Electrochemical behaviour and preconcentration of uranyl(VI) at Nafion-coated glassy carbon electrodes

145

_I.Electroanal. Chem., 324 (1992) 145-159 Elsevier Sequoia S.A., Lausanne

JEC 1863

Electrochemical behaviour and preconcentration of uranyl(V1) at Nafion-coated glassy carbon electrodes Paolo Ugo, Barbara Ballarin, Salvatore Daniele and G. Antonio Mazzocchin Department of Physical Chemistry, University of Venice, S. Marta 2137, Venice, I-30123 (Italy) (Received 24 July 1991; in revised form 2 October 1991)

Abstract

The electrochemical behaviour of UO:+ solutions at Nafion@-coated glassy carbon electrodes has been examined and compared with the behaviour observed at uncoated electrodes. At pH 2.4, two reduction processes are always observed at both electrodes; however, the mechanism involved in the first process is affected strongly by the presence of the polymeric coating. While in homogeneous solution the first reduction product UOZ is quite stable and reacts only with electrogenerated U”‘, in the presence of Nafion the local high concentration of H + inside the coating causes it to disproportionate. The relevance of the ion exchange competition between H+ and Na+ to this reaction is proved. The influence of the applied potential on the rate and extent of incorporation was also studied. The ion exchange characteristics of Nafion for the different oxidation states generated by reducing UOz+ have been compared qualitatively and the selectivity coefficients for U’” and U”’ determined. The values obtained indicate a selectivity of Nafion for U I” about 20 times higher than that for U”‘, so that preconcentration from UOi+ dilute solutions can be achieved by applying a suitable reducing potential at the modified electrode.

INTRODUCTION

Ion exchange voltammetry has been applied successfully in recent years to detect low concentration levels of electroactive cations which have been preconcentrated by ion exchange in the polymeric coating [l]. Among others, f-element cations are particularly worth studying since the application of “classical” anodic stripping techniques is generally prevented for determining trace amounts of these species [2]. To check the possibility of applying Nafion-modified electrodes to the analysis of some f-element cations, we have recently studied the electrochemical behaviour of Yb3+ and Eu3+ at Nafion-modified electrodes [3]. The results obtained prompted us to develop an ion-exchange voltammetric method which is able to detect sub-micromolar concentration levels of these two rare earths [4]. In 0022-0728/92/$05.00

0 1992 - Elsevier Sequoia S.A. All rights reserved

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this study we extend this approach to an actinide cation by examining the electrochemistry of uranyl(V1) at Nafion-coated glassy carbon electrodes. The polarographic and voltammetric behaviour of uranyl dissolved in aqueous solutions has been studied widely using mainly mercury as the electrode material [5-131, while only a pioneering paper by Zittel and Miller [14] has described the use of bare glassy carbon electrodes with this aim. More recently, a series of papers has reported the use of glassy carbon electrodes modified by adsorption of trioctylphosphine oxide to detect trace amounts of U”’ [B-18]; to our knowledge, no paper dealing with the preconcentration and detection of uranyl at a polymermodified electrode has been published yet. All these studies indicate that a quite complex electrochemical behaviour, which is strongly pH-dependent, is involved in uranyl reduction. Since, in our case, the use of the polymeric coating can further complicate the study, we carried out this investigation operating only in slightly acid solutions, where kinetic complications 1111 as well as hydrolysis and precipitation problems [6,19,20] are expected to be almost negligible, at least as far as the homogeneous solution phase is concerned. In the light of increasing interest in electroanalytical methods able to determine trace amounts of uranyl in seawater 121,221, we carried out the present study in sodium chloride-containing media. EXPERIMENTAL

Chemicals All chemicals used were of reagent grade quality and UCI, was prepared according to ref. 23; it was stored and manipulated under a nitrogen atmosphere. 2.5% w/v Nafion solutions were prepared by 1: 1 dilution with methanol of commercially available 5% w/v Nafion solutions (Aldrich, solution prepared from Nafion 117, equivalent mass 1100 g). Apparatus and procedures All electroanalytical measurements were made at room temperature (22 + 1°C) under a nitrogen atmosphere using a single-compartment, three-electrode cell. Electrode potentials were measured and are referred to a saturated calomel electrode (SCE). Common commercially available electroanalytical instrumentation was employed. UCl, solutions were prepared directly in the electrochemical cell by adding a weighed amount of the salt to the supporting electrolyte solution which had been previously degassed with nitrogen. Electrochemical measurements on these solutions were carried out as quickly as possible and the real concentration of U’” was checked at the end of the experiment by spectrophotometric measurements using the molar extinction coefficient E = 29 1 mol-’ cm-’ measured at 648 nm at pH 2 [241.

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Nafion-coated electrodes were prepared by droplet evaporation of 5 ~1 of a 2.5% w/v Nafion solution deposited on a mirror-polished glassy carbon electrode (area 0.2 cm*); drop evaporation was carried out in a closed vessel in the presence of P,O, as desiccant according to a recently described dry-curing procedure [25]. Preliminary experiments, carried out by us, on the influence of curing humidity on the performance of the Nafion-modified electrodes indicated that the best incorporation and reproducibility for uranyl(V1) are obtained when solvent evaporation is carried out in an anhydrous atmosphere. This evidence suggests that incorporation of hydrophilic cations, such as UOi+ , is favoured by the increase in the permselectivity of the coating related to dry evaporation and demonstrated in ref. 25. Moreover, comparison of our result with the evidence reported in ref. 25 that hydrophobic cations, such as Ru(bipy)i+ , are better incorporated in wet-cured Nafion coatings supports the hypothesis that dry or wet-curing could play opposite roles as far as the incorporation of hydrophilic or hydrophobic cations is concerned. Calculations

The number of electrons, nx, involved in the first reduction step of UOi+ at bare glassy carbon electrodes (see later) was determined by combining voltammetric and chronoamperometric responses [26] according to the equation

(1) where Z, is the chronoamperometric current at time t, Zp is the voltammetric peak current recorded at scan rate u, the subscript X indicates data relevant to the process under study (i.e. UOi+ reduction) and the subscript R refers to data relevant to a reference reduction process which in our case was the Ru(NH&’ one-electron reduction. Data relevant to both X and R were recorded at the same electrode and in the same solution; care was taken in combining experimental results obtained in the same time-scale [27]. The distribution coefficients, k,, were calculated as k, = [M”+l,/[M”+],, where the subscripts p and s refer to the concentration of the electroactive species M”+ in the polymeric coating and in solution, respectively. The selectivity coefficients, Kx”, for the generic ion exchange equilibrium M”++n

(SO;X+)

+ [(SO&M”+]

+n X+

(2)

were calculated as KkM =

W+ l,[X+l:/[M”+ IsiX+I”,

where X+ is the supporting Whenever possible, the y~/M”+lJX+ln,r, were obtained according to the

(3)

electrolyte cation. corrected selectivity coefficients Kg = [M”+],[X+]: * also calculated. Solution activity coefficients y were Bates-Guggenheim approximate equation using 0.055

148

dm3 mol-’ as the coefficient for the linear term [281 and 0.53, 0.4 and 0.9 nm as the mean ion diameters for UO g+ [29], Na+ and H+ [30], respectively. The film concentrations of the incorporated electroactive species were calculated by coulometric integration after background subtraction of the relevant voltammetric peaks (peak A and A’* of Fig. 2 for Uvr and U’“, respectively) recorded at low scan rates (5 mV s-r) and from the known film volumes (density of wet Nafion = 1.58 g cmb3 [31]). RESULTS AND DISCUSSION

Electrochemical behaviour of uranyl(M) at Nafion-coated glassy carbon electrodes The electrochemical behaviour of UOz+ at bare glassy carbon electrodes was studied in order to compare the results with those obtained from the study of

Fig. 1. Cyclic voltamrnograms of (a) 6 X 10e4 M UO,(CH,COO), and (b) 4 x 10e4 M UC14. Working electrode : glassy carbon; supporting electrolyte : 0.1 M NaCl + HCI, pH 2.4; scan rate : 50 mV s - ‘.

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UO$+ incorporated into Nafion coatings. As shown in Fig. la, for UO$+ dissolved in a slightly acid solution the reduction peaks A, at - 230 mV, and B, at - 950 mV, are observed. Coulometric potentiostatic experiments carried out at -500 mV using a glassy carbon plate showed a catalytic-like behaviour in which, even forcing the reduction over 3-4 mol of electrons per mol of uranyl, a residual reduction current constant about 10% of the initial current was always observed. This behaviour, which is probably due to the occurrence, in the coulometric time-scale, of a reaction between electrogenerated U” and water [lo], prevents the use of coulometry to determine the number of electrons involved in the reduction. As an alternative, the number of electrons involved can be determined, as described in the Experimental section, by combining voltammetric and chronoamperometric data which are obtained in a much shorter time-scale. By this method, the reduction at peak A is a one-electron process. The evidence that peak B is twice as high as peak A indicates that the second reduction is a two-electron process and that U”’ is expected to be generated in this reduction. It is worth noting that when the scan is reversed at potential values more negative than peak B, the oxidation peak A’ associated with peak A with A E, = 100 mV (which suggests the occurrence of a quasi-reversible process) shifts to the more positive peak A’* at + 200 mV. This observation, together with the value of 0.5 for the ,/, ratio relevant to the B-B’ peak system, suggests that electrogenerated U”’ can react to some extent with electrogenerated UO:, so producing the species which is oxidized at peak A’*. Quite similar behaviour was reported in the literature for the voltammetric reduction of uranylW1) at mercury electrodes [12], where the return scan was characterized by the partial irreversibility of the second reduction process as well as by the appearance of two anodic peaks corresponding roughly to peaks A’ and A’* of our voltammograms. Since U”’ and U” can cornproportionate to give II’” [12], we examined the cyclic voltammetric behaviour of authentic I-J’” solutions in order to verify the possible involvement of this species in peak A’*. As shown in Fig. lb (solid line), the direct electrochemical oxidation of UCl, solutions at bare GC takes place at a peak potential equal to that of peak A’* of Fig. la, while the reduction of the electrogenerated oxidized species takes place at a peak potential corresponding to peak A in Fig. la, but with a peak current half that relevant to the direct oxidation peak. The direct reduction of U’” (see Fig. lb, dashed line) occurs at the potential value of peak B, again with a peak current about half of that involved in the direct oxidation at peak A’* and half of that recorded at the same peak B after peak A’* has been traversed. This evidence confirms that II’” is oxidized to UOz+ at peak A’* via a two-electron process; the so generated UOz+ is reduced in a one-electron step at peak A, further two-electron reduction to II”’ occurs at peak B. In the case of the direct reduction at peak B, the process is now a one-electron reduction. It is worth noting that the comproportionation reaction between U”’ and U” appears to be faster at the glassy carbon-solution interface than at the mercury-solution interface as suggested by the complete disappearance of peak A’ in Fig. lb (extended scan) in

Fig. 2. Steady-state cyclic voltammograms recorded after dipping a Nation-coated glassy carbon electrode in 6x 10W4M UO,(CH,COO), (1 and 4~ 10T4 M UCl, (- - -_). Other experimental conditions as in Fig. 1.

comparison with the persistence of the analogous oxidation peak recorded on mercury under similar experimental conditions [12]. Figure 2 (solid line) shows the steady-state voltammogram recorded when a Nafion-coated electrode was dipped in the same solution of Fig. la. Comparison of these figures indicates clearly that uranyl is incorporated inside the Nafion coating and a signal roughly one order of magnitude higher than the one recorded at the bare electrode is now observed. When the modified electrode was transferred to pure supporting electrolyte, continuous monitoring of the peak height by scanning the potential showed a rather quick release of the incorporated species, the peak current decreasing to one half in about 5 min. Moreover, this comparison also demonstrates that other relevant changes occur in the electrochemical behaviour of the incorporated cation with respect to its behaviour at the bare electrodes. First of all, at the Nafion-coated electrode, the reoxidation peak A’* (EP = 440 mV> associated with the first reduction peak A, now at - 300 mV, is always located at potentials much more positive than those expected for a reversible or quasi-reversible charge transfer regardless of whether peak B is traversed or not. Moreover, the ,/, ratio relevant to the A-A’* peak system is 1. On the other hand, the peak system B-B’ (E,(B) = - 1040 mV, AE, = 115 mV) is more reversible at the modified electrode ((Z,),/(Z,), = 1) than at bare glassy carbon. Also in this case, examination of the voltammetric behaviour of UCl, solutions helps in understanding the behaviour of the species under study. As shown by the dashed line in Fig. 2, the steady-state voltammetric pattern relevant to U’” incorporated into Nafion keeps the same main features as those recorded starting

151

from UOz+ solutions, the main difference being a further increase in the peak currents with which a distortion of the peaks, probably due to the ohmic drop effect, is associated. In particular, at the modified electrode II’” is oxidized at peak A’* and the so generated species is reduced at peak A in the reverse scan with an (Z,>,/, ratio close to 1. As far as the case of U vr is concerned, this evidence indicates that at the modified electrode II’” is the species generated at peak A and reoxidized at peak A’*, and that the comproportionation reaction between U”’ and U”, described above for the case of the bare electrode, does not operate. The most likely pathway to generate U’” from U”’ in this case is the one-electron reduction to U” followed by disproportionation according to the reaction 2UO:

+4H+eU

4++U02++2H 2

20

(4)

It is worth noting that when reaction (4) is fully operative, the overall reduction process at peak A becomes a two-electron process. The disproportionation reaction (4) is known to take place in acid solutions at pH < 2 [7-111 and, in fact, it does not occur in homogeneous solution under our experimental conditions. However, the activity of protons in a Nafion membrane can be much higher than that of the contacting solution [32], so that the occurrence of reaction (4) cannot be completely disregarded inside the coating. For instance, a basic species such as [Rt@lH,),p~]~+ (where pz = pyrazine) incorporated into Nafion starts to be protonated when the contacting solution is at pH 5 [32], while at this pH the same basic complex dissolved in homogeneous solutions is fully unprotonated. An approximate evaluation of the concentration of protons inside the coating can be obtained from selectivity coefficient data. Considering that uranyl is only weakly incorporated into Nafion and that the solution concentration of UOg+ is much lower than that of H+ and Na+, it seems conceivable that the proton concentration inside Nafion is ruled mainly by the ion exchange competition between these last two species and can be evaluated using the KANN”value reported in the literature [33]. When the solution concentration of Na+ is 0.1 M and the solution pH is 2.4, using 1.436 M (calculated from density and equivalent mass) as the concentration of ion exchange sites in Nafion, the concentration of H+ inside the membrane is estimated to be 4.8 X lo-* M. Such a high H+ concentration inside Nafion strongly supports the hypothesis that UO: electrogenerated inside the coating is not stable and can disproportionate according to reaction (4). If this is true, a suitable increase in the membrane pH is expected to be able to restore the reversibility of the UO~‘/UO~ couple. Since our experiments were carried out in the presence of uranyl ions dissolved in aqueous solution, the possibility of changing the pH inside the membrane by changing the pH of the contacting solution was not practicable because it introduces undesired effects such as hydrolysis of uranyl as well as hydrolysis and precipitation of its reduction products [6,19,20]. However, considering that the membrane pH is also related to the concentration of Na+ in solution, an alternative way to increase the

-OS

0

o.sE

h

Fig. 3. Cyclic voltammograms recorded 1 h after dipping a Nafion-coated glassy carbon electrode in 10W3M UO,(CH$OO), solution containing HCI (pH 2.4) and 1.3 M NaCl. Scan rate:50 mV s-l.

pH inside the membrane is by increasing the Na+ solution concentration while keeping the solution pH constant. Figure 3 shows the changes in the voltammetric pattern relevant to the first reduction process involving incorporated UOi+ which occur when the Na+ solution concentration is increased to 1.3 M. It is evident that under these experimental conditions the reduction process that takes place at peak A becomes more reversible and in the return scan an increase in peak A’ and a decrease in peak A’* are observed, even if the latter peak does not disappear. Also, under these experimental conditions the degree of incorporation is very low and the signal recorded at the modified electrode is comparable to that recorded at the bare electrode. Finally, it is worth noting that under the experimental conditions of Fig. 2, i.e. at a Na+ solution concentration of 0.1 M, the occurrence of reaction (4) does not control the reduction process completely at peak A since under pure kinetic conditions peak A is expected to be a two-electron peak while peak B might become a one-electron peak. However, pure kinetic control is reached at lower Na+ solution concentrations : for instance, at 0.05 M Na+ the ratio between peaks A and B approaches 2. Zon exchange and preconcentra tion Figure 4 shows the influence of the applied potential on the rate and extent of incorporation. The peak current increase observed when incorporation was carried out under open-circuit conditions and shown by curve c is much lower than those illustrated by curves a and b, which refer to data obtained when the incorporation was carried out while scanning the potential continuously between 0 and - 1250 mV and between + 600 and - 1250 mV, respectively. In particular, the highest extent of incorporation is achieved when the reoxidation peak A’* is not traversed

153

‘%

200

..

I 0

1000

PO00

%

Fig. 4. Influence of the applied potential on the time dependence of peak B of Fig. 2 during incorporation from 6 X 10m4 M UOi+ solutions. Potential scan limits: (a) 0 and - 1250 mV, (b) + 600 and -1250 mV. Curve c refers to peak currents recorded at intervals while keeping the cell under open-circuit conditions between the measurements. Scan rate: 50 mV s-l.

in the voltammetric scan, so that the applied potential is able to enrich the coating in the reduced species. This behaviour is also confirmed by the influence of the applied potential on the release of the incorporated species. As shown in Fig. 5, a faster release is observed if the potential is scanned continuously, traversing all the peak systems from peak A’* to peak B while it is slowed down if the positive scan is limited to 0 V. This evidence indicates that Nafion displays a lower selectivity for UOz+ than for its reduction products. The relative selectivity rank for U’” and U”’ can be estimated by the influence of the applied potential on the release from coatings charged with U’” solutions. If no potential is applied after transfer of the modified electrode to pure supporting electrolyte, no decrease in the peak current is observed even 1 h after the transfer, while a 23% decrease is noted when the potential is scanned continuously over the B-B’ peak system where U”’ is generated. These results suggest that the selectivity of Nafion of U rv is higher than that for U”‘. Quantitative evaluation of the selectivity coefficients for Nafion coatings can be done by coulometrically determining the concentration of the incorporated electroactive counterion in equilibrium with different solution concentrations of the same species [34]. It should be noted that the time necessary to achieve ion-exchange equilibrium conditions can be quite long and this methodology cannot be applied to highly

0

I 500

moo ’

%

Fig. 5. Influence of the applied potential on the time dependence of peak B of Fig. 2 during release after transferring the electrode into pure supporting electrolyte. Potential scan limits: (a) 0 and - 1250 mV; (b) + 600 and - 1250 mV. Prior to transfer, the incorporation was carried out under the conditions of Fig. 4, curve c.

reactive species such as U”’ in our case. However, the above-described procedure, in principle, can be applied to UOz+ and U’“. For correct evaluation of the selectivity coefficient of Nafion for UOg+ , the possible occurrence of the disproportionation reaction (4) must be taken into account. As shown in the previous section, such a reaction takes place in the Nafion coating at relatively low Na+ solution concentrations, while its influence decreases upon increasing this last parameter. However, the determination of the selectivity coefficient of Nafion at very high Na+ solution concentrations could be affected by a decrease in the permselectivity of Nafion, as reported for other ion exchange coatings which lose their permselectivity when dipped in solutions of high ionic strength [35]. Moreover, the extent of incorporation under these experimental conditions is so low that serious experimental errors can affect the coulometric determination of the concentration of the incorporated species. An alternative way is to operate under kinetic control, where the UOz+ reduction is expected to be a two-electron reduction process. As reported above, this condition can be achieved when the Na+ solution concentration is very low. As shown in Fig. 6, the partition isotherm obtained operating at a Na+ solution concentration of 0.05 M displays a linear trend (linear regression coefficient r = 0.997). A distribution coefficient k, = [UO~‘l,/[UO~‘l, of 98.8 is calculated from the slope. In order to evaluate the selectivity coefficient Kxuv’, we assumed that, as far as the ion exchange behaviour is concerned, Na+ and H+ are equivalent [33] and the

155

Fig. 6. Partition isotherm corresponding to UOz+ ion exchange data obtained by carrying out the incorporation under open-circuit conditions. Supporting electrolyte: 0.05 M NaCI + HCl, pH 2.4.

overall ion exchange equilibrium can be represented + 2 (so;x+)

uo;+

# [(so;),uo;+]

by the following reaction :

+ 2 x+

The ion-exchange selectivity coefficient becomes K/$“’

= [uo:+],[x+l1/[~~,2+]~~~+l~

(5)

where [X’], = [Na+], = [H’lS

and

[X’],

= [SO;] p - 2[UOg+] p

By substituting the data relevant to Fig. 6, a value of Kiuvl = 0.21 is obtained; by taking into account the correction for the activity coefficients in solution, the value Kzw = 0.26 is calculated. This figure can be compared with the selectivity coefficients reported in the literature [33] for other divalent cations by converting our selectivity coefficient KFxw into the selectivity coefficient KgVI, of the kind used in ref. 33 and defined as Kg”’

_

{quo;+)}p[x+lhd’

I’*

I {x]‘,[uo:+l,rUo:+ I

(6)

where {~(UO~+&, and {3(X+>), are the equivalent ionic fractions of the polymeric coating which interact with UOg+ and X+, respectively. The two selectivity constants are related to each other by the equation, K,UW= ( K,UWx 2 x 1.436) “*

(7)

156

which contains a conversion factor involving the net charge of uranyl ion and the total molar concentration of ionic sites of the coating. This relationship allows one to calculate a value of 0.86 for the selectivity coefficient expressed by eqn. (6). Thanks to the already mentioned ion-exchange equivalency of Na+ and H+ 1331, this figure can now be compared with the selectivity coefficients reported in ref. 33; it is found to be very close to the selectivity coefficient of 0.97 relevant to Zn*+. The determination of the selectivity coefficient for Urv has to face another problem, i.e. the fact that the chemical nature and the real charge of the U”’ species incorporated into Nafion are unknown. In fact, under our experimental conditions, U I” can give the hydrolysis product U(OHj3+ [24,361 as well as chloride complexes [37] or even mixed complex species. Such uncertainty in attributing a precise net charge value to the incorporated species is reflected in the impossibility of knowing the number of ionic sites, n, involved in the ion exchange process expressed in eqn. (2). Consequently, the calculation of the selectivity coefficient by the methodology used abov for UO$+ is now prevented. However, the determination of K&” is possible if the dependence of the concentration of the incorporated species on the solution concentration of the non-electroactive counterion X+ is studied instead of that on the solution concentration of U’“. In the case of the generic ion exchange equilibrium (21, it is evident that if K;fl is small enough and [X’]: z+ [M”+],, then IX’]: can be considered constant and approximately equal to the total concentration of ion exchange sites in Nafion. Then eqn. (3) becomes K* = [M”+],[X+]:,‘[M”+], where K * = Kk”[X+];. By plotting log[M”+],/[M”+], vs. log[X+]:, a straight line is expected to be obtained, with -n as the slope and logW*) as the intercept. Figure 7 shows the relevant plot for U’” incorporation, where the Urv concentration in the coating was obtained by integrating coulometrically the two-electron peak A’* recorded at a low scan rate. This plot is linear for Na+ concentrations lower than or equal to 0.5 M (linear regression coefficient r = 0.998). The deviation from linearity observed at higher supporting electrolyte concentrations could be due to failures in the film permselectivity [32,35]. From the slope and intercept of the linear portion, the values II = 1.96 and K * = 7 are obtained. From this last figure, considering [X’], = 1.436 M, a value of Kx”‘” = 4.9 is calculated. The value obtained for n indicates that the net charge of the U” species incorporated into Nafion is about 2, so excluding the involvement of the simple solvated ion U4+ as well as the complexes U(OHj3+ and UC13+. Since the chemical nature of such U’” species having 2+ charge is unknown, the correction for taking into account the activity coefficients for the ions in solution cannot be applied in this case. The fact that the selectivity coefficient for the U” species is about 20 times higher than that for UOz+ has interesting consequences on the development of methods for preconcentrating uranium species from UOz+ dilute solutions. As

157

Fig. 7. Dependence of the partition ratio for Urv on the concentration of the NaCl supporting electrolyte. The incorporation from UCi, solutions was carried out under open-circuit conditions; the solution pH was kept constant at 2.4.

(b)

-1

Fig. 8. Incorporation from 8~10-~ M UO$’ solution as a f&&on of the potential limits applied during the cyclic v~t~e~ic-pr~n~Rtration process. Other conditionsas in Fig. 2. The time between the first and last scans recorded both in (a) and in (b1 was 1 h.

158

shown in Fig. 8a for an 8 X lob6 M UOi+ solution, when the positive potential limit of the scan is higher than the potential value corresponding to peak A’* of Fig. 2, no incorporation of electroactive species is observed even after 1 h of continuous potential scanning. On the contrary, if the positive scan limit is kept lower than the potential value of peak A’*, rapid incorporation is achieved which allows us to detect by cyclic voltammetry the solution concentrations of uranyl in the micromolar concentration range (see Fig. 8b). Such an effect shows the usefulness of knowledge of the redox and ion exchange behaviour of the different oxidation states involved in the preconcentration of electroactive cations which can be incorporated in different redox states. ACKNOWLEDGEMENTS

We wish to thank Professor Fred C. Anson (Caltech, Pasadena) for his useful suggestions and Dr. France Ossola (C.N.R.-I.C.T.R., Padua) for the preparation of UCl,. Financial support by C.N.R. and M.U.R.S.T. (Rome) is acknowledged. REFERENCES 1 J. Wang in A.J. Bard (Ed.), Electroanalytical Chemistry, Vol. 16, Marcel Dekker, New York, 1989, p. 53. 2 F. Vydra, K. Stulik and E. JuBkovl, Electrochemical Stripping Analysis, Ellis Homood, Chichester, 1976, Ch. 5. 3 P. Ugo, B. Ballarin, S. Daniele and G.A. M azzocchin, J. Electroanal. Chem., 291 (1990) 187. 4 P. Ugo, B. Ballarin, S. Daniele and G.A. M azzocchin, Anal. Chim. Acta, 244 (1991) 29. 5 W.E. Harris and I.M. Kolthoff, J. Am. Chem. Sot., 67 (1945) 1484. 6 W.E. Harris and I.M. Kolthoff, J. Am. Chem. Sot., 69 (1947) 446. 7 D.M.H. Kern and E.F. Orlemann, J. Am. Chem. Sot., 71(1949) 2102. 8 E.F. Orlemann and D.M.H. Kern, J. Am. Chem. Sot., 75 (1953) 3058. 9 K.A. Kraus, F. Nelson and G.L. Johnson, J. Am. Chem. Sot., 71 (1949) 2510. 10 KA. Kraus and F. Nelson, J. Am. Chem. Sot., 71 (1949) 2517. 11 M. Mastragostino and J.M. Saveant, Electrochim. Acta, 13 (1968) 751. 12 L. Sipos, L.J. Jeftic, M. Branica and Z. Galus, J. Electroanal. Chem., 32 (1971) 35. 13 N.P. Bansal and J.A. Plambeck, Can. J. C&em., 59 (1981) 1515. 14 H.E. Zittel and F.J. Miller, Anal. Chem., 37 (1965) 200. 15 K.H. Lubert, M. Schnurrbusch and A. Thomas, Anal. Chim. Acta, 144 (1982) 123. 16 K.H. Lubert and M. Schnurrbusch, Anal. Chim. Acta, 186 (1986) 57. 17 K. Izutsu, T. Nakamura, R. Takizawa and H. Hanawa, Anal. Chim. Acta, 149 (1983) 147. 18 K. Izutsu, T. Nakamura and T. Ando, Anal. Chim. Acta, 152 (1983) 285. 19 R.N. Sylva and M.R. Davidson, J. Chem. Sot., Dalton Trans., (1979) 465. 20 S.P. Best, R.J.H. Clark and R.P. Cooney, Inorg. Chim. Acta, 145 (1988) 141. 21 C.M.G. van den Berg and M. Nimno, Anal Chem., 59 (1987) 924. 22 M. Mlakar and M. Branica, Anal. Chim. Acta, 221 (1989) 279. 23 J.A. Hermann and J.F. Suttle, Inorg. Synth., 5 (1957) 143. 24 K.A. Kraus and F. Nelson, J. Am. Chem. Sot., 72 (1950) 3901. 25 K.A. Striebel, G.G. Scherer and 0. Haas, J. ElectroanaI. Chem., 304 (1991) 289. 26 P. Ugo, S. Daniele, G.A. Maazocchin and G. BontempeIIi, Anal. Chim. Acta, 189 (1986) 253.

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