Table I. Results from Tests Using Synthetic Samples Content of Hg in quartzFound, ppm mercuric sulfide mixture, ppm 5 4.8 5.2 5.2 5.2 3.6 10
10 10 12 12 11.2
Content of Hg in sulfidemercuric sulfide mixture, ppm 10
10.4 9.6 10.4 11.6
sulfide with (a) pure, ground quartz of minus 200 mesh and (b) sulfide minerals free from mercury were tested using the method. Results obtained are given in Table I. Spike tests were also conducted. A composite sulfide sample was analyzed for its mercury content using the proposed method. To this sample, known amounts of mercury in the form of mercuric sulfide were added and mercury de-
Table 11. Results from Tests Using Synthetic Samples Found, ppm Unspiked sample Hg content ppm 12 (average of 5 determinations) 15.2 Sample 2 ppm Hg 14.4 14.4 15.2 17.2 Sample .f 5 ppm Hg 18 16 15.2
+
terminations made. Results obtained are shown in Table 11. These figures indicate that though the method is simple and requires only the absolute minimum of laboratory equipments, it gives reasonably good results and should be useful to laboratories having no sophisticated instruments. ACKNOWLEDGMENT
The authors thank the Director, Geological Survey Department, Ministry of Agriculture and Lands, Malaysia, for permission to publish this paper. RECEIVED for review November 19, 1970. Accepted February 15, 1971.
Gas Chromatographic Studies of Surface Complexes Formed by Aromatic Molecules with Lanthanum Chloride on Silica Gel and Graphon David F. Cadogan and Donald T. Sawyer Department of Chemistry, University of California, Riverside, CaIq. 92502 A CONTINUING INTEREST in the use of salt-modified adsorbents for gas-solid chromatography (1-4) has prompted the consideration of surface complexes as a means to obtain selective separations. Because previous work (5) has established that the stability constants of weak charge-transfer and hydrogen-bonded complexes can be evaluated from gas-liquid chromatographic measurements, a similar approach has been used for the study of gas-solid complexes. The present paper summarizes the results of such studies on salt-coated silica gel and Graphon for a group of substituted aromatic molecules at three different temperatures. Coating active solids with salts like LaCls, which contain partly filled d or f shells, also may have analytical advantages. Because such salts are strong Lewis acids, they should form complexes of varying strengths with electron donor molecules. This adds another dimension to the column parameters (1) D. T. Sawyer and D. J. Brookman, ANAL.CHEM.,40, 1847, (1968). (2) D. J. Brookman and D. T. Sawyer, ibid.,p 2013. (3) A. F. Isbell, Jr., and D. T. Sawyer, ibid., 41,1381 (1969). (4) D. F. Cadogan and D. T. Sawyer, ibid., 42, 190 (1970). (5) D. F. Cadogan and J. H. Purnell, J. Chern. Soc., Sec. A , (1968) 2133.
which may be varied to achieve more selective analytical separations. EXPERIMENTAL
For one set of columns, silica gel (100-200 mesh Davison 62) was used as the absorbent. It was coated with 10% by wt of NaC1, LaCl,, and LaC13-NaC1 mixtures containing 19.4, 35.5, and 81 % LaCl,, respectively. Another set of columns was prepared with Graphon (a graphitized carbon obtained from Godfrey L. Cabot, Inc., Boston, Mass.) as the adsorbent. In addition to an uncoated column, one coated with 10% by wt of NaCl and another with 10% by wt of LaCh were prepared. The salt mixtures, which were deposited on the adsorbent from water by means of rotary evaporation, were resieved, activated at 250 "C, and packed into stainless steel columns, 3 ft X 1/8-inch0.d. The latter were thoroughly cleaned with both polar and nonpolar solvents prior to packing. After packing, the columns were installed in a modified Barber-Colman Series 5000 gas chromatograph. A Leeds and Northrup Speedomax H recorder with chart speeds of 0.5- and 3-inch/min was used to record the chromatograms. The helium carrier gas was dried before entering the columns by passage through 5A Molecular Sieve and Drierite. Sample injection was by means of a 0-1 pl Hamilton syringe, and ANALYTICAL CHEMISTRY, VOL. 43, NO. 7, JUNE 1971
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10,o
Table I. Heats of Adsorption, -AHa, on Two Salt-Coated Silica Gel Columns at 200 "C, Kcal-mole-' Adsorbate 10% NaCl 10% LaClS Hexane 7.37 7.16 1-Hexene 8.34 9.18 Benzene 10.22 15.24 Fluorobenzene 9.34 10.87 Chlorobenzene 10.42 11.69 Bromobenzene 11.19 12.71 Iodobenzene 11.73 14.09
9,O
8 #O
T O
NE 6,O
/@
Table 11. Heats of Adsorption, -AHa, of Various Adsorbates on Salt-Coated and Uncoated Graphon at 200 "C, Kcal-mole-' Graphon/ Graphon/ Adsorbate Graphon 10% NaCl 10% LaC13 Pentane 8.94 10.86 10.60 1-Pentene 8.28 10.17 10.29 Benzene 10.51 12.93 14.54 Toluene 12.99 14.75 Fluorobenzene 11.03 13.6 15:94 Cyclohexane 9.39 10.92 11.30 Cyclohexene 10.25 12.32 12.67 1,4-Cyclohexadiene 10.65 12.94 13.88
N I
/@
E I
E
5,O
by the BET equation using low temperature nitrogen adsorption data (6). The salt-coated silica gel had a surface area of 460 m2/gand the Graphon a surface area of 125 m2/g.
0
23
-9 X
RESULTS
4 .O
9
d
3.0 (
BENZENE
d
,.
k,O
9
A
Y
FLUOROBENZENE
I
I #O
I
-+-e
- HEXENE
r( A
V
+A .
"7
V
The retention volumes for a series of aliphatic and aromatic hydrocarbons have been determined for the NaC1- and L a c k modified columns at 175, 200, and 225 "C. For a given column the heat of adsorption, -AHa, of an adsorbate, can be evaluated by plotting log V R us. 1/T, where V R is the corrected retention volume and T is the column temperature in OK. The slope of such a plot is -AHa/2.3 R, where R is the gas-constant. Table I summarizes the heats of adsorption for several adsorbates on silica gel columns coated with 10% NaCl and 10% LaC13, respectively. A similar set of data for uncoated and salt-coated Graphon columns is presented in Table 11. The experimental error for the heats of adsorption is approximately +0.5 Kcal-mole-l. In gas-solid chromatography the partition coefficient of an adsorbate, KR,is related to the corrected retention volume by the expression
HEXANE
.o,o
I
1
0
I
240
( L O CI,
CONC,)
1
4,O
x
I
I
6,O
KR = V R / A( m l r 2 ) I
I
8,O
i o 7 MOLES - r n - 2
Figure 1. Specific retention volumes for a series of compounds as a function of the surface concentration of Lac&on silica gel at 200 "C. NaCl used as the diluent; all columns coated with 10 by weight of salt (NaCI and/or LaC13)
detection by a flame ionization detector. The samples, which were the purest available materials, were injected as the vapor from above the liquid phase. On each column, retention volume measurements for a series of adsorbates were made at 175, 200, and 225 "C. Each adsorbate was injected at least 3 times, both singly and in mixtures. The system dead-volume (void space) was assumed to be equal to the methane retention volume at 275 "C. The surface areas of the adsorbents were determined 942
ANALYTICAL CHEMISTRY, VOL. 43, NO. 7, J U N E 1971
(1 1
where A is the surface area of the adsorbent in the column. Hence, a plot of ( V R / A ) us. the surface concentration of LaCl8, C, illustrates the effect of this salt, relative to NaC1, on the partition coefficient of a given adsorbate. Figure 1 includes such plots for a group of aliphatic and aromatic adsorbates on silica gel columns at 200 "C. The concentration of LaC13 on the adsorbent surface has been calculated by assuming that the LaC13-NaC1 mixtures yield a monolayer or less of coverage. The bottom curve of Figure 1 indicates that hexane is essentially unaffected by substituting LaCL for NaCl on the surface. In contrast, the aromatic adsorbates interact much more strongly with LaC13. This implies some type of specific interaction between the aromatic ring and the LaCL molecule. (6) F. M. Nelson and F. T. Eggertsen, ANAL. CHEM., 30, 1387 ( 1958).
~~
The limited slope for 1-hexene indicates only a small specific interaction for the olefinic bond. DISCUSSION AND CONCLUSIONS Consideration of the data in Table I and Figure 1 indicates that the interactions of aromatic molecules are significantly greater when LaC13 is present on the silica gel surface. The linear slopes of the curves of Figure 1 also imply that the specific interactions are directly proportional to the surface concentration of LiC13, a condition indicative of complex formation. Studies of complex formation in gas-liquid systems (5) have shown that the stability constants of the resulting complexes can be evaluated by the expression
where KR and K R O are the partition coefficients of a solvate with a complexing solution and an inert solvent, respectively, K Lis the stability constant of the complex and C is the concentration of ligand in the solution. This same expression is appropriate for the present gas-solid studies, but with K R and KRO representing the partition coefficients of an adsorbate on a mixed LaC13-NaC1 column and a NaCl column, respectively. For this system Kl represents the stability constant of the adsorbate-Lacla complex and C is the surface concentration of LaC13 in the mixed coating (mole m-2). When the data of Figure 1 are analyzed by Equation 2, the stability constants of the LaC13-surface complexes can be evaluated. These are summarized in Table 111. By determining the stability constants at three different temperatures, the enthalpies of complex formation, -AH,, can be evaluated from the slopes for plots of log Kl us. 1/T (van't Hoff plots). The results of such analyses also are summarized in Table 111. Consideration of the data in Table 111 indicates that electron-donating substituents (alkyl groups) (3) enhance the interaction of the aromatic ring. The magnitude of the heats of complex formation, -AH,, also are consistent with the formation of metal-aromatic complexes. In contrast, there appears to be no specific interaction between hexane and LaC13and only a slightly enhanced interaction with 1-hexene. Thus, the heat of adsorption for benzene on Lacla-coated silica gel is increased 5 Kcal over its value on NaC1-coated silica gel while the heat of adsorption for hexane is less on LaCI3 than on NaCl (see Table I). Reference to the Graphon data on Table I1 indicates that addition of a salt-coating enhances the sorbate-sorbent heats of adscrption of all of the molecules. However, the enhancement of the benzene interaction by LaC13 relative to NaCl is much less on Graphon than on silica gel. Treatment of the experimental retention volume data for the group of test compounds by the analysis techniques used in a previous study ( I ) permits the functional group contributions to the free energy of adsorption, A(-AG),, on the Graphon and silica gel columns to be determined. Table IV summarizes the results of such an analysis and indicates that specific interactions due to pi-electron systems are nonexistent on Graphon. However, specific aromatic inter-
~
~
Table 111. Stability Constants and Heats of Formation of Aromatic-LaCla Complexes Enthalpy of complex formation, Stability constant -AH$, at K1.at 200 "C 200 "C (m 2-mole-1) (:Kcal-mole- l) Benzene 2.09 x 105 3.6 Toluene 4.37 x 106 4.8 5.06 x 105 Ethylbenzene 4.8 4.43 x 105 Isopropyl benzene 4.5 Fluorobenzene 3.00 x 106 4.6 2.17 x 105 Chlorobenzene 2.8 3.1 Bromobenzene 2.84 X lo6 6.6 2.75 X lo6 Iodobenzene 1.62 X 106 1-Hexene Table IV. Functional Group Contributions to the Free Energy of Adsorption, A( -AG),, on Various Surfaces at 500 OK, Kcal-mole-' Graphon Silica gel NaCl LaC13 NaCl LaCh 1.11 0.41 0.41 1.08 1.32 C 0.36 0.40 Terminal a bond -0.11 -0.17 -0.09 0.48 0.04 0.32 Aromatic a bond -0.13 -0.15 -0.05 -0.37 0.21 0.20 0.25 4-F &Me 1.34 1.63 . . . 0.54 ... Ring a bond (cyclohexene) 0.14 0.17 0.19 ... 0.44 Ring ir bond (1,4-Cyclohexadiene) 0.18 0.22 0.41 ... 0.64
actions with Lacla-Graphon are implied by the difference in the aromatic pi-bond data between NaCl and LaCl3 in Table IV, 0.19 Kcal. This is comparable to the difference between NaCl and LaC13 on silica gel, 0.16 Kcal. Thus, the extent of complexation between LaC13 and the benzene molecule appears to be independent of adsorbent. For LaC13-coated silica, the specific interaction of benzene is a combination of the pi-electron interaction by the silica surface ( 4 ) and of LaCla complexation. An interesting feature of the data in Tables I1 and IV is the enhanced interaction of cyclic olefins on Graphon. Apparently the six-membered ring fits the surface such that a pi-electron interaction occurs. In addition, 1,4-cyclohexadiene appears to undergo a degree of complexation with LaC13 on Graphon; this also occurs with the Laclasilica gel surface. On the latter surface, the data indicate that cyclohexene also is complexed. The ability to study surface complexation by gas-solid chromatography, in the manner illustrated by the present investigation, should prove useful for characterizing other weak surface complexes. In particular, studies of a variety of metal salts would indicate the role of complexation in relation to catalytic properties.
RECEIVED for review December 7, 1970. Accepted February 19, 1971.
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