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Anal. Chsm. 1084, 56,650-653
Metal Speciation by Donnan Dialysis James A. Cox,* Krystyna Slonawska, and Diane K. Gatchell Department of Chemistry and Biochemistry, Southern Illinois University, Carbondale, Illinois 62901
Allen G. Hiebert Department of Chemistry, Knox College, Galesburg, Illinois 61401
I n Donnan dlalysls aqueous samples are separated from recelver electrolytes by an Ion exchange membrane. The present work demonstratesthat the dlalysls of metals Into sall solutlons occurs In proportlon to the sum of the concentratlons of the free metal and the metal held In the form of labile complexes; however, wlth strongly acldlc or chelatlng recelvers, the dlalysls occurs In proportion to the total soluble metal. Hence, Donnan dlalysls provides the basls for a rapid esthnatlon of the total soluble (Le., free plus labile complexed) metal and nonlablle-complexed metal. The method Is demonstrated wlth Pb, Zn, Cu, and Cd complexes of glycine, humic acld, and nltrllotrlacetlc acld and Is applied to a lake water sample. The results are compared to values obtalned from an established approach that utlllres strlpplng voltammetry and separatlons wlth a chelating Ion exchange resln.
In most cases the uptake of metals by biological systems proceeds from the free ion state rather than directly from complexes (1). As a result the sum of the concentrations of a given metal in the free ion state and in the form of very labile complexes is an important indicator of water quality. Among the methods available to determine this value, anodic stripping voltammetry and ion exchange chromatography are probably the most widely accepted. The former functions by selecting a deposition potential that permits reduction of the free ion only; of course, metal that is in the form of complexes which can dissociate during the few milliseconds ( 2 ) that a given increment of the solution volume is in contact with the electrode surface will albo be deposited. With ion exchange chromatography free metal ions and labile-complexed metals are analogously partitioned onto the resin phase, but in this case the time scale of lability is a few seconds (2). Based upon the above, two similar schemes for metal speciation are available in the recent literature ( 1 , 3 , 4 ) . The approach by Figura and McDuffie (3)is used herein. Their method subdivides the soluble metal into the categories of very labile, moderately labile, slowly labile, and inert based upon results of anodic stripping voltammetry, column ion exchange, batch ion exchange, and quantification of the total soluble metal after digestion. Moreover, approximate dissociation rate constants are assigned to the kinetic regimes. Other approaches, such as the combination of electrodeposition at a rotating disk electrode and stripping voltammetry (5),provide more accurate kinetic information, but at present the Figura and McDuffie method provides the best compromise between the practicality of application and the reliability of quantitative information on metal speciation. Their scheme has certain limitations. The method takes 2-3 days and requires several tedious analytical steps. The sample treatment that is required introduces problems of storage without perturbing the positions of chemical equilibria. The introduction of a buffer for the stripping voltammetry experiment may also shift the positions of the chemical equilibria (6). 0003-2700/84/0356-0650$01.50/0
In the present report we suggest that Donnan dialysis, which is described in detail elsewhere (7-lo), can be used as a rapid means of performing metal speciation. Our previous work indicates that the pH of the receiver electrolyte (or stripping solution) governs the chemistry at the sample-membrane interphase (11). On that basis, our speciation scheme involves one dialysis into an acidic receiver and a second dialysis into a neutral receiver. The first dialysis occurs in proportion to the total soluble metal (except for inert forms). The second dialysis proceeds in proportion to the sum of the free metal concentration and the metal held in very labile complexes. The difference corresponds to the nonlabile-complexed metal. The assumption is that the ligand is a conjugate base of a weak acid; therefore, at the acidified interphase of the first dialysis, the complex will dissociate and the released metal will directly enter the membrane. Other types of nonlabile complexes and metals held by strong adsorption to macromolecules will be classified as inert. It is important to note that no chemical steps are performed on the sample, so perturbations of the positions of chemical equilibria are avoided (this assumes that a large sample volume is used to avoid depletion effects). Further, the receiver electrolytes are excellent storage media. The dialyses can therefore be performed in the field and the quantitative information recovered later in the laboratory.
EXPERIMENTAL SECTION The cation exchange membranes were type P-1010 (RAI Research Corp., Hauppauge, NY).They were pretreated in 1M HCl for 1h prior to initial use. Between experiments they were rinsed with 1 M HC1, with a solution of the same composition as the receiver electrolyte but five times more concentrated, and with the receiver electrolyte. Each rinse was for 15 min. The dialysis cells were plastic cylinders. The membrane was held in place by Teflon tape and an O-ring (9). The membrane area (the cross section of the cylindrical cell) was 5.0 cm2. The receiver electrolyte (5 mL) was pipetted into the dialysis cell. The experiments were initiated by placing the cell in contact with 500 mL of sample that was held in a beaker. The bottom of the beaker was rounded so that the magnetic stirrer would be reproducibly located. The dialysis time was 30 min. After dialysis, the receiver chamber contents were transferred to a 10 mL volumetric flask and diluted to volume with water. The chemicals were Reagent Grade and were used without purification except as noted below. The nitrilotriacetic acid (Sigma Chemical Co.) was recrystallized as the Cd(I1) complex (12);the Na+ or H+form was subsequently metathesized by ion exchange. The humic acid (Aldrich Chemical Co.) was technical grade that was purified by passing solutions through a Chelex-100 (Bio-Rad Laboratories) column. The glycine (Matheson, Coleman and Bell) has a reported purity of 99.5%. Quantification was generally by flame atomic absorption spectrometry. Working curves were prepared with standard solutions that contained the diluted receiver electrolytes to normalize the matrices. The receiver electrolytes used in this step were subjected to blank dialyses to correct for any losses of ionic strength that would occur because of a small amount of anion transport through the P-1010 cation exchange membrane. The anodic strippingvoltammetry experiments were performed with a PAR Model 174 system. A microburet-type hanging 0 1984 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 56, NO. 4, APRIL 1984
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presence of acid in the receiver eliminated, within experimental error, the effects of glycine and nitrilotriacetic acid on the enrichment factors of Cu and Zn. Acidification of the receiver probably eliminates the effect of the complexing agents by creating a low pH environment at the sample-membrane interphase. The metal complexes would then dissociate at this junction. The metal would enter cu 0.1 2.5 i 0.2 the membrane phase and dialyze into the receiver. Only a cu 0 2.5 i 0.2 small amount of proton actually enters the bulk sample phase; Cu, 0.5 mM GLY, pH 7.5 0.00 0 1.9 i 0.1 the pH of the sample changes by about one unit during the 0.00 0.1 2.5 i 0.1 Cu, 0.5 mM GLY, pH 7.5 0.00 0 1.7 i 0.1 Cu, 0.5 mM NTA, pH 6.0 30-min dialyses. 0.00 0.1 2.4 i 0.2 Cu, 0.5 mM NTA, pH 6.0 The percent of the metal held as a nonlabile complex can 0.00 0.3 2.5 i 0.2 Cu, 0.5 mM NTA, pH 6.0 be calculated from the Table I data. For example, based on Cu, 0.5 mM NTA, pH 6.0 0.00 0.5 2.5 i 0.2 the earlier discussion, for the Cu-glycine system the percent Zn 0.1 2.4 i 0.2 nonlabile a t pH 7.5 is lOO(2.5 - 1.9)/2.5 = 24% under the Zn, 0.5 mM GLY, pH 6.0 0.97 0 2.2 i 0.1 experimental conditions employed. With the Zn-glycine Zn, 0.5 mM GLY, pH 6.0 0.97 0.1 2.4 i 0.1 system under the stated conditions, the experimental results All samples contain 1.0 x M of the listed metal. indicate that 8% is in the form of a nonlabile complex. The Fraction of metal theoretically predicted to be free ion result is higher than expected when it is considered that under the listed sample conditions. All receivers contheoretically only 3% of the zinc is even complexed under tain 0.6 M Na,SO,. GLY, glycine; NTA, nitrilotriacetic acid. these conditions. The latter is calculated from the acid dissociation constants of glycine (4.5 x and 1.7 X and mercury drop indicator electrode was used. The saturated calomel the formation constants of the 1:l and 1:2 complexes (3.3 X reference electrode was isolated from the electrochemical cell by lo6 and 2.8 X lo4) with the assumption that the significant a luggin capillary salt bridge to minimize the introduction of forms of glycine are H,GLY, HGLY-, and GLY2-, where the chloride into the sample. latter is the ligand. The high result may be due to the exColumn ion exchange experiments were performed with Cheperimental error but also may be the result of imperfections lex-100 and with Dowex 50WX4, 100-200 mesh, resins. The in the experimental or theoretical model. Adsorption of former has iminodiacetic acid functional groups. Both sodium glycine on the membrane may be a source of such error. and calcium forms of that resin were used for the speciation work; T o establish the veracity of this approach, a series of labhowever, since the speciation results were the same in preliminary experiments, the sodium form was generally used. The latter resin oratory samples was speciated by Donnan dialysis and by ion is a conventional cation exchanger with sulfonate as the functional exchange chromatography with sulfonated and chelating regroup. The column and batch experiments were performed as sins. The results are summarized in Table 11. Where comdescribed by Figura and McDuffie (3). With the Dowex resin, parisons are possible, the data are in good agreement with a 1.3 g (dry weight) was packed into the column and converted to previous report (2). the Na+ form with NaOH prior to use. It is apparent that Donnan dialysis gives values similar to RESULTS AND DISCUSSION those from the sulfonate ion exchange column method. The The first step in the development of the Donnan dialysis results by Donnan dialysis are generally higher which is speciation scheme was to test the hypothesis that the receiver probably because the experiment is in a somewhat faster electrolyte composition could be adjusted to override complex kinetic regime. That is, the contact time of an increment of formation in the sample. The enrichment factor, EF, for the sample volume with the membrane is shorter than that with metal from a nominally noncomplexing medium into a given the column resin; hence, there is less time for the metal receiver electrolyte was first determined (EF is the concencomplex to dissociate and fall into the “labile” category. The tration of the metal in the diluted receiver after dialysis diChelex column method always gave lower results than those vided by the initial concentration in the sample). The EF was obtained with the sulfonate column even though the experredetermined when a complexing agent was present in the imental parameters were made as identical as possible. The sample. The results shown in Table I demonstrated that the data were unchanged when the Chelex resin was initially in ____ ~ _ ~ ~ - _ _ _ _ _ Table 11. Comparison of Metal Speciation by Donnan Dialysis to Column Ion Exchange Methods Table I. Effect of the Receiver Electrolyte Composition on the Donnan Dialysis of Metals from Complexing Media concn of HC1 in receiver, samplea y b M EF
samplea metal
percent nonlabile by sulfonate Chelex column column 23 0 8 5 25 1 60 3 58 5 43 5
ligand PH yb DD 1 0 pM CU 0.5 mM GLY 7.5 0.00 24 10 pM Zn 0.5 mM GLY 7.5 0.10 11 1 0 pM CU 0.5 mM NTA 7.5 0.00 29 1 0 pM Pb 0.5 mM NTA 7.5 0.00 57 10 pM Zn 0.5 mM NTA 7.5 0.06 58 10 pM Cd 0.5 mM NTA 7.5 0.00 48 0.4 pM Cu 1 mM GLY 7.5 0.00 4 3 0 (o)c 0.12 p M Pb 1 mM GLY 7.5 0.97 9 8 1 0.12 pM Cd 1 mM GLY 7.5 0.74 3 1 0 0.12 pM Pb 0.16 mM NTA 7.5 0.00 26 21 3 10 pM CU 3 mg HA/L 6 43 41 30 1 0 pM Zn 3 mg HA/L 6 25 17 0 1 mg HA/L 6 14 9 0 1 0 IJM Zn 1 0 pM Zn 0.1 mg HA/L 6 8 4 0 1 0 pM Pb 3 mg HA/L 6 32 27 15 Key: GLY, glycine; NTA, nitrilotriacetic acid; HA, humic acid; DD, Donnan dialysis. See Table I. Values in parentheses taken from ref 2; they were obtained under identical conditions except that the sample pH was 7 . 8 .
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ANALYTICAL CHEMISTRY, VOL. 56, NO. 4, APRIL 1984
~_____________~
Table 111. Speciation of Metals in Lake Water Zn
a
species total metal soluble metal insoluble metal very labilea moderately labilea slowly labilea inert a DD labile As defined in ref 2 and 3.
%
11.1 -0.3 0 0
103 -3 0 0
MglL 2.4 2.1 0.3
%
0.7
33 57 1.0 0
1.2 0.2 0
98 DD, Donnan dialysis.
the Ca2+,rather than Na’, form. This seemingly demonstrates that the iminodiacetic acid functional groups on the Chelex resin induce lability by competing with complexing agents in the sample for the metals. Metal speciation results are highly dependent upon sample conditions. With Cu-glycine complexes at 1 X M Cu(I1) and pH 7.5, the percent of nonlability by Donnan dialysis decreases from 24 to 3 when the glycine concentration is decreased from 0.5 to 0.05 mM. The change is anticipated in that the equilibrium distribution among the species (free ion, 1:l complex, and 1:2 complex of Cu:glycine) varies with concentration and pH. In fact, with 0.05 mM glycine under these conditions, 0.3% of the Cu(I1) is not complexed. Likewise, the nonlabile fraction of Zn-humate complexes (Table 11) increases with the concentration of humic acid in the sample. Such factors make it difficult to compare published results. The final step in the evaluation of Donnan dialysis as a speciation technique was to compare it to the method of Figura and McDuffie (2, 3) on a real sample. Campus Lake at Southern Illinois University, Carbondale, was the source of the sample. The metals speciated were Cu, Zn, Pb, and Cd. The Chelex resin and the stripping voltammetry methods were used as suggested (2). The Donnan dialysis speciation was performed by first determining the EF for the transport of a given metal from a laboratory standard solution into 0.6 M Na2S04. The EF was then determined with a lake water sample (using the independently determined total soluble amount of that metal and the concentration in the 0.6 M Na2S04 receiver after dialysis for the calculation). The Donnan dialysis-labile fraction is then the ratio of these EF’s. The results are compared in Table 111. In the case of Zn, both speciation schemes indicate that the soluble metal exists as either a free ion or a very labile complex. For Cd, Cu, and Pb, the “Donnan dialysis-labile”fraction is greater than the “very labile” category by the Figura and McDuffie method but less than the sum of the “very labile” and “moderately labile” groups. The “very labile” fraction is determined by anodic stripping voltammetry which functions on the millisecond time scale, whereas the “moderately labile” regime is established by column chromatography at a time scale of about 10 s (2). Consistent with our earlier statement, the Donnan dialysis kinetic regime is apparently somewhat faster than that of ion exchange chromatography but is not in the range of stripping voltammetry with our present experimental design. The Donnan dialysis-nonlabile fraction can be calculated in two ways. First, the nonlabile quantity can be taken as the difference between the total metal and the DD-labile metal in Table 111. Alternatively, a dialysis into a 0.6 M Na2S04, 0.1 M HC1 receiver can be performed. The difference between the amounts dialyzed into this receiver and into 0.6 M Na2S04 can be attributed to the nonlabile fraction. The advantage of the latter approach is that the total soluble metal in the sample does not have to be independently determined. The
.
_
_
_
PglL
%
Pg/L 3.8 2.3
0.2 0.4 0.4
20 40 40
0.1
0
0 26
2.5 1.0 1.5
84
-
-
~
-
cu
Pb
Cd
PglL 13.1 10.8 2.3
~
%
1.5 1.1 1.1 0
4 48 48 0
24
respective results for the nonlabile fraction determined by these approaches are as follows: Zn, 2%, 5%; Cd, 16%, 7%; Cu, 76%, 73%; and Pb, 74%, 63%. The agreement between these values demonstrates that reasonable results can be obtained without an independent determination of the soluble metal in the sample. The level of disagreement is a reflection of the error introduced by assuming that the acidified receiver causes dialysis in proportion to the total soluble metal. A neutral-to-basic salt solution that contains a strong chelating agent such as iminodiacetic acid or EDTA provides an alternative to the use of an acidified receiver for overriding the effect of complexing agents in the sample on the Donnan dialysis experiment. For example, of the systems that were investigated, Zn-NTA was least successfully dialyzed into an acidified receiver, especially when the sample pHs and the NTA concentrations were high. With 1.0 X low6M Zn, 5.0 X lo4 M NTA at pH 7.5 as the sample, the enrichment factor with a 0.6 M Na2S04,0.1 M HC1 receiver was only 73% of the value obtained when NTA was absent, which is a significant difference from the value anticipated from our model. The same experiment with a 0.6 M Na2S04,0.05 M EDTA receiver at pH 7.5 yielded only a 3% decrease in the enrichment factor when NTA was included in the sample. Iminodiacetic acid was equally useful as a receiver component. The 0.6 M Na2S04,0.05 M EDTA receiver at pH 7.5 was tested on the lake water samples. The E F s obtained by using lake water samples were the following fractions of those obtained with laboratory standards prepared at the 1.0 pg/L level: Pb, 0.92; Zn, 0.90; Cd, 0.96; and Cu, 0.85. The values are less than unity indicating some fouling of the cation exchange membrane and/or that part of the metal exists as an adsorbed state on macromolecules. Nevertheless, these results suggest that it is not necessary to directly determine the total soluble metal in a grab sample. Instead, the sample can be dialyzed into the EDTA-containing receiver. Subsequently, the soluble metal data can be recovered by quantification of metals in the receiver and the use of enrichment factors obtained with laboratory stock solutions. Further work is in progress in this regard. In conclusion, Donnan dialysis has been demonstrated to provide estimates on metal speciation. The method does not at this time provide the detailed information of the Figura and McDuffie approach; however, the estimates can be made in less than 1h. The dialyses can be performed in the field. Chemical steps are not required. The cells can be made so inexpensively that they can be considered disposable. Because it is a membrane transfer process, Donnan dialysis may simulate natural phenomena; however, it can also be argued that in order to simulate the action of protein-containing systems in the aquatic environment, functional groups such as thiols should be used (13). The limitations of the Donnan dialysis method are the following: (a) the kinetic regime is presently limited, (b) the behavior of irreversibly bound metals, such as those adsorbed on macromolecules, is unknown, (c) the precision is about 6-8% under laboratory conditions, (d) the
~
Anal. Chem. 1984, 56,653-657
method as described cannot be used on saline samples, and (e) the classification as nonlabile of complexes which cannot be Donnan-dialyzed into an acidified or ligand-containing receiver as “inert” may not be justified. The latter is a general problem with metal speciation schemes.
Registry No. Pb, 7439-92-1;Zn, 7440-66-6;Cu, 7440-50-8;Cd, 7440-43-9; water, 7732-18-5. LITERATURE CITED (1) Florence, T. M.; Batley, G. E. CRC Crit. Rev. Anal. Chem. 1980, 9 , 219-296. (2) Figura, P.; McDuffie, B. Anal. Chem. 1979, 51, 120-125. (3) Figura, P.; McDuffle, B. Anal. Chem. 1080, 52,1433-1439. (4) Batley, G. E.; Florence, T. M. Anal. Left. 1978, 9 , 379-388. (5) Shuman, M. S.;Michael, L. C. Envlron. Scl. Techno/. 1978, 12, 1069- 1072. (6) Skogerboe, R. K.; Wilson, S.A.; Ostetyoung, J. G. Anal. Chem. 1080, 52,1980-1962.
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(7) Wallace, R . M. Ind. Eng. Chem. Process Des. Dev. 1987, 6 , 424-431. (8) Blaedel, W. J.; Haupert, T. J. Anal. Chem. 1988, 38, 1305-1308. (9) Cox, J. A.; DiNunzio, J. E. Anal. Chem. 1977, 4 9 , 1272-1275. (10) Cox, J. A.; Carnahan, J. W. Appl. Spectrosc. 1981, 35, 447-446. (11) . . Cox, J. A.; Olbrych, E.; Brajter, K. Anal. Chem. 1981, 53, 1308-1309. (12) Shuman, M. S . ; Shain, 1. Anal. Chem. 1989, 4 1 , 1818-1825. (13) Florence, T. M.; Batley, G. E. Anal. Chem. 1980, 52, 1962-1963.
RECEIVED for review August 1,1983. Resubmitted November 28,1983. Accepted December 5,1983. Although the research described in this article has been funded in part by the United States Environmental Protection Agency under assistance agreement number CR-809397 to Southern Illinois 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.
Determination of Dissolved Hydrogen and Effects of Media and Electrode Materials on the Electrochemical Oxidation of Molecular Hydrogen William C. Barrette, Jr., and Donald T. Sawyer* Department of Chemistry, University of California, Riverside, California 92521
Linear-sweep and rotateddlsk voltammetry have been used to characterlze the effects of media and electrode activation on the electrochemical oxidatlon of molecular hydrogen In water, dhnethylformamlde(DMF), dhrethyi sulfoxide (Me,=), pyrldine (pyr), and acetonitrile (MeCN). When a platinum electrode Is systematlcaily peactlvatedby anodlzatlon to PtO, H, is oxidized at It by a reproduclble, dlffusion-controlled process and the linear sweep voltammetric peak current is directly proportional to the partial pressure of dissolved hydrogen. The optimum preanodiratlon potentials vary from +1.0 V vs. SCE (pyr) to +3.0 V (MeCN). The reproduclbiiky of the anodic peak currents is good (
