Anal. Ch8m. 1983, 55, 1165-1168
sizes, the largeir samples presented fewer problems and provided individually more reliable data than the 100-mL samples. Not only could the larger samples be counted for shorter periods of time and still result in the same uncertainty (counting statistics) but also the blank concentrations, which were critical for some elements in the 100-mL samples, were greatly reduced for the larger samples. The detection limits for some elements were significantly improved in the larger samples. Thorium concentrations could be determined in the 200- and 500-mL samples but not in the 100-mL sampleia. Aluminum and tin could only be determined in the 500-mL samples. The main problem with A1 was the high blank values for the polyethylene bags. One 100-mL sample was counted for A1 after first removing both bags. A value of 1.2 f 0.5 ng/mL was observed, which agrees with the value determinied for the 500-mL samples (1.1f 0.2 ng/mL). The relatively high analytical uncertainty for this 100-mL sample was due in part to Al decay during the additional time required to remove the bags. Although the 500-rnL samples produced better analytical results, they took considerably longer to prepare, and required increased quantities of resin and reagents, as well as more sophisticated equipment. The 200-mL samples, however, required no additional effort or materials compared to the 100-mLsamples. The blank influence was reduced by a factor of 2 since no adlditional resin or reagents was required. This reduced blank influence, as well as the improved counting statistics, enabled the determination of Th in the 200-mL sample and reduced the uncertainty for several other elements.
CONCLUSION The application of the Chelex-100 resin separation preconcentration, vvith the direct use of the resin itself as the fiial sample for analysis, is an extremely useful technique for NAA. The elements dlemonstrated to be analytically determinable from samples of high Ralinity waters are Al, Co, Cr, Cu, Eu, Fe, Mn, Mo, Ni, Sc, Sn, Th, U, V, and Zn. The 500-mL samples, although invallving more reagents and manipulation, gave the greatest sensitivity and accuracy, although a significant increaeie in time and effort was, required. A sample size of 200 mL was found to be the optimum, doubling the sensitivity over the 100-mL samples but requiring no additional effort, however, the 500-mL sample size may be necessary to further reduce the blank for some elements when analyzing open-ocean water samples. The determination of Cr and V by this technique offers significant advantages over methods requiring aqueous final forms, in view of their poor elution reproducibility. The removal of Na, C1, and Br prior
1165
to irradiation allows the determination of other elements having short and intermediate half-lives, and greatly reduces the radiation dose received by personnel. This procedure has been successfully applied in a study of more than 100 samples collected throughout the entire length of the Chesapeake Bay. The salinity of these samples varied from that of freshwater to that of Atlantic Ocean water. Registry No. Al, 7429-90-5; Co, 7440-48-4; Cr, 7440-47-3;Cu, 7440-50-8; Eu, 7440-53-1; Fe, 7439-89-6; Mn, 7439-96-5; Mo, 7439-98-7; Ni, 7440-02-0; Sc, 7440-20-2; Sn, 7440-31-5; Th, 7440-29-1; U, 7440-61-1; V, 7440-62-2; Zn, 7440-66-6; Chelex-100, 11139-85-8;water, 7732-18-5.
LITERATURE CITED Riley, J. P.; Skirrow, G. “Chemical Oceanography”; Academic Press: New York, 1975: Volumes I and 111. Riley, J. P.; Taylor, D. Anal. Chim. Acta 1988, 40, 479. Davey, E. W.; Soper, A. E. I n “Analyticai Methods in Oceanography”; Gibbs, T. R. P., Jr., Ed.; American Chemical Society: Washington, DC, 1975; Adv. Chem. Ser., No. 147. Florence, T. M.; Batiey, 0. E. Tebnta 1978, 2 3 , 179; 1977, 2 4 , 151. Lee, C.; Kim, N. B.; Lee, I.C.; Chung, K. S. Talanta 1977, 2 4 , 241. Kingston, H. M.; Barnes, I. L.; Brady, T. J.; Rains, T. C.; Champ, M. A. Anal. Chem. 1978, 50, 2064. Kuehner, E. C.; Alvarez, R.; Pauisen, P. J.; Murphy, T. J. And. Chem. 1972, 4 4 , 2050. Useller, J. W. NASASP-5074, Office of Technology Utilization NASA, Washington, DC, 1969. Moody, J. R.; Lindstrom, R. M. Anal. Chem. 1977, 4 9 , 2264. Maienthai, E. J.; Becker, D. A. NBS Tech. Note (US.) 1978, No. 926. Moody, J. R.; Rook, H. L.; Paulsen, P. J.; Rains, T. C.; Barnes, I. L.; Epstein, M. S. NBS Spec. Pub/. (US.) 1977, No. 464. Kingston, H. M.; Peiia, P. A. Anal. Chem. 1981, 53, 223. Kelly, W. R.; Fassett, J., personal communication, 1981. National Bureau of Standards (U.S.), Certificate of Analysis, SRM 1643a, 1960. Greenberg, R. R. Anal. Chem. 1979, 51, 2004. Becker, D. A.; LaFieur, P. D. J . Radloenal. Chem. 1974, 19, 149. Jones, J. W.; Capar, S. Q.; O’Haver, T. C Analyst (London) 1982, 107, 353. Strachen, D., personal communication, 1981. Kingston, H. M. “Quantitative Ultratrace Metal Analysis of High Salinity Water Utilizing Chelating Resin Separation”, Interagency Energy-Environmental Research and Development Program, Report, EPAINBS, EPA-60017-79-174, 1979. Murphy, T. J. NBS Spec. Pub/. (US.)1976, No. 422. Lindstrom, R. M. Internal NBS Communication, Report of Analysis 5192, 1979.
RECEIVED for review October 27, 1982. Accepted March 10, 1983. Certain commercial equipment, instruments, or materials are identified in this paper in order to adequately specify the experimental procedure. Such identification does not imply recommendation or endorsement by the National Bureau of Standards, nor does it imply that the materials or equipment identified are necessarily the best available for the purpose.
CORRESPONDENCE Time-Dependent Selectivity Behavior and Dynamic Response of Silver Halide Membrane Electrodes to Interfering Ions Sir: During ,the past few years a considerable number of theoretical treatments have been devoted to the selectivity behavior of silver halide solid-state- or precipitate-based membrane electrodes (1-12) (see also the references found therein). These! descriptions agree in that the emf response of AgX membrane electrodes to the primary ions X- in the presence of interfering halide ions Y-is given by the Nicolsky 0003-2700/83/0355-1165$0 I.50/0
equation (1) (13)
E = ExO - RT - In [ax’ F
+ KxYay’]
(1)
where E is the emf of the electrochemical cell, Exo is a reference potential, and R, T,and F have their usual meaning. The measured activities of the electroactive species, ax’ and 0 1983 American Chemical Society
1166
ANALYTICAL CHEMISTRY, VOL. 55, NO. 7, JUNE 1983
ay’, refer to the boundary zone of sample solution. This boundary is in direct contact and in thermodynamic equilibrium with the membrane surface. The theoretical selectivity factor Kxy of the membrane material therefore corresponds to the equilibrium constant of the basic ion-exchange reaction AgX + Y- AgY + X- (see eq 2a). Accordingly, K x y is commonly identified with the ratio of solubility products (eq 2b) ax’[AgYl - -ax‘ s (24 KXY = ay’[AgX] ay’ 1 - s
The apparent coverage factor s (9) evidently represents the molar fraction of AgY that is formed in the surface layer of the AgX membrane by ion-exchange or dissolutionfprecipitation reactions. In practice, the following expression is used to describe the observed electrode response (14):
E
= E x O - RT - In [ a x
F
+ kXyPotay]
(3)
where ax and a y are the respective bulk sample activities and kXyPOt is the apparent selectivity coefficient. Diffusion-controlled differences between boundary (a9 and bulk activities (a)may result in striking discrepancies between expected and observed selectivities. The same phenomena were reported (15) and treated theoretically (8,1619) for ion-exchange liquid membrane electrodes. Generally, the experimental kXyPot values reflect the ion selectivity of the membrane material only if an equilibrium is reached between the electrode surface and the bulk of sample solution (17). A formal interpretation of the apparent selectivity behavior of solid-state membrane electrodes was offered recently by Hulanicki and Lewenstam (9). The authors implicitly made use of the Nernst approximation to describe the ionic fluxes Ji across the aqueous diffusion layer (thickness 6, diffusion coefficients Di, activity coefficients n)at zero-current steady state
where D[ = D i / y i . From this one obtains the relation ax’ (Dy’/Dx’)uy’ = ax (Dy’/Dx’)ay (5)
+
+
Combination of eq 1,2a, and 5 then leads to the fundamental result for the electrode response (9),given here in terms of ion activities instead of concentrations
Equation 6 clearly predicts that the apparent selectivity coefficients, as defined in eq 3, vary with the decisive factor s. For example, if a pure (i.e., not pretreated) AgX-membrane electrode is exposed to a solution of ions Y-, the initial selectivity factor is determined as
k XYPot = DY‘ - 1 sg=o (7) DX‘ whereas the final selectivity after electroactive mixed-phase formation is given by the equilibrium value ~
Although the variability of the selectivity coefficients of solid-state membrane electrodes was documented and discussed in great detail by Hulanicki and Lewenstam (5,9,18),
the authors did not treat this phenomenon theoretically and quantitatively by a time-dependent model. Indeed, the nature of the rate-controlling process is controversial. In the 1940’s and 1950’s Schwab (21) and Jaenicke (22,23) found that the conversion reaction of silver chloride in bromide solution leads to loosely deposited reaction products on the solid phase. Thus a considerable amount (ntot)of silver salt can be converted, the rate of this reaction being controlled by anion diffusion across the aqueous boundary layer (21-23). Similar results were presented recently by Sandifer (10). A somewhat different mechanism was suggested by Rhodes and Buck (7) who found evidence for diffusion of bromide ions into the bulk of the silver chloride membrane phase. Correspondingly, two limiting cases are considered in the present theoretical treatment: (1)A solid-state membrane approach which is based on the assumption that a given total number of moles of silver salt, nbt, is involved in a mixed-phase formation on the membrane surface (cross section area A ) . (2) A so-called liquid membrane approach (16, 18, 19) that allows for unrestricted diffusion of interfering ions into the bulk membrane phase. With either of these two models, the time dependence of the pivotal coverage factor s in eq 6 can be rigorously formulated. In the solid-state membrane approach, external film diffusion of primary and interfering ions is rate controlling. Conservation of mass then requires that
ntot ds -_ = Jy
A dt where the mass flux is given by eq 4. After substitution of the unknown boundary activities using eq 2a and 5, one obtains the following differential equation: ds = -DY’A s(ax + K X Y ~ Y- )K X Y ~ Y dt nbt6 SWXY - (DY’/Dx’))- K X Y
(10)
This expression can be integrated to yield an implicit solution for s ( t ) . For an initially pure silver salt, which is the case discussed below, the limiting values of s ( t ) are given by eq 7 and 8, and the final result reads
In
(
;>
1--
= (ax
+ K x y a y ) C t (11)
with the experimental parameter C Dy‘A
c=-
(12) ntot6 By use of a liquid membrane approach (16, 18, 19), an explicit solution was derived for the total activity that is measured by the ion-selective electrode (see eq 3)
This relation holds for diffusion-type anion-exchanger membranes that initially contain the species X- only. The timedependent function f ( t ) involves the mean diffusion coefficient D and the total concentration, X,of anions in the membrane (16, 18, 19).
RESULTS AND DISCUSSION The time-response functions of an AgCl membrane electrode to Br- solutions of different concentrations are shown
ANALYTICAL CHEMISTRY, VOL. 55, NO. 7, JUNE 1983 EMF
[mv]
1
7
I
I
A ~ M E M B R A N E, ~ o - ~I- M +001M Br-
200
O
O
O
O
O
O
f
-100
L
0
0 7 - t lh
2h
h--J 24h
TIME
Figure 1. Timedependent response of a silver chloride membrane electrode to bromiide solutions of different concentrations: (solid lines) theoretical curves calculated from eq 6 and 11 uslng DB,/Dc, = 1, Y ~ ~ =/ 1, Y Kcre, ~ ~ = 355 ( 4 ) , and C = 10 miin-‘ M-‘; (polnts)experimental values (constant ionic strength) taken from Figure l b in ref 9; (dotted line) theoretical curve calculated from eq 3 and 13 using 7. = mln M2.
in Figure 1. The curves, as calculated from eq 6 and 11,were fitted to experimental values reported by Hulanicki and Lewenstam (9) by adjusting the value of the experimental parameter C. Both theory and experiment document that an initial time period exists where dissolution/precipitation (anion exchange) on the membrane surface dictates the selectivity behavior. In this stage, the electrode measures the boundary activity of C1- ions released from the membrane material. Hence, the apparent selectivity coefficient kCIBrPot becomes approximately unity (see eq 7). Only after this period, depending on the bulk activity of interfering ions and on the magnitude of C, a rapid approach to the final equilibrium selectivity jtCmrpt 355 (see eq 2b and 8) is observed. Thus the results in Figure 1 suggest thak the interference in chloride electrodes by bromide, especially at lower concentrations, may be much less serious than is expected from pure solubility considerations. This effect can even be magnified if the anion permeability of the external diffusion barrier is lowered (reduction of By’/6 and of C). In fact, Sandifer (11) reported that overcoating a silver/silver clhloride eledrode with cellulose acetate successfully reduces its sensitivity to interferents. Attempts to fit the experimental data in Figure 1by a liquid membrane approach (see dotted curve), allowing for diffusion of bromide ions into the bulk of the silver chloride membrane (7), were quite unsuccessful. In contrast, the agreement with the f i s t theory is evident, which indicates that transformation of the membrane material is restricted to a surface layer (coverage by silver bromide). This conclusion is in accordance with Sandifer’s findings (10). Since C is here on the order of lo2cm3 mol-Ls-l, Dy’ cm2 s-l, A 1cm2,and 6 cm, one obtains ntot mol. Correspondingly, the average thickness of the silver bromide layer formed on the chloride electrode rnmt be -1 pm. The mechanism underlying the dynamic response behavior depicted in Figure 1is comparable to the one suggested earlier for glass membrane electrodes (24). This multilayer membrane model accounted for differences in the selectivity behavior existing between the bulk of the membrane and the surface layer. In the present treatment, the corresponding selectivity terms are given by Kxy and Dy’/Dx’. A so-called sluggish response to interfering ions was predicted for cases where the bulk membrane strongly prefers these species and the surface layer is chmacterized by a poor ion selectivity (see eq 10 in ref 24). By the same arguments, a transient response to interfering ions should result if the bulk membrane rejects
--
-
-
1167
10
20
30
-----
40s
I TIME
Figure 2. Timedependent response of a silver Iodide precipitate based M iodide and different bromide electrode to solutions containing concentratlons: (solid lines) theoretical curves calculated from eq 6 and 11 using D B r l D I:= 1, Y ~ ~ = / Y1,~KIBr= 1.66 X ( 4 ) ,and C = 800 s-’M-’; (points)experimental values (constant ionic strength) taken from Figure 4 in ref 12.
these ions. The reason is that, theoretically, a stepwise change from a conditioning solution (anion activities axo and a Y o ) to a new sample (ax and ay) produces the following initial emf excursion of an anion sensor (see also eq 6 with s = so)
RT ax + (DY’/Dx’)~Y AE(t-O)a-In (15) F axo + (Dy’/Dx’)ayo whereas the final potential change evidently is given by
AE(t
-+
m)
RT
= --
F
In
ax + K X Y ~ Y axo + Kxyayo
(16)
Equations 15 and 16, which agree with the limiting cases derived earlier (24),permit easy rationalization of transient response phenomena. Thus a transient response to an activity change of interfering ions (Kxy > ax, Kxyay C ax (activity increase)
-
or for ay
= 0, ayo >> axo, KXYaYo < axo
(activity decrease)
Indeed, Lindner, T6th, and Pungor (12) reported on such transient responses of AgI-precipitate-based electrodes to Brions in the presence of I- ions. For this system, the equilibrium selectivity is specified as kf,O; = KXY= 1.66 X loe4(see eq 8 and 2b and ref 4). Pungor and co-workers also offered an informal explanation of the observed effects. According to their model, the initial emf excursion is due to diffusion of Br- ions to the electrode surface and subsequent desorption of I- ions by a fast ion-exchange reaction. The decay of the transient potential is attributed to diffusion of iodide ions into the bulk solution. This model, although qualitatively correct, appears to be at variance with common physical laws, however. Firstly, it would imply strongly disparate diffusion rates for the two ionic species and, secondly, it would be in conflict with the zero-current condition. The mechanisms proposed in the present theory are more logical in these respects. They invoke simultaneous (counter-) flows of primary and interfering ions coupled by the zero-current condition and dissolution/precipitation reactions. Figure 2 demonstrates that the experimental emf vs. time curves obtained by Lindner et al. (12) roughly conform to the present solid-state membrane theory for initially pure silver halide electrodes. Other kinetic processes seem to be of minor importance since the response time in the absence of interfering ions was usually below 1 s (25). The value of the parameter C used for the calculations in Figure 2 is on the order of lo6 cm3 mol-l s-l. This high value reflects the nearly ideal experimental conditions (6 lo4 cm)
-
Anal. Chem. 1983, 55, 1168-1169
1168
-
and the favorable quality and structure of the ion-selective membrane surface (nbt 10-7 mol). The evident superiority of precipitate-based silicon-rubber-supported membranes relative t o other membrane configurations was also documented by Pungor's group (12). Registry No. AgC1, 7783-90-6; AgI, 7783-96-2.
LITERATURE CITED Pungor, E.; TBth, K. Anal. Chlm. Acta Ig69, 4 7 , 291. Buck, R. P. Anal. Chem. 1968, 4 0 , 1432. Wuhrmann, H A . ; Morf, W. E.; Simon, W. He&. Chlm. Acta 1973, 56,
1011. Morf, W. E.; Kahr, G.; Simon, W. Anal. Chem. 1974, 46, 1538. Hulanickl, A.; Lewenstam, A. Talanta 1977, 24, 171. Frelser, H., Ed. " Ion-Selective Electrodes in Analytical Chemlstry"; Plenum: New York, 1978;Chapters 1 and 2. Rhodes, R. K.; Buck, R. P. Anal. Chlm. Acta 1980, 113, 67. Morf, W. E. "The Prlnciples of Ion-Selectlve Electrodes and of Membrane Transport"; AkadBmlal Kladb: Budapest; Elsevler: Amsterdam,
(12) Llndner, E.; TBth, K.; Pungor, E. Anal. Chem. 1982, 5 4 , 202. (13) Nicolsky~B. p. zh. F1z. Khlm. l9s79 ' 0 , 495. (14) IUPAC Recommendations for Nomenclature of Ion-Selective Electrodes Pure ADD/.Chem. 1978. 48. 127. (15) Huianicki, A,; kwandowski, R. C h e k . Anal. (Warsaw) 1974, 19, 53. (16) Jyo, A.; Ishibashl, N., ref 8, pp 246-258. (17) Pungor, E., BuzSs, I., Eds. Ion-Selective Electrodes"; AkadBmiai KladB: Budapest, 1981. (18) Morf, W. E., ref 17, p 267. (19) Senkj?, J.; Petr, J., ref 17. (20) Hulanickl, A.; Lewenstam, A. Talanta 1978, 23, 661. (21) Schwab, G.-M. Kolloid-2. 1942, 101, 204. (22) Jaenicke, W. 2.Nektrochem. 1953, 57, 843. (23)Jaenicke, W.; Haase, M. 2.Nektrochem. 1959, 63, 521. (24) Morf, W. E. Anal. Lett. 1977, 10, 87. (25) Lindner, E.; TBth, K.; Pungor, E. Anal. Chem. 1982, 5 4 , 72.
Werner
E. Morf
1981.
Department of Organic Chemistry Swiss Federal Institute of Technology CH-8092 Zurich, Switzerland
Hulanicki, A.; Lewenstam, A. Anal. Chem. 1981, 53, 1401. Sandifer, J. R. Anal. Chem. 1981, 53,312. Sandlfer, J. R. Anal. Chem. 1981, 53, 1164.
RECEIVEXI for review July 23,1982. Accepted January 27,1983.
Behavior of Cations in Nonsuppressed Anion Chromatography Sir: The behavior of cationic species during anion analysis of aqueous samples by nonsuppressed chromatography has received little attention. The presence of selected cations could affect the stability and performance of the columns and through interaction with the column materials coelute with the anions of interest. The interaction of alkali and alkaline earth metals with separator columns used in suppressed ion chromatography has been reported (1). For nonsuppressed systems, where silation is not 100% efficient, there are sufficient SiOz.aq sites available to interact with the metals (2-4). The separation of Au3+, Hg2+,and Cu2+by quaternary ammonium based anion separator columns has been reported (5) and presumably is due to the formation of amine complexes. This paper presents the interaction behavior of selected cations with a nonsuppressed anion separator column, and the analytical implications are discussed. EXPERIMENTAL SECTION Chromatographic Procedure. The chromatographic system employed consisted of a Perkin-Elmer Series 3B liquid chromatography, a Vydac Model No. 3021 C4.6 anion separator column, a Vydac Model 6000CD conductivity detector, a Sargent Welch XKR strip-chart recorder, and a Hewlett-Packard Model 3390A integrating recorder. The column separator group was amine based. Injector sample loop volume was 0.100 mL, and samples of 0.5 mL were introduced with a Hamilton Co. Model No. 750 microliter syringe. Laboratory temperature was maintained at 22.5 & 2.0 "C. Phthalate eluents were prepared by dissolving potassium hydrogen phthalate (KHP) in 900 mL of doubly distilled water, adding sufficient 0.1 M KOH to obtain the desired pH, and diluting to a final volume of 1.0 L with the distilled water. Distilled water eluents had their pH adjusted with either 0.01 M KOH or concentrated "0% Cation stock solutions, 0.10 M, were prepared by dissolution of their nitrate salts in distilled water. Standard solutions were prepared for analysis by dilution of the appropriate stock with the eluent of interest thus minimizing the magnitude of the solvent response in the chromatogram. All salts used were reagent grade. Fraction Analysis. In some cases, eluent fractions were collected for further chemical characterization. Fractions were collected manually for each 20-9 period over the entire course of the determination. All analyses were repeated a minimum of three times and all equivalent fractions were combined prior to analysis. The fractions were stored in closed Pyrex test tubes for less than 24 h prior to analysis. Cation content of the fractions was obtained by standard flame atomic absorption. Sample pH was measured
Table I. Retention Characteristics of Ionic Speciesa anions
3.6
Br NO;
4.7
so,2-
s,o,za
RT,min
c1-
5.4 9.4 13.1
cations Pb2+ Znz+ cu2+
R,, min
3.5 4.4
5.1
Eluent is 2.5 mM KHP; pH 5.0; flow rate, 2 mL/min.
with a Microelectrodes, Inc. Model MI-I110 combination pH probe, standardized at pH 4 and 10, and monitored with a Radiometer Model 26 pH meter. Total phthalate in the fractions was determined spectrophotometrically at 281 nm in a 1/50 (v/v) HCl matrix with 1-cm quartz cells (6). RESULTS AND DISCUSSION During a chromatographic study of factors that control the separation of anions, it was observed that certain cations were retained by the anion exchange column. Of the cations studied there appear to be three general groupings. The first of these includes the cations that do not interact with the column, Na+, K+, Ca2+ Mg2+9 Ni2+, Mn2+, and Cd2+. The second group interach to such a degree as to be chromatographed at times similar to those observed for certain anions, Cu2+,Pb2+,and Zn2+. The third group, Fe3+,A13+,and Hg2+,is strongly retained by the column and did not elute. Figure 1 presents a typical chromatogram for NaN03 (graph A) and what is observed when C U ( N O ~and ) ~ Pb(N03) are chromatographed (graph B). Atomic absorption spectrophotometry was necessary to identify the analyte for each metal peak. At low pH values, -4, the cations elute more rapidly than at higher pH and conversely anions elute less rapidly since there is a smaller fraction of the completely deprotonated phthalate anion present. The retention times demonstrated by the cations are comparable to those observed for some anions and thus incorrect peak assignments could be made. This problem is demonstrated in Figure 2 where chromatograms of copper, lead, and zinc nitrate are superimposed upon a chromatogram of NaC1, KBr, and NaN03. Table I summarizes retention times for the analytes. As the pH of the eluent changes from 4 to 5, the retention time of the cations increases by approximately 25% whereas the retention time of the anions decreases by 9
0003-2700/83/03551 168$01.50/0 0 1983 American Chemical Society
