ANALYTICAL CHEMISTRY, VOL. 50, NO. 4, APRIL 1978
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Enrichment of Anions of Weak Acids by Donnan Dialysis James A. Cox" and Kuo-Hsien Cheng Department of Chemistry and Biochemistry, Southern Illinois University, Carbondale, Illinois 6290 1
The rate of Donnan dialysis enrichment of weak acids was found l o be influenced by the pH of the sample and of the receiver electrolyte; however, when the receiver pH is much less than the pKof the most acidic species, the transfer rate is independent of sample pH over a wide range. Under optimum conditions, the enrichment rate of several anions is identical. These results suggest that diffusion of the sample anion is not the rate-determining step in these enrichments and, further, that the rate of transfer across the membraneheceiver interface is especially important.
Donnan dialysis enrichment involves separation of a sample solution from a relatively high ionic strength receiver electrolyte by an anion-exchange membrane ( I , 2). The approach to Donnan equilibrium results in transfer of ions of the appropriate charge sign from the sample to the receiver; thus. if t h e latter volume is smaller, enrichment of those ions is accomplished. Since the transfer rate is time independent (as long as t h e system is not near Donnan equilibrium) and is directly proportional t o t h e concentration of t h e selected ion in t h e sample over a wide range of conditions ( 3 ) ,preconcentrations for prescribed times result in linear relationships between readout and initial sample concentration when metal ions or anions of strong acids are subsequently determined in t h e receiver (3, 4). Donnan dialysis of conjugate bases of weak acids presents a special problem because the distribution of these species according to charge is a function of p H , thereby possibly complicating the enrichment procedure. T h e present study was initiated t o determine the factors which primarily influence their transfer rat.e and to establish conditions which would permit preconcentrations in direct proportion to their initial sample concentration. Because of their general importance and polybasic nature, H3P04 and H3As0, were selected as t h e major test species. EXPERIMENTAL The anion-exchange membranes were Permion P-1025 obtained from RAI Research Corporation, Hauppauge, Long Island, N.Y. They were successively rinsed in 0.1 F HC1, H20, 0.1 F NaOH, HzO, 1F KN03 and 0.1 F KNOBprior to use. The acid-base cycle was repeated three t,imes; they were soaked at least 24 h in the 0.1 F KNOBprior to use. When receiver electrolytes other than K N 0 3 were used, the latter two solutions of the sequence were altered accordingly. The analytical methods employed in this work were the following: phosphate and arsenate, molybdenum blue method ( 5 ) ; chloride, differential pulse polarography; sulfate, chloranilate spectrophotometric method (6);pyruvate, linear scan voltammetry; chloroacetate, linear scan voltammetry. The electrochemical experiments were performed with a PAR 170 system. Reagent grade chemicals and doubly deionized water were used throughout. RESULTS AND DISCUSSION T h e primary difference between the Donnan dialysis behavior of anions which are conjugate bases of weak acids and previously studied ions is the effect of p H , the importance of which is demonstrated by the experiments summarized in 0003-2700/78/0350-0601$01 .OO/O
Table I. Influence of pH o n t h e Donnan Dialysis Enrichment of Phosphate Receiver pH Sample pH Enrichment factor" 1.5 1.5 3.5 5.2
2.0
2.8
3.0-10.0 7.0 2 0.03 4.0 6.3 4.0 4.9 5.2 6.9 3.9 5.2 9.2 3.3 a Ratio of receiver concentration after enrichment to the original sample concentration. Receiver: 2 mL of 0.1 M KNO, adjusted with HNO, or KOH. Membrane: 4 . 9 cm2 P-1025. Enrichment time, 30 min. Table 11. Effect of pH on t h e Donnan Dialysis Enrichment of Selected Weak Acids" Receiver Sample Enrichment Acid, pK PH PH factor Chloroacetic (2.8) Pyruvic (2.2)
1.5 3.0 5.6 1.5 1.5 3.0
Arsenic (2.6, 7.0, 11.5)
5.6 1.5 1.5 1.5
4.0 4.0 4.0
4.2 6.0 4.2 4.2 1.3 4.9 9.0
3.0 5.7 4.0 Sulfuric (pK,, 1.9) 1.5 4.0 a Conditions the same as reported in Table I. 5.7
6.8 6.9
5.4 7.0 6.9
6.8
5.4 0.4 7.2 7.0 5.1 2.5 2.9
Table I. T h e data indicate that if the receiver p H is much less than the pK(s), the rate of Donnan dialysis transfer, which establishes the enrichment factor, is independent of sample pH; it is also necessary that the sample not be so acidic that the major fraction of the weak acid system is a neutral species. T h e hypothesis was substantiated by comparable studies with other weak acids (Table 11). Except for the case of sulfate, the weak acids which were investigated all gave enrichment factors of about 7 for a 30-min preconcentration into a p H 1.5 receiver. This fact, along with the observation that a very acidic receiver yields the greatest enrichment rate, suggests t h a t the rate of transfer across t h e membrane/receiver boundary may be the rate-limiting step of a Donnan dialysis of weak acids with anion-exchange membranes. Otherwise, it would be required t h a t the anions have similar diffusion coefficients in t h e membrane. T h e relatively low enrichment rate for sulfate may be due to strong interaction with t h e fixed sites of the membrane. This interpretation is consistent with t h e results of experiments on the Donnan dialysis of samples of mixtures of weak acids. T h e results are Summarized in Table 111. Although the tabulated data are for p H 5.2 samples, the results were unchanged over t h e p H range 4-9. Mixtures which did not contain sulfate exhibited the same enrichment as the single-component cases in Table I and 11. T h e decrease of the Donnan dialysis enrichment rate in the presence of sulfate (or bisulfate) could not be eliminated by changing the receiver 6 1978 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 50, NO. 4 , APRIL 1978
Table 111. Donnan Dialysis Enrichment of Mixtures of Weak Acids EF' Test anion Added anion 7.1 Phosphate lo-'M sulfate 6.7 M 10-3 M 4.8 6.3 Pyruvate 10-4M 10-3M 4.7 Phosphate M pyruvate 7.2 M acetate 7.2 a Enrichment factor of the test anion. Sample pH, 5 . 2 ; other conditions the same as Table I. electrolyte p H or anion; for the latter, chloride, bromide, acetate, chloroacetate, nitrate, and citrate were used. T o establish which of the above observations are related t o the weak acid nature of the investigated systems, selected experiments were repeated with chloride-containing samples. Using the conditions outlined in Table I, l@-4-10-5M C1samples were enriched by a factor of 7.0 in 30 min, and the results were independent of sample p H over the investigated range of 2-9. Unlike t h e weak acid cases, variation of t h e receiver p H over the range 2-6 did not change the enrichment factor (7.02 f 0.03, 8 points). With mixed samples, t h e enrichments of phosphate and chloride were mutually independent. In addition, the presence of sulfate does not influence t h e enrichment of chloride. These results are generally consistent with the previous observations. Chloride would be expected to weakly associate
with the ion-exchange sites, so pH effects should be negligible. Likewise, the presence of competing anions should not alter the enrichment. T h a t the enrichment factor for chloride is the same as those for weak acids when the latter are transferred into a low-pH solution supports a model for Donnan dialysis transfer in which the primary factor which determines the rate is the Donnan potential; with weak acids the primary rate can be decreased by t h e exchange reaction a t t h e membrane/receiver interface but not by diffusion of the test species; Le., the electric field gradient predominates over the diffusion gradient. Further study is in progress to substantiate and refine this model. T h a t conditions can be defined which permit Donnan dialysis of weak acids to be independent of sample p H is important. Applications to chemical analysis such as for matrix normalization and/or enrichment of trace samples are feasible.
LITERATURE CITED (1) R. M. Wallace, Ind. Eng. Chem., Process Des. Dev., 6, 423 (1967). (2) W. J. Blaedel and T. J. Haupert, Anal. Chem., 38, 1305 (1966). (3) G. L. Lundquist, G.Washinger, and J. A. Cox, Anal. Chem., 47, 319 (1975). (4) J. A. Cox and J. E. DiNunzio, Anal. Chem., 49, 1272 (1977). (5) "Standard Methods for the Examination of Water and Wastewater", 13th ed., American Public Health Association, Inc., Washington, D.C., 1971. (6) R. J. Bertolacini and J. E. Barney, Anal. Chem., 29, 281 (1957).
RECEIVED for review November 21, 1977. Accepted January 16, 1978. This work was supported in part through the Water Resources Center, University of Illinois, Project A-087-ILL, in the OWRT Allotment Program.
Simultaneous Multielement Determination by Atomic Emission with an Echelle Spectrometer Interfaced to Image Dissector and Silicon Vidicon Tubes Hugo L. Felkel, Jr., and Harry L. Pardue" Department of Chemistry, Purdue University, West Lafayette, Indiana 47907
Silicon target vidicon and image dissector camera systems have been coupled to an echelle grating spectrometer for multielement determinations with a dc plasma excitation source. Spectral resolution expressed as full width at half height ranges from 0.4 to 0.9 A for the vidicon system and 0.2 to 0.7 A with the image dissector system, and wavelength calibration procedures permit locations of lines to be predicted to between 0.03 and 0.2 A for both systems. Effects of peak height and peak area measurements on signal-to-noise ratios for both detectors are evaluated. Results for multielement samples of alkali and alkaline earth and of transition metals obtained with the echelle spectrometer/image dissector are comparable in most respects to single element data obtained with conventional optics and detectors with the same plasma. Some interelement effects associated with the dc plasma are discussed.
Numerous recent reports have discussed the application of imaging detectors for a variety of applications involving atomic spectrometry (1-10). One of the promising areas of applications is for t h e simultaneous determination of metallic 0003-2700/78/0350-0602$01 0010
elements by atomic absorption and atomic emission spectrometry. Most applications in this area have been based on imaging detectors used with conventional optics that disperse spectra in only one dimension. These applications impose a severe tradeoff between spectral range and spectral resolution. Some recent reports have demonstrated the feasibility of using echelle grating spectrometers t o take advantage of t h e two-dimensional character of some imaging detectors to obtain good resolution over spectral ranges of several hundred nanometers ( I , 9-11). Applications reported to date have included both atomic emission and atomic absorption spectrometry. In this paper we describe and compare results obtained with an image dissector and a silicon target vidicon tube used with an echelle grating spectrometer and a direct current plasma excitation source. Items included in the study are: useful spectral range and resolution, wavelength accuracy, effects on signal to noise of peak height and peak area measurements, some interelement effects in the d c plasma, comparisons of sensitivities in single- and multielement mixtures, comparisons of sensitivities and other characteristics of the detectors, and detection limits for several elements. T h e image dissector system is up to 25 times more sensitive than the silicon target C 1978 American Chemical Society
