the magnesium with an excess of this reagent equivalent to 0.1Mfree TTA. In all cases the recovery of these two radionuclides, which were present in millicurie amounts, was found to be higher than 99.9% and their radiochemical purity was virtually 100%. The addition of concentrated hydrochloric acid to the uranium fission product solution and the subsequent reduction of the volume of the solution by evaporation as described in the working procedure, ensures that only less than 0.03z of the total niobium-95 activity is contained in the cesium eluate. The water wash following the removal of TTA with pyridine (see working procedure) is necessary because otherwise the 6 N hydrochloric acid, which is subsequently used to elute the cesium, would exothermically react with pyridine and form air bubbles in the resin bed. Strong interference in the separation of cesium-137 is caused by the presence of fluoride ion. A coadsorption of bismuth from 0.1M TTA-pyridine solution (see Table I) can be prevented by the addition of a small amount of hydroiodic acid-e.g., 0.1 ml of the concentrated acid in 10 ml of the feed solution. It is expected that the separation procedure described in this
paper is also applicable to the isolation of larger quantities of alkali metals from other materials such as trace impurities contained in alkali metal reactor coolants, soils, rocks, and organic matter. Because, however, macro amounts of the chlorides and other salts of sodium, potassium, alkaline earths, and other elements show a limited solubility in the TTApyridine media-e.g., only 0.07 mg of sodium chloride is soluble in 1 ml of 0.1M TTA-pyridine-a modified working procedure has to be employed. This consists of adsorbing the alkali metals on Dowex 50 from a 0.1M TTA solution which is 50 vol % in pyridine and 50 vol in water. Under these conditions the solubility of sodium chloride is 75 mg NaCl/ml of this mixture. If more sodium chloride is present, two phases are formed: an upper phase consisting mostly of pyridine in which TTA is dissolved, and a lower phase mainly consisting of water saturated with sodium chloride plus undissolved crystals of this compound.
z
RECEIVED for review January 18, 1968. Accepted March 28, 1968. Based on work performed under the auspices of the U. S. Atomic Energy Commission.
Sorption Isotherms on Liquid-Coated Adsorbents Acetone on Diatomaceous Earth Supports Coated with Tri-0-tolyl Phosphate and Squalane Paul Urone and Yoshihiro Takahashi Department of Chemistry, University of Colorado, Boulder, Colo. 80302
George H. Kennedy Department of Chemistry, Colorado School of Mines, Golden, Colo. 80401 Acetone static sorption isotherms were determined on three series of liquid-coated diatomaceous earth supports. The amounts sorbed by Chromosorb P and W coated with increasing percentages of tri-o-tolyl phosphate (TOTP) were equal to the sum of: the amount sorbed by the surface modified by the immediate’covering layer of TOTP molecules, and the amount sorbed by the remainder of the TOTP. The first (about 0.3% w/w) depended upon its own isotherm and was independent of the total amount of TOTP. The second depended upon the amount of excess TOTP and the isotherm for the bulk TOTP liquid. The squalane-coated Chromosorb P series showed that the surface of Chromosorb P was only slightly modified by squalane with respect to acetone, and the amount sorbed up to 2% squalane was essentially the same as that for the uncoated Chromosorb P.
THESORPTION of substances from gaseous or liquid media is of interest in a large number of disciplines. The sorption (or solution) of gases and vapors in liquids is described by Raoult’s law under ideal conditions and by Henry’s law at high dilution under nonideal conditions. The adsorption of gases by solids is described by a number of theoretical relationships, such as the Langmuir and Freundlich isotherm equations, and is characterized by a nonlinear relationship between partial pressure and amount adsorbed ( I ) . For liquid-coated ad(1) D. M. Young and A. D. Crowell, “Physical Adsorption of
Gases,” Butterworth, Washington, D. C., 1962.
1 130
ANALYTICAL CHEMISTRY
sorbents, there is a question as to whether adsorption, solution, or both are present. In gas-solid and liquid-solid chromatography, distribution coefficients frequently are not constant, and the resulting elution peaks usually tail for inconveniently long periods of time. In many cases this situation is also encountered in gas-liquid chromatography, where it is generally attributed to adsorption at the liquid-solid interface. The need to alleviate this situation coupled with the possible advantages of the greater selectivity afforded by a solid with a large surface area make it highly desirable that the sorption characteristics of coated supports be better understood. There have been a number of gas chromatographic studies of liquid surface adsorption on liquid-coated supports (2-5). Martire ( 4 ) summarized the studies and found that the effect of adsorption on the liquid surface was small compared to the solution effects except for those instances when a highly polar liquid coating was used and the solute was only slightly soluble in it. Pecsok and Gump (6) used static methods to study polar solutes in nonpolar solvents coated on several types of sup(2) R. L. Martin, ANAL.CHEM., 33, 347 (1961). (3) D. E. Martire, R. L. Pecsok, and J. H. Purnell, Trans. Faraday SOC.,61, 2496 (1965). (4) D. E. Martire, ANAL.CHEM.,38, 244 (1966). (5) R. L. Pecsok, A. DeYllana, and A. Abdul-Karim, ibid., 36, 452 (1964). (6) R. L. Pecsok and B. H. Gump, J . Phys. Chem., 71,2209 (1967).
port. They felt that both the gas-liquid interface and the solid support interfered with the determination of activity coefficients. For several years our laboratory has been concerned with studying the effects of the solid support upon the retentive characteristics of gas chromatographic columns. A method for highly deactivating a support surface was developed to provide a relatively inert reference surface (7). In addition a method of coating a support was developed to give a uniform dispersion of the liquid phase (8). This was important for studies of columns coated with less than 1 % liquid phase. Comparative studies of surface active and nonsurface active supports were unable to show liquid surface effects but did show that support effects were present even in columns coated with 20% or more liquid phase (9). Studies of the variation of distribution coefficients with concentration or sample size gave distribution isotherms which obeyed either a Freundlich or a Temkin adsorption equation but not a Langmuir equation (10). A number of workers have reported that distribution isotherms obtained by gas chromatographic methods duplicated sorption isotherms obtained by static methods (11,12). However, it was felt that with a sensitive microbalance more precise and more direct measurements of the isotherms could be obtained. For these purposes an apparatus was built which could measure the sorption isotherms of coated or uncoated supports in the partial pressure ranges approaching those encountered in gas chromatography (Figure 1) and of one to two orders of magnitude more sensitive than those previously reported. Two types of diatomaceous earth supports were selected as being representative of a large percentage of the type of supports used (13). They were Chromosorb P and Chromosorb W. Acetone was chosen as the adsorbate because it was intermediate in its relative polarity and because a considerable amount of related work has been done with it (9, IO). Two representative gas chromatography liquid phases were used to coat the supports. They were tri-o-tolyl phosphate (TOTP) and squalane, a C30 hydrocarbon. They were chosen to represent moderately polar and nonpolar type liquids, respectively. Distribution isotherms were obtained for each batch of coated support and for each of the bare supports at 55 “ C and at several other temperatures. The partition isotherms for each of the liquids were also determined. In a number of instances, the isotherms were determined with increasing as well as descreasing partial pressures of acetone. There were few to no differences in the isotherms, and in general hysteresis was absent. EXPERIMENTAL
Apparatus. Sorption isotherms were determined gravimetrically using a sensitive, fused quartz microbalance manufactured by the Ruska Instrument Corp., Houston, Texas. The balance was connected to a standard vacuum ( 7 ) P. Urone and J. F. Parcher, J. Gas Chromarog., 3, 35 (1965). (8) J. F. Parcher and P. Urone, ibid.,2, 184 (1964). (9) P. Urone and J. F. Parcher, ANAL.CHEM., 38, 271 (1966). (10) P. Urone, J. F. Parcher, and E. N. Baylor, Separation Sci., 1 (5), 595 (1966); “Separation Techniques in Chemistry and Biochemistry,” R. A. Keller, Ed., Marcel Dekker, New York, 1967, p 193. (11) S. J. Gregg and R. Stock in “Gas Chromatography 1958,” D. H. Desty, Ed., p 90, Butterworths, London, 1958. (12) R. H. Perrett and J. H. Purnell, J . Chromarog., 7, 455 (1962). (13) D. M. Ottenstein in “Advances in Chromatography 111,” Vol. 111, J. C. Giddings and R. A. Keller, Eds., Marcel Dekker, New York, 1965, p 137.
\
I\
Id*
L.---.-J
Figure 1. Experimental apparatus system equipped with a two-stage mercury diffusion pump and a mechanical fore-pump (Figure 1). The balance was capable of measuring weight changes as small as 10-6 gram for a sample weighing up to 2 grams. Acetone pressures were read with a manometer filled with Apiezon A oil. The vacuum side of the manometer required constant pumping to prevent pressure build-up caused by the diffusion of acetone through the oil. Pressures were read to the nearest 0.1 mm of oil (0.01 torr). The sample was contained in a quartz bucket and its temperature was controlled by a conventional oil bath to *O,l “C. No special precautions were taken to control the temperature of the microbalance as its temperature coefficient was negligible over the temperature range of the laboratory. Materials and Reagents. Chromosorb P (60- to 80-mesh, acid washed) obtained from Matheson Coleman & Bell and Chromosorb W (60- to 80-mesh) obtained from Wilkens Instrument and Research, Inc., were used throughout this work. Their surface areas (N2, BET) were approximately 4 meter2 per gram and 1.0 meter2 per gram, respectively (13). The supports were dried at 105 “C for several days before they were used. The squalane and TOTP were used as received from the above suppliers without further treatment. Spectrograde acetone and chloroform were used. Coating Supports. T o assure uniform coating, the method previously described by Parcher and Urone was used (8). About 20 grams of the support were placed in the coating chamber. A known concentration of TOTP-acetone (or squalane-chloroform) solution was prepared and poured into the coating chamber. After the support particles were well wetted by the solution, the excess solution was removed by suction. The chamber was then heated while dry nitrogen gas was passed through to remove the remaining solvent. When the sample was nearly dry, it was placed in a vacuum oven to complete the drying process. After coating by the above technique, each sample was gravimetrically analyzed to determine its exact per cent coating. Procedure. Twenty-gram batches of the well mixed supports were coated with each liquid in amounts ranging from 0.1 to 2 O x (w/w). The range of 0.1 to 1% was of special interest and several batches of each of the supports were coated in this range. The solid support material was weighed into quartz sample buckets which were then suspended from the beam of the microbalance. The sample weight was balanced by glass counterweights and the balance sensitivity adjusted depending upon the combination of solvent and solute used. After sealing, an oil bath was placed around the microbalance chamber and thermostated at the desired temperature. The sample was degassed at and 10-6 torr for uncoated supports and between torr for the coated supports. When no further weight change was observed, a small increment of acetone was added to the VOL 40, NO. 7 JUNE 1968
1131
0.8
-p
0.6
P
1
0.4
0.2
0.3k
0
P/Po x IO2
Figure 2. Sorption isotherms at 55 "Cfor acetone on Chromosorb P coated with 0, 0.11,0.30, and 0.49 % TOTP Q is the amount of acetone sorbed in milligrams per gram of coated support
system from a reservoir (Figure 1). After equilibrium was reached (when the microbalance reading became constant; within 5 minutes at most), the pressure was read on the oil manometer and the change in weight of the sample recorded. Further increments of acetone were then added, and the procedure was repeated for each datum point. The sorption isotherms were obtained for each sample by plotting the weight of acetone sorbed per gram of sample (milligram per gram) L ; S . the relative saturation pressure of acetone, (P/Po), where P is the measured pressure of acetone and Po is the saturation vapor pressure of acetone at the experimental temperature. Buoyancy effects were checked with nitrogen and were negligible at the partial pressures used. To simulate more closely conditions in a gas-liquid chromatographic column, the effect of helium as a carrier gas was also investigated by the static sorption method. Before any amount of acetone had been added to the system, purified helium (passed through a series of molecular sieve, activated charcoal, and silica gel filters) was added at a pressure of 20 torr. No weight change was observed. Acetone isotherms were then determined as described above and were identical to those determined without the presence of helium, although equilibrium was attained at a much slower rate. The slower rate was probably caused by the slower diffusion of the acetone through the helium to the sample. Consequently, all of the data presented here are isotherms determined without helium being present. RESULTS AND DISCUSSION
Figure 2 shows representative sorption isotherms for acetone on Chromosorb P coated with 0, 0.11, 0.30, and 0.49% TOTP. In addition to those shown, distribution isotherms were determined for 0.21, 1.0, 2.2, 3.5, 4.9, 15.8, and 19.8% TOTP and 0.37,0.68,2.0,7.6,10.2, and 14.6% squalane coated on Chromosorb P. It is important to note that the amount of acetone sorbed, Q, drops from a maximum on the uncoated support to a minimum at approximately 0.3 after which the amount sorbed increases as the amount of TOTP is increased. This minimum had been observed previously by an indirect gas chromatographic technique (9). It has not until now been reported by investigators using static methods of determining isotherms. Figure 3 shows the sorption minimum more clearly. The amount of acetone sorbed at a given partial pressure (PIP0 = 2.0 x lo-*) was obtained from the isotherms obtained for each of the TOTP-coated supports and was plotted against the per cent of coating on the support. The plot graphically shows that a minimum amount of sorption occurs at slightly more than 0.3 % coating of TOTP on Chromosorb P. Beyond
z
1132
e
ANALYTICAL CHEMISTRY
I 1.0
0
I 3.0
I 2.0 Wi.%
I
I
4.0
5.0
TOTP
Figure 3. Dependence of amount of acetone sorbed at 55 OC and P/Po = 2.0 x 10-2 on increasing percentages of TOTP coated on Chromosorb P
the minimum, the amount of sorption increases linearly with the per cent of TOTP. To explain these observations, it was reasoned that the sorptive capacity of the support was reduced to a minimum by the small amount of TOTP needed to deactivate (or more actively compete for) the strong adsorption sites on the surface of the support. The figure 0.3% TOTP on Chromosorb P corresponds to the amount required for monolayer coverage as determined by a rough calculation using 100 as the crosssectional area of a TOTP molecule. The minimum sorption capacity exhibited at this per cent coating could also be caused by the inability of TOTP to cover completely the surface because of stearic hinderance. The smaller acetone molecule could still squeeze its way to the unoccupied adsorption sites (14). It was also reasoned that the increased sorption above 0.3 coating was caused by the sorptive or solution capacity of the excess TOTP. Because the sorption isotherm of the bulk liquid had also been measured, it was easily possible to calculate what the sorption isotherms should be for the various coated supports assuming the above hypothesis. Thus, at each given partial pressure and at a constant temperature
Az
z
Qtotal
=
Qmin
+
QTOTP
where Qminis the amount sorbed by the deactivated support and Q T O Tis~the amount sorbed by the TOTP in excess of the 0.3 both obtainable from their respective isotherms. In more general terms for a constant temperature:
z,
QP
=
Ws
dQmin P + W L dQL -P dP dP ~
where Qp is the total amount of solute sorbed at a given partial pressure, P; dQmin/dPand dQLjdP describe the sorption isotherms of the deactivated surface and bulk liquid, and Ws and W Lare the weights of the support and liquid coating (above that needed to deactivate the surface), respectively. When the liquid isotherm is linear (as was the case for TOTP and squalane), the slope of the isotherm, k L , may be used and Equation 1 becomes : QP
=
W , dQmin P dP ~
+ WLkLP
Equation 2 is similar t o a gas chromatographic equation de(14) Sydney Ross and Ian J. Wiltshire, J. Phys. Chem., 70, 2107 (1966).
2.01
C
/ 0.4-
c
I
P m
-
E 0.3-
0
0.2
0.8
-
m 0.6
-
-
0
1.0
\
-F
______------2.0
3.0
4.0
5.0
P/Po x 102 0.4-
Figure 5. Calculated sorption isotherm c and experimental isotherm data points (circles) for acetone on Chromosorb W coated with 2.9 TOTP
0
Curve identities are the same as those for Figure 4
o,61 0.4
0
I.o
LOL
b
-____---I I 2.0
3.0
4.0
5.0
0 A
P/Po x 102
0
0.4p
Figure 4. Calculated sorption isotherms and experimental isotherms data points for acetone on Chromosorb P coated with 2.2 (I), 3.5 (II), and 15.9% (1II)TOTP
I
I 1.0
I 3.0
I
2.0
0.68 %
2.02 % 10.2 1;
I 4.0
I 5.0
P/Po x I 0 2
Dashed line a is the sorption isotherm for the deactivated support and dashed line b is the isotherm for the excess TOTP. Solid line c is the calculated isotherm, and small circles are observed data points
Figure 6. Sorption isotherms at 55 "C for acetone on Chromosorb P coated with 0,0.68,2.02, and 10.2 % squalane
rived for the retention volume, VAT,,of a solute on a liquidcoated surface active support (10).
(3) (4)
where C,, CL, and C, are the concentrations of the solute in the mobile, liquid, and support, and A , is the surface area of the support. Figure 4 shows the isotherms calculated with Equation 2 and observed data points for Chromosorb P coated with 2.2, 3.5, and 15.9% TOTP, respectively. Curve a in each shows the amount of acetone calculated t o have been sorbed by the deactivated support [ = W,(dQ,i,/dP)P]. On a per gram basis, curve a remains the same for all supports coated with 0.3 TOTP or more. Curve b shows the amount of acetone calculated to be sorbed by the TOTP in excess of that required to deactivate the support ( = WLICLP). The b curves depend upon the amount of TOTP present and are obtained from the isotherm for acetone in pure TOTP. Because the isotherm for acetone in TOTP is linear, all the b curves are linear. Curve c represents the sum of curves n and b. The experi-
mental points are represented by small circles. They coincide with the calculated line c so well as to seem t o define the line. The same type of curves and coincidence of observed data points was obtained for the TOTP-coated columns without exception. Figure 5, for example, shows that the same relationship holds for 2.9% TOTP coated on Chromosorb W, a support which has a different type of surface, Because Chromosorb W has a lower specific surface area, Qmin occurred a t 0.16% TOTP, and curve a was calculated on this basis. Figure 6 shows the sorption isotherms obtained for acetone on squalane-coated supports. The sorption isotherms are identical with the isotherm of the uncoated support up to a t least 2.0% squalane. At 7.6% squalane (not shown) and 10.2% squalane, the amount of acetone sorbed is approximately the same but less than the amount sorbed by the more lightly coated supports (Figure 6). At 14.6% squalane the sorption isotherm increases slightly, and a minimum is assumed t o exist somewhere between 3 and 10% liquid coating. It is apparent from Figure 6 that squalane at low concentrations does not deactivate the support surface within the solute partial pressure range used. Sorption occurs at a slightly slower rate than with TOTP, but is ultimately independent of the presence of squalane, although anomalies have been observed at the low partial pressures found in gas chromatography (9). At higher squalane concentrations sorption is decreased for lower P/Po pressures but is increased at higher PIP0 pressures. This could be caused by a number of VOL. 40, NO. 7,JUNE 1968
1133
indicates that the presence of a modified adsorbent surface is a more logical assumption and is consistent with previous studies involving moderately polar solutes and substrates
(4-6). 0.8
0.6
I .o
I
2.0
I
3.0 PIPo x 102
I 4.0
I
5.0
Figure 7. Sorption isotherms at 55 “C for methanol on Chromosorb P coated with 19.8 TOTP Curve identities are the same as those for Figure 4 reasons, among which may be a mass action effect or a partial blocking off of the support surfaces in the finer capillaries. Unsaturated impurities in the untreated squalane may have caused some of the anomalies, especially at the higher liquid loads. Figure 7 represents our most recent work and shows that the additive relationship also holds for methanol on TOTPcoated Chromosorb P. A number of other solutes, solvents, and supports are being studied and their relationship will be reported at a later date. CONCLUSIONS
The new details of sorption processes on liquid-coated adsorbents discovered in these studies should help develop new avenues of approach to applied and research activities in the fields of chromatography, catalysis, adsorption, and biochemistry where the special properties of large specific surface areas are of concern. The existence, under the conditions of the experiments reported here, of a new, apparently independently acting, adsorbing surface formed by a small fraction of the coating liquid should help in the understanding and development of surfaces with special physical or chemical properties. At this point, the new relationships seem t o be more easily recognized in those systems where a polar liquid solvent competes more strongly for the active adsorption sites than does the solute molecule. A minimum in the sorptive capacity of the system which occurs at low percentages of liquid coating remains constant. As the per cent coating is increased, the sorptive capacity of the increasing layers of the covering liquid adds to the minimum sorptive capacity. At no level of increasing per cent liquid load does this primary sorptive capacity appear to decrease, nor is there evidence that the sorptive capacity of the system as a whole decreases or increases as the surface area of the coating liquid becomes smaller or larger with per cent liquid coating. This eliminates the possibility of adsorption by the liquid surface as a significant factor. It
1134
ANALYTICAL CHEMISTRY
The data for the squalane-coated supports are interesting in at least two aspects. They move vividly show the competitive nature of the liquid-coated support for a solute in that the amount of acetone sorbed is not largely affected by the presence of the larger nonpolar squalane molecules. The data also show that acetone, which is only slightly soluble in solulane, easily penetrates the squalane to be sorbed by the underlying surface. Empirically, practitioners in gas chromatography have long used small percentages of subcoating substances such as stearic acid and polyethylene glycols or trimethyl silanization t o reduce the tailing effect of surface active supports (13). Such procedures lead to the formation of a modified and less active sorbent surface. Whether the magnitude of the additive effect of such a surface is large enough to be recognized in ordinary gas chromatographic methods or in measuring thermodynamic functions is a matter of conjecture and of continuing interest to us at this point. RECEIVED for review February 1, 1968. Accepted March 29, 1968. This investigation was made possible by a National Science Foundation Grant No. GP-5400.
Correct ions Potentiometric Measurements with CalciumSelective Liquid-Liquid Membrane Electrodes In this article by G . A. Rechnitz and Z . F. Lin [ANAL. CHEM., 40,696 (1968)l an error appears on page 699, column 2, just preceding Reaction 8. The value of k111 is given as 1.3 x 101lM-’ sec-1 but should have the units of 1.3 X 10 lM- sec-1.
A Technique for the Evaluation of Systematic Errors in the Activation Analysis for Oxygen with 14-MeV Neutrons In this article by S. S. Nargolwalla, M. R. Crambes, and J. R. DeVoe [ANAL.CHEM.,40, 666 (1968)l an error appears on page 667, column 1, line 6. The line should read “NO= Avogadros Number 6.023 X loz3.” Also, on page 667, “ Wt” “ Wt” Equation 3, for __ read -. U V
