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Separation of Mixtures of Cations by Donnan Dialysis Sir: Donnan dialysis is typically performed by using an ion exchange membrane to separate a relatively low ionic strength sample from a concentrated receiver electrolyte. In this mode sample ions with the appropriate charge sign are transported into the receiver electrolyte at nearly equal rates. This method is made into an enrichment technique by using a receiver electrolyte volume which is less than that of the sample (1, 2). The general characteristics of the receiver electrolyte are unchanged by the introduction of the sample ions. Further, the transport rate is proportional to the test ion concentration and independent of other sample components over a wide range of conditions. Hence, Donnan dialysis provides matrix normalization (3-5). In our earlier work we observed that the presence of EDTA in the receiver electrolyte promoted the transport of Cu(I1) when the receiver was a t a pH which favored chelation (5). Other of our studies have also shown that the transport of ions across an ion exchange membrane is governed by the receiver rather than the sample composition. For example, Cu(I1) was dialyzed from humate media if the receiver pH was in the range where the copper humate complex was dissociated (5); under such a condition the transport was independent of sample pH (and hence the equilibrium state of Cu(I1) in the sample). The above results suggested a method of ion separation by Donnan dialysis. The receiver electrolyte concentration could be lowered to cause a decrease in transport rate for all ions. The electrolyte could be selected to be a poor promoter of Donnan dialysis to further decrease the general transport rate. Now a complexing agent for the cation of interest could be added to the receiver and the electrolyte adjusted to a favorable pH for complex formation. The present paper demonstrates that this scheme will result in preferential Donnan dialysis of the selected cation.
Table I. Effect of the Receiver Electrolyte on the Donnan Dialysis of Cu(I1) sample M Cu, pH M Cu, pH M Cu, pH M Cu, pH M Cu, pH
5 5 5 5 5
M Cu, pH 5 a
receiver
EF a
0 . 2 M MgSO,, pH 2 0.1 M MgSO,, pH 2 0.05 M MgSO,, pH 2 0.01 M MgSO,, pH 2 0.01 M MgSO,, 2 mM IDA, pH 6 0.01 M MgSO,, 2 mM IDA, pH 2
4.8 5.0 0.7 0.3 4.5 0.4
([Cu(11)] in receiver after dialysis)/([Cu(II)] sample).
Table 11. Donnan Dialysis Separation of Cu(I1) from Co(I1) and Ca(I1) sample
receivera pH
EF Cu
EF Co
3.0
1.1
0.1
7.0
3.5
3.2
7.0
3.6
10-5 M CU, 10-5 M CO, PH 5 10-5 M CU, 10-5 M CO, PH 5 10-5 M CU, M Ca, pH 5 a
EF Ca
0.2
0.02 M LiCl, 2 mM IDA.
EXPERIMENTAL SECTION The work was performed with P-1010 cation exchange membranes (RAI Research Corp., Hauppauge, Long Island, NY)which were soaked in 0.1 M HC1 and in the receiver electrolyte for a few hours prior to use. The complexing agent was not added to the electrolyte during this step. The membranes were attached across the bottoms of ca. 2 cm diameter glass tubes with Teflon tape and O-rings. The receiver electrolyte, 5.00 mL, was pipetted into the cylinders. The dialyses were initiated by dipping these cells into 100 mL volumes of magnetically stirred samples. After 0.50 h the contents of the receiver were transferred to 10-mL volumetric flasks and diluted to the mark with a solution of the same composition asthe receiver electrolyte. The metals were determined by atomic absorption spectrometry (Varian Model 475). The proposed separation method was tested with iminodiacetic acid (IDA) as the chelating agent in the receiver electrolyte. Either MgS04 or LiCl was used as the receiver electrolyte. The pH was adjusted with dilute HC1 or NHIOH prior to dialysis.
RESULTS AND DISCUSSION Our previous studies indicated that with a high concentration of Mg(I1) in the receiver, the Donnan dialysis of cations approaches diffusion control (6). The Table I data demonstrate that by decreasing the Mg(I1) concentration a point is reached a t which a kinetic barrier to Donnan dialysis apparently is introduced. The rate of Cu(I1) transport across the cation exchange membrane is thus decreased. Addition of the IDA restored the rate to near the diffusion limit. That a constant E F was observed above 0.1 M MgS04 demonstrated
pr Figure 1. Donnan dialysis of cations into a complexing medium: reM single ceiver, 0.02 M LiCI, 2 mM IDA at stated pH; samples, component solutions of the metal at pH 4; (a) Cu(II), (b) Co(II),(c)
Ca(I1).
that the trend was not simply due to a change in the Donnan potential. This interpretation is also supported by the effect of pH; Cu(I1) transport was not promoted by IDA when the receiver pH was too low to permit significant chelation of Cu(I1) by IDA. The effect of the receiver pH on the EFs for Cu, Ca, and Co into dilute electrolytes is summarized in Figure 1. The data are consistent with the relative values of the IDA complex formation constants for these metals. The data display suggests that at pH 3 preferential transport of Cu from a Cu, Co mixture should be observed and at pH 7 , Cu should be likewise separated from a Cu, Ca mixture. The results in Table I1 substantiate this prediction. Compared to the values in Table I the EFs are lower. This is the result of using LiCl rather than MgS04 as the electrolyte in order to obtain a higher separation factor. The results in Table I1 are independent of sample pH over the range 2-6. Further, the use of Ndion 117 cation exchange membrane (DuPont Polymer Products, Wilmington, DE) yielded the same general trend in the results. The above results indicate that Donnan dialysis may have a capability for performing separations similar to solvent
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extraction. Because some transport of the uncomplexed ion will occur, separation factors will not be as great as by extraction; however, Donnan dialysis has some potential advantages. It can be adapted to continuous monitoring of streams (7), and the aqueous receiver system can be coupled to certain analytical methods more readily than the organic phase of solvent extraction. Presently the use of specific complexing agents is being explored to extend the range of application. LITERATURE C I T E D (1) Lundquist, G. L.; Washinger, G.; Cox, J. A. Anal. Chem. 1975, 4 7 , 319-322. (2) Blaedel, W. J.; Kissel, T. R. Anal. Chem. 1972, 44, 2109-2111. (3) Cox, J. A.; Cheng, K. H. Anal. Lett. 1978, 11, 653-660. (4) Cox, J. A.; Twardowski, 2 . Anal. Chim. Acta 1980, 119, 39-45. (5) Cox, J. A,; Twardowskl, 2 . Anal. Chem. 1980, 52, 1503-1505.
(6) Cox, J. A,; DiNunzio, J. E. Anal. Chem. 1977, 49, 1272-1275.
James A. Cox* E w a Olbrych Department of Chemistry and Biochemistry Southern Illinois University at Carbondale Carbondale, Illinois 62901 Krystyna B r a j t e r Department of Chemistry University of Warsaw Warsaw 02-093, Poland
RECEIVED for review February 17,1981. Accepted April 10, 1981. This work was supported by the National Science Foundation under Grant CHE-7908660.
AIDS FOR ANALYTICAL CHEMISTS Teflon FEP Coil as Reactor for the Photochemical-Conductivity Detection in Liquid Chromatography Paolo Clccioll, Rem0 lappa, and Alfred0 Guiducci Istltuto sull’lnauinamento Atmosferico del C.N.R., Area della Ricerca di Roma, Via Salaria Km 29.300, C.P. 10. 000 16 Monterotondo Scalo, Italy
In the last few years, many specific detectors have been developed for high-performance liquid chromatography (HPLC). Among them, photochemical-conductivity detector (PCD) (1,2)seems to possess some unique features which may make it particularly valuable for the solution of trace organic problems, especially in the environmental, biological, and pharmaceutical areas. These features can be summarized as follows: (1) high selectivity for the detection of halogenated as well as many nitrogen- and sulfur-containing compounds; (2) high sensitivity allowing the determination of several compounds in the nanogram or, in some instances, low picogram range; (3) simple design which makes the PCD inexpensive and largely accessible to any laboratory. However, the achievement of the optimal PCD working conditions presents some operational problems which render its use difficult and time-consuming. The reasons of these difficulties can be understood by analyzing in detail the PCD working mechanism. Basically the PCD is comprised of a splitting device, a postcolumn reactor, and two conductivity cells placed in parallel. The sample emerging from the column is split in two streams. On the analytical side of the detector, the sample passes through a quartz reaction coil where it is irradiated by an intense UV lamp functioning as ionizing source. This effluent then passes into the analytical conductivity cell. On the reference side, the sample passes through a Teflon delay coil having the same dimensions as the quartz reaction coil and then reaches the reference conductivity cell. The signal is obtained as an output of the difference in conductivity of the irradiated and nonirradiated flow paths of the sample. Optimal sensitivity and stable base line result when the amount, band spreading, and delay time of the samples passing through the analytical and the reference cell are the same. Since these parameters are difficult to control when different materials are employed for the construction of the UV reactor and the delay coil, the flow rates passing through the two cells need frequent adjustments. This
operation is very critical and time-consuming because no flow controllers connected to the cells outlet can be employed. As such devices generate an overpressure within the detector, the disconnection (or even the breakage) of the quartz reaction coil might occur. For the same reason no flow rates exceeding 3 mL/min can be employed with the PCD, and it must be placed as the last component when used as a part of a multidetection unit. Since the devices developed for reducing the formation of gaseous bubbles into the liquid stream cannot be employed, it is difficult to carry out analyses with volatile eluants (such as methanol) when flow rates lower than 0.5 ml/min are needed. The aim of the present work is to suggest a simple and inexpensive way to overcome the above limitations and to improve the operability and the versatility of the PCD without affecting its overall performances. EXPERIMENTAL SECTION A Tracor PCD Model 965 (Austin,TX) in tandem with a Varian 5000 liquid chromatgraph (Varian, Walnut Creek Division, CA), was used for our investigations. A Variscan UV absorbance detector was used as an additional component for the multidetection unit. Two different columns were employed for the chromatographic analyses: a 25 cm X 4 mm i.d. stainless steel column packed with Micropack MCH (10 pm) supplied by Varian and a homemade 25 cm X 2 mm i.d. glass column packed with Carbopack B (Supelco, PA) 25-33 pm (3). Teflon FEP tubing 1/16 in. 0.d. X 0.015 in. i.d.) was employed for the construction of the reaction coil. It was supplied by Tracor as a part of the accessory kit of the Tracor 965 PCD. RESULTS A N D DISCUSSION In a recent work, Frei et al. (4)have tested several materials for making postcolumn reactors for HPLC. According to these authors, Teflon TFE works better than quartz when UV-induced fluorescence is used as the detection method in liquid chromatography. Because significant advantages can be gained by avoiding the use of the quartz postcolumn reactor for the photochemical-conductivity detection in liquid chro-
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