. The background current was less than 2 pa. from f1.0 to -0.7 volts. The peak currents obtained for an o-dianisidine solution did not show appreciable variation with immersion time. Cyclic voltammograms obtained in this solvent showed no irregularities, nor does any other aspect of the performance of this electrode material in this solvent warrant special attention. The electrode material behaved in a similar fashion in propylene carbonate. A typical background is shown in Figure 1. The slight waves noticeable before background are apparently solvent impurities. The background current was less than 3 pa. in the region $0.86 to -0.6 volts.
+ 1.b Figure 2.
E IVOLTSI
o.'o
Cyclic voltammogram of
1 .O X 1 O-3M trianisylamine in acetoVI.
a ratur-
nitrile (0.1M TEAP) at carbon paste (E vs. S.C.E.)
Table 1.
Effect of Surfactant Concentration on i,
Sodium lauryl sulfate (w./w. %)
i, (ra.1
14 25 33 40 46
38.1 36.6 35.0 35.4 37.8
This electrode material apparently possesses all of the advantages of the original carbon pashe electrode and has no obvious disadvamtages. The presence of a surfactant does not noticeably alter the electrochemical behavior of the compounds studied. One would not expect the variations of peak current shown in Table I to be easily predictable since several variables such as charging current, electrode area, and electrode resistance are involved. The possible effect of the surfachant upon electrode kinetics should be studied more thoroughly. The greatest advantage of this electrode would seem to be the very low background currents which are ob-
tained. These background currents are much lower and more reproducible than those obtained a t platinum. Studies at low sensitivity indicated that the current increase noted in all solvents studied, a t about 1.0 volt was not the true background current. The true background potential varied, of course, but was a t about 1.5 volts for most solvents studied. The exact nature of this prebackground wave is a t this time unknown, but it appears to be an impurity in Nujol. Totally satisfactory electrode materials are presently not available for use in DMF, dimethyl sulfoxide (DMSO), benzonitrile, and acetic acid. I n the case of D M F and DMSO, when the amount of surfactant necessary to avoid electrode decomposition is used, the electrode resistance becomes too great. Benzonitrile is miscible with Nujol, and sodium lauryl sulfate undergoes an acid-base reaction with acetic acid. The electrode composition chosen for this work was somewhat arbitrary, as was the choice of surface active agent and pasting liquid. I n general, the surfactant must be nonreactive, chemi-
cally and electrochemically, and the pasting liquid must be solvent immiscible as well as nonreactive. The paste must also contain enough graphite to avoid high resistance. Thus, the possibility exists that an electrode material for any specific solvent may be obtained by the careful manipulation of these variables. The carbon paste electrode described above is highly functional and has advantages not possessed by other electrodes in current use. Studies are in progress which are directed toward extending the usefulness of this electrode. LITERATURE CITED
(1) Adams, R. K., ANAL. CHEM. 30, 1576 (1958). (2) Adams, R. N., Rev. Polarog. (Kyoto) 11, 71 (1963). (3) Alden, J. R., Chambers, J. Q., Adams, R. N., J . Electroanal. Chem. 5, 152 (1963). (4) Olson, C., Adams, R. N., Anal. Chim. Acta. 22, 582 (196IO). SUPPORTED in part by the Petroleum
Research Fund administered by the American Chemical Society and by National Science Foundation undergraduate research fellowships.
Simplined Puriflication of Acetonitrile for Electroanalytical Applications George A. Forcier and John W. Olver, Department of Chemistry, University of Massachusetts, Amherst, Mass.
S
of purification for acetonitrile are (desirable and necessary because of its current extensive use as a solvent in analytical and electrochemical studies. Currently the commercially available acetonitrile is produced as a by-product of acrylonitrile production ( I ) . Consequently, acrylonitrile as well as water and hydrolysis products of acetonitrile such as ammonia, acetic acid, and acetamide are present as impurities. The potentials attainable in acetonitrile are severely limited in both the anodic and cathodic directions by the presence of some of these impurities. For example, ammonia and amines drastically limit the available anodic potential range, and acetic acid or acrylonitrile, the cathodic range. The conventional purification method (6) consists of shaking with potassium hydroxide and distilling repeatedly from phosphorus pentoxide. This method causes extensive polymerization and solvent loss, but does not remove acrylonitrile effectively nor reproducibly dry the solvent. Alternative procedures recommended by Coetzee, et al. (2) yield solvent with higher and more predictable purity. However, removal IMPLE METHODS
of acrylonitrile is both difficult and time consuming. We have devised purification procedures which are simple and rapid and yield solvent of equivalent purity in greater yield than previously possible. EXPERIMENTAL
Apparatus. All polarograms were obtained with a Sargent Model XV recording polarograph. The polarographic cell was a modified H-type cell with a n unfused Vycor plug separating the working compartment from the reference compartment. All potentials are referred to the aqueous saturated calomel electrode. All conductances were measured using a Fisher Scientific “Low Conductivity” cell (cell constant = 0.11808) and an Industrial Instrument Conductivity Bridge, Model RC-18. The amount of water present was determined by Karl Fischer titrations with a n amperometric end point detection method. The current was measured between two platinum microelectrodes with 20 mv. applied using a Sargent Co. “Ampot” amperometric titrator. Reagents. Reagents used in the purification procedures included : acetonitrile (Fisher, Union Carbide, and Eastman practical grade yielded
essentially the same results) ; sodium hydride dispersed in mineral oil (Metal Hydrides Inc.); calcium hydride, -40 mesh (Metal Hydrides Inc.); phosphorus pentoxide (B. and A. standard of purity, J. T. Baker, purified, and Mallinckrodt, A. R.; all gave essentially the same results); potassium bisulfate, reagent grade, (Fisher); iodic acid (B. and A. standard of purity); and sulfuric acid (Baker, A. R.). Tetraethylammonium perchlorate, used as supporting electrolyte in all polarograms, was prepared from the tetraethylammonium bromide and sodium perchlorate. This salt was recrystallized from water three times and dried a t 65’ C. Procedures for Purification. Step 1. Reflux over sodium hydride (1 gram per liter of acetonitrile) for 10 minutes followed by rapid distillation. Step 2. (a) Reflux over phosphorus pentoxide (2 grams per liter of acetonitrile) for 10 minutes followed by rapid distiIlation; or (b) Reflux over potassium bisulfate for twenty minutes (2 grams per liter of acetonitrile) followed by rapid distillation; or (c) Reflux over iodic acid (2 grams per liter of acetonitrile) for 20 minutes, followed by rapid distillation; or (d) Reflux with concentrated sulfuric acid (1 mi. sulVOL 37, NO. 11, OCTdBER 1965
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