Anal. Chem. 1985. 57, 385-387 385

The dual ion-exchange pretreatment is similar to Donnan dialysis. With the latter, an acid solution is used instead of the resin slurry. Dual ion-exchange has a major advantage, however. Because of imperfect exclusion of anions by cat- ion-exchange membranes, Donnan dialysis results in the contamination of the sample by the conjugate base of the acid that is employed (6-8).

The method was applied to matrices other than carbonate and NaOH. For example, by using an anion-exchange resin slurry in the hydroxide form and a sheet of anion-exchange membrane, the chloride in a 20 mL sample of 0.123 M HC1 was decreased to 4.1 X lo4 M in 5 h. In this case, a mag- netically stirred sample and slurry were used rather than a flowing sample. The less favorable sample volume to mem- brane area ratio, relative to use of a tubular membrane, re- sulted in the slow reaction.

Sulfate in a 0.1 M NaCl matrix was determined with the tubular Nafion membrane system and the cation-exchange resin slurry in the proton form. The NaCl was converted to HC1. The product was collected, evaporated to near dryness, and diluted with water. The sulfate was then determined by ion chromatography. Quantification required a modification of the procedure described in the Experimental Section be- cause of osmotic dilution.

Unlike the applications to carbonate, NaOH, and HC1 matrices, with NaCl samples the product, HC1, is also an electrolyte. Because the sample is at a high ionic strength during its entire excursion through the Nafion tube, osmotic dilution becomes significant. For example, a 5-mL sample of 0.5 M NaCl is diluted to 7.5 mL when it is pumped at 0.29 mL/min. Hence, in order to obtain quantitative results, the pump must be used in a mode that processes a known volume of sample. The product solution is then diluted to a fixed volume, so the analytical results are corrected for dilution. By this procedure with 100 mL of 0.5 M NaCl containing 0.030 M Na2S04, a 97% recovery of sulfate is obtained. These concentrations approximate the levels in seawater.

With matrices that are not readily decomposed or evapo- rated, the procedure is far more tedious. In addition, when

the product is an electrolyte, the resin slurry gets contami- nated. In the above conversion of NaCl to HC1, sufficient chloride diffuses through the Nafion membrane to contami- nate the slurry after a single experiment. Moreover, as the pH decreases during the conversion, anions that are conjugate bases of weak acids will protonate to form neutral species which can then diffuse through the membrane (7,8).

A final precaution that must be taken is that if a chemically stable, nonvolatile nonelectrolyte is formed, there is a potential for it to convert back into an electrolyte during the ion chromatography experiment. For example, if the product is a weak acid, the pH of the eluent must be less than the pK of the weak acid.

NOTE: The dual ion-exchange apparatus that is employed in this paper is the subject of a pending US. patent.

Registry No. Na2C03, 497-19-8; NaHC03, 144-55-8; NaOH, 1310-73-2; NaC1,7647-14-5; HCL, 7647-01-0; C1-, 16887-00-6; SO4*,

LITERATURE CITED 14808-79-8; NOS-, 14797-55-8; Po43-, 14265-44-2.

(1) Smith, F. C., Jr.; Chang, R. C. “The Practice of Ion Chromatography”; Wlley: New York, 1983; Chapter 7.

(2) Serlkova, L. I . ; Davydenko, T. F. Mefody Anal. Kontrolya Proizvod. Khim. Prom-sti. 1977, 3, 48; Chem. Absfr. 1978. 89, 35910m.

(3) Serikova, L. I.; Davydenko, T. F. Mefody Anal. Kontrolya Kach. Prod. Khim. Prom-sfi. 1978, 7, 16: Chem. Abstr. 1977, 90, 197105a.

(4) ”American Society for Testlng and Materlals Annual Book of Standards”; ASTM: Philadelphia, PA, W83; Voi. 11.02, pp D4130-82.

(5) Cox, J. A.; Tanaka, N. Anal. Chem., following paper in this issue. (6) Yeager, H. L. “Perfluorinated Ionomer Membranes”; Eisenberg, A,,

Yeager, H. L., Eds.; Amerlcan Chemical Society: Washington, DC, 1982; ACS Symp. Ser. No. 180.

(7) Dasgupta, P. K. Anal. Chem. 1984, 56, 96. (8) Dasgupta, P. K. Anal. Chem. 1984, 56, 103.

RECEIVED for review June 29,1984. Accepted September 4, 1984. Although the research described in this article has been funded in part by the U.S. Environmental Protection Agency under assistance agreement No. CR-809397 to Southern 11- linois University, it has not been subjected to the Agency’s required peer and administrative review and therefore does not necessarily reflect the view of the Agency and no official endorsement should be inferred.

Dual Ion-Exchange Method for the Controlled Addition of a Prescribed Ionic Species to a Solution

James A. Cox* and Nobuyuki Tanaka

Department of Chemistry and Biochemistry, Southern Illinois University, Carbondale, Illinois 62901

Modification of the composition of a solution by addition of an ionic species is a common laboratory practice. Ion-ex- change methods are generally used if the concomitant intro- duction of a conjugate ion cannot be tolerated (I). These methods are not without problems. Direct introduction of an ion-exchange material into a sample can change the con- centration of the analyte in solution because of the uptake or release of solvent and/or adsorption losses. The process is difficult to monitor unless a technique such as the gradual incursion of a resin-filled basket (2) is used. These problems are especially severe if a column rather than a batch method is used or if the sample volume is small.

The merits of the ion-exchange method are retained while the above problems are minimized by the dual ion-exchange apparatus described herein. An ion-exchange resin slurry acta

0003-2700/85/0357-0385$01.50/0

as an ion reservoir in the same manner as in batch experi- ments; however, it is isolated from the sample by an ion-ex- change membrane that contains fixed sites of the same charge sign as those of the resin slurry. Ions from the reservoir exchange for ions of the same charge sign in the sample by a process that is identical with Donnan dialysis in its net effect.

Dual ion-exchange differs from Donnan dialysis in that a resin slurry is used instead of an electrolyte as the ion reservoir. Ion-exchange membranes do not perfectly exclude ions of the same charge sign as the fixed sites on the membranes (co-ions). When Donnan dialysis is used, co-ions from the reservoir are therefore introduced into the sample to some extent along with the desired species. With the described apparatus, the co-ions are the fixed sites on the resin which are physically precluded from entering the sample.

0 1984 Amerlcan Chemical Society

386 ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985

n n 13r

Flgure 1. Dual ion-exchange apparatus: A, ion-exchange membrane; B, Plexiglas cell; C, sample; D, resin slurry; E, stirring bars; and F, magnetic stirrer.

EXPERIMENTAL SECTION The apparatus can be constructed in the same manner as

Donnan dialysis assemblies (3) with the resin slurry used in place of the receiver electrolyte. For example, a glass cylinder with one end closed by an ion-exchange membrane can be used as the sample holder, and a beaker can be used to contain the resin slurry. In the present case, the body of the sample holder was a Plexiglas pipe (73 mm long, 51 mm o.d., and 38 mm id.) with a reduced 0.d. portion threaded on the outside at one end. A female threaded Plexiglas retainer ring (51 mm o.d., 38 mm i.d., 22-mm length) was adapted to screw over the end of the pipe. A Plexiglas O-ring (41 mm o.d., 38 mm i.d. and 3.2 mm thick) was adapted to slide into the retainer ring and seal the space between the end rim of the pipe and the lip of the retainer ring.

The cell was assembled by placing a sheet of membrane inside of the retainer ring so that it laid on the inner lip of the retainer ring. The O-ring was then dropped in so that the membrane was held between it and the inner lip. The ring was screwed onto the Plexiglas pipe. One layer of Teflon tape was used over the threads. The effective membrane area was 11.3 cm2.

For the dual ion exchange of cations, two types of sulfonated cation-exchange membranes (sheet form) were used, Nafion 117 (Du Pont Polymer Products Division, Wilmington, DE), 0.19-mm thickness, and R-1010 (RAI Research Corp., Hauppauge, NY), 0.051-mm wet thickness. A slurry of cation-exchange resin was prepared by mixing wet Dowex 50WX4,100/200 mesh, in either the sodium or the proton form (80 g) with water (34 g) in a 250-mL beaker (cut to a 50-mm height for easy access).

For the dual ion exchange of anions, R-1035 anion-exchange membrane (RAI Research Corp.), 0.051-mm wet thickness, was used. The resin slurry was made by mixing 80 g (wet) of Dowex 1x4, 50/100 mesh, chloride form, with 34 g of water.

The cation-exchange resins were regenerated with either 4 M HC1 or 4 M NaC1. The anion-exchange resin was put into the chloride form with 4 M HCl. It is very important to thoroughly rinse the resins prior to use in the slurry. About 150 volumes of water per volume of resin was used. The resin was placed in a column for this step.

The initial ionic state of the membranes was matched to the cation of the sample in the reported experiments. For most applications, this step is not critical. In fact, the reported acid- ifications were initially more rapid when the counterion of the cation-exchange membrane was proton.

The cell containing a sample solution to be treated was dipped into the magnetically stirred ion-exchange resin slurry. The sample was also magnetically stirred by using a small bar that lies on the membrane. A typical setup is shown schematically in Figure 1.

In the present study, either pH or conductance was generally measured in the sample to monitor the dual ion-exchange process. The pH was measured with an Orion Research Model 701A meter using an Esterline Angus Speed Servo 11 strip chart recorder. The conductance measurements were made with a YSI Model 32 conductance meter, No. 3403 cell (Yellow Spring Instrument Co., Yellow Spring, OH), and a Hewlett-Packard Model 7101B re- corder. The experiments were performed at 24 f 1 "C.

A 7 -

5

I

40 60

3 , L, 0 20

T i m e , m i n

Figure 2. Dual lon-exchange acldiflcatlon of Na3P04 and NaOH solu- tions: curve A, 15 mL of 0.01 M Na3P04; curve B, 15 mL of 0.01 M Na3P04 and 0.1 M NaOH; bath, Dowex resin slurry In the proton form: cation-exchange membrane, Naflon 117.

d 1 7

Time, m i n

Figure 3. Metathesis of H2CO3 from Na,C03: curve A, Dowex resin slurry bath In the proton form; curve B, 1 .O M H2S04 bath; sample, 50 mL of 2 mM Na2C03; cation-exchange membrane, R-1010.

The quantification of sodium was done by atomic absorption spectrometry with a Perkin-Elmer 2280 atomic absorption spectrometer. Certain comparative experiments were performed with an acid bath instead of the resin slurry. Here, an ion chromatograph (Dionex Model 201Oi, Sunnyvale, CA) was em- ployed to monitor the diffusion of the conjugate base through the cation-exchange membrane into the sample.

All chemicals used were ACS reagent grade. House-distilled water that was doubly deionized with Cole-Parmer Research Grade cartridges was used.

RESULTS AND DISCUSSION The acidification of a solution with the dual ion-exchange

system is demonstrated with samples of 0.01 M Na3P04 alone and in a solution with 0.10 M NaOH. The results are shown in Figure 2. In both cases the time to go from pH 7.0-3.0 is 8.1 min which demonstrates that there is no measurable loss of phosphate onto or through the Nafion 117 cation-ex- change membrane. The position of the curves on the pH axis agrees with points for a simple titration; however, these plots cannot be considered as titrations since the rate of incursion of proton depends on the cation content of the sample. The rate, therefore, can vary as sodium, for example, is depleted. The pH change is easily stopped at any value transversed by the curves by removing the sample cell from the slurry bath.

The rate of the process also depended on the thickness of the cation-exchange membrane. When the original sample was 20 mL of 0.1 M Na3P04, the time required to form NaH2P04 when the Nafion 117 membrane (0.19-mm thick- ness) was used was 68 min. With the R-1010 membrane (0.051-mm wet thickness) the comparable time was 58 min. With samples of lower concentration, the cation content seemingly became rate-limiting, so the acidification rates with Nafion 117 and R-1010 were the same. For example, with 20 mL of 0.01 M Na3P04 as the sample, the time to form Na- H2P04 from Na2HP04 was 11 min in both cases.

The acidifciations shown in Figure 2 can be performed by using an acid bath instead of a resin slurry; however, the utility of the former method is limited by introduction of the con-

ANALYTICAL CHEMISTRY, VOL. 57, NO. 1, JANUARY 1985 387

fil,oh

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L

0 20 40 60 T i m e , min

Flgure 4. Comparison of dual ion exchange and Donnan dialysis acidificatbn of Na,PO,: curve A, Dowex resin siuny bath in the proton form: curve B, 500-mL bath of 0.001 M H2S04; curve C, 100-mL bath of 0.005 M H,SO,; curve D, 100-mL bath of 0.01 M H,SO,; sample, 20 mL of 0.01 M Na3P04; cation-exchange membrane, R-1010.

A

1

1

4min. T i m e -

Figure 5. Ion chromatograms of samples acidified by dual ion-ex- change and Donnan dlaiysls. Curves A D correspond to samples A-D in Figure 4; the ion chromatograms are taken after 60 min in the Flgure 4 experiments. The initial 2 min of chromatograms B-D Is identical with that of A.

jugate base of the acid into the sample (4,5). For example, when a resin slurry in the proton form is used, the conductance of a 2 mM Na2C03 solution decreases to a constant, low value, 8.8 pmhos (Figure 3). The experimental value cannot be compared to a calculated one because of trace impurities in reagent grade Na2C03. When the experiment is repeated with a 1 M H2S04 bath rather than the resin slurry, the conduc- tance initially decreases but then increases because of sulfate incursion (Figure 3). The transport of sulfate into the sample is expected because of the nonzero transport number for an- ions in cation-exchange membranes.

With lower concentrations of sulfuric acid as the bath, the incursion of suulfate is less, as expected, but it is still sig- nificant. Further, the rate of acidification is decreased. Figure 4 contains a comparison of the rate of acidification of 0.01 M Na3P04 with the resin slurry bath and baths prepared with various concentrations of sulfuric acid. After 60 min, ion chromatograms of the samples were obtained (Figure 5). Even with a 0.001 M H2S04 bath, a significant sulfate peak, which corresponds to 4 X M SO:-, was observed. With the 0.005 M and 0.01 M H2S04 baths, the peaks corresponded to 2 x lo4 M and 1.2 X lo4 M SO:-, respectively. Experiments with

L 40 80 120 160 O O

T i m e , min

Flgure 6. Dual ion-exchange metathesis of AgCl from AgNO,: bath, Dowex anion-exchange resin In the chloride form: sample, 50 mL of 0.01 M AgNO,; anion-exchange membrane, R-1035.

HC1 and 2-naphthalenesulfonic acid baths also showed in- cursion of the conjugate bases. There was no evidence of sulfate when the resin slurry was used.

Dual ion exchange can be performed with anions by using the appropriate slurry and membrane. For example, with an anion-exchange resin slurry in the chloride form and an an- ion-exchange membrane, the precipitation of Ag(1) can be performed. Figure 6 illustrates the decrease in conductance of a sample that is initially 0.01 M AgNOB during such a dual ion-exchange experiment. The value after 180 min, 6.7 pmhos, approaches the conductance of a saturated AgCl solution in the cell that is used, 3.4 pmhos.

With an aqueous system, the sample does not need an added electrolyte to initiate and sustain the dual ion-exchange reaction. An experiment was performed with a distilled water sample (75 mL) and a cation-exchange resin slurry in the sodium form. The proton in the sample from the autopro- tolysis of water exchanged with sodium from the slurry. A plot of sodium ion in the sample vs. time was a straight line; the concentration of sodium in the sample went from below the detection limit of flame atomic absorption spectrometry to 1.6 X

Although pH adjustment of samples to which the addition of an anion is not desired is the most apparent application of this device, several other uses can be suggested. For ex- ample, we we presently using it in a Donnan dialysis sampling system for ion chromatography. Anions are preconcentrated into a NaHC03/Na2C03 receiver electrolyte; the Na+ is sub- sequently exchanged for H+ by dual ion-exchange so that the treated dialysate, which will have a low ionic strength, can be injected into an ion chromatograph. The details will be the subject of a future report.

NOTE The apparatus described herein is the subject of a pending patent.

M in 200 min.

LITERATURE CITED (1) Smith, F. C., Jr.; Chang, I?. C. “The Practice of Ion Chromatography”;

Wiley: New York, 1983; Chapter 7. (2) Japanese Patent 80157331: Chem. Abstr. 1981, 95, 64210n. (3) Lundquist, G. L.; Washinger, G.; Cox, J. A. Anal. Chem. 1975, 47,

319. (4) Monet, G. P. In “Adsorption, Dialysis, and Ion Exchange”; American

Institute of Chemical Engineers: New York, 1959; Chem. Eng. Prog. Symp. Ser. No. 55 pp 191-198.

(5) Dasgupta, P. K. Anal. Chem. 1984, 56, 96.

RECEIVED for review June 4, 1984. Accepted September 4, 1984. Although the research described in this article has been funded in part by the U.S. Environmental Protection Agency under assistance agreement No., CR-809397 to Southern 11- linois University, it has not been subjected to the Agency’s required peer and administrative review and therefore does not necessarily reflect the view of the Agency no official en- dorsement should be inferred.