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Purif ication of Acetonitrile Marc Walter and Louis Ramaleyl Department of Chemistry, Dalhousie University, Halifax, Nova Scotia, Canada
THEUSE OF ACETONITRILE as a solvent for acid-base, electrochemical, and spectroscopic studies has markedly increased over the last decade. Its high dielectric constant (37.5 at 20 "C), low UV cutoff (less than 200 nm), and low proton availability contribute to its favorable properties. However, it is difficult to purify, particularly with respect to compounds such as acrylonitrile and benzene which have boiling points close to that of acetonitrile. The classical method for the purification of acetonitrile consisting of repeated fractional distillations from phosphoric oxide gives a relatively pure product. However, this treatment is time-consuming and causes extensive polymerization of the solvent. Coetzee et a f . ( I ) have developed a procedure which produces a solvent suitable for polarographic use. They remove the acrylonitrile by treatment with potassium hydroxide. O'Donnell, Ayres, and Mann (2) point out that this procedure does not eliminate the oxidizable impurities that absorb in the UV. They obtain a product free from these impurities by treatment of the acetonitrile with benzoyl chloride and alkaline permanganate. Other purification procedures have been reported. Forcier and Oliver (3) reflux with sodium hydride to destroy the acrylonitrile, while Sherman and Olson (4) use a dinitrogen tetroxide treatment to obtain a solvent suitable for polarographic use. Moe ( 5 ) has obtained a polarographic-spectroscopic grade solvent by means of a slow azeotropic distillation with 95 ethanol. We have developed a purification procedure which gives a product of purity higher than those previously reported. However, we have found that the obtainable purity of the acetonitrile is dependent upon its method of manufacture. EXPERIMENTAL
Apparatus. The voltammetric instrumentation used consisted of a general purpose operational amplifier system of conventional design. Current voltage curves were recorded on a Heath EUW-20A servo recorder. All potentials were measured against an aqueous sodium chloride saturated calomel electrode (SSCE). The salt bridge which separated the SSCE from the working compartment of the electrochemical cell was plugged at the reference side with an agarsodium chloride gel and filled with an acetonitrile solution of the supporting electrolyte. At open circuit in a 0.05M tetraethylammonium perchlorate (TEAPtacetonitrile solution, a typical drop time for the dropping mercury electrode
used for the cathodic studies was 7.8 sec,/drop at a flow rate of 0.87 mglsec. The area of the platinum indicator electrode used for the anodic studies was 0.10 cm2 as measured from the linear voltage sweep oxidation of ferrocene. Conductance data were obtained with a commercial conductance bridge, B221 Wayne Kerr Universal Bridge. The cell constant was 2.13 x 10-2 cm. Gas chromatographic data were obtained with a Hewlett-Packard 700 gas chromatograph using a flame ionization detector for organic determinations and a thermal conductivity detector for water determinations. Reagents. Tetraethylammonium perchlorate was prepared by the addition of 70 % perchloric acid to an aqueous solution of tetraethylammonium bromide. T h e precipitate was recrystallized five times from water, dried at 60 "C in cucuo, and stored over anhydrous magnesium perchlorate. Sodium hexafluorophosphate, obtained from Ozark Mahoning Company, was recrystallized from water, dried at 60 "C in cacuo, and stored over anhydrous magnesium perchlorate. Matheson, Coleman and Bell (MC/B) and Eastman practical grades acetonitrile were primarily used in this study. All other chemicals used in the purification procedures were of reagent grade quality. Procedure. Two new purification procedures were investigated. METHODA. Step 1, reflux over anhydrous aluminum chloride (15 grams per liter of acetonitrile) for one hour followed by rapid distillation. Step 2, reflux over lithium carbonate (10 grams per liter of acetonitrile) for one hour followed by rapid distillation. Step 3, reflux over calcium hydride (2 grams per liter of acetonitrile) for one hour followed by a careful fractionation from a helice packed column at high reflux ratio. Retain the middle 80% fraction. Acetonitrile purified by method A is recoverable in approximately a 70% yield. METHODB. Step 1, same as method A, step 1. Step 2, reflux over alkaline permanganate (10 grams potassium permanganate and 10 grams lithium carbonate per liter of acetonitrile) for fifteen minutes followed by rapid distillation. Step 3, reflux over potassium bisulfate (15 grams per liter of acetonitrile) for one hour followed by rapid distillation. Step 4, same as method A, step 3. Acetonitrile purified by method B is recoverable in approximately a 60% yield. All distillations were carried out in borosilicate glass stills fitted with Teflon @u Pont) stopcocks and protected from atmospheric moisture with magnesium perchlorate drying tubes. RESULTS AND DISCUSSION
Author to whom correspondence should be addressed. ~
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(1) J. F. Coetzee, G. P. Cunningham, D. K. McGuire, and G. R. Padmanabban, ANAL.CHEM., 34, 1139 (1962). (2) J. F. O'Donnell, J. 7.Ayres, and C . K. Mann, ibid., 37, 1161 (1965). (3) G. A. Forcier and J. W. Oliver, ibid., p 1447. (4) E. 0. Sherman, Jr., and D. C. Olson, ibid.. 40,1174 (1968). (5) N. S. Moe, Actu. Cltem. Scald., 21, 1389 (1967).
The experimental data together with the purification procedures used are summarized in Table I. G a s chromatographic data i r e given in Table 11. With all methods, the specific conductance of the purified solvent is less than mho/cm and the water content is less than 10 ppm (0.5 millimolar). Method A yields a solvent suitable for most polarographic studies irrespective of the source of acetonitrile. The small
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Table I. Comparison of Purification Methods Anodic current Cathodic UV Source density, current, cutoff, Absorbance and methoda pAIcm2b pAc nmd at2OOnm Eastman, none 159 48.0 227e ... 88 MC/B, none 40.0 232 ... Eastman, A 125 1.04 218 ... MC/B, A 80 0.80 222 ... 60 Eastman, B 0.25
