For example, the A-15 resin, although qualitative, requires more energy for swelling than for the D-50 resin. Consequently, the A-15 resin must possess a more rigid resin matrix or must swell to a greater degree compared to the D-50 resin. It appears that both contribute to the net result. Since the D-50 resin collapses upon drying and must solvate before becoming solvent permeable, while the A-15 resin can be used in solvents which provide poor swelling properties, it is necessary to assume that the A-15 resin is more rigid and resistant to contraction (or swelling). Bead swelling for the A-15, although smaller than the D-50, also plays a significant role even though the resin is a porous type resin. The difference in rate of heat evolution for the two types of resins is consistent with their porous and gel character. Solvent must penetrate the dried, shrunken D-50 bead, and included in the overall process is a swelling rate. The A-15,
although shrunken, is still porous enough so that a more rapid rate is observed initially. Similarly, for the different M-form resins the more viscous n-propanol (in comparison to methanol) is able to penetrate the A-15 resin. For the D-50 resin in n-propanol the rate of heat evolution is so slow that precise A& measurements are not possible within the time permitted for the measurement. RECEIVED for review August 12, 1971. Accepted December 27, 1971. Research supported by National Institutes of Health under Grant GM15851 and by a National Aeronautics and Space Administration Traineeship (A.D.W.). Taken from the Ph.D. thesis submitted to the University of Iowa by A.D.W. in January 1970. Initial experiments were presented at the 154th National Meeting, ACS, September 1967, Chicago, Ill.
Apparatus for Supercritical Fluid Chromatography with Carbon Dioxide as the Mobile Phase R. E. Jentoft and T. H. Gouw Chewon Research Company, Richmond, Calif. 94802 An apparatus designed for supercritical fluid chromatography for operation at ambient temperatures and pressures up to 5000 psi is described. This particular unit has been especially developed for operation with C 0 2 as the mobile phase. An improved version of an earlier described mercury displacement pump ( 1 ) is used to drive the mobile phase. High pressure nitrogen is still the primary pressure source, but pistons having spring-loaded Teflon seals are used instead of mercury as the pressure transfer medium. By incorporating the pressure intensification principle in the unit, high output pressures can be attained with a regular 2000-psi nitrogen source. To prevent problems associated with the decompression of the supercritical phase to a gas, we have constructed a high pressure ultraviolet detector cell which has been successfully used up to 4300 psia. A high pressure fraction collector system is used to collect the eluant from the chromatographic system. The analysis of alkylbromides and of some metal organic compounds is given as examples of the separations which can be obtained with this unit.
MOSTOF THE NOTEWORTHY separations by supercritical fluid chromatography (SFC) reported so far are based on the use of mobile phases with critical temperatures above 100 "C (2-5). This aspect limits the applicability of the technique to compounds which are stable at these elevated temperatures. For the analysis of thermally labile compounds, it is desirable to employ an eluant with lower critical temperatures because the chromatographic separations can then be carried out at ambient temperatures. (1) R. E. Jentoft and T. H. Gouw, ANAL.CHEM., 38, 949 (1966). (2) R. E. Jentoft and T. H. Gouw, J. Chromatogr. Sci., 8, 138 (1970). (3) N. M. Karayannis and A. H. Corwin, ibid., p 251. (4) N. M. Karayannis and A. H. Corwin, J . Chromafogr., 47, 247 ( 1970). (5) S. T. Sie and G W. A. Rijnders, Separ. Sci., 2, 699, 729, 755 (1967).
Substances with a critical temperature below 100 "C are also gaseous at ambient conditions. This is advantageous in preparative separations because the solvent can be readily removed from the solutes of interest. The third major advantage is that under these conditions we can make use of the specialty packings, such as the Durapaks (6) and the Permaphase Zipax (7,8) which may not be very stable for extended operation above 150 "C even though some of these phases have been reported to be usable above 250-300 "C (8). These packings have superior chromatographic characteristics in comparison to regular supports. Sie and Rijnders have already observed in one of the earliest papers on this technique that at high fluid velocities, the major contribution to band spread with regular porous packing materials is due to the intraparticle resistance to mass transfer. This term is proportional to the average particle diameter of the packing. A drastic reduction in particle diameters can, however, lead to excessive pressure drops. This is deleterious to the chromatographic separation process because, in a supercritical fluid, the partition coefficients are strongly dependent on the pressure of the system. Specialty packings are usually spherical in shape and are available in very narrow mesh range sizes. Columns with much lower pressure drops can therefore be obtained in comparison to when regular packings with the same average particle diameter are used. Some specialty products such as the pellicular sorbent (9) or the controlled surface porosity supports (7, 8) have additional advantages when used in SFC. These packings have a fluid-impermeable core which results in a lower resistance to mass transfer in comparison to when the particles are completely porous. The use of these ( 6 ) I. Halasz and I. Sebestian, Angew. Chem., Znf. Ed., 8, 453 (1969). (7) J. J. Kirkland, J. Chromatogr. Sci., 7, 7, 361 (1969). (8) J. J. Kirkland and J. J. DeStefano, ibid., 8, 309 (1970). (9) C. Horvath and S. R. Lipsky, ibid., 7, 109 (1969). ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
681
Hlghpu$+p!Pressure
Tptube
I
i‘iI
Pressi Regula
I
Motor
t
y - R i n g Seal
High
Fraction Collector
Pressure N,
Figure 1. Apparatus for supercritical fluid chromatography at ambient temperatures packings can therefore result in a substantial increase in separation speed. OPERATION AT AMBIENT TEMPERATURES Several papers have already described the details of SFC for operation at somewhat elevated temperatures (2, 5, 10). The construction of a unit where the critical temperature of the mobile phase is close to ambient levels is not as straightforward as one might expect. The largest problem in this assignment is related to the area of component detection. The detectors in current use have found previous application in gas or in liquid chromatography and can generaly be operated only at atmospheric or not too elevated pressures. In SFC we are bound by a generalized phase relationship (Guldberg’s Rule) (II), in which the critical temperature, T,, has been found to be related to the boiling point at atmospheric pressure, Tb, by the following approximate function: Tb = 213 T,
(1)
where the temperatures are given on an absolute scale. Those mobile phases which are liquid at ambient pressure and temperature have critical temperatures of around 200 O C or higher. Compounds with critical temperatures below 150 O C are generally gaseous at atmospheric pressure and room temperatures. Thermally stable solutes are conveniently analyzed with eluants with a T, above 200 OC. After decompression to atmospheric levels, the chromatographic effluent can be introduced into a regular liquid chromatographic detector. If decompression of the mobile phase results in a gas (T, 7 150 “C), one would expect to be able to use the regular gas chromatography detectors for detection purposes. However, because of their lack of volatility, high molecular weight solutes tend to agglomerate or to condense on the walls of the system in this decompression process, making detection extremely difficult. For the same reason, the same phenomena can be expected to take place with lower molecular weight solutes which are ionic in nature. If the solute is gaseous under these conditions, true solutions will result; and detection should pose no particular (10) N. M. Karayannis, A. H. Corwin, E. W. Baker, E. Klesper, and J. A. Walter, ANAL.CHEM.,40, 1736 (1968). (11) J. R. Partington, “Treatise on Physical Chemistry,” Longmans, Green, and Company, New York, N.Y., Volume 1, 1949. 682
ANALYTICAL CHEMISTRY, VOL. 44, N O . 4, APRIL 1972
problems. However, for these compounds, gas chromatography would have sufficed to carry out the desired analysis. In the approach described in this paper, a high pressure ultraviolet (UV) detector cell and a high pressure sample collection system are used. These components obviate the problems associated with the decompression of the mobile phase to a gas at ambient pressure. INSTRUMENTATION A schematic diagram of the system is shown in Figure 1. High pressure nitrogen is used as the primary pressure source. A pressure programmer, consisting of two pressure regulators and a variable speed motor (Z),is used to control the inlet pressure to the pump. This pump is a recently constructed, improved version of the mercury displacement pump described earlier (1). The other units in the system are the chromatograph oven, a thermostat to control the temperature of the mobile phase prior to its entering the detector compartment, a high pressure UV absorption detection system, and a high pressure fraction collector. Pump. Of major importance in SFC is the ability of this technique to process wide-boiling range mixtures successfully by pressure programming the system during the chromatographic run (2). In the region close to the critical point, a sharp decrease in partition ratios is observed with only a slight increase in pressure. Pressure programming can, however, be successfully applied in a wide range of pressures and temperatures above the critical conditions. The pumping system for a supercritical fluid chromatograph should, therefore, be able to generate high pressures, be pulseless, and have an output which can be pressure programmed. It should be noted that the newer pumps for high resolution liquid chromatography, such as the multiple-stroke piston pump or the large-volume syringe-type pump, are designed to operate with adjustable flow and with the pressure as the dependent variable. These pumps are, therefore, somewhat unsuitable for pressure-programmed applications. For pumps in SFC, the output pressure should be adjustable with the flow observed as the dependent variable. The high pressure pulseless mercury displacement pump, described several years ago ( I ) , was very useful for SFC applications because it fulfilled the criteria described above. The large amount of mercury involved was a disadvantage; the output pressure was also limited to the nitrogen source pressure. Another problem was the instability of the unit observed in handling fluids, such as COz and fluorocarbon refrigerants, which have high vapor pressures at ambient temperatures.
The new pump retains the capabilities of the old unit but does not have any of the three drawbacks cited. A schematic diagram of this pump is shown in Figure 2. The unit consists of two identical stainless steel compound cylinders which are connected to each other by a system of check valves and solenoid valves. Each syringe consists of a I/&. i.d. by 1l/a-in. 0.d. by 8-in. long top cylinder connected to a 1112-in.i.d. by 3-in. 0.d. by 8-in. long bottom cylinder. A double-headed piston is used to transfer the pressure from the bottom cylinder to the top portion of the syringe. Because of the difference in diameters, an approximately 340-1 pressure intensification effect is obtained. To ensure good sealing and yet maintain a low friction between the piston and the cylinder walls, spring-loaded Teflon seals are used around the pistons. Teflon (Du Pont) is one of the very few elastomers impervious to liquid COS. Other elastomers tend to dissolve COS at high pressures. Dissolution during the decompression cycle results in blisters and cracks in the polymeric material. Graphite-filled Teflon is used around the top piston. Since the bottom piston does not come into contact with the mobile phase, lubrication can be applied to reduce friction; and seals of regular virgin Teflon can be used. High pressure tubing fittings, and accessories are used in the pump. Swagelok fittings are used throughout the rest of the apparatus. In the “normal” mode, high pressure nitrogen is introduced into the bottom compartment of the Compound Cylinder A. The position of the three-way solenoid valves is shown on the bottom of Figure 2. Piston A is forced upward to pump the mobile phase into the chromatographic system. Check Valve CI prevents the mobile phase from entering Cylinder B. During the same period, the piston of Cylinder B is forced downward because of the fresh mobile phase which is being fed into this syringe from the main solvent reservoir. The 200-pound pressure relief valve at the outlet of the bottom cylinder of Syringe B acts as a brake; it prevents the piston from being slammed down as the solenoids are activated into this “normal” mode. During this process, the nitrogen in the bottom cylinder of this syringe is vented into the atmosphere through a capillary bleed. A micrometering valve is mounted at the outlet of the capillary to provide control of the bleed rate. To facilitate the transfer of mobile phase from the main reservoir to the top portion of Syringe B, the latter is thermostatically controlled at a temperature below the critical temperature of the mobile phase and also the temperature of the main reservoir. This reservoir can be a regular high pressure gas cylinder; the fluid of choice is preferably drawn from this container through a dip tube to ensure that liquid instead of gas is being transferred into the lines. Proximity switches are mounted close to the junction of the two different diameters of each syringe. Magnets are mounted on the top of each bottom piston. As soon as the bottom piston in Cylinder A has almost reached its highest point, the magnet activates Proximity Switch PA; the solenoids then go into the configuration shown as the “filling” mode in Figure 2. Now the bottom half of Cylinder B is connected to the high pressure nitrogen source. This configuration allows Cylinder B to fill Cylinder A with fresh mobile phase while still pumping that phase into the chromatographic system at the controlled high pressure. The Check Valve C1 prevents the mobile phase from being forced back into the main reservoir. During this process, the gas in the bottom half of Cylinder A is bled off to the atmosphere through another controlled capillary bleed. Proximity Switch PBsenses the end of the travel of Piston B and activates a circuit to switch the solenoids back to the “normal” mode. The duration of this normal mode is obviously dependent on the flow rate in the system. The “filling” mode takes about 1.5-2 minutes at a pressure of around 1000 psi to just a few seconds at pressures above 3000
Proximity Switches
Nz
Vent
e
Three-way Solenoid Valves Cycling Mode
“Filling”
,
Solenoid Configuration
+
,
Proximity Switch A t End of Cycle PA Activated At End of Cycle Pg Activated
Figure 2. High-pressure pulseless pump. The three-way solenoid configurationsare shown on the bottom Injector
-2-f
-
6” x 0.006” Capillary :
FromPump
Bath Fluid
Heater
Alternate Columns
Figure 3.
Insulation
Chromatograph oven for two-column operation
psi. The pressure gauges, PI to P4, are included to give a visual indication of the performance of the pump. This pump has now been in successful operation for many months. With a 2000-psi N2 pressure source, the unit will recycle through both the “normal” and “filling” mode at output pressures up to 5000 psi. In this respect, the term “pulseless’’ should be considered from a practical point of view in relation to a regular piston pump. When either of the proximity switches are activated, a momentary deflection can usually be observed on the pressure gauges, PI and P2. If our chromatographic detection system is operated at full sensitivity (0.23 absorbance unit per inch), a momentary excursion of the pen of l/An. or less is sometimes observed. Since the volume of mobile phase which can be displaced between two cycles is approximately 60 ml, this amount is generally sufficient for a complete chromatographic run. The pump can hence be adjusted not to cycle during the run or to cycle outside the regions of interest. Chromatograph Oven. The chromatograph oven, shown schematically in Figure 3, is essentially a thermostated box in ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
683
,No. 2 'U"-Rinq
Figure 4 shows the injector assembly in current use. It is basically a liquid chromatograph inlet system obtained commercially (Precision Sampling Corporation, Baton Rouge, La.), which has been modified for our particular needs. A '/An. by l/&n. Swagelok reducing union has been substituted for the septum retainer fitting. Two "0"-rings are used instead of a septum to seal the syringe needle. The Swagelok bottom portion has been modified to accept a 1/~6-in, fitting. Detector. Almost all continuous detection systems in chromatography are constructed to operate a t ambient or only slightly elevated pressures. We have already touched upon the problems associated with the decompression of a supercritical mobile phase to a gas. In the detection system described here, the temperature of the mobile phase is decreased to below the critical temperature in a 2441-1.by 0.01-in. capillary tubing between the outlet of the chromatographic column and the inlet of the detector (Figure 1). This section is cooled to, e.g., 25 "C by the use of a heat exchanger jacket connected to a thermostat. By maintaining the pressure in the system at sufficiently high levels, the mobile phase will be a liquid under these conditions. For C02 at 25 "C, the minimum pressure is 930 psia; a t 20 "C, the vapor pressure of liquid CO, is about 830 psia. Above these minimum pressures, no solutes are expected to precipitate out of solution. Even though many compounds will show decreased solubilities a t lower temperatures, the density of liquid C o n , and hence its solubilizing power, is at least two to three times that of the corresponding supercritical phase. The latter phenomenon should be more than adequate to maintain a homogeneous solution. It should be pointed out that if it were possible to maintain the detector cell above T,, then one would connect the column outlet directly to the detector [as in the unit described by Karayannis et ai.( l o ) ]to reduce band spread caused by condensation in the capillary. Work is in progress to achieve this capability. The detection proper is carried out in a high pressure UV detector cell. Liquid C02 is quite transparent in the UV down to 210 nm. This class of detector cells with pressure ratings up to 1000 psi is now available commercially. Karayannis et al. reported a cell which can withstand pressures up to 2000 psi (10). The cell described in this paper has been successfully used up to 4300 psia. A capillary valve (Precision Sampling Corporation, Baton Rouge, La.) is mounted between this detector and the thermostated capillary tubing to provide for a n additional pressure drop, should much higher column outlet pressures be encountered in this unit. The high pressures capability of the cell is, however, important to obtain good chromatograms. Large pressure drops over this capillary valve would result in a noticeable Schlieren effect in the detector cell because of the difference in
Aluminum Disk
nlet
Figure 4. Injector Assembly which the columns are maintained at the desired operating temperatures. The unit has been designed especially with operational versatility in mind to facilitate study of the technique. The ideal form of the oven/column arrangement for routine operation would be different. The mobile phase first goes through a 30-ft by lts-in. preconditioning coil where it acquires the same temperature as the columns and becomes supercritical. Note that two alternate columns can be mounted in this unit. In the drawing shown, the right-hand column is being used. The mobile phase flow is split into two streams a t Junction JA,with one portion going through a 6-in. by 0.006-in. capillary and another portion going through the normally closed Valve VA to a tee a t the bottom of the injector. The injected sample is deposited between T1 and T2 by a syringe having a long needle. Valve VA is opened during the injection period to ensure that all sample is flushed into the column. It is closed during the regular portion of the run. Valve VB is always slightly cracked open to ensure a very small stream of the mobile phase going through the alternate column to prevent the injected sample from going into this section. By reversing the connections a t VB and CA,the right-hand column becomes the alternate column; and the left-hand section becomes the operational unit. This configuration allows us to test two different columns by a simple change of two fittings.
1.59 mrn 0.D. 0.56mm I . D . Inlet and Outlet Tubes
Figure 5. High pressure UV detector cell
-
The cell is symmetrical. The exploded portion of the drawing refers to the front half of the cell only Channels
I 684
1 mrn I.D. 15 mm tong
ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
Spacer Teflon
To Back Pressure Regulator and V e n t 7 1
f-
From Detector
Outlet
I
I
I
I
I
10
20
30
40
50
T i m e , Minutes
Figure 7. Separation of Cs-Czon-alkylbromides
I
L3.5”
I ---
Dicyclopentadienyl Fe (Ferrocene)
1
Figure 6. High-pressure fraction collector density of the mobile liquid phase before and after this valve. Density differences are enhanced by the cooling which is incident t o this process because there is no opportunity for thermal equilibration. A schematic diagram of the split flow high pressure UV detector cell is shown in Figure 5 . It is mounted in a Cary 10-11 spectrophotometer. The detector volume is 14 pl. The split flow concept to decrease the dead volume without a concomitant increase in sensitivity toward flow and temperature fluctuations has been reported earlier by Felton (12). The UV radiation source is focused through the bottom channel of the cell only. Teflon spacers are used between the quartz lenses and the metal body of the cell to provide the channels between the two parallel holes and to prevent the lenses from corning into contact with the metal. When the mobile phase is COz, the inner Viton “0”-ring is replaced by a Teflon “0”-ring. A second capillary valve for flow control is located a t the outlet of the detector. If the effluent is to be vented into a n environment a t lower pressure, the necessary pressure drop is then taken over this valve. It can be closed for stopped-flow chromatography if one wishes to obtain the UV absorption spectrum of a n unknown substance in the cell with the Cary 10-11 spectrophotometer. Sample Collection System. Workers in the field of preparative gas chromatography are familiar with the problems of trapping high molecular weight solutes emerging from a hot column outlet. It is very difficult to achieve quantitative recoveries because of the tendency of these materials t o form a fine fog. This fog formation is also observed in SFC during the decompression of the mobile phase to a gas. In our sample collection system, the environment is maintained at sufficiently high pressure to allow the chromatographic effluent to remain liquid. A schematic diagram of this fraction collector is shown in Figure 6. It is essentially a 3-in. i.d. by 7-in. high stainless steel pressure vessel with a high pressure polycarbonate window through which one can observe the inside of the unit. A light is mounted inside the vessel t o facilitate viewing the operations. The turntable in the unit is coupled magnetically to a movable ring mounted outside the unit. In this pressure vessel, we can place up to six test tubes or vials of 20-ml capacity each held in position around a short glass cylinder by a rubber band. By adjusting (12) H. Felton, “Advances in Chromatography 1969,” A. Zlatkis, Ed., Preston Technical Abstracts Company, Evanston, Ill., 1969, p 3 1 5 ,
Cyclopentadienyl Mn (CO),
c
.-z
:.P
8 ‘ .-.0
Dicyclopentadienyl
.-1lc
-
I
10
I
M
I
I
30
40
J
50
Tine. M i n u t e s
Figure 8. Separation of ferrocene (dicyclopentadienyl iron), cyclopentadienyl Mn(C0)3,and dicyclopentadienyl TiClz the backpressure regulator a t the outlet of the unit, the emerging mobile phase can be maintained in the liquid state. By allowing a greater bleed through this regulator, the solvent can be vaporized off carefully without bumping. The solutes of interest are then left behind in the test tubes. The unit can be dismantled quite readily by turning off the bottom cap. If there is still residual pressure in the system, the cap is “locked” in place by the friction of the “0”-ring. This safety feature prevents the unsuspecting worker from opening the unit while it is still under pressure. The flowmeter which is mounted a t the outlet of the regulator is used to give a n indication of the flow rate in the system. This collection system has proved to operate very well with, e.g., liquid COz as the mobile phase. APPLICATIONS
Figure 7 shows a chromatogram of a synthetic mixture of Cs-C20 n-alkylbromides. The first major peak corresponds t o n-octylbromide; the two small deflections a t the tail end of the chromatogram are probably dibromoparaffins. Alkylbromides are generally rather difficult t o analyze by gas chromatography because of their thermal instability during the volatilization process in the hot injector. The analysis shown was carried out on a 1-m by ’/&ne 0.d. stainless steel column packed with a commercially obtained Carbowax 400 “brush” packing on 100/200 mesh Porasil F. COz was used as the supercritical mobile phase. The column temperature was maintained isothermally a t 40 “C; the column inlet pressure was programmed from ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
685
1070 psi to 2000 psi at 20 psi/min. The sample size was 2.5 pl. Detection was carried out at 217 nm. This large sample size was necessary because alkylbromides have a low extinction coefficient in the UV, even at a wavelength as low as 217 nm. Some overloading of the column may be the result. Figure 8 shows a chromatogram of a synthetic mixture of ferrocene (dicyclopentadienyl iron), cyclopentadienyl Mn(CO)a, and dicyclopentadienyl Tic&. These metalloorganic compounds are not very stable at elevated temperatures. Dicyclopentadienyl TiClz (as well as other cyclopentadienyl compounds) is reported to be a 1 :1 electrolyte according to conductivity measurements in various ethers (13). The
separation shown was carried out on the same column described previously. The temperature was maintained at 33 “C. Detection was carried out at 250 nm. The pressure was held isobarically at 1000 psi for 30 minutes to resolve the iron and manganese compounds and then rapidly increased in pressure to 3000 psi at 130 psi/minute to elute the titanium compound. This particular program type is especially beneficial for the separation of similar substances in the presence of a compound with a very much longer elution time.
(13) W. Strohmeier, H. Landsfield, and F. Gernert, Z . Electrochem., 66, 823 (1962).
RECEIVED for review September 13, 1971. Accepted December 7,1971.
Synergistic Effects in Ion Exchange in Mixed Solvents-C hloride A. P. Rao and S. P. Dubey Chemistry Department, Indian Institute of Technology, Delhi, New Delhi-29, India
The anion-exchange behavior of Co(ll), Cu(ll), and UOz(ll) in ketone-alcohol-HCI mixtures was studied using Dowex L X 8 . The synergistic enhancement of Kd values while using mixtures of two solvents has been attributed to increased formation of the species undergoing exchange equilibria in the mixture as compared to the individual solvents. From the absorption spectral measurements, the species taking part in the exchange equilibria are shown to be CoCI3-, CuCI3-, and U02C12in the solution phase. They are present as CoCI2-, CuCI42-, and as a mixture of UOzClzand UO2CI3on the resin.
Solvents. Reagent grade methanol, ethanol, n-propanol, acetone, methyl ethyl ketone, and diethyl ketone were purified by standard methods ( 4 ) and used. Determination of the Elements. Co(I1) and UO,(II) were determined spectrophotometrically ( 5 ) while Cu(I1) was determined using micro EDTA titrations (6). The distribution coefficients ( K d )were calculated from; mg of the element/g of the resin K d = mg of the element/ml of solution
KORKISCH (I) HAS REPORTED that the adsorption of uranium onto the anion-exchange resin Dowex 1-X8 from methanol or ethanol solutions, increases when part of the alcohol is replaced by dibutyl-carbinol or butoxyethanol. The distribution values are higher in the mixture than in individual solvents. Subramanyam and Sastri ( 2 ) made similar observations during their studies on the adsorption of cobalt from alcohols and acetone. Recently we (3) have also reported similar enhanced distribution values for some elements in acetone-methanol-HC1 mixtures, using the anion exchange resinDowex 1-X8. This work was undertaken to investigate the mechanism for this synergistic enhancement in Co(II), Cu(II), and UO2(II) in ketone-alcohol-HCl mixtures.
The distribution coefficients were determined by the batch equilibrium method (batch method). Each equilibrium was performed with 50. ml of a mixture containing sufficient HCl to give the required overall acid normality, and 5 mg of the element. To this mixture, 1 gram of the resin was added and the solution was agitated for 12 hours at room temperature (30 f 1 “C). The resin was then separated by filtration and the element was determined in the filtrate, after evaporating the organic solvent and the excess acid. The experimental eiror for the determination of distribution coefficients was =k5 for K d values below 1000 and f10 for Kd values above 1000. Spectrophotometric Studies. The absorption spectra were taken on Unicam SP 500 and SP 700 spectrophotometers using 1-cm cells. For taking the spectra of the metal complexes adsorbed on the resin, a procedure similar to that of Ryan (7) was employed using 2-mm cells.
EXPERIMENTAL
RESULTS AND DISCUSSION
Materials. The strongly basic anion exchanger Dowex 1-X8 (chloride form, 20-50 mesh), dried at 30 OC was used for the experiments. Standard Solutions. Exactly weighed amounts of Co(II), Cu(II), and U02(II) as chlorides were dissolved in 0.01M HCl to give solutions containing 5 mg of the element per ml.
The variation of Kd values of Co(II), Cu(II), and UOz(I1) with the composition of the solvent mixtures at various HC1 concentrations is given in Figures 1, 2, and 3 . It can be seen from the Figures that the Kd values pass through a maximum. On the other hand, other ions which were studied previously (3) [e.g., Zn(II), Cd(II), Ni(II), and Th(1V)I did not
(1) J. Korkisch, Progress Report to IAEA and U.S. Atomic Energy Commission under Contract No. At (30-1),2623, Oct.
(4) A. Weissberger, “Technique of Organic Chemistry, Organic Solvents,” Vol. VII, Interscience, New York, N.Y., 1955. ( 5 ) G. Charlot, “Colorimetric Determination of Elements,”
1964.
(2) J. Subramanyam and M. N. Sastri, J. Inorg. Nucl. Chem., 31,
199 (1969). (3) A. P. Rao and S. P. Dubey, hid. J . Chem., 7 , 396 (1969). 686
ANALYTICAL CHEMISTRY, VOL. 44, NO. 4, APRIL 1972
z
z
Elsevier, Amsterdam, 1964. (6) H. Flaschka and H. Abdine, Chemist-Analyst, 45, 2 (1956). (7) J. L. Ryan, Inorg. Chem., 2, 348 (1963).
