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Anal. Chem. 1987, 5 9 , 534-536
Determination of Anions in Polyelectrolyte Solutions by Ion Chromatography after Donnan Dialysis Sampling James A. Cox* Department of Chemistry, Miami University, Oxford, Ohio 45056 E w a Dabek-Zlotorzynskal
Department of Chemistry and Biochemistry, Southern Illinois University, Carbondale, Illinois 62901 The direct injection of polyelectrolyte solutions onto an ion chromatographic column cannot be performed because of deleterious effects on the column lifetime and performance. Even dilution of such samples w i l l not alleviate these problems. The common means of isolating complicated samples from the analytical column is to use an ion-exchange precolumn, but it will fail to separate analytes from matrix components of the same charge sign. Donnan dialysis provides a means of separating typical ions from matrix components of large size regardless of their charge signs. In this procedure, the sample is separated from a receiver electrolyte of relatively high ionic strength by an ion-exchange membrane. As long as the receiver volume is much less than that of the sample, high enrichment factors can be attained (1, 2). By proper selection of the receiver electrolyte and/or the ion-exchange membrane, the enrichment factors are independent of sample composition over a wide range of conditions ( 3 , 4 ) . Quantification is typically on the basis of a fixed-time kinetic model. Because of the high ionic strength of the receiver, direct injection of the dialysate into an ion chromatograph is generally not appropriate unless detection is based on something other than conductivity (5). Recently we developed a dual ion exchange system to alleviate this limitation (6-8). Here, if the analyte is an anion, the receiver is selected to be a conjugate base of a weak acid. The dialysate is passed through a cation-exchange hollow fiber which is bathed in a cationexchange resin slurry in the proton form. The nonelectrolyte that is formed will not influence the chromatographic separation or detection unless a basic eluent is employed. By use of a carbonate receiver, the subsequent eluent selection is not important since the H2C03which is formed in the dual ionexchange process decomposes to HzO and COz. A resin slurry is used rather than an acid bath (9) to obviate complications which can occur because of the incursion of the conjugate base of the latter. The incursion especially occurs when high ionic strength solutions are in contact with both sides of the membrane due to the breakdown of the Donnan exclusion principle (IO). This phenomenon is not very significant when the acid-bathed hollow fiber is between the analytical column and the detector since the ionic strengths of the solutions are quite low. Moreover, the result is only an increase in the base line. With our application, introduction of even a trace level of anion during the dual ion exchange step can lead to error. A key to coupling Donnan dialysis with dual ion exchange is the ability of a carbonate solution to serve as the Donnan dialysis receiver. In general, the nature of the receiver determines whether the Donnan dialysis enrichment factor for a given species is independent of the sample composition (3, 4). For the Donnan dialysis of anions, the pH of the receiver must be less than the pK of the conjugate acid of the anion when typical membranes are employed (3). With a membrane, such as the type used in this study, which yields transport 'On leave from the Department of Chemistry, University of Warsaw, Warsaw 02-093, Poland.
by a site-to-site mechanism, the pH of the receiver electrolyte is not critical, so the carbonate receiver is satisfactory ( 8 , I I ) . EXPERIMENTAL SECTION The Donnan dialysis experiments were performed as previously r e p o d (8). A 5-mL receiver containing 0.04 M Na2C03and 0.16 M NaHC03 was isolated from a 100-mL sample by a sheet of anion-exchange membrane (11.3 cm2)that was prepared in our laboratory (11). Unless otherwise stated, the dialysis time was 30 min. Convection was provided by magnetic stirring. The dialysate was diluted to 10 mL in a volumetric flask. Dual ion exchange was then performed, also in a manner previously described (6-8). The dialysate was passed at 0.29 mL/min through a 2-m length of Nafion cation-exchange tubing which was immersed in a slurry of 55 g of Dowex 50W-X4 resin (proton form), 100/200 mesh, in 24 g of HzO at 70 "C. The product was brought to room temperature, and a 1OO-pL portion was injected into a Dionex 2010i ion chromatograph. A Dionex HPIC-AG4 guard column and HPIC AS4 analytical column were used along with a hollow fiber suppressor. The eluent, a solution containing 1.0 mM Na2CO3and 4.0 mM NaHCOS,was flowed at 2.0 mL/min. Solutions of the polyelectrolytes, poly(sodium 4-styrenesulfonate) and polyacrylic acid (Aldrich Chemical Co.), were prepared by adding a weighed portion to water (in-house distilled which was further purified with a Sybron/Barnstead Nanopure I1 system) and mixing at room temperature for 5 h. All other chemicals were ACS Reagent Grade; they were used without purification. Between experiments, the membranes were soaked for 1 h in a stirred solution of the same composition as the receiver electrolyte and rinsed with water for 2 h. Fresh solutions were used after each 30-min period. RESULTS AND DISCUSSION It was anticipated, and herein demonstrated, that the polyelectrolyte would decrease the Donnan dialysis enrichment factors from the values obtained with simple aqueous stock solutions (the enrichment factors were calculated as the ratio of the analyte concentration in the diluted dialysate to the initial concentration of the analyte in the sample). Hence, quantification on the basis of the sequential standard addition and the internal standard methods was used. The concentrations of the analytes in the dialysates, after dilution and dual ion exchange treatment, were determined by fitting the peak heights to calibration curves. These curves were generated by using standards prepared in a 0.02 M Na2C03,0.08 M NaHC03 matrix and treating them by dual ion-exchange prior to performing ion chromatography. The data for the determination of chloride in poly(sodium 4-styrenesulfate), PSS, by sequential standard addition are summarized in Table I. These data are evaluated by either linear least-squares analysis or plot of the concentration of chloride in the treated dialysate vs. the concentration of chloride added to the PSS sample. It follows from the typical sequential standard addition method that the intercept yields the concentration of chloride in the diluted dialysate from the original sample. The slope is the Donnan dialysis enrichment factor, which is defined above. Dividing the former by the latter yields the concentration of the chloride in the PSScontaining sample. With 0.1 g PSS/L, (4 & 2) X lo-' M C1-
0003-2700/87/0359-0534$01.50/00 1987 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 3, FEBRUARY 1, 1987
Table I. Data for the Determination of Chloride in PSS"by Donnan Dialysis after Sequential Standard Addition
sample g of PSS/L C1- added, pM 0.1 0.1 0.1 0.1 0.1 1.0 1.0 1.0 1.0 1.0
0.0 5.0 10 15 20 0.0
5.0 10
15 20
Table 11. Sequential Standard Addition4 Determination of Anions in Polyacrylic Acid by Ion Chromatography of the Dialysates after Treatment by Dual Ion Exchange
dialysate contentbCl-, pM
analyte
not detected (
