Table II. Analysis of Diverse Samples at 35.0 "C Sample
Heat evolved, cal SO4 found,
Series (i)
(ii)
(iii)
Material
SO4, mmole
Volume, ml
A
B
A - B
mmole
Error, %
Na~S04, aqueous
0.518 0.518 0.566
0.250 5.000 0.500
0.90 3.40 1.66
-2.52 -0.08 -2.05
3.42 3.48 3.71
0.520 0.529 0.564
0.3 2.1 -0.4
MsSO~, aqueous
0.817 0.511 0.544
0.400 0.250 0.500
2.35 1.40 1.97
-3.02 -1.95 - 1.58
5.37 3.35 3.55
0.816 0.509 0.540
-0.2 -0.4
0.511 0.564 0.716 0.409
0.250 0.500 0.350 0.200
1.30 1.57 1.41 0.77
-2.07 -2.17 -3.32 -1.95
3.37 3.74 4.73 2.72
0.512 0.568 0.719 0.413
0.2 0.8 0.4 1.1
0.359 0.257 0.513
0.350 0.250 0.500
1.31 0.93 1.83
-1.03 -0.75 -1.57
2.34 1.68 3.40
0.356 0.255 0.517
-0.9 -0.6 0.7
Na~S04, 10%
ethanolic (iv)
Na2S04
+ NaZCO3,
aqueous
The effect of ionic strength, or of coexisting electrolytes, is seen in a comparative survey of Table I and Series (i) in Table 11. In the second experiment in Series (i) where 20 times as large a volume of solution as in the first experiment was used for the same amount of sulfate, the heats shown in A and B were remarkably different from each other, but the difference, A - B, gave a consistent value. The unusually large error, 2.1%, of this experiment was a result of the decrease in concentration of the reaction solution which was the direct consequence of adding a large volume of dilute solution. The decrease was considered so serious that the relation expressed by Equation 2 no longer held true. Effect of different cations on the heat was exemplified in Series (i) for sodium sulfate and in Series (ii) for magnesium sulfate. The differences in A and in B were cancelled out by subtraction.
-0.8
~-
Series (iii) demonstrated the effect of a different solvent system other than simply aqueous. In spite of using a 10% ethanolic solution, the procedure gave appropriate values of sulfate. Series (iv) was a n example in which the precipitation reaction was accompanied by another drastic reaction. In this case, coexisting sodium carbonate gave rise to an acid-base and a gas-evolving reaction in the presence of nitric acid. In the discussion above, the dual-step procedure was examined with the reaction of barium sulfate formation as a n example. For other reactions, however, the composition of the reactant solution, hence the blank solution, should be modified so that the particular reaction takes place specifically, if possible, depending on the nature of the species of concern. Received for review April 9, 1973. Accepted July 26, 1973.
Separation of Monosubstituted Phenol Isomers Using Liquid Crystals Lawrence E. Cook' and Ronald C. Spangelo2 Department of Chemistry, University of North Dakota, Grand Forks. N . D 58207
The separation of several isomeric monosubstituted phenols by gas chromatographic means using liquid crystal stationary phases has been studied. While the separation of the isomeric phenols was partially successful, complete separation of the isomers was obtained when the phenols were converted to substituted phenyl n-alkyl ethers. Separation factors for the meta- and para-substituted ether isomers ranged from 1-06 to 1.25. The effect of increasing the length of the n-alkyl chain was studied and found to increase the separation factor as the chain length increased. The polarity and the molecular shape of the solutes were major factors in determining the separation of the isomers. Converting the phenol to an ether and running the GC analysis required only l l h hours. IPresent address. Michigan Department of Public Health, Division of Crime Detection, Lansing, Mich.. 48912. 2Present address, Upjohn Co.. Kalamazoo. Mich.. 49001. 122
Environmental awareness has become increasingly important in recent years. One class of compounds recognized as a major source of pollutants is the phenols. Phenols are introduced into the environment through the discharge of industrial wastes and the decomposition of various pesticides and herbicides. A fast, simple, and effective method of analysis for phenolic compounds would be useful. Identification of isomeric phenols is sometimes difficult by gas chromatographic means using conventional stationary phases, because meta- and para-substituted isomeric phenols often have similar boiling points and polarities. A stationary phase which separates on a basis other than boiling point or polarity may possibly separate such isomers. Kelker ( I , 2) and Dewar (3, 4 ) have shown that disubstituted benzene isomers can be efficiently separated by gas chromatography on liquid crystal stationary phases.
A N A L Y T I C A L CHEMISTRY, VOL. 46, NO. 1, J A N U A R Y 1974
The selectivity shown by liquid crystal phases is a consequence of the high degree of molecular order found in the liquid crystal mesophase. Originally, the retention of solutes on this type of stationary phase was thought to be almost solely dependent upon the molecular shape of the solute. Recently, however, Chow and Martire ( 5 ) have demonstrated that the selectivity observed is actually a result of several factors of which molecular shape is only one of several major contributors. Several of the other important factors are the polarity, the polarizability, and the flexibility of the solute molecule. Approaches to the analysis of phenols by gas chromatography have been made by several groups of workers. Kusy (6) and Zarazir and coworkers (7) have used conventional GLC methods with only partial success. Usually separation of phenols on long, conventionally packed GLC columns results in impractically long analysis times or incomplete resolution. Paterson (8) has shown the separation of ortho-, meta-,and para-cresols, but was unable to completely resolve meta and para isomers. Guiochon and Zulaica (9) appeared to separate the methyl phenols, but retention times became very long. Hrivnak and coworkers (10-12) have achieved a base-line separation of isomeric phenols using high efficiency open tubular columns. Another problem in the separation of phenols is peak asymmetry due to nonlinear adsorption on the solid support on the column wall. Therefore, derivatizing the phenols, a technique widely used by gas chromatographers to improve the analysis of solutes, may correct the peak asymmetry problem and improve separation factors. Several types of derivatives of phenols have been prepared to eliminate tailing, and these vary from trimethylsilyl ethers (13, 14) and alkoxy derivatives (15, 16), to acetate and trifluoroacetate esters (17, 18). A number of these derivatives give good base-line separation, but there are always one or two pairs of isomers that are incompletely separated. We felt that an alternative procedure for the gas chromatographic analysis of phenols was needed. Consequently, the method described herein utilizes both a derivatization technique, to improve peak symmetry; and a liquid crystal stationary phase to increase resolution. The method of preparation of derivatives is simple, fast, and uses only reagents commonly found in the laboratory. The method is amenable to flame ionization and thermal conductivity detectors.
EXPERIMENTAL
Kelker and H . Winterscheidt, Fresenius' Z. Anal. Chem., 220, 1
(1966). H. Kelker and A . Verhelst, J. Chromatogr. Sci.. 7 ( 2 ) . 79 (1969). M. J . S. Dewar and J. P. Schroeder, J. Amer. Chem. Soc.. 86, 5235
(1964) M . J . S. Dewar and J . P. Schroeder, J , Org. Chem.. 30, 3485 ( 1 965). L. C . Chow
Column I
Loading,
YO 5.7
p-[(p-Methoxybenzy1idene)-
5.9
p-(p-Ethoxypheny1azo)phenyl
Stationary phase
Temperature range of the mesoghase, C 80-1 08
amino]phenyl acetate II
65-1 10
undecylenate 7.4
p-(p-Eth0xyphenylazo)phenyl
60-1 19
IVa
2.7
heptanoate p-[(p-Methoxybenzy1idene)aminolphenyl benzoate
118-168
IVb
7.9
p-[(p-Methoxybenzy1idene)-
118-168
Ill
amino]phenyl benzoate V VI
5.0 5.1
SE-30
Carbowax 20M
Ail columns were 6 ft by '/a-in. i.d. copper tubing packed with 60/100 mesh, A.W., DMCS treated Chromosorb W previously coated with the apa
Dropriate stationary Dhase.
thrup Speedomax X/L 680 recorder. The GLC columns used were 6 ft by yd-in. i.d. copper tubing packed with 6O/lOO mesh A . W., DMCS treated Chromosorb W previously coated with the liquid crystal phase to be studied. Table I lists the various columns studied. The liquid crystal phases are all nematic and exhibit thermal stability in the temperature range a t which they were studied. Helium was used as the carrier gas in all studies. Columns I, 11, IVa, VI, and VI1 all had a corrected flow rate of approximately 100 ml of carrier gas per minute. Columns I11 and IVb had a corrected flow rate (19)of approximately 75 ml per minute. Reagents. The alkyl bromides and phen6ls were obtained from Matheson, Coleman and Bell of Norwood, Ohio. The alcohols were obtained from various sources. The liquid crystals were purchased from Eastman Organic Chemicals of Rochester, N.Y. All reagents were used as received. The sodium alkoxides were prepared by the reaction of sodium metal with the appropriate alcohol. Derivatives. Phenol and the substituted phenols were derivatized by reaction with the appropriate alkyl bromide in the presence of a sodium alkoxide. The reaction mixture was placed in a sealed tube and heated to 100 "C for $ hour. The samples were then cooled and the salts formed during the reaction were removed. No further attempt t o isolate the ether from the reaction mixture was made. The chromatogram of the prepared n-alkyl ether reaction mixtures showed no trace of phenolic contamination. The chromatograms of the 2-propyl and 2-butyl ether reaction mixtures indicated some phenolic contamination.
RESULTS AND DISCUSSIONS
Instrumentation. The study was conducted on a Beckman GC-2 gas chromatograph equipped with a thermal conductivity detector. The chromatograms were recorded on a Leeds and NorH.
Table I. Stationary Phases Studieda
and D. E. Martire, J. Phys. Chem.. 75, 2005 (1971). Vladimir K u s y , J . Chromatogr.. 57, 132 (1971) D. Zarazir, P. Chovin. and G . Guiochon, Chromatographia, 3 (4), 180 (1970). A . R. Paterson, in "Gas Chromatography," lnstrum. SOC.Amer. Symposium, June 1959, H . J . Noebels, R. F. Wall, and N . Brenner, Ed., Academic Press, New York. N . Y . . 1961, p p 323-6. J . Zulaica and G . Guiochon, J. Polymer Sci., 4, 567 (1966). Jan Hrivhak, J. Chromatogr. Scl., 8, 602 (1970). Jan Hrivhak and Emanuel Beska, J. Chromatogr., 5 4 , 2 7 7 (1971). Jan Hrivhak and Jiri Macak, Anal. Chem.. 43, 1039 (1971). R . W. Freedman and G . 0. Charlier. Anal. Chem.. 36, 1880 (1964). R. W. Freedman and P. P. Croitoru,ibid.. p , 1389. H. G . Henkei, J. Chromatogr., 20, 596 (1965) G . A. L. Smith and D. A . King, Chem. lnd. (Londonj, 1964, 540. E. R. Adland and J . W . Roberts, J . lnst. Petroi., 51, 376 (1965). A. T. Shulgin, Ana/ Chem.. 36, 920 (1964).
Table I1 lists the separation factors ( a ) obtained for the substituted phenols and several of their alkyl ether derivatives on SE-30. The values of apImindicate that the meta and para isomers of the methyl phenyl and methoxyphenyl systems are difficult to separate on the basis of boiling point. For the chlorophenyl alkyl ethers, a p I m is large enough to suggest separation is possible. However, in this system cym/, suggests difficulty in separating the ortho and meta isomers. The ortho-substituted phenols are capable of intramolecular hydrogen bonding, which reduces their boiling points far below those of their meta and para isomers. Derivatizing the phenol eliminates intramolecular hydrogen bonding and the boiling point of the ortho isomer becomes comparable to those of the meta and para isomers. For the methyl phenyl and the methoxyphenyl ethers, the ortho isomer's boiling point is still slightly lower than those of the meta and para isomers. In the case of the chlorophenyl ethers, the para isomer's boiling point is (19) S. Dal Nogare and R. S. Juvet in "Gas-Liquid Chromatography, Theory and Practice," Interscience, New York, N.Y.. 1962, p 77.
A N A L Y T I C A L C H E M I S T R Y , VOL. 46, NO. 1, J A N U A R Y 1974
123
Table II. Separation Factors for the Isomeric Phenols and Substituted Phenyl Ethers Obtained on 5 % SE-30 Ethers Separation factor
Phenol
Ethyl
n-Propyl
rl-Butyl
m-/o-Methyl phenol p-lo-Methyl phenol p-/m-Methyl phenol m-/o-Methoxyphenol p-lo-Methoxyphenol p-/m-Methoxyphenol m-lo-Chlorophenol p-/p-Chlorophenol p-/m-Chlorophenol
1.14 1.14 1.oo 2.12 2.02
1.10 1.11 1 .oo 1.31 1.28 0.97 1.oo 1.06 1.06
1.09 1.12 1.02 1.32 1.30 0.98 1.01 1.07 1.06
1.10 1.12 1.02 1.37 1.35 0.99 1.03 1.09 1.06
0.95 3.04 3.08 1.01
n-Pentyl 1.11 1.15 1.04 1.39 1.38 1 .oo 1.04 1.11 1.06
2-Propyl 1.09 1.12 1.02 1.36 1.36 1.oo 1.05 1.11 1.06
2-Butyl 1.08 1.12 1.04 1.33 1.35 1.01 1.02 1.10 1.08
Table 111. Separation Factors for the Isomeric Phenols and Substituted Phenyl Ethers Obtained on 5.1 % Carbowax 20M Ethers Separation factor
Phenol
Ethyl
n-Propyl
n-Butyl
m - l o - M e t h y l phenol p-lo-Methyl phenol p-/m-Methyl phenol m-/o-Methoxyphenol p-lo-Methoxyphenol p-/m-Methoxyphenol m-lo-Chlorophenol p-lo-Chlorophenol p-/m-Chlorophenol
1.40 1.36 0.97 8.44 7.13 0.84 8.71 8.71 1.oo
1.25 1.24 0.99 1.35 1.29 0.95 0.86 0.97 1.13
1.22 1.22 0.99 1.38 1.33 0.96 0.86 0.99 1.14
1.23 1.22 0.99 1.42 1.38 0.97 0.89 1.01 1.14
-
raised, compared with the meta isomer, but the meta and ortho isomers have similar boiling points. Table I11 shows the separation factors obtained by chromatographing the compounds on a Carbowax 20M column. Again, a p I m suggests difficulty in separating the meta and para isomers, with the exception of the methoxyphenols and the chlorophenyl ethers. However, a p l 0for the chlorophenyl ethers shows the ortho and para isomers to be difficult to separate. It would appear that the opposing effects of the greater polarity of the ortho isomer and the higher boiling point of the para isomer result in both isomers having similar retention times. Liquid crystals show selectivity on the basis of molecular shape, with preferential retention shown for rigid, rodlike molecules. A derivative which accentuates the structural differences between isomers should also increase the preference of the liquid crystal phase with regard to the most linear molecule. We chose the n-alkyl ethers for several reasons. Their preparation by the Williamson ether synthesis is relatively simple. The alkyl chain can be easily forced into a linear conformation upon dissolution. And, finally, the effect of increasing chain length on the separation of the compounds can be easily studied. Table IV shows that the isomeric meta- and para-substituted phenols are not easily separated on liquid crystals. The separation factors obtained on column I11 show the methyl phenols to be partially separated. The chlorophenols and methoxyphenols are also partially separated by columns IVa and IVb. In all cases, the phenols exhibit asymmetric peak shapes, either leading or tailing. The separation factors for the ethers show several trends. The separations obtained for the ethers are much better than those of the corresponding phenols. Also, the separation factor generally increases from approximately 1.1 to as much as 1.28 as the alkyl chain length increases. Table V shows that the 2-propyl and 2-butyl ethers have net retention volumes roughly equal to those of the ethyl and n-propyl ethers. Finally, the para isomer in all the sets of ether isomers studied is the last component eluted from a liquid crystal stationary phase. 124
n-Pentyl 1.25 1.26 1.01 1.45 1.40 0.97 0.91 1.04 1.14
2-Propyl 1.27 1.28 1.01 1.52 1.48 0.98 0.92 1.08 1.16
2-Butyl 1.22 1.23 1.01 1.47 1.48 1 .oo 0.89 1.06 1.19
Since liquid crystals show a selectivity partially based on molecular structure, the assumption that meta- and para-substituted isomeric phenols could be separated would seem logical. The -OH substituent of a phenol is not a large group and does not emphasize structural differences greatly. By converting the phenol to an ether, an increase in separation factor values would be expected with increasing alkyl chain length. The longer the alkyl chain becomes, the greater the resemblance of the solute molecule to that of the liquid crystal. Also, lengthening the alkyl chain allows a greater number of points of interaction between solute and solvent molecules. Because a linear solute molecule allows the greatest interaction with the solvent, interactions will be maximized if the alkyl chain is restricted to one of the linear conformations as the molecule dissolves. Such a restriction means a loss of entropy in the solute molecule, which requires energy. The energy necessary to force a n-alkyl chain into a given conformation is less than the solvation energy obtained upon dissolution (5, 20). As the alkyl chain length increases, more energy is required to force it into the preferred conformation and finally the solvation energy gain is less than the energy required to force the solute molecule into a linear conformation. The selectivity of the liquid crystal stationary phase will decrease a t this point. Therefore, an optimum chain length will be observed in a series of derivatives of increasing carbon chain length. Increasing the alkyl chain length also increases the boiling point of the solute and its retention time. Reaching the optimum chain length may result in an undesirably high boiling point. Also, nonlinear adsorption on the solid support or the column wall causes extreme peak broadening so that even if peak maxima are separated, the resolution obtained may be poor. Converting the hydroxyl group to an ether reduces hydrogen bonding and retention time, improves peak symmetry, and emphasizes structural differences in the isomers. These factors combine to effect a
(20) D. G. Willey and G.H. Brown,J. Phys. Chern., 76, 99 (1972)
ANALYTICAL CHEMISTRY, VOL. 46, NO. 1, JANUARY 1974
Table IV. Separation Factors Obtained for the Isomeric Phenols and Their Ether Derivatives on Liquid Crystal Stationary Phases Ethers Te,m p ,
Column
C
Separation factor
Phenol
Ethyl
n-Propyl
n-Butyl
I I II
99 102 99 106 95 100 106 122 113 122 144 122 23 35 54 68 22 22 22 22 35 35 154 154 168 168
p-/m-Methyl phenol p-/m-Methyl phenol p-/m-Methyl phenol p-/m-Methyl phenol p-/m-Methyl phenol p-/m-Methyl phenol p-/m-Methyl phenol p-/m-Methyl phenol p-/m-Methyl phenol p-/m-Methyl phenol p-/m-Methyl phenol p-/m-Methyl phenol p-/m-Methoxyphenol p-/m-Methoxyphenol p-/m-Methoxyphenol p-/m-Methoxyphenol p-/m-Chlorophenol p-lo-Chlorophenol p-/m-Chlorophenol p-lo-Chlorophenol p-/m-Chlorophenol p-lo-Chlorophenol p-)m-Chlorophenol p-lo-Chlorophenol p-/m-Chlorophenol p-lo-Chlorophenol
1.08
1.10 1.06 1.14 1.07 1.16 1.13 1.12 1.14 1.19 1.16 1.13 1.20 1.19 1.18 1.11 1.06 1.33 1.16 1.34 1.12 1.26 1.09 1.23 1.07 1.18 1.03
1.11 1.08 1.14 1.08
1.10 1.08
1.10 1.10
... ...
...
...
...
1.16 1.13 1.15 1.21 1.19 1.15 1.20 1.21 1.19 1.12 1.08 1.32 1.14 1.35 1.16 1.31 1.14 1.24 1.08 1.20 1.06
1.17 1.14 1.18 1.23 1.20 1.13 1.22 1.26 1.21 1.14 1.10 1.34 1.17 1.38 1.21 1.34 1.18 1.26 1.12 1.20 1.08
...
I1 Ill Ill Ill
IVa IVb IVb IVb
IVa IVb IVb IVb
IVb
IVa IVa IVb
IVb IVb IVb IVb IVb IVb IVb
...
1 .oo 1.Od 1.06 1.03 1.04 1.01 1.03 1.02 1.02 0.97 0.93 0.90 0.93 0.93 1.08 9.61 1.08 9.33 1.06 8.18 1.os 7.03 1.04 6.19
n-Phentyl
... ...
...
2-Propyl
2-Butyl
1.10 1.08 1.14 .08
1.12 1.10
.. .15 .14
...
..
1.24 1.22 1.17 1.19 .28 .21 .14 .10 .34 .20 .38 .23 .34 .20 1.26 1.14 1.21 1.10
.20 .17 .15
.. .21 .19 .13 .09
.. .. .36
.. .31 .24 1.26 1.15 1.21 1.15
... .. .. .17 .14 .16 .20 .18 .16 .22 .24 .19 .15 .12 .32 .16 .35 .19 .31 .16 1.25 1.11 1.22 1.09
Table V. Net Retention Volumesa of the Methyl Phenols and the Methyl Phenyl Ethers Obtained on Column IVb at 122 "C Ethers Solute
Phenol
Ethyl
n-Propyl
n-Bury1
n-Pentyl
2-Propyl
2-Butyl
2-Methyl phenol 3-Methyl phenol 4-Methyl phenol
305.8 434.0 445.0
83.7 104.3 121.6
127.6 153.1 181.8
21 1.a 255.8 308.4
357.2 440.2 538.7
72.2 89.6 105.2
111.9 134.6 159.1
_ a_All net retention volumes are expressed in terms of milliliters of carrier gas. better separation. In fact, it was possible to completely separate at least one set of ether isomers on each column studied. The retention time of the pentyl ethers was long enough and the separation obtained was good enough to make increasing the size of the alkyl chain further appear unnecessary. An arbitrary maximum of 30-min retention for the most strongly retained isomer was chosed and met by the pentyl ethers. Longer alkyl chains may also meet this limit but, since the pentyl ethers were capable of complete separation, larger ethers were not investigated. Table V shows that the 2-propyl and 2-butyl ethers are retained approximately as long as the ethyl and rz-propyl ethers, respectively. This phenomenon indicates that the longer substitutent in a branched ether determines the relative retention time of the solute. Since the smaller substituent, in this case the methyl group, on the alkyl chain decreases the linearity of the molecule and cannot be compensated for by conformational restrictions, the separation factors obtained for these ethers would be expected to be less than for the normal alkyl ethers. Instead, the separations obtained for the branched ethers are comparable or slightly better than for the normal alkyl ethers. A possible explanation may be the difference in polarity of the two types of ethers. Since the branched ether is more polar, and liquid crystals are polar phases, the greater po-
larity of the branched ethers compensates for the loss in structural preferability. Except in the case of the methoxyphenols, the para isomer is the last component eluted in every set of isomers studied. The para-substituted isomer is the most linear, therefore, the isomer which physically resembles the solvent molecules most. As such, the para isomer fits into the ordered structure of the liquid crystal mesophase with greater ease than the ortho or the meta isomer. In addition, the para isomer can approach the solvent more closely than the other isomers because no major substituent lying off the longitudinal axis of the solute molecule is present to hinder the approach of solute and solvent molecules. This property increases the strength of solute-solvent interactions and results in longer retention times for the para isomer than for an isomer with weaker solutesolvent interaction forces. The anomalous behavior of the methoxyphenols may be due to a combination of lower volatility and greater polarity of meta-methoxyphenol as compared to para-methoxyphenol. The results in Table IV indicate that when using liquid crystal phases, the greatest selectivity is obtained at temperatures near the solid-mesophase transition temperature (21) Hans Kelker, in "Advances in Chromatography," Vol. 6, J. C. Giddings and R. A. Keller. Ed., Marcel Dekker, New York. N.Y., 1968, p 247.
ANALYTICAL C H E M I S T R Y , VOL. 46, NO. 1, J A N U A R Y 1974
125
Ethyl
I
3
L Y
r-BJtyl
I
3
4
5
6
7
0
5
7
8
9
'0
11
'2
Time
m nutes
'
1
2
3
4
Figure 1. Separations of the n-alkyl ether derivatives of the methyl phenols on column IVb at 122 "C
(21). For example, the separation factor for meta- and para-methoxyphenyl alkyl ethers decreases as the temperature increases when using column IVb. The mesomorphic transition temperature, from Table I, for column IVb is 118°C.
1
NOTES
In the case of these phenols and their ether derivatives, all of which have high vapor pressures, even far below their boiling points, the temperature a t which the column is used should be 75 to 100 "C below the boiling point of the most strongly retained isomer. Figure 1 illustrates the separation obtained for several of the n-alkyl derivatives of the isomeric methyl phenols on column IVb. The figure shows the degree of separation obtained :or the various sets of isomers, the increase in retention time with increasing alkyl chain length, and the symmetrical peak shapes obtained for the ether derivatives. I n summary, the identification of various phenols is possible on liquid crystal phases. The separation of isomers is best accomplished by modifying the structure of the solutes so as to emphasize any differences in the shapes of the molecules and by eliminating groups causing extreme molecular interactions, such as inter- and intramolecular hydrogen bonding. Received for review April 18, 1973. Accepted August 1, 1973. This work was supported in part by a Faculty Research Grant administered by the University of North Dakota.
I
Calorimetric Investigation of Adsorption of Aromatic Compounds by Linde Molecular Sieve 13X Delbert J. Eatough, Sedigheh Salim, Reed M. Izatt, James J. Christensen, and Lee D. Hansen
Departments of Chemistry and Chemical Engineering, Brigham Young University, Provo, Utah 84602
Titration calorimetric data have been used to calculate the thermodynamic quantities log K , AH", and AS" for many different systems, e.g., proton ionization and metal ligand complex formation (1-4). Titration calorimetry can be used to determine log K values under conditions where other methods are sometimes difficult to apply with accuracy, e.g., the study of reactions in nonaqueous, highly acidic, or highly basic solutions, or the study of the formation of weakly associated complexes in which the concentrations of participants are not easily measured or in which the ligand concentrations have no pH dependence. In the present study, an additional application of titration calorimetry is presented. Log K, AH", AS", and adsorption capacity values have been determined for the adsorption of aromatic compounds by Linde Molecular Sieve 13X (LMS 13X). The calorimetric log K value and the capacity for the adsorption of aniline by LMS 13X were (1) J. J. Christensen, J. Ruckman, D. J. Eatough, and R. M. izatt, Thermochimica Acta, 3, 203 (1972). (2) D. J. Eatough, J. J. Christensen, and R . M. Izatt, Thermochimica Acta, 3, 219 (1972). (3) D. J. Eatough, R. M. Izatt, and J. J. Christensen, Thermochimica Acta, 3, 233 (1972). (4) D. P. Wrathall and W. Gardner, "Temperature, its Measurement and Control in Science and Industry," H. Plumb, Ed., Vol. 4 , part 3, 1973, p 2223. 126
checked by an independent method of analysis based on the potentiometric titration of aniline in the supernatant solution.
EXPERIMENTAL Reagents and Solutions. Solutions of aniline (vacuum distilled); toluene (Wasatch Chemical, analytical); nitrobenzene (Fisher, certified); and LMS 13X (Linde Division of Union Carbide, powder) in n-hexane (Baker, "Analyzed," dried over molecular sieves) were prepared by dissolving or suspending weighed amounts of the compounds in n-hexane. A solution of perchloric acid in glacial acetic acid was prepared by dissolving HClOl (SO7270, ACS grade) in glacial acetic acid (ACS grade). The resulting solution was standardized against potassium hydrogen phthalate (National Bureau of Standards. acidimetric standard.) The molecular sieve was used directly as received from Linde. All compounds were prepared and stored in a dry nitrogen atmosphere. Procedure. The isothermal titration calorimeter and operational procedures have been described (2, ,5, 6). In the calorimetric experiments. 0.4.44 solutions of each aromatic compound in n-hexane were titrated into the reaction vessel which contained a suspension of 0.5 g LMS 13X in 50 ml of n-hexane. The temperature of all runs was 25.00 " C . Three or more duplicate runs were made for each system studied. Heats of dilution of reactants were (5) J. J. Christensen, H. D. Johnston, and R. M. Izatt. Rev. Sci. lnstrum., 39, 1356 (1968). (6) D. J. Eatough, J. W. Gardner. J. J. Christensen. and R. M . Izatt, unpublished data, 1973.
ANALYTICAL CHEMISTRY, VOL. 46, NO. 1, JANUARY 1974
