Laplace transform variable.) In addition, the coupling circuit acts as a crude, but effective, filter for power-line frequency noise, the time constant of the RC network being appropriate for this purpose. The two reference electrodes should in general be placed at essentially the same distance from the working electrode ; otherwise the ohmic drop resulting from two different reference electrode positions may introduce an error as control shifts from the wire to the standard reference electrode. The above circuit has been successfully employed in a study of film formation at a Mg electrode in 2N Mg(C104)z using a platinum gauze counter electrode. The dual-reference system for this work consisted of a saturated HgzS04 standard reference electrode (SMSE) and a smooth platinum wire.
The potentiostat employed is the very fast rise Tacussel Model PIT-20-2A coupled with the Tacussel GITP pulse generator specifically designed for electrochemical studies. An example of the response of the potentiostat to a 50-nsec input pilot pulse is shown in Figure 2. Curve A represents the output pulse obtained using the dual-electrode system; curve B that obtained with the saturated HgzS04 reference; and curve C that obtained with the platinum wire alone. It can be seen that there is a significant improvement in rise time of about one order of magnitude, which, for this system, allows double-layer effects to be studied. RECEIVED for review February 9, 1968. Accepted March 21, 1968.
Purification of Acetonitrile for Voltammetry E. 0. Sherman, Jr., and D. C. Olson Shell Development Co., Emeryville, Calf. ACETONITRILE has gained importance over the past few years as a useful solvent for electrochemical studies. It has a favorable dielectric constant (37.5 at 20 "C) and a low viscosity (0.345 cp at 25 "C). Being an aprotic solvent, acetonitrile can be employed for the study of radical intermediates in the electrochemical reduction of organic compounds. It is a useful solvent for the study of metal complexes which are unstable in other solvents such as water. It is, however, a difficult solvent to purify for electrochemical application. The most bothersome impurities are unsaturated nitriles, in particular acrylonitrile, which require drastic treatment for complete removal. Many methods for purifying acetonitrile have been reported in the literature (1-4). A summary and critical evaluation of purification methods are given in ( I ) and ( 4 ) . Although Coetzee et al. describe a method which removes the impurities from acetonitrile, it requires a long and involved procedure and may introduce ammonia into the final product. However, the elimination of ammonia can easily be accomplished by step 2 of the procedure of Forcier and Olver (3). A simpler method recommended by Coetzee (4) for most purposes reduces the unsaturated nitriles to approximately 10-3M. We have developed a relatively short and simple method for the purification of large batches of acetonitrile which gives a product of comparable or higher purity with respect to electrochemically active impurities than those previously reported. EXPERIMENTAL
Purification Procedure. Six milliliters of liquid N204 were added to 3.8 liters of Nanograde (Mallinckrodt Chemical Co.) or Pesticidequality (Matheson Coleman & Bell) acetonitrile contained in a 5-liter round-bottomed flask fitted with a calcium chloride drying tube. The resulting bright green solution was then heated at approximately 50 "C for 2 hr during which the solution became a deep yellow-brown. Calcium hydride (10 grams/liter) was added and the mixture
(1) J. F. Coetzee, G. P. Cunningham, D. K. McGuire, and G. R. Padmanabhan, ANAL.CHEM., 34, 1139 (1962). (2) J. F. O'Donnell, J. T. Ayres, and C. K. Mann, ibid., 37, 1161 (1965). ( 3 ) G. A. Forcier and J. W. Olver, ibid., p 1447. (4) J. F. Coetzee, Pure Appl. Chem., 13, 429 (1967).
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ANALYTICAL CHEMISTRY
was purged at the boiling point with nitrogen to remove excess Nz04. The calcium hydride mixture used in this step 40 mesh (Metal was made from equal parts of -40 and -4 Hydrides, Inc.). The solvent was then flashed through a short column while maintaining a nitrogen atmosphere in the distillation system. A 250-ml forecut and the final 200 ml were discarded. An additional amount of calcium hydride (10 grams/liter) was added to the distillate and the mixture refluxed for 1 hr. Finally, the acetonitrile was fractionally distilled from calcium hydride under a nitrogen atmosphere through a Nester/Faust (Model NF-136) 36-inch spinning band column. The 1-inch delivery tube of the distillation column was packed for a length of 20 inches with F-20 chromatographic alumina (Aluminum Co. of America) which was freshly activated for 12 hr at 225 'C. This chromatographic column served to remove weak acids present in the starting material or produced during the preceding steps. The first 400-ml fraction obtained with a reflux ratio of 15 to 1 was discarded. After reducing the reflux ratio to 5 to 1, the major portion was collected. The final 200-ml fraction was also discarded. The purified solvent was stored in a dry box after being thoroughly purged with dry nitrogen. Apparatus. The polarographic cell was conventional in design and was thermostated at 25.00 =t0.05 "C. The reference electrode compartment was separated from the test solution by a medium porosity fritted glass disk which was positioned near the indicator electrode to minimize the uncorrected IR drop. Pure nitrogen presaturated with acetonitrile was used to remove dissolved oxygen from the test solution and to blanket the solution during the measurements. Tetraethylammonium perchlorate (0.10M) served as supporting electrolyte and was prepared by the method of Kolthoff and Coetzee (5). A three-electrode configuration was employed. An acetonitrile silver, 0.100M silver nitrate electrode was used as the reference electrode. Contact between the test solution within the reference electrode compartment and the silver nitrate solution was made with a porous Vycor plug The potential of this reference electrode was $0.352 V cs. an aqueous saturated calomel electrode. All potentials are reported with respect to the silver-silver ion electrode. The dropping mercury electrode (DME) had a drop time of 3.37 sec at
+
(5) I. M. Kolthoff and J. F. Coetzee, J. Am. Chem. Soc., 79, 890 (1957).
-1.0
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I 3.6
Figure 1. Polarograms of purified acetonitrile with 0.1M Et4NC104at DME and Pt electrodes
0 V and a mercury pressure of 76.1 cm. The capillary constants for the DME were 1.57, 1.58, 1.48, and 1.38 mg2j3 sec-”* at 0, -1.000, -2.000, and -2.500 V, respectively. A spherical platinum electrode with an area of 0.134 cm2 was also employed as an indicator electrode. The auxiliary electrode was a platinum wire spiral. Current-voltage curves were obtained with an ORNL Model 1988-A controlled potential polarograph and displayed on an Electro Instruments Model 500 x - y recorder. Linear voltage sweep rates of 200 and 1000 mV/min were used with the DME and platinum electrode, respectively. Water and .4cid Determination. The water content of the purified acetonitrile was determined by a micro Karl Fischer titration in a gas-tight cell. Acid impurities were measured by potentiometric titration on a microscale with 0.2N tetran-butylammonium hydroxide in isopropyl alcohol as titrant. RESULTS AND DISCUSSION
The acrylonitrile in the starting material produced a wave of 15 to 20 pA at -2.4 V. Two additional unknown impurities gave waves at ca. - 1.6 (0.4 pA) and -2.7 V (5 MA). Polaro-
grams obtained with both the DME and a spherical platinum electrode of acetonitrile purified by the method described in the experimental sections are shown in Figure 1. Only two small waves due to impurities remain. The combined height of these waves was 0.2 p A . The shape of the current-time curves for single drops of the DME revealed that the largest portion of the residual current over the entire potential range was due to charging of the double layer at the electrode interface with only a small fraction arising from electron transfer reactions. Coetzee er al. have employed the residual current observed near the potential of the supporting electrolyte discharge as a
measure of acetonitrile purity. Acetonitrile purified by the method recommended by them for most purposes (requiring two fractional distillations) gave a current of 2.6 MAat a potential of - 2 . 5 V CS. SCE (-2.84 V us. the silver-silver ion electrode), A more involved method requiring three fractional distillations yielded a purer product with 0.3 pA of current at the same potential. The acetonitrile purified by the method described here gave 0.4 pA of current at -2.84 V us. the silver-silver ion electrode. This method is, therefore, not only more rapid than those of Coetzee and coworkers but also yields a product comparable in freedom from electroactive impurities t o that resulting from their most stringent treatment and superior to that resulting from their recommended method. A direct comparison with the method reported by O’Donnell et af. ( 2 ) or method F of Forcier and Olver (3) is difficult since they do not give the necessary electrochemical data. The water content of the purified acetonitrile was found to be
