Porous Thin-Layer Modified Glass Bead Supports for Gas Liquid Chromatography J. J. KIRKLAND lndustrial and Biochemicals Department, E. I.du Pont de Nemours & Co., Experimental Station, Wilmington, Del. Glass beads modified with thin layers of finely divided diatomaceous earth or silica show significant advantages over untreated glass beads as gas chromatographic supports. Altering particles are tightly anchored to the glass bead surface by means of a fibrillar boehmite (alumina). Modified bead columns exhibit lower HETP and higher optimum linear gas velocities, when compared with columns of untreated beads. The significant improvements are believed to be primarily the result of thinner liquid film coatings, leading to more efficient solute mass transfer. HETP of 0.8 mm. have been obtained with silica solmodified 60- to 80-mesh glass beads coated with a silicone oil.
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efficiency of gas chromatographic columns with glass bead supports can be improved by eliminating the relatively large liquid pools which collect a t the glass bead contact points, and distributing the liquid evenly over the beads (4, 6). A support composed of hard, nonporous granules with a porous, inactive layer should show the desired effects (6). The average stationary film thickness on a porous inactive layer would be less than that on conventional glass beads; therefore, one would expect the liquid mass t-ansfer process to be much faster. The nonporous properties of the solid core would also result in improved gas mass transfer characteristics over conventional diatomaceous earth supports. The advantages of supports with solid cores and porous surfaces have been pointed out by others (2, 11). Dewar and Maier (3) have prepared columns with glass beads coated with stationary phase containing a very fine diatomaceous earth suspended to the glass beads only by surface tension forces. The adhering particles apparently produced sufficient roughness to give more efficient spreading of the liquid. Columns containing 60- to 80mesh glass beads with finely divided diatomaceous earth performed with a height equivalent theoretical plate (HETP) of 0.8 mm., compared with 4 to 5 mm. for columns with uncoated beads. No evaluation was made regarding the effect of distribution-homoHE
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ANALYTICAL CHEMISTRY
geneity of the loose particles on column efficiency. Halhsz and Horvitth have reported on the use of microbeads coated with a por‘ous thin layer as a column support (8). The “porous layer glass beads” were prepared in the manner used for coating of capillary columns (7, 9). Particles of various materials in the particle size range of 1 micron or less were deposited from solvents onto the surface of the nonporous glass beads. The finely divided particles are apparently held to the surface by van der Waals forces. A trial-and-error approach was used in developing procedures for dispersing and suspending the various porous thin layers. Another procedure for producing porous surfaces on nonporous cores has been to etch or roughen the surface of the glass beads. While mechanically roughened glass beads show no improvement in column performance (S), a preliminary study by Ohline and Jojola ( l a ) has indicated that beads etched chemically with gaseous hydrogen fluoride lead to more efficient columns. The chromatography of polar solutes with such columns presents some problems, however, since etching of the surface tends to increase the peak tailing of polar compounds. A previous study has shown that 100m,u silica particles can be strongly
Figure 1 . Diatomaceous earth-modified glass bead surface Electron photoinicrograph at 7500 X
anchored to nonporous glass beads with the use of a fibrillar boehmite, Baymal colloidal alumina (E. I. du Pont de Nemours & Co.), as a binding agent (IO). Beads coated in this manner were used for gas solid or modified gas solid chromatography by packed or capillary column techniques. It was pointed out that this approach should be useful for preparing a variety of column packings composed of dense cores coated with finely divided thin layers of porous solids. This report describes the preparation and characteristics of gas liquid partition columns consisting of partitioning liquids deposited on finely divided diatomaceous earth or silica particles tightly bonded to glass beads with fibrillar boehmite. These modified glass beads produce columns which are significantly more efficient than those prepared from untreated glass beads. EXPERIMENTAL
Apparatus and Reagents. Gas chro-
matographic measurements were made with a n F & 11 Scientific Corp. Model 300 gas chromatograph equipped with a conventional flame ionization detector, electrometer, and 1-mv. recorder. Columns mere prepared from 1-meter, 1/4-inch-o.d., 3/lsin~h-i.d., lengths of U-shaped stainless steel tubing. A mixture of 15% dibutyl ether, 26% decane, 34% methyl benzoate, and 25% naphthalene was used to obtain all data. Aliquots were injected into the gas chromatograph with a 0.10-p1. Nano-jector (Scientific Kit Co., Washington, Pa.). Preparation of Diatomaceous Earth-Coated Glass Beads. One hundred and sixteen grams of 60- to 80-mesh glass beads (Microbeads, Inc., Jackson, Miss.) were soaked in 10% methanolic potassium hydroxide for 2 hours, washed thoroughly with distilled water t o neutrality, and dried in a vacuum oven at 110’ C. for 1 hour. The cleaned beads were placed in a 50 X 275 mm. glass chromatographic tube fitted with a coarse sintered-glass retainer plug. Approximately 125 ml. of 2% Baymal sol (0.1% in nitric acid) were added to the beads in the column. The contents were mixed thoroughly and allowed to stand for about 5 minutes. The excess liquid was drained off by gravity, and the bead bed washed by passing tap water upward through the bed until
the effluent mas clear. Excess water was drained off by applying a slight pressure of nitrogen on top of the tube. About 125 ml. of a 5% 0.1- to 1-micron dispersion of diatomaceous earth were added and allowed to drain through the bed by gravity. [The dispersion was prepared by dry-ballmilling Chromosorb W (Johns Xanville Corp.) overnight. The resulting powder was treated with concentrated hydrochloric acid and washed to neutrality. A fraction of the desired particle size range was obtained by sedimentation.] Escess dispersion was eliminated b y backwashing. X vacuum was placed on the outlet of the tube to air-dry the treated beads. The beads were then heated in a circulating air oven for 2 hours at 250" C. Preparation of Silica Sol-Coated Glass Beads. Silica sol-treated glass beads were prepared by a similar procedure (10). After treatment with 2% l3aymal solution, the beads were soaked for 5 to 10 minutes with a 2Yc 100-mp silica sol ( I ) , adjusted to p H 3.5 with dilute HCI. Excess sol was eliminated by backnashing, as described above, and the treated beads were dried at 110" C. for 1 hour. Five successive layers of Baymal and silica were placed on the beads in this manner. The final washed beads were dried for 2 hours a t 250" C. in a circulating air oven. Preparation of Columns. T h e desired concentration of liquid phase was deposited on untreated a n d silica sol-treated glass beads by evaporating a dichloromethane solution of Dow Corning 200 silicone oil (100 centistokes) on a steam b a t h while stirring. T h e resulting coated beads were packed into columns using vibration. Diatomaceous earth-treated glass bead columns were prepared by an on-column coating procedure. The treated glass beads were packed into a n empty straight column with a minimum of handling and vibration. The column was then closed off with glass woo1 plugs, and a solution of Dow Corning 200 silicone oil in dichloromethane was forced u p the column. To accomplish this, the column of untreated beads was pushed through a one-hole stopper into a filter flask containing the coating solution. The end of the column was introduced under the surface of the liquid. Nitrogen pressure was placed on the filter flask outlet until the coating liquid appeared in a reservoir attached to the top of the column. About 50 ml. of solution were allowed to pass up through the column to ensure adsorption equilibrium of the liquid phase. The pressure was then removed, and the solution allowed t o drain back into the flask. Nitrogen pressure mas then applied to the top column t o remove the remaining solvent. When treated in this manner, beads exposed to a solution of 10% liquid phase in dichloromethane resulted in about 0.5Tc liquid coating. Actual liquid loadings were measured by determining the amount of silicone oil in the dichloroI
methane before and after treating the beads. The weight per cent liquid loadings determined with this technique closely agreed with values measured by eluting the liquid phase from coated columns with dichloromethane, and weighing the oil residue after evaporating the solvent. All columns contained 27 f 0.5 grams of total packing. Columns were preconditioned at 225' C. for a minimum of 1 hour with the carrier gas flowing. DISCUSSION
The Characteristics of fibrillar boehmite-silica sol-treated glass beads have been described ( I O ) . The surface of successively treated glass beads represents a multilayer structure of silica particles, each tightly anchored by a very thin fibrillar boehmite film. Discrete 100-mp silica particles can be seen by electron microscopy on the surface of glass beads treated with this material. These particles cannot be removed easily by mechanical forces. The surface area of untreated 60- to 80mesh beads increased from about 0.03 to approximately 0.5 sq. meter per gram, with five treatments of Baymal-100-mp silica sol ( I O ) . Glass beads coated with a thin film of diatomaceous earth show distinct physical differences when compared with silica sol-treated glass beads. When examined under an optical microscope, sol-treated beads show a frosted appearance, since the adhering silica particles are too small to be resolved. However, aggregate particles can easily be seen on the surface of the diatomaceous earth-coated glass beads. The gross surface of these beads appears to be about 80% covered by diatomaceous earth particles. The diatomaceous earth film on the glass beads is not as tightly anchored as the silica particles on 100-mp silica sol-treated beads. Electron photomicrographs of the surface of diatomaceous earth-treated glass beads show discrete particles in the 0.1- to 1-micron range, as pictured in Figure 1. Because of the relatively large particle size range involved, the diatomaceous earth film can be removed by excessive abrasive action. Diatomaceous earthtreated beads have a surface area of about 0.10 sq. meter per gram, compared with 0.03 sq. meter per gram for untreated beads. Although nonporous glass beads are superior to porous diatomaceous earthtype supports in their gaseous mass transfer characteristics, the smooth surface of the beads gives rise to the collection of partitioning liquid in capillaries a t the points of contact (4). Therefore, glass bead columns demonstrate poor liquid mass transfer characteristics. as a result of the
relatively deep liquid pools which are formed. Pools of liquid can be seen at the contact points of- the unmodified glass beads coated with 0.22% DC-200 silicone oil. However, the liquid phase is not evident at the points of contact of silica sol-treated beads coated with the same amount of liquid. Presumably, the liquid phase is dispersed over the porous film surface. Beads modified with thin films of diatomaceous earth or silica particles combine the desirable characteristics of the higher surface area porous diatomaceous earth supports (optimum suspension of the partitioning liquid) with those of the nonporous core (optimum gas diffusional characteristics). Unmodified glass beads treated with 0.22% silicone oil tend to stick together. Diatomaceous earth- or silica-coated beads coated with the same amount of oil appear dry, and are free-flowing. Modified beads coated with 0.85% silicone oil tend to stick together slightly, indicating that excess liquid is present. The gas chromatographic characteristics of the modified beads verify that the porous thin films can significantly improve the performance of glass beads as gas chromatographic supports. As shown by the plots of HETP us. flow rate for decane in Figure 2, columns with diatomaceous earth (DE) or silica sol-modified glass bead supports are more efficient than corresponding columns of unmodified beads. (Partition ratios for decane range from 3 to 19, depending on liquid concentration.) Improved liquid mass transfer effects resulting from the porous layer are demonstrated for the modified beads by the significantly smaller H E T P increase with increasing linear gas velocity, and by the higher optimum linear gas velocity characteristics. The efficiencies of the diatomaceous earth-modified bead columns rival those of columns with supports composed entirely of the type of diatomaceous earth used in coating the beads. The HETP of silica-modified bead columns are even smaller than those of the diatomaceous earth-modified bead columns, as demonstrated in the bottom plot of Figure 2. Presumably the partitioning liquid on the sol-treated beads is more homogeneously dispersed. The optimum HETP for decane on silica sol-treated beads coated with 0.22% silicone oil was 0.8 mm., the same value obtained by Dewar and Maier using 60- to 80-mesh liquid coated beads containing impregnated diatomaceous earth (3). The H E T P us. flow rate differences between unmodified and diatomaceous earth-modified beads are similar, but of a lower magnitude, for high partition ratio solutes. Figure 3 shows the plots obtained for naphthalene (partition VOL. 37, NO. 12, NOVEMBER 1965
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I
UNMODIFIED
4.0
I
I
I
7.0
BEADS
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UNMODIFIED
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I
I
I
0.22 %
BEADS
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6.0 E 2.0 E
w’ z
1.0
L
0.0+-----4
:
-
4.0
-
E E 3.0
w’
50
DIATOMACEOUS E A R T H MODIFIED B E A D S
4.0
5.0
w
0 .
d c 1.85%
1.0 -
2.0
1 AND SILICA SOL-MODIFIED
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2.0 0‘
I
I
IO
5
LINEAR
Figure 2.
I
I
15
20
1
25
I
30
I .o
GAS V E L O C I T Y , cm/sec.
HETP vs. flow rate plots for decane
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Column 75’ C.
2
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t
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,lo
15
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30
HETP vs. flow rate plots for naphthalene
of liquid required just to fill the bead contact points is a function of the surface energy of the liquid, the conditions of the beads, and the temperature of the system. These factors could account for the wide divergence of “optimum” liquid loadings for glass bead suvvorts .. which have been reported.
BEADS
4
1
1
Column 75’
3 I
I
L I N E A R GAS V E L O C I T Y , cmysec.
Figure 3.
ratios of 10 to 53) for varying stationary liquid concentrations. The data for untreated glass beads showing a decrease in H E T P with amount of DC-200 silicone oil are unlike previously published information of this nature. The efficiency of glass beads is normally expected to increase with decreasing liquid loading. The apparent anomaly may be explained by variances in the posture of the liauid phase on the glass beads. The amdunt
SOL- MODIFIED I
EARTH-MODIFIED BEADS
Is b
8
5
C.
Diatomaceous earth- (and silica-) coated glass beads demonstrate H E T P patterns similar to those of the conventional diatomaceous earth supports-Le., H E T P increases with increasing amounts of liquid phase. This similarity is additional evidence of the porous nature of the modified glass bead surface. Columns prepared with diatomaceous earth-modified glass beads show small or no differences in their adsorption of polar compounds, compared with unmodified glass beads, even though colloidal alumina particles are used as binding agents. Therefore, diatomaceous earth-modified beads appear suitable for the analysis of polar compounds, and have the advantage of increased efficiency and/or speed of analysis. Figure 4 shows chromatograms of a mixture containing both polar and nonpolar solutes carried out on columns prepared from unmodified and diatomaceous earth-modified glass beads. Even though a nonpolar liquid phase was employed, no appreciable difference in the symmetry of the peaks of polar compounds is shown.
4 Figure ‘4. columns
Separation of compounds with glass bead
Components. 1 -Dibutyl ether, 2-decane, 3-methyl benzoate, 4-naphtha. lene Conditions. 0.85% silicone oil on beads; column temperature 75’ C.j Row rata 50 cc./min. He
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ANALYTICAL CHEMISTRY
Silica sol-treated beads can demonstrate adsorptive effects (IO). When coated with Of ’Ow Corning 200 silicone oil, sol-modified beads adsorb polar compounds such as dibutyl ether and methyl benzoate, causing tailing peaks. Nonpolar compounds, such as decane, elute as symmetrical peaks. With a 0.85% silicone oil loading, peak symmetries for all compounds in the test mixture are identical to those obtained with unmodified glass beads. Use of polar stationary liquid phases in smaller concentrations also results in the production of symmetrical peaks for polar compounds.
LITERATURE CITED (1) Bechtold, M. F., Snyder, 0. E. (to E. I. du Pont de Nemours & GO.), u. s. Patent 2,574,902 (Nov. 13, 1951). (2) Bohemen, J., Purnell, J. H., J . Chem. SOC.1961, 360.
matography 1960, Edinburgh,” R. P. W. Scott, ed., p. 139, Butterworths, London, 1960. (7) HalAsz, J., H o d t h , C., ANAL.CHEM. 35, 499 (1963). (8) Ibid., 36, 1178 (1964). (9) HalAsz, J., HorvAth, C., Nature 197, 171 (1963). (10) Kirkland, J. J., 5th International Symposium of Gas Chromatography, Brighton, England, September 1964. (11) Norem, s. D., ANAL. CHEM. 34, 40 (1962). w., Jojola, R.9 1bid.j 36, (12) Ohline, 1681 (1964).
(3) Dewar, R. A., Maier, C. E., J. Chromatog. 11, 296 (1963). (4) Giddings, J. c., ANAL. CHEM. 34, 458 (1962) ( 5 ) Ibid., 35,‘ 438 (1963). (6) Golay, M. J. E., “Gas Chro-
RECEIVEDfor review March 3, 1965. Accepted June 17, 1965. Division of Analytical Chemistry, Lab-Line Award Symposium, 149th Meeting, ACS, Detroit, Mich., April 1965.
ACKNOWLEDGMENT
I a m indebted to Ralph K. Iler for his helpful suggestions and to Glenn J, Wallace for his assistance with the experimental work,
Hexa mminecobalt(lll) Tricarbona toco balta te(lll) A N e w Analytical Titrant
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JAMES A. BAUR and CLARK E. BRICKER Department o f Chemistry, University o f Kansas, Lawrence, Kan.
b A new reagent for titrimetric oxidations is described and evaluated. is easy This reagent, CO(NH~)BCO(CO&, to prepare and, in the dry state, appears to be stable indefinitely. This reagent is a very weak oxidant in bicarbonate media but when added to an acid solution, the cobalt(lll) from the tricarbonato portion of the compound is released. The cobalt(ll1) thus generated is a very strong oxidant and is capable of reacting quantitatively with iron(ll), vanadium (IV), cerium(lll), and other reducing agents. The equivalence point of these titrations can be determined by redox indicators, by potentiometric methods, or by photometric detection.
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POTENTIAL OF hydrated cobalt(II1) when it is reduced to cobalt(I1) is well known to be about 1.8 volts us. the standard hydrogen electrode ( 8 ) . Because cobalt(II1) reacts spontaneously with water to liberate oxygen, it is extremely difficult to keep solutions of this strong oxidant. Bricker and Loeffler (2) found that cobalt(II1) sulfate was stable in 18N sulfuric acid if the solution was stored at temperatures below 0’ C. With standardized solutions of cobalt(II1) sulfate, it was possible to oxidize quantitatively such weak reducing agents as cerium(II1). If a complexed cobalt(II1) salt could be prepared which would give stable solutions in neutral or alkaline media, it may be possible, on acidification of the solution, to generate the powerful oxidant in situ. Most complex cobalt (111) salts are either too stable to
HE HIGH
liberate the free cobalt(II1) ion on acidification or else the ligand or a moiety of the compound is a reducing agent. One exception is the tricarbonatocobalt(II1) anion which should liberate carbon dioxide and the hydrated cobalt(II1) cation upon acidification. Job (4) prepared a green compound from potassium carbonate and cobalt(II1) to which he assigned the formula, KaCoOa. Blanchetiere and Pirlot (1) used the green complex of cobalt(II1) and carbonate as an indirect colorimetric method for the determination of potassium after precipitating NaK2Co(NOz)6. I n a similar way, Willard and Ayres (9) used this same complex for the colorimetric determination of cobalt, and Laitinen and Burdett (6) developed an iodometric method for the determination of cobalt based on this green complex. McCutcheon and Schuele (6) prepared the compound Co(N&)&o(COs)3 and measured its solubility in water as well as some other physical properties. Mori, et al. ( 7 ) claim to have precipitated K&o(CO&. There is, however, no apparent reference in the literature where tricarbonatocobalt(II1) has been used as a titrant. This work describes the preparation of solid hexamminecobalt(II1) tricarbonatocobaltate(II1) and the use of this reagent as an analytical titrant. EXPERIMENTAL
Preparation of Co (NH3)&o( C o d 3 . T o 50 ml. of 1M cobalt(I1) chloride, add 100 ml. of water and sufficient
sodium bicarbonate to saturate the solution. Cool the solution in ice water and then, with constant stirring, add dropwise 10 ml. of 30y0 hydrogen peroxide. Continue to stir the solution for 5 to 10 minutes to allow any excess hydrogen peroxide to decompose. Filter the resulting mixture through a fritted glass funnel. T o the clear, dark green filtrate add solid hexamminecobalt (111) .chloride until the supernatant liquid develops a n orange tinge. The resulting solution is again filtered through a fritted glass crucible and the solid material is washed repeatedly with water until the washings are colorless. The solid is then kept in a vacuum desiccator over magnesium perchlorate until dry. The dry solid is then stored a t room temperature in a screw cap bottle. The hexamminecobalt (111) chloride is prepared essentially by the method described in reference (3). This method consists of mixing 73 grams of cobalt(I1) nitrate hexahydrate in 100 ml. of water with 80 grams of ammonium nitrate, 2 grams of activated charcoal, and 180 ml. of concentrated ammonia. The resulting solution is oxidized with hydrogen peroxide or by bubbling air through the solution until the brown precipitate ceases to form. The precipitate is filtered through paper, washed with water, and then dissolved in 1300 ml. of hot water containing sufficient hydrochloric acid to give an acid reaction. The hot solution is then filtered to remove the charcoal, and 400 ml. of concentrated hydrochloric acid are added to the filtrate. The solution is allowed to cool and the solid Co(NH3)&13 is removed by filtration, washed successively with 60% and 95% ethanol and then dried in a vacuum desiccator. VOL. 37, NO. 12, NOVEMBER 1965
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