Liquid-Liquid Extraction with Ammonia and Diethyl Ether Alan M. Phipps’ and Joseph X. McDermott Department of Chemistry, Boston College, Chestnut
Hill,Mass. 02167
Use of the liquid-liquid extraction system, anhydrous liquid ammonia-diethyl ether has been investigated. Mutual solubilities are similar to those observed in the water-ether system. Partition of ten metal cations as a function of the concentration of four ammonium salts showed quantitative extraction into the ammonia phase in all cases. The partition of 2,4-dinitroaniline as a function of ammonium ion concentration is that expected for a weak acid. Above 1M N H 4 N 0 3this compound as well as 4-methyl-2-nitroaniline and 2-methyl5-nitroaniline, i s rapidly salted out of the ammonia phase. 2- and 4-Nitroaniline do not behave as acids but have differing partition characteristics both from each other and from 2- and 4-nitroacetanilide. In each case partition is markedly different from that observed in the water-ether system.
carbons from petroleum products, usually requiring temperatures above the boiling point of ammonia and the addition of a cosolvent. Use of 2,2,4-trimethylpentane as a diluent for different types of extractant a t -74 “C is being investigated in this laboratory. Diethyl ether has been tabulated as miscible since the early work of Franklin and Kraus ( 4 ) . This is true above approximately -43 “C, but at -74 “C we have found uide infra that the mutual solubilities are similar to those of the ether-water system. This paper describes the use of this solvent pair with several inorganic and organic solutes for liquid-liquid extraction of the type governed by competitive solvation of neutral species.
SINCETHE STUDIES of Franklin, Kraus, Cady, and others at the turn of the century, there has been considerable interest in the use of liquid ammonia as a solvent medium for a wide range of analytical methods. Polarography, spectrophotometry, and acid-base titrations to mention a few, have been advantageously investigated. Several review articles and chapters covering solvent applications and physical properties have appeared in the past 20 years; particular reference could be made to the very extensive monographs of Jander (1) and Smith (2). The ready solubility of many inorganic and organic electrolytes and nonelectrolytes in liquid ammonia suggests that the solvent would be a valuable one for separation processes. This could conceivably afford the opportunity to separate in situ reaction products of some of the large variety of organic reactions which are carried out in liquid ammonia. The ion exchange behavior of several cations and anions in liquid ammonia has been investigated (3). Selectivities widely different from those observed in aqueous solution were found, but the rate of exchange was very slow, as has been observed in all strictly anhydrous solvents employing polyvinylstyrene-divinylbenzene resins. Because the exchange rate is determined by diffusion into and out of the resin, it was felt that more rapid equilibration would be achieved in liquid-liquid extraction systems. It is convenient to work at a pressure of 1 atm using a nonpolar solvent which is a liquid from the freezing point of ammonia (-77.7 “C) to above room temperature. This limits the number of available solvents. Suitable chlorohydrocarbons are miscible over the liquid range of ammonia, as are the lower aliphatic esters. The lower aliphatic hydrocarbons are quite insoluble and there are numerous patents describing the concentration of aromatic and olefinic hydro-
EXPERIMENTAL
Present address and address to which correspondence should be sent, The Gillette Co., Gillette Park 2T, So. Boston, Mass. 02106. (1) J. Jander, “Anorganische und Allgemeine Chemie in Fliissigern
Arnrnoniak,” “Chemistry of Nonaqueous Ionizing Solvents,” Volume I, Part I ; Interscience Publishers, New York, 1966. (2) H. Smith, “Organic Reactions in Liquid Ammonia,” ibid., Part 11, 1963. (3) A. M. Phipps and D. N. Hurne, ANAL.CHEM., 39, 1755 (1967). 530
ANALYTICAL CHEMISTRY, VOL. 43, NO. 4, APRIL 1971
Reagents. ACS Reagent grade anhydrous diethyl ether and 99.99 minimum purity anhydrous ammonia were used as the two liquid phases without further treatment. Reagent grade anhydrous ammonium and metal salts or ammoniate salts were stored over P 2 0 5and used as is. The hydrated nitrates of U02(II), Mg(II), and Th(1V) were maintained in a vacuum desiccator over P205for several days, dissolved in ether and dried with molecular sieve (Linde 4A). Highest Purity aniline derivatives were recrystallized from methanol after treatment with activated charcoal, dried, and stored over P205. Procedure. Equilibration of the two liquid phases was carried out in 50-ml graduated centrifuge tubes immersed in a n acetone4ry ice bath which maintained a temperature of - 74 + 1 “C. In general, the same procedures were followed as described previously for the study of ion-exchange processes (3). Organic solutes were added to the tube in ether solution by syringe or pipet after condensation of the ammonia. Approximately 10 ml of each phase were equilibrated for at least 1 hour with periodic swirling. It appeared that only a few minutes were required for the complete extraction of solutes giving colored solutions. After equilibration, a measured sample of approximately 5 ml of each phase was analyzed for solute concentration. The systems reported here appeared free of emulsion to visual inspection. The several cases of quantitative extraction into the ammonia phase support this observation and justify the sample handling methods [e.g., Mg(N03)2 originally 0.03M in ether, could not be detected after equilibration in the residue of the ether phase sample using eriochrome black TI. The concentration of the substituted aniline compounds was determined spectrophotometrically with a Cary 14 R o r Beckman DU spectrophotometer after evaporation of the solvent and dilution of the residue to a n appropriate volume with diethyl ether or methanol. The invasion of the ether phase by ammonia, was determined by transferring a measured volume of the ether phase directly into an excess of 4 z aqueous boric acid and titrating with standard hydrochloric acid. The invasion of the ammonia phase by diethyl ether was determined by a gas chromatographic procedure to be described elsewhere.
(4) E. C. Franklin and C. A. Kraus, Amrr. C/zem. J., 20, 820
(1898).
I
I
I
I
60 -
.E
n
30-
.-C0 c
C
$ 10-
4 01
0
4 I
0
I
I
2
I
L
3
4
Concentration of Ammonium Salt i n Ammonia
Figure 1. Effect of ammonium salt concentration on extraction of ammonia into diethylether 0 BrA Nos0 SCN
RESULTS AND DISCUSSION
The invasion of the ether phase by ammonia is shown in Figure 1 for several ammonium salts (acids in the ammonia system). At low salt concentrations the ammonia phase is the upper (less dense; phase. An inversion of the two phases takes place at approximately 1.5Mammonium nitrate and thiocyanate and 1.OM ammonium bromide. It was not possible to obtain reliable samples of either phase at salt Concentrations in the immediate vicinity of the point of equal density. The invasion of the ammonia phase by ether is shown in Figure 2 as a function of ammonium nitrate concentration. Using the values, 0.73 and 0.79 for the densities in grams/ milliliter at -74 "C of ammonia and diethyl ether, respectively, the weight per cent mutual solubilities of the pure solvents can be calculated to be 9.5 % ammonia in ether and 8.0z ether in ammonia. This is felt to be comparable to the aqueous-ether system for which the corresponding solubilities at 20 " C are 6.9% ether in water and 1.3% water in ether. The ammonium acids however, unlike the aqueous counter parts, are not noticeably extracted into the ether phase. The UV spectrum of ether which has been equilibrated with solutions of ammonium nitrate in liquid ammonia shows no evidence of the presence of nitrate. The ether extraction of nitric acid from aqueous solution is appreciable; about 25 from 4 M nitric acid (initial) with an equal volume of ether at 19 "C (5). A concentration of 4 M represents the approximate solubility of ammonium nitrate in liquid ammonia at -74 "C. Inorganic Ions. Partition between these two solvents was investigated for the following metals originally made up in liquid ammonia solution as the anhydrous nitrates or nitrate ammoniates at a concentration of approximately 0.1M ; Cu(II), Ni(II), Zn(II), Cd(II), Pb(II), and Ce(1V). Equilibrations were carried out over a concentration range of zero to saturation in additional ammonium acid NH,X, where X = NOs-, Br-. I-, o r SCN-. [Pb(lI) was investigated with NO -and I -only, Ce(IV) with N O i - only.]
z
( 5 ) R . Bock and E. Bock, 2. Auorg. Cl7rm.: 263, 146 (1950).
I
0 0
I
I 2 Concentration Ammonium
I
I
3
4
Nitrate
M
Figure 2. Effect of ammonium nitrate concentration on extraction of ether into ammonia
In no case could these metal ions be detected in the ether phase after equilibration. With the exception of Ni(1I) all of these ions are extractable into ether to a significant extent from one or more of the analogous aqueous systems. [Tabulations are given in (6).] Mg(II), UO2(II), and Th(IV) originally 0.01-0.1M in ether solution as nitrates, and Fe(II1) in ether solution as the perchlorate were all found to be quantitatively extracted from the ether phase over the full range of ammonium nitrate concentration. The same result was obtained for Fe(II1) with ammonium bromide and thiocyanate. I n the case of UOu(II) and Th(IV), the appearance of a third phase was observed. In each case the ether solution contained some water as determined by Karl Fischer titration: approximately 0.5 and 1.0 mole of water per mole of U02(II) and Th(IV), respectively. [Less than 0.1 mole of water per mole of Mg(I1) was found.] This is not a sufficient amount of water for complete precipitation of (NH4)?U2O7 or T h o ? as has been observed on treating U 0 2 ( N 0 & . 2 H z 0 or T h ( N 0 J I . 4 H 2 0 with liquid ammonia (7). At concentrations of ammonium nitrate above the point of inversion, there was no precipitate from the Th(1V) solutions. It can be reasonably suggested that these ten cations and four anions represent a wide enough variety of ion-solvent interaction to preclude the separation of inorganic salts by liquid-liquid extraction between ether and liquid ammonia. Ammonia is perhaps too basic to allow significant solvation competition with a n oxygenated solvent. The familiar ion-pair extraction from aqueous solution into ether is probably impossible in this case because of the insolubility in ether of the solvated proton. Nitroaniline Compounds. In general, organic nitro compounds of all types have been found to conduct a current to some extent in liquid ammonia. The cation is assumed to be the ammonium ion in all cases and the acid dissociation is assumed to be a two-step, ionization-dissociation process caused by the relatively low dielectric constant (27 at -70°C). N 0 2 R N H 2 N H 3 -1: (N02RNH-,NH4+) /, 2
+
-
N02RNH-
+ NHi'
(1)
(6) G. H. Morrison and H. Freiser, "Solvent Extraction in Analytical Chemistry," J . Wiley and Sons, New York. N. Y . . 1957. (7) G. W. Watt, W. A. Jenkins. Jr., and J. M. McCuiston, J . Amer. Cliem. Soc., 72, 2260 (1950). ANALYTICAL CHEMISTRY, VOL. 43, NO. 4, APRIL 1971
531
,
I
I Concentration
2
3
I
0‘ 0
4
I
0.01
Ammonium Nitrate
Figure 3. Dependence of partition ratio on concentration of ammonium nitrate 0 1.7 X 10-4M 4-nitroaniline A 2.2 X 10M4M 2-nitroaniline 0 1.42 X lO-4M 2,Cdinitroaniline
The partition behavior of some nitro aniline compounds has been investigated in terms of the experimentally determined partition ratio defined (8) as the ratio of the total analytical concentration of the solute in the organic phase to the total analytical concentration in the polar phase. The partition ratios of 2- and 4-nitroaniline and 2,4dinitroaniline as a function of ammonium nitrate concentration are shown in Figure 3. It can be seen that neither 2- nor 4-nitroaniline is a strong enough acid in liquid ammonia to exhibit a partition dependence on ammonium ion concentration which would be explainable with recourse to Equation 1. Differences in hydrogen bonding may account for the opposite effect observed for these two isomers, inasmuch as hydrogen bonding between solvent and solute has been convincingly cited (9) as the main factor regulating the partition of organic solutes between ether and water. Intramolecular hydrogen bonding of 2-nitroaniline has been described as moderately strong in organic solvents on the basis of infrared spectra (10) but other interpretations have been made (11, 12). It would be anticipated that the closed structure would show a preference for the ether phase. Intra-molecular hydrogen bonding would be decreased with increasing ammonium ion concentration which could lead to partition behavior similar to that of the para isomer. It is perhaps more likely that the ortho isomer forms an ammoniate which is decreasingly soluble in the ether phase as the ammonia is removed (Figure 1). 2,4-Dinitroaniline is a strong enough acid in liquid ammonia for the influence of the ammonium ion concentration to be observable in solvent extraction. This can be seen in an expanded concentration scale in Figure 4. Franklin and Kraus (13) found (8) L. B. Rogers, “Principles of Separations,” in “Treatise on Analytical Chemistry.” Part I, Vol. 2, Interscience, New York, N. Y . , 1961, Chap. 22. (9) R . Collander, Acta Chenr. Scaiid., 3, 717 (1949). (10) P. J. Krueger, Curl. J. Chem., 41, 363 (1963). (11) L. K. Dyall, Spectrochim. Acta, 17, 291 (1961); ibid., 22, 483
I
,
1
0.03 0.06 Concentrallon of Ammonium Ion
0 09
M
Figure 4. Dependence of partition ratio of 1.42 X lO-3M 2,4dinitroaniline on concentration of ammonium nitrate 0
Without additional electrolyte
A Nitrate concentration maintained at 0.1M with KNOI
Table I. Acid Dissociation Constant for 2,4-Dinitroaniline KU x 103 KU x 103 M NHaN03 X lo3 (Equation 2) (Corrected) 0.839 1 ,67 1.39 2.64 1.89 1.34 2.05 1.39 3.48
2,4-dinitroaniline to be a fairly good electrolytic conductor in liquid ammonia while 4-nitroaniline exhibits only slight conductivity. Fohn et al. have suggested, on the basis of spectral data, that 2,4-dinitroaniline ionizes spontaneously in pure liquid ammonia while 2- and 4-nitroaniline do not (14).
By assuming that only the neutral species (N02)2RNH2 is present in the ether phase, the extrapolated value shown in Figure 4 can be taken as Kc, the partition coefficient for this species. Solutions containing the ionized form or the ion pair have an intense purple color. There was no visible evidence of this in the ether phase. It has previously been mentioned that the investigation of inorganic ions produced no evidence that ammonium salts or ion pairs are extractable into ether from liquid ammonia. An estimation of an acid dissociation constant Ka (corresponding to the product klk2 in Equation 1 can be obtained from the relationship:
a combination of expressions for Ka and K p . (NHlf) represents the equilibrium concentration of ammonium ion contributed by ammonium nitrate and 2,4-dinitroaniline. Calculated values of Ka for the three intermediate concentrations of ammonium nitrate shown in Figure 4 are listed in Table I. The values listed in the second column were obtained o n the assumption that ammonium nitrate is completely dissociated. However, incomplete dissociation at this concentration is indicated by the conductivity studies
(1966). (12) A. G. Moritz, ihid., 18, 671 (1962). (13) E. C. Franklin and C. A. Kraus, J. Amrr. Clzem. SOC.,27, 191 ( 1905). 532
ANALYTICAL CHEMISTRY, VOL. 43, NO. 4, APRIL 1971
(14) E. C. Fohn, R. E. Cuthrell, and J. J. Lagowski, Znorg. Clzern.
4, 1002 (1965).
0
0
I Concentration
2 Ammonium N i t r a t e
3
fi
Figure 5. Dependence of partition ratio on concentration of ammonium nitrate 0 1.46 X 10-4M 4-methyl-2 nitroaniline 0 1.46 X 10 -3M 4-methyl-2 nitroaniline A 1.46 X 10-3M 2-methyl-5 nitroaniline
of Franklin and Kraus (15). Using this data to calculate a degree of ionization allows the calculation of the “corrected” Ku listed in Table I. The true value would be somewhat smaller because the conductivity data were obtained at -33 rather than -74 O C . Suppression of acid dissociation at higher salt concentration is shown by the lower curve in Figure 4 obtained at a constant nitrate concentration of 0.1M. This does not represent constant ionic strength because different degrees of ionization are involved; 0.26 and 0.39 for potassium and ammonium nitrate, respectively, from the data in reference 15. 2,4,6-Trinitroaniline, which is a very strong conductor in liquid ammonia, is quantitatively extracted from ether solution into ammonia over the 0 to 4 M range of ammonium nitrate concentration. The extensive salting-out of 2,4-dinitroaniline from the ammonia phase is in distinct contrast to the extraction of the mono substituted compounds but is observed for methyl substituted nitroanilines as is shown in Figure 5. Neither 4-methyl-2-nitroaniline nor 2-methyl-5-nitroaniline show extraction evidence of acid dissociation. It is also indicated in Figure 5 that for 4-methyl-2-nitroaniline the solute preference for the ether phase increases with dilution. In the case of 2,4-dinitroaniline, the solute preference for the polar (liquid ammonia) phase increases with dilution as would be expected for a weakly dissociated acid. Cuthrell, Fohn, and Lagowski ( 1 6 ) determined the acid dissociation (and ionization) constants of the N-substituted nitroanilines, 2- and 4-nitroacetanilide, by spectrophotometric means. Values of 4.8 X and 8.3 X respectively, were determined for the product klk?of Equation 1 . Spectral data indicate that these two compounds ionize spontaneously in liquid ammonia but are weaker acids than 2,4-dinitroaniline (14). The increase in Kp over the range 0 to 0.1M ammonium (15) E. C. Franklin and C. A . Kraus, Amer. Cliem. J . , 23, 277 (1900). (16) R . E. Cuthrell, E. C. Fohn, and J. J. Lagowski, Dzorg. Cltem., 5 , 111 (1966).
”
0
3
2
I
Concentration
Ammonium
Nitrate &j
Figure 6. Dependence of partition ratio on concentration of ammonium nitrate 0 1.38 X 10-3M 4-nitroacetanilide A 1.37 X 10-3M 2-nitroacetanilide
nitrate observed for 2-nitroacetanilide (Figure 6) is not large enough to permit a significant calculation of an acid dissociation constant. Any evidence of the dissociation of 4-nitroacetanilide would be obliterated by the extensive salting out effect which was not observed for either nitroaniline isomer (Figure 3). The reason for the pronounced preference of 2-nitroacetanilide for the liquid ammonia phase is not apparent. This compound has been unambiguously reported as strongly intramolecular hydrogen bonded in organic solvents (17-19), but the extraction data do not support this for this system, assuming the closed structure to be more soluble in the ether phase. Both 2-nitroacetanilide and 2-nitroaniline are more soluble in pure ether than the para isomers. It is more likely that 2-nitroacetanilide forms a specific solvate or complex more or less independent of the ammonia or ammonium ion activity. The relatively large difference in the partition ratio for these two compounds in the absence of ammonium salt is in marked contrast to the ether-water system for which a Kp value of 15 was obtained for both isomers at the concentrations indicated for Figure 6. Similarly, ether-water K p values have been reported to be 150 and 110 for 2- and 4nitroaniline, respectively (9). CONCLUSION
It has been demonstrated that liquid-liquid extraction between diethyl ether and liquid ammonia may be applied to organic substances which are neutral or weak acids in ammonia. It is reasonable to assume on the basis of the compounds studied that in many cases of both types, greater discrimination would be observed in the ammonia-ether system among compounds for which near quantitative extraction into the ether phase is observed from the aqueous system. (17) A. E. Lutskii and B. P. Kondratenko. Z h r . Obshchei Khim., 29, 2073 (1959); Cliem. Abstr., 54,818411. (18) E. J. Forbes, K. J. Morgan, and J. Newton, J. Cliem. SOC., 1963, 835. (19) M. Gomel and H. Lumbroso, Bull. SOC.Chim.Fr.. 1962, 1203. ANALYTICAL CHEMISTRY, VOL. 43, NO. 4, APRIL 1971
533
In any specific case an alternative separation method would probably be available without recourse to the low temperature techniques required for this procedure. However, if the sample were already in liquid ammonia solution, as for example the reaction products of the very common reduction employing elemental sodium in liquid ammonia, the possibility would exist of extracting a specific product directly. This would, moreover, not necessarily require conversion of the excess sodium to a sodium salt because alkyl ethers are inert to this reduction mixture (20). It would be expected (20) C. A. Kraus and G. F. White, J . Amer. Chem. Soc., 45, 768 (1923).
that the distribution phenomena reported here would apply to liquid partition in general and in particular to paper chromatography or thin-layer chromatography on cellulose. Liquid ammonia is strongly absorbed by cellulose and can be expected to perform adequately as a stationary phase with ether as the mobile phase. This aspect of the ammonia-ether system is presently being investigated.
RECEIVED for review October 20, 1970. Accepted December 30, 1970. This work was supported in part by Grant GP 7501 of the National Science Foundation.
Tandem Thermogravimetric Analyzer-Gas Chromatograph-High Resolution Mass Spectrometer System Teh-Liang Chang and Thomas E. Mead Central Research Laboratories, American Cyanamid Company, Stamford, Conn. 06904
A thermogravimetric analyzer-gas chromatographhigh resolution mass spectrometer system has been coupled in series. An unique stainless steel system consisting of two valves and two traps joins a thermogravimetric analyzer (TGA) to a gas chromatograph (GC). This arrangement has the capability of making multiple trapping at several desired portions of a TGA thermogram. The TGA effluent gas is collected in a cold trap and then directly injected into the GC for separation. The separated components are individually introduced into the high resolution mass spectrometer (HRMS) for unequivocal identification. To demonstrate the efficiency of the system, a polystyrene foam and an ethylene-vinyl acetate copolymer were studied. Results indicated excellent separation between the contents of the two traps. GC resolution and peak shape were comparable to conventional injection systems. Sixteen effluent compounds from the thermal degradation of polystyrene foam were identified by the HRMS. The elemental compositions of the molecular ions of 31 components were obtained from the pyrolysis of ethylene-vinyl acetate copolymer. IN RECENT YEARS,with the introduction of modern thermal analytical instruments, such as the thermogravimetric analyzer (TGA), differential thermal analyzer (DTA), differential scanning calorimeter (DSC), etc., thermal analysis has become a major instrumentation tool in many laboratories ( I , 2). Among these instruments, the T G A determines the weight loss of samp!e as a function of temperature. This is especially useful for the study of the thermal degradation of polymers (3). By itself, of course, TGA does not have the capability of identifying the thermal degradation products. Coupling of a TGA to a mass spectrometer has been reported ( 4 , 5). The effluent gas from a TGA is directly introduced into a mass spectrometer for analysis. However, the TGA effluent gas, in many cases, is a highly complex mix(1) C. B. Murphy, ANAL.CHEM., 42, 268R (1970). (2) P. F. Levy, Amer. Lab., 1970 (l), p 46. (3) J. Mitchell, Jr., and J. Chiu, ANAL.CHEM., 41, 248R (1969). (4) F. Zitomer, ibid., 40, 1091 (1968). (5) D. E. Wilson and F. M. Hamaker, “Thermal Analysis, Vol. 1,” R. F. Schwenker, Jr.. and P. D. Gam, Ed.. Academic Press, New York, N. Y., 1969, p 517. 534
ANALYTICAL CHEMISTRY, VOL. 43, NO. 4, APRIL 1971
ture. Identification of less abundant components is rather difficult and often overlooked. In addition, quantitative estimation of the constituents is very difficult. Another approach is to collect the pyrolysis products and then analyze this collected material by a gas chromatograph (GC). This method has been applied to the analysis of thermal degradation products of polyvinyl chloride (6), vinyl plastics (7), polysulfone (8), etc. A technique coupling TGA and GC has been described (9) and applied to polymer characterization (10). This techn’que eliminates additional handling and transferring of sample from a TGA effluent collector to a GC sample injector. We have successfully coupled three instruments, namely, TGA, GC, and MS (mass spectrometer), in a tandem fashion which has not been previously reported. This series connection of three instruments permits thermal degradation of nonvolatile materials by the TGA, separation of the effluent components by the GC, and their unequivocal identification by the HRMS (high resolution mass spectrometer). This series coupling of three analytical instruments results in great convenience, distinct time saving, and more accurate analysis. Although effluent components from a thermal degradation in a TGA could be trapped and later injected into a GC for their analysis, the possibility exists in losing information with this additional handling step. Of course, the same problem would exist in transferring the effluents from the GC to the MS. A unique stainless steel valving unit with two cold traps is employed to interface the TGA and GC units. This interface unit is capable of collecting the TGA effluent gas in one trap while analyzing the collected material of a second trap by GC and HRMS. Multiple trapping is also possible at several desired portions of a TGA thermogram. ~~
(6) E. A. Boettner and B. Weiss, Amer. Irid. H y g . Assoc. J . , 28, 535 (1967). (7) E. A. Boettner, G. Ball, and B. Weiss, J . Appl. Polym. Sci., 13, 377 (1969). (8) W. F. Hale, A. G. Farnham, R. N. Johnson, and R. A. Clendinning, J . Polym. Sci., Part A-I, 5, 2399 (1967). (9) J. Chiu, Thermoclzim. Acm, 1, 231 (1970). (10) J. Chiu, ANAL.CHEM., 40, 1516 (1968).
