LITERATURE CITED
(1) Archer-Daniels-Midland Go., Minneapolis, Minn., “Composition and Consia& of Natural F a 6 and Oils.” (2) Fieser, L. F., ~ieser,M,, “Organic Chemistry~”pa 4 0 8 ~ Reinhold, New York, 1956. (3) Hopkin% c. y., Bernskin, H. J., Can.J . Chem. 37, 775 (1959).
(4) Mikasch, J. D. von, Frazier, C., IND. ENG. CHEM., ANAL. ED. 13, 782 (1941). (5) Pack, F. C., Planck, R. W., Dollear, F. G., J . Am. Oil Chemists’ SOC.29, 227 (1952). (6) PoPle, J. A.7 Schneider, w. G.7 Bern-
stein, H. J., “High Resolution Nuclear Magnetic Resonance,” p. 98, McGrawHill, New York, 1959. (7) Shoolery, J. N., J . Chem. Phys. 31, 1427 (1959).
(8) Stiihli, H., Mitt. Gebiete Lebensm. Hyg. 46, 121 (1955). (9) Varian Associates, Palo Alto, Calif., Tech. Info. Bull. Vol. 3, No. 1 (1960).
RECEIVEDfor review March 5, 1962. Accepted May 21, 1962. Pittsburgh Conference on Analytical Chemistry and Applied Spectroscopy, Pittsburgh, Pa., March 5, 1962.
Purification of Acetonitrile as a Solvent for Exact Measurements J. F. COETZEE, G. P. CUNNINGHAM, D. K. McGUIRE, and G. R. PADMANABHAN Deportment of Chemistry, University o f Pittsburgh, Pittsburgh 7 3, Pa.
b Acetonitrile is attracting increasing attention as a solvent for electrochemical and other reactions. Since it is a comparatively inert solvent, several possible impurities are sufficiently reactive to modify its propertiessignificantly, even in very low concentrations. The conventional method for the purification of acetonitrile (repeated distillation from phosphorus pentoxide) is not entirely satisfactory. Two alternative methods that give superior results are described. The results of polarographic and gas chromatographic tests for logical impurities are given. The persistent polarographic wave at -2.2 volts vs. the saturated calomel electrode is not caused by acetic acid, as previously assumed, but by unsaturated nitriles.
D
the last several years there has been a rapidly increasing interest in the use of acetonitrile as a solvent for electrochemical and other reactions. Various theoretical studies of a quantitative nature have been carried out in acetonitrile. URING
TYPICAL EXAMPLES. Polarography of inorganic substances (11, 12, 16) and various classes of organic compounds (18), as well as solid electrode voltammetry of inorganic substances (11, 16). Measurement of electrode potentials of common inorganic couples (16). Conductometry of salts, particularly substituted ammonium salts (4, 17). Quantitative studies of dissociation equilibria of acids (6, 10) and aminetype bases (14), using conductometric and spectrophotometric methods, Measurement of the autoprotolysis constant of acetonitrile (7). Electron spin resonance studies of electrolytically generated transient free radicals (9). Aromatic chlorination rate studies (9)
-
In addition to these quantitative theoretical studies, numerous empirical and semiempirical investigations (particularly acid-base titrations) have provided results that are useful for practical and comparative purposes (14). Since acetonitrile is a comparatively inert solvent, the presence of many possible impurities, even in very low concentrations, would make it unsuitable for exact studies. Several authors have commented on difficulties encountered in the purification of acetonitrile for such purposes. EFFECT OF IMPURITIES
Numerous methods are available for the preparation of nitriles (3, 13). We were unable to obtain information from the manufacturers about the methods actually used in the commercial preparation of acetonitrile and other nitriles. Technical data reports are available from the Eastman Kodak Co. for n- and isobutyronitrile (8), but not for acetonitrile. Polarographic and other tests have been carried out for a number of substances that logically may be present as impurities in acetonitrile, particularly isonitrile, water, and the various hydrolysis products of acetonitrilenamely, acetamide, ammonium acetate, ammonia, and acetic acid. Acetonitrile is a relatively weak base, much weaker than water (11,12), and an extremely weak acid (7,14). Several possible impurities are acids or bases sufficiently strong to modify the properties of the solvent significantly, even when present in very low concentrations. The presence of acetic acid would be particularly objectionable in exact studies of bases in unbuffered solutions, and that of ammonia in similar studies of acids, even if the concentrations of these impurities are as low as 10-6M. Although the effect of water is less
marked, the presence of relatively low concentrations of this persistent impurity can cause large errors in certain measurements. EXAMPLES. A. The polarographic half-wave potential of the solvated proton (as 1 X 10-*M perchloric acid) in acetonitrile as solvent becomes 0.15 volt more negative on adding 1 X 10-2M water (11). This large shift is mainly the result of the pronounced increase in the solvation energy of the proton which occurs when the relatively strong base water converts the species CH&NH+ into H2OH+ (11,12). B. The dissociation constant of the protonated form of the Hammett indicator 4chloro-%nitro-N-methylaniline increases by a factor of 20 when 2 X 10-2M water is added (as .shown in unpublished results, this laboratory). The cause of this large effect is basically the same as for example A. C. The conductance of 0.1M nbutylamine is doubled by adding 8 X 10-2M water ( I C ) , because the acid properties of water (proton donor strength, as well as solvation of anions by hydrogen bonding) are much stronger than those of acetonitrile. CONVENTIONAL METHOD FOR PURIFYING ACETONITRILE
The conventional method for the purification of acetonitrile generally involved some kind of pretreatment to remove acetic acid-for example, shaking with aqueous potassium hydroxide, solid potassium carbonate, or aluminafollowed by repeated distillations from phosphorus pentoxide until the residue was no longer colored (orange or black). Certain authors included an additional preliminary drying operation (shaking with calcium chloride, magnesium sulfate, etc.) and an additional (final) distillation of the product, either alone or with added potassium carbonate or barium oxide. Repeated distillation of acetonitrile from phosphorus pentoxide has a major VOL 34, NO. 9, AUGUST 1962
11 39
disadvantage in that it usually causes extensive polymerization of the solvent. When polymerization (formation of an orange gel) occurs, the yield may be seriously decreased (in extreme cases by as much as 50% in a single distillation) and the quality of the product is unpredictable-for example, the water content may be high and acetic acid may be present. It is then necessary to distill the product again. The entire process is time-consuming and the over-all yield may be very low. In view of these disadvantages of the conventional method, we have devised and studied several alternative methods.
Table 1.
Polarographic Tests for Logical Impurities in Acetonitrile
Expt.
Origin and Treatment of Solvent
1
Eastman practical grade, unpurified
2
Eastman ractical grade, urified by Metgods A through b - 1 (see LLExperimental")
3-1
Eastman practical grade, purified by Method B Same aa 3-1, but concn. of su porting electrolyte doublerfio
3-2
0.10M
EXPERIMENTAL
Apparatus. Polarograms were obtained with a Leeds & Northrup Electrochemograph Type E recording polarograph. The polarogra hic cell was similar to that described berore (11), with an aqueous saturated calomel electrode as reference and an aqueous agar-potassium chloride salt bridge, and a special device to prevent accidental introduction of water into the electrolysis solution. In a discussion of the applicability of this arrangement (7') it is pointed out that an aqueous agar-potassium chloride salt bridge must not be kept immersed in acetonitrile for more than a few minutes. All polarograms were recorded as quickly as possible after immersion of the salt bridge. Gas-liquid chromatograms, run at the Mellon Institute, Pittsburgh, were obtained with a 5-meter X 1/4-inch column containing 10% Ucon fluid on 60- t o 80-mesh Chromasorb. Column conditions are specified in Table I11 and in the text. Reagents for Purification of Acetonitrile. The following reagents were used: silica gel, 6- to 16-mesh (Fisher); Molecular Sieves, Type 4 A (Linde); Drierite, 10- t o 20-mesh (W. A. Hammond Drierite Co.), dried overnight a t 220' C.; adsorption alumina, 80- to 200-mesh (Fisher), freshly activated for 4 hours a t 400' C.; phosphorus pentoxide (Mallinckrodt analytical reagent grade) ; calcium hydride, mixture of equal parts of -40 and -4 40 mesh (Metal Hydrides Inc.). Other Reagents. Tetraethylammonium perchlorate was prepared by mixing hot 1M aqueous solutions of tetraethylammonium bromide and sodium perchlorate, followed by the treatment described elsewhere (11). All other chemicals were of reagent quality, unless specified otherwise. Preparation of Methyl Isocyanide. Methyl isocyanide was prepared by reaction of silver araentocvanide with methyl iodide. Add slowlv and with stirring 170 grams of silv& nitrate dissolved Yn 200 ml. of water to 65 grams of potassium cyanide dissolved in 500 ml. of water. Filter on a Buchner funnel, and wash the silver argentocyanide precipitate three or four times with water. Make a slurry of the precipitate with 120 grams
+
3-3 3-4 3-5 3-6 3-7 3-8 3-9
ANALYTICAL CHEMISTRY
Approx. lOmM HACadded to solvent 3-1 Approx. 50mM HACadded to solvent 3-1 Solution 3-6 passed through 36 X !/z inch alumina column Solution 3-7 passed through alumina column Approx. 5mM n-BuNHzadded to solvent 3-1
3-10
Approx. 0.3mM Et4NOH*added to solvent 3-1
3-1 1
Approx. 40mM (300% excess) Et4NOHb>c added to solution
3-12
Approx. lOmM NH,Ac added to solvent 3-1
3-13
Approx. 20mM CHsNC (isocyanide) added to solvent 3-1
3-14
Approx. 20mM CHsCONBz added to solvent 3-1 Ap rox. 1OmM each of MeOH, &,OH, ethyl acetate, and diethyl ether added to solvent 3-1 Approx. 500mM water added to solvent 3-1
3-15 3-16
3-5
4-1
Fisher Spectrograde solvent, no further purification
4-2
Approx. lOmM isobutyraldehyde added to solvent 4 1 Approx. 3mM acrylonitrile added to solvent 4-1 Water added to solution 4-3
4-3
4-4
5
Eastman ractical grade, purified by met\od E
6
Matheson Chromatoquality reagent, no further purification Sohio, unpurified
I
1 140
Same as 3-2, but supporting electrolyte chan ed to Et4NBr Approx. 1mM N g a d d e d to solvent 3-1
7
Polarographic Properties" Drawn-out cathodic wave starting a t -2.0 volts, no true diffusion plateau, i d = 12 pa. at -2.5 volts, and 19 pa. at -2.7 volts. Small anodic wave starting a t $0.1 volt (see expt. 3-4). See BF. Figure 1 In each caae, 2 very incompletely resolved waves starting at -2.05 volts (hereafter referred to aa wave XU), with id values given in Table 11. See AE, Figure 1 Wave XY, id = 6.0pa. (Table 11). See AE, Figure 1 Wave XY, unchanged Wave XY, unchanged Well-defined anodic wave of NH, starting a t + O . l volt, E I / Z.= +0.25 volt, id = 4 pa. See CE, Figure 1 Similar to solvent 1, with id = 24 pa. a t -2.5 volts and 31 pa. at -2.7 volts Very drawn-out wave starting a t -2.0 volts, id > 90 pa. at -2.7 volts WaveXY, id = 8pa. Wave XY, no further decrease from 3-7 Wave XY unchanged. Anodic wave of +0.2 volt, id = 17 BuNHz, E112 pa. Wave XY unchanged. Anodic wave of OH-, starting a t -0.8 volt, id = 1 pa. at 0 volt Wave XY, id = 9 pa. (actually increase from 3-1 : see 3-16 and 4-4). Large anodic wave of OHVery drawn-out cathodic wave starting a t - 1.8 volts, but with well-defined diffusion plateau, id = 45 pa. at -2.6 volts (reduction of "If,). Two incompletely resolved anodic waves starting at +0.1 volt, id = 40 pa. a t +0.6 volt (probably due to free NH3and Ac-) Wave XY, id = 9 pa. Anodic discharge at +0.3 volt, id > 80 pa. a t +0.5 volt No change from 3-1 N o change from 3-1
Double wave XY becomes single wave, Eliz = -2.17 volts; id increases to
10.6 p a . No significant wave up to discharge potential (-2.73 volts). See A , Figure 2
Small, incompletely developed wave starting a t -2.5 volts Wave XY, id = 15.7 pa. See B, Figure 2
Double wave XY becomes single wave; = -2.10, -2.10, and -2.19 volts, and id = 19.5, 23.5, and 23.0 pa., for 170, 510, and 850mM water added, respectively. See Figure 2 No wave up to,dischargepotential (-2.82 volts). See Table I1 and AD, Figure 1 Cathodic wave at - 1.6 volts (not identified), id = 1.2 pa. a t - 1.8 Volts. No significant wave a t -2.2 volts Wave XY, id = 6 pa. Small anodic wave, id = 0.5 pa.
Supporting electrolyte of 0.05M Et4NC10a,unless specified otherwise. All potentials aqueous S.C.E. Diffusion currents measured at -2.5 volts, unless specified othermse. Characteristics of dropping mercury electrode given under Table 11. b Eastman a ueous 10% Et4NOHsolution. c Approx. 2&? water introduced by EtnNOHsolution. us:
f
I
t2
a t8
i I
I
-18
-2 0
-22
-24
-26
-28
EDM..V M S C E
o l p x j
-4
*06
+04
102
0
*crlB - 2 0
-22
-24
-26
-28
-30
E D Y E . V vs S C E
Figure 1. Polarographic waves in acetonitrile with 0.05M Et4NC104 as supporting electrolyte
Figure 2. Polarographic waves of acrylonitrile in Fisher Spectrograde acetonitrile with 0.1 M Et4NClO4 as supporiing electrolyte A.
Residual current curve 6. 3.0mM acrylonitrile present C,D,E. 0.17, 0.51, and 0.85M water added to 6
AD. Acetonitrile purified by Method E AE. Acetonitrilie purified b y Methods A-1 and B AG. l O m M acetic acid added to acetonitrile purified by Method B BF. UnpuMed acetonitrile CE. 1 m M NHa added to acetonitrile purified by Method B
graphic wave in acetonitrile, corresponding to the over-all reduction process: 2HA+2e-+2A-+H*
Method D. 1. Intermittently shaken or stirred (magnetic stirrer) with calcium hydride (10 grams per liter) for 2 days, then decanted. 2. Fractionally distilled from phosphorus pentoxide (5 grams per liter). Method D-1. 1. Product from D refluxed over calcium hydride (5 grams per liter) for several hours. 2. Fractionally distilled (ver slowly). Method E. 1. Refluxed w i d aqueous 1% potmiurn hydroxide solution (1 ml. per hter) for several hours. 2. Fractionally distilled. Procedure. METHODS FOR PURIFY3. Intermittent1 shaken or stirred ING ACETONITRILE.Eastman pracwith calcium hydriak (10 grams per liter) tical grade acetonitrile was purified for several hours, then decanted. by the following methods. 4. Fractionally &tilled from phosphorus pentoxide (5 grams per liter). Method A. 1. . Shaken successively 5 . Refluxed over calcium hydride with 2 batches of silica gel (50 grams per (5 grams per liter) for several hours. liter), 2 batches of alumina (20 grams per 6. Fractionally distilled (very slowly). liter), and phosphorus pentoxide (20 grams er liter). In all cases the solvent was distilled 2. gractionally distilled from fresh under a high reflux ratio through a phosphorus pentoxide (5 grams per liter). column 4 feet long packed with Method A-1. 1. Product from A distilled again from fresh phosphorus inch i.d. borosilicate glass helices. pentoxide (5 grams per liter). The first and last 10% fractions were Method B. 1. Shaken for a maximum discarded. of '/2 hour with silica gel (50 grams per The origin of other grades of acetoliter), then decanted. nitrile tested in the present study is 2. Silica gel treatment repeated for 8 given in Tables I, 11, and 111. hours. The solvent was stored and handled 3. Passed through alumina column. ~ t described 9 before (11 ) . 4. Fractionally distilled from Drierite (25 grams per liter). Method B-1. Product from B fracRESULTS AND DISCUSSION tionally distilled from calcium hydride ( 5 grams per liter). Method C. 1. Shaken intermittently Acetic Acid and Other Logical with calcium hydride (10 grams per liter) Impurities in Acetonitrile. Considerfor 2 days, then decanted. able attention was devoted t o the 2 . Fractionally distilled from fresh removal of acetic acid from acetocalcium hydride (5 grams per liter). nitrile, since acetic acid is a logical Method C-1. Product from C distilled again from fresh calcium hydride impurity in acetonitrile and even ( 5 grams per liter). minute traces may be very objectionMethod C-2. Product from C fracable in certain studies. Acetic acid tionally distilled from phosphorus pentgives a somewhat drawn-out polarooxide ( 5 grams per liter). of methyl iodide, and heat in a securely stoppered pressure bottle for 4 to 6 hours a t 60" f 5" C. (pressure ca. 2 atm. At lower pressures the yield is small.) Add the product to 65 grams of potassium cyanide dissolved in a suitable volume of water, and carry out a careful fractional distillation t o remove excess methyl iodide (b.p. 42.4' C.); then collect the methyl isocyanide fraction (b.p. 59" C.). Caution: Isocyanides are very toxic.
t
The apparent half-wave potential for a 1mM solution in 0.lM tetraethylammonium perchlorate as supporting electrolyte is - 2 . 3 volts us. S.C.E. Unpurified acetonitrile, as well as that purified by the conventional method described above, generally gives a polarographic wave a t - 2 . 2 to - 2 . 3 volts. This wave is very difficult to eliminate, sometimes even by four or five distillations from phosphorus pentoxide. It has been assumed rather generally that this wave is caused by acetic acid. That this is not the case is shown by the series of experiments listed in Table I, which also includes tests for other logical impurities in acetonitrile. CONCLUSIONS.The presence of isocyanide can be detected polarographically (expt. 3-13). At concentrations lower than that used, a true anodic wave undoubtedly will be produced. Ammonia (expt. 3-4) and ammonium acetate (expt. 3-12) can be detected and determined easily. Alumina is very effective in removing acetic acid from acetonitrile (expt. 3-71,
The impurity giving rise to the double wave a t -2.2 volts (wave X U ) is not acetic acid (expts. 3-6 through 3-11), but probably an unsaturated nitrile, such L-LS acrylonitrile (expt. 4-3). The effect of added water (cf. expts. 3-16 and 4-4) is typical of that observed before [in studies listed by Wawzonek and Duty ( I @ ] , when proton donors were added to solutions of unsaturated compounds in acetonitrile and dimethylformamide. VOL 34, NO. 9, AUGUST 1962
1141
Values for the magnitude of the impurity wave, XY,and for the cathodic discharge potential of acetonitrile purified by the various methods tested, are listed in Table 11. Unsaturated Nitrile Impurities in Acetonitrile and Isobutyronitrile. Eastman practical grade isobutyronitrile, after purification by essentially Methods A and D, gave the same wave XY at.-2.2 volts, but with a much larger diffusion current than for acetonitrile (unpublished results, this laboratory). Furthermore, addition of isobutyronitrile (purified by Method A) to acetonitrile (from Method B) increased wave XY. Product information supplied by the manufacturers (8) lists methacryloni-
Table 11. Variable Polarographic Properties of Acetonitrile Obtained by Different Purification Methods Cathodic - - _-.__ .-
Diffusion Discharge Method” Current* Potentiale Unpurified 17 ( - 2 . 7 volts) ca. - 2 . 7d A 7 (-2.5 volts) -2.80 A-1
Be B-1 C
c-1 (2-2 D D-1
E’
6 6.0 5.5 12.5 8.4 8.8 4.5 2.6
-2.m -_ ~
-2.75 -2.65d -2.75 ~a.-2.6~
ca. - 2 . 6d -2.75 -2.87 -2.82
0.3
For description of methods, see “Ex-
perimental.” In all case8 starting material waa Eaatman practical grade acetonitrile. Diffusion current in microamperes obtained with 0.05M EtcNClOc a8 supporting electrolyte at a dropping mercury electrode with following characteristics m = 1.30 mg./second, t = ca. 2 seconds at -2.5 volts. All values at -2.5 volts, unless specified otherwise. e A ll potentials us. aqueous S.C.E. Discharge very gradual. Could be due to formation of solid plug of KCl in salt bridge, in which case these values would be uncertain (7). Water content of product unduly high (2 X 10-*M) aa compared to other methods (always below 2 X 10-aM for Methods A and A-1, and well below 1 X 10-*M for remaining methods; too low to determine by Karl Fischer titration). I Product contained 5 X 10-6M NHI.
Table 111.
trile (2% by weight maximum) ,aldehydes (1% maximum, as C=O), and water (0.570 maximum) as the major impurities in their isobutyronitrile. We have verified the presence of methacrylonitrile in our purified isobutyronitrile by gasliquid chromatographic analysis (Table 111). Furthermore, addition of methacrylonitrile to isobutyronitrile enhanced the impurity peak with a retention time of 0.780. Finally, since the retention time of methacrylonitrile varies with its concentration, a soluticn of methacrylonitrile in cyclohexane also was tested, a t such a concentration that its peak height was approximately equal to that of the peak a t a retention time of 0.780 in isobutyronitrile. Under slightly different conditions (column temperature 54’ C., helium flow rate 55 ml. per minute), the retention times for equal peak heights were 0.720 (major peak for methacrylonitrile in cyclohexane) and 0.739 (impurity peak in isobutyronitrile) . It is evident from Table I11 that under our conditions traces of acrylonitrile could not be detected in acetonitrile by gas chromatography. However, the polarographic results described above appear to be conclusive. The gas chromatograms gave no sign of the presence of acetic acid in our acetonitrile purified by Method B or of isobutyric acid in our isobutyronitrile purified by Methods A and D. Recommended Methods for Purifying Acetonitrile. It is evident from Tables I and I1 that i t is difficult to remove unsaturated nitrile impurities from acetonitrile, unless drastic methods are used, such as the “cyanoethylation’, reaction ( I , 6) incorporated in Method E :
+
OH CHz:CH.CN Hz0 -+ HO.CHz*CHz.CN+ NC.(CHz)z-0-(CH2)z.CN The boiling point of the final product is 161-3’ C. a t 5.5-mm. pressure (6), and distillation separates acetonitrile very effectively. However, as may be expected, the drastic treatment resorted to in this method may introduce some ammonia into the final product.
Gas Chromatography of Various Samples of Nitriles Origin Retention Timesalb Fisher Spectrograde 0.662 Eastman practical grade 0.288, 0.667 (major) Eastman practical grade dis0.238, 0.709, 0.780, 1 .OOO tilled from Pz06 (major), 1.599, 1.755 Methacrylonitrile Shell Chem. Corp. (with 0.229, 0.360, 0.474, 0.683, 0 . 1yostabilizer) 0.821 (major), 0.922 [Isobutyraldehyde Eastman Red Label 0.204, 0.282 (major)] 4 Column tem erature, 55” C.; helium flow rate, 50 ml. per minute.
Sample Acetonitrile Acrylonitrile Isobutyronitrile
a
Based on iso!utyronitrile
1142
=
1.OOO.
ANALYTICAL CHEMISTRY
For certain purposes small concentrations of ammonia will present no problem, but for others-e.g., studies involving weak acids in unbuffered solutions-even traces of ammonia are very objectionable. In concentrations greater than approximately 5 x 10--6M, ammonia can be detected and determined polarographically (see Table I). Even smaller concentrations of ammonia (down to 10-6M) can be detected from the increasing absorption of 10-4M picric acid a t 420 mp (IO). For studies in which such small concentrations of ammonia will interfere, Method E must not be used. For most purposes Method D-1 is recommended. Under our conditions it usually left approximately 5 x lO-4M unsaturated nitrile in the product. Small concentrations of unsaturated nitriles have very little effect in ordinary acid-base studies in saturated nitriles as solvents. For example, addition of as much as 0.1M methacrylonitrile to a 2 X 10-3M solution of perchloric acid in isobutyronitrile as solvent, shifted the half-wave potential for the reduction of the solvated proton by only 0.02 volt (unpublished results, this laboratory). However, reagents such as mercaptans and halogens will be affected by the presence of unsaturated nitriles. If necessary, the unsaturated nitrile content of acetonitrile can be reduced further by Method D-1 by additional fractional distillation from calcium hydride. The unsaturated nitrile is concentrated in the early fractions, as shown by the diffusion current values obtained a t -2.5 volts for several fractions of a typical (first) distillation from calcium hydride: 10 to 25%, 4.6 pa.; 25 to 50%, 3.0 pa.; 50 to 75%, 1.5 pa; 75 to 90%, 1.9 pa.; combined 10 to 90%, 2.6 pa. Efficient fractionation is essential, because the boiling points of acetonitrile (81.6’ C.) and acrylonitrile (77.3’ C.) are close together. Moreover, azeotropes may be formed. It also is essential to shake or stir effectively in the initial drying operation with calcium hydride, because the hydrogen which is evolved has a tendency to adhere to the surface of the calcium hydride and thereby prevent further reaction. A magnetic stirrer is particularly effective. Method D-1 has the inherent advantage that both an acidic (phosphorus pentoxide) and a basic (calcium hydride) drying agent are used. Hence basic as well as acidic impurities should be removed. Calcium hydride is very effective in removing the last traces of acetic acid, and of water as well (since the reaction is irreversible). The yield of Method D-1 is superior to that of the conventional method, since only one distillation from phosphorus pentoxide is involved, and the product is drier and absolutely free of acetic acid. It is our opinion that reDeated distillation
of acetonitrile from phosphorus pentoxide serves no useful purpose beyond the first distillation, apart from the fractionation involved. The presence of phosphorus pentoxide during further distillation is actually objectionable, since it generally causes extensive polymerization. Although i t is possible to recover a fair amount of acetonitrile by prolonged heating of the polymer, unavoidable bumping of the gel results in irregular distillation and less efficient fractionation. Furthermore, in the presence of the gel phosphorus pentoxide becomes less effective as a drying agent. Lithium and lithium aluminum hydrides cause polymerization of acetonitrile. If the original solvent contains a relatively high concentration of water, preliminary with silica gel Or Molecular Sieves (Type a), before addition of calcium hydride, is recommended.
can be determined by Karl Fischer titration. For such low water concentrations the titration must be carried out directly in the acetonitrile sample, although the end point is by no means ideally stable. For higher water concentrations i t is preferable to titrate the acetonitrile sample in dry methanol assolvent. An alternative method for the determination of water in acetonitrile and other nitriles, as well as in ketones, is based on measurement of the volume of hydrogen liberated by calcium hydride. This method will be described elsewhere. LITERATURE CITED
(1) American Cyanamid CO., “The Chemlstry of Acrylonitrile,” 2nd ed., 1959. ( 2 ) Andrews, L. J., Kee‘er, R. M., J . Am. Chem. SOC.81, 1063 (1959). (3) Astle, M. J., “Industrial Organic
Nitrogen Compounds,” Reinhold, New York, 1961. (4) Berns, D. S.:Fuoss, R. M., J . Am.
Chem. SOC.82, 5585 (1960). (5) Bruson, H. A., Riener, T. W., Zbid., Determination of Water in Aceto65 23 (1943). &de. Water, in CO~Ce~trations (6) &hantooni, M. K., Jr., Ph.D. thesis, down to approximately 1 X lO+M, University of Minnesota, 1960.
(7) Coetzee, J. E’.,.Padmanabhan, G. R., J . Phys. Chenz., m press. (8) Eastman Chemical Products, Kingsport, T ~ ~$“ormal ~ , , Butyronit,rfle,” Tech. Data Sheet N-111: “Isobutyronitrile,’’ Tech. Data Rept. N-102. (9)Jura, Hoffmann, A. K*, w. G., W. H., J. Am. Chem. SOC. 83, 4675 (1961). (IO) Kolthoff, I. M., Bruckenstein, S., Chantooni, M. K., Jr., Ibid., 83, 3927 (lg61). (11) Kolthoff, I. M., Coetzee, J. F., Ibid., 79, 870, 1852, 6110 (1957). (12) Kolthoff, I. M., &e&, S.,J. Phys. Chem. 65, 1020 (1961). D. T., them. Revs*42, lS9 (14) M ~ W. ~s.,coetaee, ~ ~ J. F., , J. Phys. Chem. 66, 89 (1962). (15) Pleskov, V. A., J . Phys. Chem. (U.S.S.R.) 22, 351 (1948). (16) Popov, A. I,, Geske, D. H., J. Am. Chem. SOC.79, 2074 (1957); 80, 1340 (1958). (17) Walden, P., Birr, E. J., Z. physik. Chem. 144,269 (1929). (18) Wawzonek, s., Duty, R. C , , J. Electrochem. SOC.108, 1135 (1961).
(l?/ggyYj
RECEIVEDfor review April 18, 1962. Accepted June 7, 1962. Work supported by the National Science Foundation under grant number NSF-G14502.
Techniques for the Mass Spectrometric Analysis of Volatile Compounds Isoluted from Natural Sources M. 1. BAZINET and
CHARLES MERRITT, Jr.
Pioneering Research Division, Quartermaster Research and Engineering Center, U. S. Army, Natick, Mass.
b The mass spectrometric analysis of trace amounts of volatile compounds isolated from natural sources is frequently complicated by the presence of large amounts of water and carbon dioxide. Water and carbon dioxide are removed by a method of vacuum manipulation of gaseous samples in equilibrium with Molecular Sieves or Ascarite. Procedures are described for high vacuum-low temperature fractional distillation of minute gaseous samples on a mass spectrometer inlet system and the use of low voltage ionization spectra to deduce the composition of multicomponent mixtures.
vious procedures, techniques.
and
some
new
REMOVAL OF CARBON DIOXIDE WITH ASCARITE
I n a great majority of samples of volatile components isolated by vacuum distillation from natural products the
* n )
TO VACUUM
PRESSURE
OAUOL
OME
Figure 1. Schematic diagram of apparatus for removal of carbon dioxide from vapor samples prior to mass spectrometric analysis A, B, C.
Vacuum stopcocks
The total condensate contained in a to a vacuum manifold is cooled to -80’ c. (dry iceethanol). Another previously evacuated gas bottle containing Ascarite, which had been prepared by removal of volatile compounds a t -80” C., is also attached to the vacuum manifold (Figure 1). After the manifold has been evacuated, stopcocks A and B, which connect both gas bottles, are left open to allow carbon dioxide to be absorbed by the Ascarite. When the system has come to equilibrium, the pressure drops to a nearly constant value. The final pressure depends on the size of the total condensate sample and the relative amount of the compounds volatile a t -80’ C. in the sample. Stopcock B, which isolates the total condensate bottle, is then closed and the bottle is replaced by a clean evacuated gas bottle which is cooled to gss bottle attached
S
of the techniques for collecting and separating the volatile components of foodstuffs and subsequently ident,ifying them by mass spectrometry have been described (4). These methods of fractionation have employed low temperature-high vacuum distillation and have been generally useful not only for foodstuffs but for a variety of other natural materials. This paper describes some improvements in pre-
“total condensate” (4) is composed mainly of water and carbon dioxide, and contains only traces of the compounds to be identified. The following procedure has been devised for removing carbon dioxide from the mixture and simultaneously fractionating the remainder into a “center cut” and a wakr fraction.
VOL 34, NO. 9, AUGUST 1962
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