Anal. Chem. 1980, 52, 1032-1035
1032
Gas-Solid Chromatographic Properties of Alkali-Metal Modified Silica M. M. KopeEni and S. K. Milonjii? Chemical Dynamics Laboratory, The Boris Kidrlc Institute of Nuclear Sciences: Box 522, 1 100 1-Belgrade, Yugoslavia
R. J. Laub' Department of Chemistry, The Ohio State University, Columbus, Ohio 432 10
The properties of alkali-metal modified adsorbents obtained from colloidal silica have been investigated by gas chromatography. Alteratlon of the surface activity caused by lncorporatlon of LiCI, NaCI, KCI, and CsCl is illustrated by the variation of retention of several aliphatic, alicyclic, chlorinated, and aromatic hydrocarbon solutes. These are dlscussed in terms of adsorbate-adsorbent Interactions. Thermodynamic data for the solutes wlth each adsorbent are also reported.
Although t h e introduction of porous polymers (e.g., Poropak) in gas-solid chromatography (GSC) has provided a solution to many analytical problems, there remains considerable interest in inorganic adsorbents, both for GSC and for "high-performance'' liquid chromatography (HPLC), with regard t o their practical applications. Thus, studies continue t o be devoted t o temperature-stable inorganic oxides which offer surface properties covering a wide range of basicity, homogeneity, a n d specific surface area since such materiais can generally be modified easily by heat or by special chemical treatments, or by the incorporation of inorganic salts or organic compounds, leading i n each case t o different adsorptive properties. These treatments at times result in materials which offer the additional advantages of lower retention times, reduced peak asymmetry, a n d improved separations. Applications of modified silica, alumina, alumina-silica, graphitized thermal carbon black, and zeolites have been described a n d discussed thoroughly by Kiselev ( I , 2 ) and Phillips and Scott ( 3 ) . Subsequent work has characterized these materials in detail. Other studies (4-15) have also been devoted to analytical applications, theoretical considerations, a n d various pre-treatment techniques employed with silica gel. W e describe, in this report, properties of new adsorbents obtained from t h e coprecipitation of silica with LiC1, NaC1, KC1, and CsC1.
EXPERIMENTAL Apparatus. A Perkin-Elmer Model 881 gas chromatograph equipped with dual hydrogen flame ionization detection was used. The flow rate of dried carrier (nitrogen) was 9-20 mL/min. A Hitachi/Perkin-Elmer Model 159 stripchart recorder with a chart speed of 10 mm/min and 2 mV fullscale response recorded the chromatograms. Adsorbate samples were injected as vapors with a 10-pL Hamilton syringe. Retention times were determined to within 0.2 s with a stopwatch. Specific surface areas were obtained with a Strohlein area meter using the single-point nitrogen adsorption method. A Siemens Kristalloflex 4 GM was used for X-ray analyses of the silicas which were in all cases amorphous. Materials. Collodial silica was obtained from liquid glass by a n ion-exchange method (16)described previously. Coagulation of the colloidal material was carried out by adding t o it 1 M
*
0003-2700/80/0352-1032S01. O O / O
solutions of LiC1, NaC1, KC1, or CsCl a t pH 10. The solutions were allowed to equilibrate for 4-5 h after which the (disperse) phases were separated by filtration, dried in air, crushed, and washed with distilled water until no further reaction with chloride ion was observed. The materials were then dried in an air oven a t 110 OC for 24 h. For protonated silica, coagulation was carried out with NaCl, followed by immediate filtration. The solid phase was then transferred to a polyethylene bottle containing 200 mL of 0.15 M HC1, and allowed to equilibrate for 24 h. The pH of the solution after equilibration was 2.2, which corresponds (17) to the pH (zero-point coverage) of fully-protonated silica. Dried materials were crushed and sieved, and the 60-120 mesh fractions were retained for use as column packings. The specific surface area and metal-ion uptake of each of the fractions were: unmodified silica (Si02-H),239 m2 g-'; silica modified with LiCl (SiO,-Li), 204 m2 g-', 0.12 mequiv Li+/g S O 2 ; silica modified with NaCl (SiO,-Na), 152 m2 g-', 0.35 mequiv Na+/g Si02;silica modified with KC1 (Si02-K),159 m2 g-l, 0.27 mequiv K+/g SiO,; and silica modified with C,C1 (Si02-C,),124 mz g-l, 0.45 mequiv Cs+/g S O 2 . All solutes (obtained from various commercial sources) were of analytical-reagent grade. All of the columns (2 m X 2.2 mm i d . ) were stainless steel, and were cleaned with both polar and nonpolar solvents prior to packing. After packing, each column was conditioned overnight with nitrogen at 250 "C. Retention volumes for each solute with each of the columns were measured a t 70, 100, 130, 160, 200, and 230 O C , with the exception of adsorbates with retention times exceeding 90 min. The system dead-time was assumed to be equal to the retention time of methane at the column temperature, which introduces an error of no more than ca. 370 for the first-eluting solutes a t 70 "C. The error becomes progressively smaller a t higher temperatures ( 1 8 ) . Each adsorbate was chromatographed a t least three times with each column, the net retention volume Vs being calculated for each solute from the usual relation:
where t k is the adjusted retention time, F, is the carrier corrected flow rate, and j is the James-Martin gas compressibility correction factor. The net retention volumes were then converted to partition coefficients, K R , by dividing V u by the total surface area of the adsorbent, As, in the column. These data correspond to the initial slope of the adsorption isotherm (19). The variation of KR from one adsorbent to another for the same adsorbate was then used to quantitate changes in the properties of the adsorbents. The solute peaks were symmetric and were independent of sample size, which indicated that the measurements were being carried out in a linear region of the adsorption isotherm. The isosteric heats of adsorption M, were calculated by plotting In KR/ T vs. 1 / T ,where the linear least-squares slopes of the plots were taken to be equal to - M / R (Le., assuming that AH is invariant over the temperature range examined here) (I). Gibbs free energies of adsorption, AG, were calculated from the relation AG = -RT.ln KR, where R is'che gas constant. The corresponding entropies of adsorption, A S , were obtained from A S = ( A H AC)/T. ~
C 1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 7, JUNE 1980
1033
Table I. K , Values ( m L / m 2 )for Listed Solutes with Indicated Silicas at 1 3 0 " C solute
bP, " C
SO,-H
Si02-Li
Si0,-Na
SiO, -K
SiO, -Cs
n-pentane n-hexane n-heptane n-octane cyclohexane isooctane
36.2 69.0 98.4 125.8 81.4 116.0
0.0251 0.0551 0.1234 0.2814 0.0552 0.1683
0.0214 0.0425 0.0853 0.1630 0.0458 0.1144
0.0189 0.0346 0.0628 0.1136 0.0377 0.0813
0.0194 0.0367 0.0673 0.1253 0.0409 0.0890
0.0245 0.0475 0.0906 0,1706 0 0502 0 1179
1-pentene 1-hexene
30 63.5
0.0804 0.1834
0.0429 0.0874
0.0384 0.0734
0.0301 0.0573
0.0380 0.0749
carbon tetrachloride chloroform methylene chloride
76.8 61.3 40.1
0.0761 0.0935 0.0655
0.0592 0.0730 0.0567
0.0476 0.0587 0.0466
0.0496 0.0594 0.0416
0.0635
80.1 132 156 110.6
0.3808 0.8316 1.480 1.212
0.3008 0.4846 0.7658 0.8099
0.2725 0.3201 0.5143 0.7233
0.1842 0.2960 0.47 89 0.4638
0.2255 0.4311 0.6842 0.5595
benzene chlorobenzene bromobenzene toluene
0
ot
-I
-501
i
I Ob
0.0627
, i
L d 20 30 40 Molar
Figure 1. Plots of In K, vs.
r e f r a c t i o n , cm3
- I 2 01
mol-'
molar refraction for indicated solutes with
lo3
unmodified silica
RESULTS AND DISCUSSION Table I presents the partition coefficients of a number of adsorbates a t 130 "C with all adsorbents. T h e influence of the metal-incorporation treatment is evident from the data: the largest partition coefficients were those obtained with unmodified silica and, in all cases, modification of the silica surface with the alkali-metal ions resulted in decreased partition coefficients as expected, since modified surfaces have fewer active sites available for interaction with an adsorbate. Retentions of adsorbates are thereby decreased relative to those with unmodified silica. Cycloalkanes are generally retained longer than the corresponding normal alkanes in gas-liquid chromatography (GLC), but the converse is usually true in GSC (20). This trend was observed with unmodified silica a t t I 100 "C, and with NaC1and CsC1-treated silicas a t t = 70 "C. However, in the case of unmodified silica a t t 2 130 O C , LiC1- and KC1-treated materials a t t 2 70 "C, and NaCl- and CsC1-treated adsorbents a t t 2 100 "C,cyclohexane was retained more strongly than n-hexane. Conversely, partition coefficients of representative chlorinated hydrocarbons showed an increase with increasing number of chlorine atoms u p to chloroform and thereafter exhibited a decrease. I t appears from Table I that for compounds of analogous structure (e.g., n-alkanes), the partition coefficients increase linearly with molar refraction of the solutes. Plots of In KR vs. molar refraction are shown, for example, in Figure 1 where the olefin data lie between the normal alkane line and the aromatic line, implying that polarization effects for the olefins
,
2 0
Figure 2.
I
25 T-',"K-~
30
Representative plots of In K,l T v s . lo3/ Tfor indicated solutes
with lithium-treated silica
Table 11. Isosteric Heats of Adsorption, -AH ( k J mol-' ), of Listed Solutes with Indicated Silicas Si0,H
Si0,Li
Si0,Na
Si0,-
solute
K
Si0,cs
n-pentane n-hexane n-heptane n-octane cyclohexane isooctane
40.74 41.57 49.88 52.38 41.57 51.55
36.35 41.07 47.39 50.72 41.07 47.39
34.92 39.91 45.73 49.05 39.91 45.73
33.26 40.74 45.73 50.72 39.08 48.22
33.26 37.41 43.23 48.22 37.41 44.90
1-pentene 1-hexene
39.91 41.07 39.91 39.91 37.41 44.90 47.39 44.06 47.39 42.40
carbon tetrach!oride chloroform methylene chloride
42.40 33.26 39.91 39.91 39.08 46.56 41.57 40.74 44.06 41.57 39.91 39.91 39.41 39.08
benzene chlorobenzene bromo benzene toluene
54.04 60.69 63.19 62.36
49.88 53.21 56.54 59.03
55.70 55.70 51.55 57.37
48.22 49.05 52.38 54.87
49.88 54.87 57.37 58.20
are less dominant in determining retention than is the case for the aromatic adsorbates. Plots of In KR/ T vs. T'of representative compounds with LiC1-treated silica are shown in Figure 2. The values of Al? (calculated from such plots) are compiled in Table 11. [Comparison of adsorbate isosteric heats of adsorption with the same column are valid, but comparison of the values for
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 7 , JUNE 1980
Table 111. Free Energies (kJ mol-') and Entropies (J mol-' K - ' ) of Adsorption of Listed Solutes with Indicated Silicas at 160 " C Si0,-H
Si0,-Li
Si0,-K
Si0,-Na
compound
AG
-4F
AG
-AB
AG
-AF
AG
n-pentane n-hexane n-heptane n-octane cyclohexane isooctane 1-pentene 1-hexene carbon tetrachloride chloroform methylene chloride benzene chlorobenzene bromobenzene toluene
15.69 13.32 10.93 8.50 13.10 9.88 11.65 9.28 12.09 11.47 12.50 7.30 5.29 3.17 4.22
130.32 126.77 140.45 140.60 126.26 141.87 119.06 125.11 125.85 134.02 124.88 141.68 152.38 153.25 153.75
16.05 13.96 11.96 9.43 13.69 10.98 14.04 11.90 12.82 12.21 12.96 7.75 6.54 4.95 4.65
120.79 127.08 137.06 138.91 126.48 134.81 127.28 136.92 106.40 124.20 122.09 133.10 137.99 142.00 147.06
16.51 14.60 12.76 10.96 14.23 11.88 14.55 12.60 13.33 12.76 13.48 8.51 8.27 6.08 5.30
118.78 125.88 135.07 138.60 125.02 133.05 118.08 130.85 122.94 123.55 123.31 148.31 147.74 133.08 144.72
16.33 14.44 12.57 10.75 14.02 11.69 15.17 13.21 13.34 12.87 13.90 9.67 8.07 6.62 7.00
n- Cg
+
Si0,-Cs
aG
-4S
15.56 13.47 11.50 9.56 13.21 10.62 14.24 12.14 12.46
112.73 117.51 126.39 125.76 116.91 128.22 119.30 125.94 119.02
12.63 8.75 6.62 5.18 6.01
119.42
-43114.51 127.43 134.64 141.96 122.62 138.36 127.20 139.94 122.98 131.48 123.11 133.69 131.92 136.25 142.90
135.42 142.02 144.56 148.29
Cyclohexane n - C,
7
1
1
I*-
-501
Benzene
i
x Chlorobenzene 'Bromobenzene A
-60t L ' 5
6 Carbon
0 Toluene
7 number
i 8
Figure 3. Plots of A G vs. carbon number for indicated solutes with cesium-treated silica
the same adsorbate with different adsorbents is generally regarded as not meaningful since the entropies of adsorption vary from one column to another ( I 5 ) . ] It is evident from the data that the aromatic hydrocarbons exhibit greater negative isosteric heats than those given by aliphatic and alicyclic hydrocarbons with the same number of carbon atoms. T h e more negative the heats, the greater is the interaction between the adsorbate and adsorbent. T h e data given in Table I1 and Figure 3 show that the isosteric heats of adsorption of the n-alkanes increase linearly with the number of carbon atoms. The slopes of the linear functions may be taken to represent the increment in the isosteric heat of adsorption contributed by each CH2 group. Thus, contributions due to the three P bonds of the benzene ring could be evaluated by measuring the vertical distance between the point for n-hexane on the normal alkane line and the point for benzene. Similar measurements have been made with regard to functional group contributions to the thermodynamic properties of a wide variety of solutes with saltcoated adsorbents by Sawyer and co-workers (7-12); however, this approach was not warranted in this initial work because of the small number of solutes employed. We hope soon to report data for a much larger number of adsorbates for which such correlations will be attempted. Data for the free energies and entropies of adsorption are compiled in Table 111. The former indicate that the adsorbate-adsorbent interactions of all adsorbates increase in the following sequence of adsorbents: Si02-H,Si02-Li,Si02-Na, Si02-K. Si02-Csdoes not, however, follow this trend. T h e adsorption properties of the normal aliphatics exhibit the same behavioral pattern with each column as do the aromatics. Dependence of the free energy of adsorption with the number of carbon atoms of the n-alkanes is shown in Figure 4;similar trends were also found for other classes of adsor-
901
8 80 0
l
l
l
5
6
7
\I1; 8
Carbon number
Figure 4. Plots of A G v s . carbon number for the n-alkane solutes of Table I with unmodified (0)and with sodium-treated (e)silicas at 160 OC
bates. From the figure, it appears that the free energy decreases linearly with the number of carbon atoms. (Since the slopes of the lines represent incremental contributions to the free energy of adsorption for each CH2 group, functional-group free-energies of hydrocarbons and substituted aromatic hydrocarbons can therefore be calculated analogous to those for aH as described above.) According to the classification scheme of Kiselev ( I ) , silica is an adsorbent of the second type, that is, it can interact specifically with molecules which contain T-electron bonds. Thus, heats of adsorption of solute molecules of group A which react nonspecifically, do not depend upon the population of OH groups on the S i 0 2 surface, that is, the highly-protonated groups on the Si02surface do not cause a noticeable change in the enthalpy of adsorption (the adsorbent interactions are thus said to be nonspecific). A different picture emerges, however, in the case of adsorption of molecules of group B, since these depend strongly upon the degree of hydroxylation of the S i 0 2 surface. Currently under investigation are the properties of the salt-modified silicas with respect to volatile solvents in view of the findings of Al-Thamir, Laub, and Purnell (21),wherein silicas and aluminas were found to catalyze the decomposition of common organic solvents at room temperature to 50 "C. Combination of a specific salt-modified silica with one or more
Anal. Chem. 1980, 52, 1035-1039
organic compounds may in fact lead to "tailor-made" adsorbents which offer a range of selectivities not currently available in gas-solid chromatography nor, for that matter, in liquid chromatography.
LITERATURE CITED (1) A. V. Kiselev and Ya. I . Yashin, "Gazo-adsorbtsionnaya Khromatografiya", Nauka, Moscow, 1967. (2) A. V. Kiselev, J . Chromafogr.,49, 84 (1970). (3) C . S.G. Phillips and C. G. Scott, in "Progress in Gas Chromatography", J. H. Purnell, Ed., Wiley-Interscience, New York, 1988, pp 121-152. (4) C . L. Guillemin, M. LePage, R . Bean, and A. J. de Vries, Anal. Chem., 39, 940 (1967). (5) C . L. Guillemin, M. Deleuil, S. Cirendini. and J. Vermont, Anal. Chem., 43, 2015 (1971) (6) C L Guillemin, M Le Page, and A J de Vries, J Chromatogr S o , 9 470 f19711 (7) 6'. T.~Sawye; and D. J. Brookman. Anal. Chem., 40, 1847 (1968). (8) D. J. Brookman and D. T. Sawyer, Anal. Chem., 40, 2013 (1968). (9) A. F. Isbell, Jr., and D. T. Sawyer, Anal. Chem., 41, 1381 (1969). (10) D. F. Cadogen and D. T. Sawyer, Anal. Chem., 42, 190 (1970). (11) D. F. Cadogan and D. T. Sawyer, Anal. Chem.: 43, 941 (1971).
1035
(12) J. P. Okamura and D. T. Sawyer, Anal. Chem., 43, 1730 (1971). (13) L. Feltl and E. Smolkovl, J . Chromafogr.. 65, 249 (1972). (14)L. Feltl, P. Lutovskp, L. Sosnovl, and E. Smolkovl, J . Chromatog.,91 321 (1974). (15) N. H. C. Cooke, E. F. Barry, and B. S. Solomon, J . Chromatogr., 109. 57 (1975). (16) Yu. G. Frolov, N. A. Shabanova, V. V. Leskin, and A. I . Pavlov, Kolloid. Zh., 38, 1205 (1976). (17) G. A. Parks, Chem. Rev., 65, 177 (1965). (18) W. K. AI-Thamir. J. H. Purnell. C . A. Wellinaton. and R. J. Laub. J , , , Chromatogr., 173, 388 (1979). (19) R. L. Gale and R. A. Beebe, J . Phys. Chem., 68, 555 (1964). (20) R. F. Hirsch, H. C. Stober, M. Kowblansky, F. N. Hubner, and A. W. O'Connell. Anal. Chem.. 45. 2100 119731. (21) (1977) W K AI-Thamlr, R J Laub, and J H Purnell, J Chromatogr , 142, 3 I
RECEIVED for review October 30, 1979. Accepted March 5, 1980. R.J.L. gratefully acknowledges support from the National Science Foundation, grant no. CHE-7820477, and from the Graduate School of the Ohio State University.
Determination of Niobium with p -Arsonophenylazochromotropic Acid and Cetylpyridinium Bromide A. Sanz-Medel"' and C. C6mara Rica Departmento de Q h i c a Analj'tica, Facultad de Q h i c a s , Unlversidad Complufense, Madrid-3, Spain
J. A. Perez-Bustamante Departamento de QGmica Analj'tica, Facultad de Ciencias de Cidiz, Universidad de Sevilla, Cidiz, Spain
Color stabilization and modification of the visible spectrum of the p-arsonophenylazochromotropicacid (L) and Nb(V) binary complex by the addition of surfactants has been studied In the course of a search for improved spectrophotometric methods for niobium. Addition of cationic surfactants resulted in an effective means of color stabilization and brought about a drastic improvement in the intensity of the absorption band maximum (sensitized reactions). The surfactant which gave the most intense absorption band maximum was cetylpyridinium bromide (CPAB). Spectrophotometric measurements indicate the formation of two Nb-L-CPAB ion-association ternary complexes: a complex with the stoichiometry 1:2:4 was formed in approximately 1 M HCI. The other complex had the stoichiometry 1:l:Z and formed in 2-4 M HCI mediums. Both ternary complexes mentioned can be applied to the spectrophotometric determination of niobium and two different analytical methods have been established: the complex formed in a 1.0 M HCI medium provides a method less selective than the complex formed in 2-4 M HCI. The method provided by the former complex is sensitive ( t = 1.87 X lo4 L mol-' cm-I), has a working range of 0.5-5 ppm Nb, and has a relative standard deviation of 0.5 YO. The complex formed in the higher acidity range provides a less sensitive method. The working range is 1-9 ppm and t = 7.40 X l o 3 L mol-' cm-I. This latter method is most adequate for a selective spectrophotometric determination of niobium (relative standard deviation being 0.5 Y O ) .
Present address: Departamento de Qujmica Analitica, Facultad de Ciencias, Universidad de Oviedo. Oviedo, Spain. 0003-2700/80/0352-1035$01 OO/O
Recent research into increasing the absorptiometry sensitivity for trace metal analysis in the visible region has investigated the use of colored metal chelate systems "sensitized" by the presence of a third component (ternary systems). The use of binary metal ion complexes of organic dyestuffs such as Catechol Violet, "sensitized" by the addition of long-chain cationic surfactants such as cetyltrimethylammonium bromide (CTAB) is claimed to be a mode of analytical reaction more generally applicable to the determination of tin ( 1 ) and one of the most sensitive methods available for the absorptiometric determination of' metal ions. Although some attempts have been made to define the mechanisms and associated color changes of such reactions, much more work is needed to elucidate them. Bayley et al. ( I ) concluded from their study that ternary complexes are formed by ion association of the quaternary ammonium group with the metal dyestuff chelate. However. in aqueous solution, this occurs only when micelles are formed. It is thus necessary to form micelles t o obtain the ternary system because the critical micelle concentration (CMC) of the surfactant turned out also to be a critical reaction concentration ( I ) . The work of Ashton et al. (2)on the Sn(1V)-Catechol Violet system in the presence of surfactants of different nature confirmed that the Sn(1V)-Catechol Violet-CTAB system is a ternary ion association system which is probably formed in colloidal solution and dispersed by the excess surfactant. The method for determining tin with Catechol Violet and CTAB was first introduced by Dagnall, West, and Young ( 3 ) and it was applied by Ashton et al. (2) to the determination 1980 Amertcan Chemical Society
