- Email: [email protected]
Voltammetric study of uranyl–selenium interactions
Analytica Chimica Acta 404 (2000) 159–166
Voltammetric study of uranyl–selenium interactions R. Djogi´c, I. Pižeta, M. Zeli´c ∗ Center for Marine and Environmental Research, Rudjer Boškovi´c Institute, POB 1016, 10001 Zagreb, Croatia Received 11 March 1999; received in revised form 24 August 1999; accepted 4 September 1999
Abstract Uranyl–selenium interactions were studied at two different ionic strengths (I = 0.1 and 3.0 mol/l) using square wave voltammetry. The uranyl(+6) reduction signal is only slightly affected by selenium(+4) reduction to HgSe, which appears in virtually the same potential range. On the other hand, the height and shape of the selenium stripping peak (which corresponds to dissolution of accumulated HgSe) can be significantly changed by the uranyl(+6) concentration in the solution. The type and magnitude of such effects are highly dependent on the ionic strength. As a result of uranyl(+6) interaction with selenium(+6), a coordination species UO2 SeO4 is formed, whose stability constant at I = 3 mol/l (log β 1 = 1.57 ± 0.01) is in very good agreement with the literature value based on spectrophotometric data. The presence of higher complexes could not be confirmed unambiguously. ©2000 Elsevier Science B.V. All rights reserved. Keywords: Uranyl(+6); Selenite; Selenate; Voltammetry; Complexation; Stability constants
1. Introduction Interactions of selenium containing ions and molecules with metal ions may be important for different purposes. The resulting formation of complexes and precipitates [1] is a basis for: 1. understanding of the selenium influence on mercury and cadmium toxicity [2] 2. determination of selenium(+4) at trace levels by cathodic stripping voltammetry after accumulation of HgSe [3], Cu2 Se [4], Rh2 Se3 [5] or As2 Se3 [6]. 3. production of metal selenides [7–9] which exhibit specific conducting properties etc.
∗ Corresponding author. Tel.: +385-1-4561181; fax: +385-1-4680242 E-mail address: [email protected] (M. Zeli´c)
In the compilation of critically selected stability constants [1] data for even 52 selenium containing ligands are given. The list of cations contains 30 simple and alkylated ions corresponding to different groups of metals. Actinides are, however, lacking, the only exception being UO2 2+ which is ‘presented’ by the solubility product of UO2 SeO4 ·4H2 O (log K = −2.25) at the ionic strength I = 0.0 mol/l. Lubal and Havel [10], while studying the UO2 2+ –SeO4 2− system by spectrophotometry and potentiometry, concluded that two complexes could be formed at I = 3 mol/l (log β 1 = 1.57, log β 2 = 2.42). Taking into account that polarographic behaviour of uranium [11–14] and selenium species [15,16] were previously studied in this laboratory, as well as problems of complex equilibria [17–19], we found it attractive to (re)investigate the interactions of these two elements using voltammetric techniques. It is important to stress that the whole problem is not of pure
0003-2670/00/$ – see front matter ©2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 3 - 2 6 7 0 ( 9 9 ) 0 0 6 8 1 - 9
160
R. Djogi´c et al. / Analytica Chimica Acta 404 (2000) 159–166
academic interest. Recent results [20] point to the pronounced influence of selenate on uranium uptake by yeast cells (i.e. to the antagonistic interaction) and less pronounced opposite effect (i.e. increased uranium uptake) when selenite was applied instead.
2. Experimental All electrolyte and ligand solutions were prepared using chemicals of reagent grade produced by Kemika (Zagreb), Fluka (Buchs) and Merck (Darmstadt) and doubly distilled water. For preparation and standardization of the uranyl (+6) stock solution, a classical procedure given in Gmelin’s handbook [21], was applied. Some of the selenate solutions were additionally purified (by electrolysis) in order to evaluate the role of impurities. Voltammograms were recorded using the Autolab System (Eco Chemie, Utrecht) attached to a modified [22] static mercury drop electrode PAR 303A (Princeton Applied Research). Platinum wire served as a counter electrode whereas all potentials were given with respect to the saturated Ag/AgCl (NaCl) reference electrode. For pH measurements an ATI Orion pH meter (model 320) equipped with a small Metrohm combined pH electrode was used. Square wave (SW) voltammograms were recorded under the following conditions: frequency (f) = 100 s−1 , scan increment (Es ) = 2.44 mV, amplitude (a) = 50 mV (25 mV when the peak corresponding to dissolution of HgSe was followed). Uranyl complexation with selenate ions was followed in a titration procedure which consisted of two parts, both characterized by constant values of ionic strength, metal concentration and pH. In the first part perchlorate medium in the polarographic cell was titrated with a selenate solution whereas in the second part the roles were changed (in order to cover the widest possible ligand concentration range). Voltammograms corresponding to individual additions were recorded in triplicate using mercury drops of the medium size (surface area: 1.76 mm2 ). Before each measurement, the solution in the polarographic cell was deaerated by high purity nitrogen and a nitrogen blanket was maintained over the surface during the measurements. The room temperature was maintained in the range 23 ± 2◦ C.
For testing of the whole measuring system (electrodes, instrument etc.) the whole procedure for the determination of stability constants was applied to the well-defined cadmium complexation with chloride ions (at I = 4 mol/l) before the start of the present study. 2.1. Data treatment All measured experimental data (primary curves) were recorded using GPES 3.4 or GPES 4.6 software (General Purpose Electrochemical System by Eco Chemie, Utrecht) and stored in files. For obtaining relevant parameter values (peak height, position and half-width) the mentioned software was used, i.e. linear and polynomial baseline subtraction, peak search and fitting facilities. In such a way the peak position reading with a precision of 0.1 mV was achieved. For the determination of stability constants, a program for non-linear fitting was used [19] as well as Microsoft’s Excel 97 software facility.
3. Results and discussion 3.1. Uranyl–selenite interactions From the polarographic point of view the most important form of selenium is the selenite ion, i.e. oxidation state +4. It gives two reduction signals [3,15] which correspond to the formation and dissolution of the HgSe deposit on the mercury electrode surface. The first process takes place in (or near) the potential range where UO2 2+ transforms to UO2 + . Consequently, overlapping of selenium(+4) and uranyl(+6) reduction signals should be expected. If the selenite concentration is gradually increased while the uranyl(+6) level (5 × 10−5 mol/l), acidity (pH = 3) and ionic strength (0.1 mol/l) are kept constant, the signal corresponding to uranyl reduction changes its shape i.e. looses the symmetry, especially for [SeO3 2− ] ≥ 10−5 mol/l (Fig. 1). Under such conditions two new peaks, which partially overlap with the signal of interest, gradually develop. SW voltammograms recorded in the absence of uranium but under otherwise identical conditions also exhibit these two peaks. An example given in the inset of Fig. 1
R. Djogi´c et al. / Analytica Chimica Acta 404 (2000) 159–166
Fig. 1. The influence of selenite on uranyl(+6) reduction peak in square wave voltammetry. Peak shape corresponding to (in ascending order) selenite concentration of 0, 2 × 10−5 , 7 × 10−5 , and 1 × 10−4 mol/l. Inset: (1) Voltammogram recorded in uranium free selenite solution (1 × 10−4 mol/l) and (2) the difference between the highest composite curve and uranium signal unaffected by selenite. [UO2 2+ ] = 5 × 10−5 mol/l, pH = 3, I = 0.1 mol/l.
shows that they are in qualitative but not quantitative agreement with the difference between composite signal and the uranium(+6) response obtained from selenite free solution. The main difference appears in the form of a small peak near −210 mV especially if the selenite concentration is higher than 10−5 mol/l. At a higher ionic strength (I = 3 mol/l) the increased selenite concentration affects only the cathodic side of uranyl(+6) reduction peak. It is again mainly the consequence of the selenite reduction signal, although the half-peak width (W1/2 = 110 mV) of the response, obtained by subtraction of the independently recorded ‘basic current’ from the composite signal, is significantly smaller than in the case of nondisturbed uranyl(+6) reduction (125 mV). Addition of selenite also affects the uranyl peak potential but in a somewhat erratic way. For I = 0.1 mol/l the pronounced influence appears at very low selenium level (10−8 mol/l) with peak shifts in both directions of up to 8 mV (with respect to the initial position) whereas at a higher selenite concentration (10−5 mol/l)
161
the effect disappears. At I = 3 mol/l such dispersion of values is not obtained although a cathodic shift of even 14 mV appears when the selenium(+4) concentration increases from 5 × 10−7 to 1 × 10−4 mol/l. From the above results it is obvious that selenite can change the shape, height and potential of the uranyl(+6) reduction peak because of its own signal. Complex formation between them cannot be confirmed or rejected without difficulties but it can be expected to occur if the analogy between selenium and sulfur compounds is taken into consideration. The stability constant for UO2 SO3 (log K = 6.7 at I = 0 mol/l) has been published [1] indicating that complexation starts at relatively low concentrations of free, i.e. unprotonated, sulfite but at pH 3 the total ligand concentration should be even 1 × 10−4 mol/l to coordinate 10% of the dissolved uranyl(+6). Taking into account that protonation constants of selenite are higher [1] than the corresponding values of sulfites, whereas higher complexation constant can hardly be expected, pronounced formation of the coordination species should appear only in more concentrated selenite (than sulfite) solutions. In other words, the effects are possible outside the studied concentration range, i.e. under conditions which are highly inappropriate for voltammetric experiments because of the pronounced selenium signals. In the ‘opposite’ type of measurements in which selenite concentration is kept constant at 1 × 10−7 mol/l whereas uranyl level is gradually increased up to 7.2 × 10−5 mol/l, the change in the selenium stripping peak obtained after accumulation during 15 s is given in Fig. 2A. The peak becomes lower whereas some kind of hump or poorly resolved second signal develops on its cathodic side. In similar experiments with copper ions [16], after initial appearance of the hump, the peak corresponding to reduction of HgSe completely disappeared whereas a new peak with different characteristics developed at more negative potentials. This was taken as an indication that copper ions were included in the deposit on the mercury surface and nowadays it is generally accepted [23] that selenium can be accumulated in the form of Cu2 Se. Results obtained with uranyl ions at two different deposition times (15 and 60 s) could not be interpreted in this way because a clearly defined new signal was not obtained. The whole effect, however, could not be taken as a simple peak splitting (which often appears
162
R. Djogi´c et al. / Analytica Chimica Acta 404 (2000) 159–166
during reduction of accumulated HgSe [15] under the influence of different factors). At a higher electrolyte concentration (I = 2.8 mol/l) and longer deposition time (120 s) up to 250% peak height enhancement was obtained in the uranyl(+6) concentration range 0–6 × 10−5 mol/l (Fig. 2B). The appearance of a poorly pronounced, i.e. overlapping, signal on the cathodic side of the selenium stripping peak was obvious again but interpretation of the whole effect remains questionable. The change with respect to the previous results (Fig. 2A) is not a consequence of the prolonged deposition time (120 instead of 15 s). After 30 s of accumulation (at I = 2.8 mol/l) during inspection of the peak characteristics at different frequencies (up to 600 s−1 ) the selenium signal was always higher and wider in the presence than in the absence of uranyl ions (although significant change in the form of the forward and reverse signals could not be observed). Formation of a new deposit which includes UO2 2+ or UO2 + and selenide (produced at the electrode surface) is not to be expected if analogy with sulfide [1] is taken into account.
3.2. Selenite as an impurity
Fig. 2. The influence of uranyl(+6) on selenium stripping peak (A) I = 0.1 mol/l, selenite concentration: 1 × 10−7 mol/l, uranyl concentration (in the descending order): 0.1, 10.5, 22.8, 35.1, 47.4, 59.7 and 72.0 mol/l, accumulation time: 15 s; accumulation potential: −0.5 V; (B) I = 2.8 mol/l, selenite concentration: 3 × 10−7 mol/l, uranyl(+6) concentration (in the ascending order): 0, 1.5, 2.5, 3.5, 4.5, 5.5, 7.5, 9.5, 10.5, 22.8, 35.1, 47.4 and 59.7 mol/l, accumulation time: 120 s. In (A) and (B) the inset gives corresponding dependence of selenium (stripping) peak area (in arbitrary units) on uranyl concentration.
Sodium selenate usually contains some selenite. According to the specification Na2 SeO4 ·10H2 O (analytical reagent by Hopkin & Williams) contains maximally 0.1% of selenite. If so, in a solution containing 1 mol/l of sodium selenate the selenite concentration could be as high as 2 × 10−3 mol/l. In reality, however, it is more than an order of magnitude lower. Newer Na2 SeO4 by Fluka officially does not contain selenite but if the perchlorate solution is titrated by selenate at a constant acidity and fixed ionic strength, pronounced signals corresponding to the reduction of HgSe appear again (Fig. 3). As already mentioned, the presence of selenite could influence the shape and potential of the uranyl(+6) reduction peak. The effect could also be observed in selenate solutions which contain selenite as an impurity. An example is given in Fig. 4. It shows the cathodic shift of uranium reduction signal (as a result of complexation) and the change of its shape in the potential range in which reduction of selenite takes place.
R. Djogi´c et al. / Analytica Chimica Acta 404 (2000) 159–166
163
Fig. 5. Elimination of selenite from selenate solution by prolonged electrolysis on a large mercury electrode (46.6 cm2 ) at potential of (a) −0.1; (b) −0.4 and (c) −0.8 V. Selenate concentration: 0.0333 mol/l, pH = 3. Fig. 3. Signals obtained during titration of a sodium perchlorate solution by selenate in the absence of uranyl ions. I = 3 mol/l, pH = 3, selenate concentrations (in the ascending order): 0.032, 0.080, 0.211, 0.286 and 0.40 mol/l. Inset: signals recorded under the same conditions but presented on a different scale. Selenate concentrations (in the ascending order): 0.60, 0.79 and 1.00 mol/l.
It follows that selenium(+4) could be a problem during investigation of uranyl(+6) complexation with selenium(+6) i.e. selenate ion. Analysis of environmental samples usually includes transformation of selenate to selenite because only the latter can be measured at trace levels. For this purpose UV irradiation [24] or heating of the sample in highly acidic chloride solutions [25,26] are usually used. In the present study it was necessary to induce the opposite reaction or to eliminate the selenite in some other way. Previous experience [15] with cathodic stripping voltammetry and potentiometric electrolysis of selenite inspired us to use a similar procedure here taking into account that selenate ion is practically electroinactive. Results presented in Fig. 5 show the procedure by which the elimination of selenite was achieved using prolonged electrolysis on a large (46.6 cm2 ) mercury electrode. The purified solution did not give any reduction signal in experiments with a short (15 s) accumulation time or without it. Uranyl(+6) signals obtained in the presence of purified and nonpurified selenate, however, were only slightly different as described in the following paragraphs. 3.3. Uranyl–selenate interactions
Fig. 4. Uranyl(+6) reduction peak in the absence (a) and presence (b) of 0.51 mol/l of Na2 SeO4 ‘spiked’ with Na2 SeO3 . I = 3 mol/l, pH = 3, [UO2 2+ ] = 1 × 10−4 mol/l.
Reduction of noncomplexed uranyl(+6) to uranyl (+5) is generally [11] recognized as a reversible process what can easily be confirmed by different
164
R. Djogi´c et al. / Analytica Chimica Acta 404 (2000) 159–166
voltammetric techniques. The measurements should be performed in acidic media because hydrolysis starts at about pH = 3.5 [14,27] being more pronounced at high ionic strengths and/or high uranyl(+6) concentrations. In concentrated acids disproportionation, of uranyl(+5) to uranyl(+6) and uranyl(+4) occurs [11]. Therefore, all experiments were performed at pH = 3.00 ± 0.05 to maximally suppress both types of possible complications. At this acidity, according to the formation constant of HSeO4 − at I = 0 mol/l (K = 1.7 [1]), 95.2% of dissolved selenate should be free. At the applied ionic strengths (0.1 and 3 mol/l) the same constant is expected to be even lower resulting in virtually complete dissociation of hydrogen selenate. Buffers were not used in order to prevent possible side reactions. By gradual increase of selenate concentration the reduction peak of interest shifts cathodically. Reversibility of the process was checked by following of the square wave half-peak width (W1/2 = 122 ± 2 mV) and the form of the forward and reverse signal over the whole ligand concentration range. Measurements were performed at two different ionic strengths (3 and 0.1 mol/l) using two kinds of unpurified chemicals as well as solutions from which selenite was eliminated by electrolysis. One of such series for I = 3 mol/l is presented in Fig. 6. The curve giving 1Ep versus log [SeO4 2− ]
Fig. 6. Dependence of uranyl(+6) peak potential shift on selenate concentration. Solid line corresponds to the shifts expected for the literature [10] values of stability constants (log β 1 = 1.57, log β 2 = 2.42) I = 3 mol/l (maintained with NaClO4 ). For clarity, only some of totally 32 measuring points are given.
seems to indicate the formation of UO2 SeO4 as the only complex species because dEp /dlog [SeO4 2− ] = 59 mV/d.u. Additionally, the overall shift at the highest ligand concentration is significantly lower (40 mV) than calculated from the stability constants given by Lubal and Havel [10]. Numerical treatment of our experimental data indicates that they can be explained by formation of one or two complexes but in the latter case β 2 < β 1 . As pointed out by Bond [28] such cases in which the ultimate constant (β N ) is (significantly) lower than the preceding one (β N−1 ), are problematic because the appearance of a small β N value could result from systematic errors in data such as non-constancy of activity coefficients at a constant ionic strength or other experimental factors. Taking into account that results given in Fig. 6 were obtained using nonpurified selenate solution, we supposed that this could be the reason for not finding UO2 (SeO4 )2 2− (although the treatment of signals included subtraction of basic current before peak potential reading). When a purified ligand solution was used the final conclusion was the same. Moreover, all results obtained in several titration series using solutions of different purity with respect to the interfering selenium(+4) could be included in the same graphical presentation (1Ep versus log [SeO4 2− ]) although some dispersion of experimental points becomes obvious. They indicate formation of only the lowest complex species whose value (log β 1 = 1.58 ± 0.01) is in good agreement with the β 1 value published by Lubal and Havel [10]. At the same time the impression is that in the presence of selenate, uranyl(+6) reduction signal is less sensitive to selenite interferences. The results corresponding to different series of measurements (and different levels of interfering selenite), in which the second constant is also given (its existence being statistically approved in three of five series by the F-test of additional term and the F-test of two variances [29,30]), are presented in Fig. 7. Taking into account that reduction of uranyl(+6) gives uranyl(+5) (but not amalgam) as a product one should keep in mind that the peak potential shift can be used for the determination of stability constants only if the product does not coordinate to the ligand. Otherwise, only the ratio of the corresponding constants could be calculated [31]. Although the assumption that uranyl(+5) is not involved in the complex formation seems reasonable [1] the opposite situation cannot
R. Djogi´c et al. / Analytica Chimica Acta 404 (2000) 159–166
165
4. Concluding remarks From the analytical point of view the uranyl– selenium interactions can be important if they appear as: (i) interferences during selenium or uranium determinations or (ii) basis for development of new methods. Taking into account that all measurements were made at relatively high concentrations of reacting species it stays questionable if any of the effects could be obtained at trace levels which are of practical importance. The only uranyl(+6)–selenium interaction which seems to be evident is the formation of UO2 SeO4 , but UO2 (SeO4 )2 2− could not be confirmed although its formation constant has been published [10]. In this system, in which results are highly dependent on the applied electrolyte concentration some problems should be studied further (effects produced when singly charged perchlorate ion is substituted with doubly charged selenate ion at a constant ionic strength [32], the influence of complexation and/or disproportionation of uranyl(+5) on the voltammetric signal of interest etc.). Fig. 7. Formation constants of uranyl(+6) complexation with selenate. Results based on measurements with solutions containing (1) ≈10−4 (2), (3), (4) 8 × 10−5 mol/l of selenite and (5) purified solutions. For comparison the average values (AV) of those five sets of constant are given together with the values calculated from all experimental points (AP) belonging to all series, whereas (LD) are published [10] values of stability constants.
be excluded. Comparison with the corresponding sulfur species indicates that two uranyl(+6) sulfato complexes were reported [1] at several ionic strengths with β 2 > β 1 . It is interesting, however, that for I = 3 mol/l only the first constant is given. At I = 0.1 mol/l results are different. Peak height increases with increasing selenate concentration (20%) whereas at I = 3 mol/l it decreases for less than 10%. Peak potential shift indicates that the β 1 value at I = 0.1 mol/l is higher (log β 1 = 2.0 ± 0.1) than obtained at I = 3 mol/l, as in the uranyl–sulfate system [1]. Additionally, experimental results can be described by the formation of two complexes with β 2 > β 1 although the higher constant could not be determined precisely because of the short accessible ligand concentration range.
Acknowledgements The financial support of the Ministry of Science and Technology of the Republic of Croatia is gratefully acknowledged. References [1] NIST Critically Selected Stability Constants of Metal Complexes. Version 3.0, US Department of Commerce, Gaithersburg, 1997. [2] Joint Group of Experts on the Scientific Aspects of Marine Pollution, Review on Potentially Harmful Substances-Arsenic, Mercury and Selenium, Reports and Studies No. 28, World Health Organization, Geneva, 1986. [3] G. Jarz˛abek, Z. Kublik, J. Electroanal. Chem. 137 (1982) 247. [4] G. Mattsson, L. Nyholm, Å. Olin, U. Örenmark, Talanta 42 (1995) 817. [5] J. Wang, J. Lu, Anal. Chim. Acta 274 (1993) 219. [6] I. de Gregori, H. Pinochet, M. Potin-Gautier, M. Astruc, Bol. Soc. Chil. Quim. 41 (1996) 383. [7] N. Myung, C. Wei, K. Rajeshwar, Anal. Chem. 64 (1992) 2701. [8] L. Thouin, S. Massaccesi, S. Sanchez, J. Vedel, J. Electroanal. Chem. 374 (1994) 81.
166
R. Djogi´c et al. / Analytica Chimica Acta 404 (2000) 159–166
[9] S. Massaccesi, S. Sanchez, J. Vedel, J. Electroanal. Chem. 412 (1996) 95. [10] P. Lubal, J. Havel, Talanta 44 (1997) 457. [11] T. Mueller, M. Petek, in: A. J. Bard (Ed.), Encyclopedia of Electrochemistry of the Elements, vol. IX, Dekker, New York, 1986, p. 439. [12] R. Djogi´c, M. Branica, Electroanalysis 4 (1992) 151. [13] R. Djogi´c, D. Krznari´c, M. Branica, Electroanalysis 9 (1997) 936. [14] R. Djogi´c, M. Branica, Anal. Chim. Acta 281 (1993) 291. [15] M. Zeli´c, L. Sipos, M. Branica, Croat. Chem. Acta 58 (1985) 43. [16] M. Zeli´c, M. Branica, Electroanalysis 2 (1990) 455. [17] M. Branica, I. Pižeta, G. Branica-Jurkovi´c, M. Zeli´c, Mar. Chem. 28 (1989) 227. [18] M. Zeli´c, M. Branica, Anal. Chim. Acta 268 (1992) 275. [19] D. Pižeta, I. Pižeta, M. Branica in: Z. Haznadar (Ed.), Proceedings of the IXth International Symposium on Computer-aided Design and Computer-aided Manufacturing, Faculty of Electrical Engineering, University of Zagreb, Zagreb, 1987, p. 291.
[20] M. Czauderna, E. Makowska, S. Smolinski, J. Radioanal. Nucl. Chem. 177 (1994) 393. [21] Gmelins Handbuch der Anorganichen Chemie, No. 55, 8th edn., Verlag Chemie, Weinheim, 1936, p. 136. [22] I. Pižeta, D. Omanovi´c, M. Branica, Anal. Chim. Acta, in press. [23] G. Mattsson, L. Nyholm, Å. Olin, J. Electroanal. Chem. 379 (1994) 49. [24] J.D. Burton, W.A. Maher, C.I. Measures, P.J. Statham, Thalas. Jugoslav. 16 (1980) 155. [25] J. Pettersson, Å. Olin, Talanta 38 (1991) 413. [26] K. Pyrzy´nska, Chem. Anal. 40 (1995) 677. [27] G. Meinrath, Radiochim. Acta 77 (1997) 221. [28] A.M. Bond, Coord. Chem. Rev. 6 (1970) 377. [29] P.R. Bevington, Data Reduction and Error Analysis for the Physical Sciences, McGraw-Hill, New York, 1969, p. 200. [30] J.C. Miller, J.N. Miller, Statistics for Analytical Chemistry, Ellis Horwood, Chichester, 1989, p. 60. [31] D.R. Crow, Polarography of Metal Complexes, Academic Press, London 1969. [32] J.O’M. Bockris, A.K.N. Reddy, Modern Electrochemistry, vol. 1, Plenum Press, New York, 1998.
Voltammetric study of uranyl–selenium interactions R. Djogi´c, I. Pižeta, M. Zeli´c ∗ Center for Marine and Environmental Research, Rudjer Boškovi´c Institute, POB 1016, 10001 Zagreb, Croatia Received 11 March 1999; received in revised form 24 August 1999; accepted 4 September 1999
Abstract Uranyl–selenium interactions were studied at two different ionic strengths (I = 0.1 and 3.0 mol/l) using square wave voltammetry. The uranyl(+6) reduction signal is only slightly affected by selenium(+4) reduction to HgSe, which appears in virtually the same potential range. On the other hand, the height and shape of the selenium stripping peak (which corresponds to dissolution of accumulated HgSe) can be significantly changed by the uranyl(+6) concentration in the solution. The type and magnitude of such effects are highly dependent on the ionic strength. As a result of uranyl(+6) interaction with selenium(+6), a coordination species UO2 SeO4 is formed, whose stability constant at I = 3 mol/l (log β 1 = 1.57 ± 0.01) is in very good agreement with the literature value based on spectrophotometric data. The presence of higher complexes could not be confirmed unambiguously. ©2000 Elsevier Science B.V. All rights reserved. Keywords: Uranyl(+6); Selenite; Selenate; Voltammetry; Complexation; Stability constants
1. Introduction Interactions of selenium containing ions and molecules with metal ions may be important for different purposes. The resulting formation of complexes and precipitates [1] is a basis for: 1. understanding of the selenium influence on mercury and cadmium toxicity [2] 2. determination of selenium(+4) at trace levels by cathodic stripping voltammetry after accumulation of HgSe [3], Cu2 Se [4], Rh2 Se3 [5] or As2 Se3 [6]. 3. production of metal selenides [7–9] which exhibit specific conducting properties etc.
∗ Corresponding author. Tel.: +385-1-4561181; fax: +385-1-4680242 E-mail address: [email protected] (M. Zeli´c)
In the compilation of critically selected stability constants [1] data for even 52 selenium containing ligands are given. The list of cations contains 30 simple and alkylated ions corresponding to different groups of metals. Actinides are, however, lacking, the only exception being UO2 2+ which is ‘presented’ by the solubility product of UO2 SeO4 ·4H2 O (log K = −2.25) at the ionic strength I = 0.0 mol/l. Lubal and Havel [10], while studying the UO2 2+ –SeO4 2− system by spectrophotometry and potentiometry, concluded that two complexes could be formed at I = 3 mol/l (log β 1 = 1.57, log β 2 = 2.42). Taking into account that polarographic behaviour of uranium [11–14] and selenium species [15,16] were previously studied in this laboratory, as well as problems of complex equilibria [17–19], we found it attractive to (re)investigate the interactions of these two elements using voltammetric techniques. It is important to stress that the whole problem is not of pure
0003-2670/00/$ – see front matter ©2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 0 3 - 2 6 7 0 ( 9 9 ) 0 0 6 8 1 - 9
160
R. Djogi´c et al. / Analytica Chimica Acta 404 (2000) 159–166
academic interest. Recent results [20] point to the pronounced influence of selenate on uranium uptake by yeast cells (i.e. to the antagonistic interaction) and less pronounced opposite effect (i.e. increased uranium uptake) when selenite was applied instead.
2. Experimental All electrolyte and ligand solutions were prepared using chemicals of reagent grade produced by Kemika (Zagreb), Fluka (Buchs) and Merck (Darmstadt) and doubly distilled water. For preparation and standardization of the uranyl (+6) stock solution, a classical procedure given in Gmelin’s handbook [21], was applied. Some of the selenate solutions were additionally purified (by electrolysis) in order to evaluate the role of impurities. Voltammograms were recorded using the Autolab System (Eco Chemie, Utrecht) attached to a modified [22] static mercury drop electrode PAR 303A (Princeton Applied Research). Platinum wire served as a counter electrode whereas all potentials were given with respect to the saturated Ag/AgCl (NaCl) reference electrode. For pH measurements an ATI Orion pH meter (model 320) equipped with a small Metrohm combined pH electrode was used. Square wave (SW) voltammograms were recorded under the following conditions: frequency (f) = 100 s−1 , scan increment (Es ) = 2.44 mV, amplitude (a) = 50 mV (25 mV when the peak corresponding to dissolution of HgSe was followed). Uranyl complexation with selenate ions was followed in a titration procedure which consisted of two parts, both characterized by constant values of ionic strength, metal concentration and pH. In the first part perchlorate medium in the polarographic cell was titrated with a selenate solution whereas in the second part the roles were changed (in order to cover the widest possible ligand concentration range). Voltammograms corresponding to individual additions were recorded in triplicate using mercury drops of the medium size (surface area: 1.76 mm2 ). Before each measurement, the solution in the polarographic cell was deaerated by high purity nitrogen and a nitrogen blanket was maintained over the surface during the measurements. The room temperature was maintained in the range 23 ± 2◦ C.
For testing of the whole measuring system (electrodes, instrument etc.) the whole procedure for the determination of stability constants was applied to the well-defined cadmium complexation with chloride ions (at I = 4 mol/l) before the start of the present study. 2.1. Data treatment All measured experimental data (primary curves) were recorded using GPES 3.4 or GPES 4.6 software (General Purpose Electrochemical System by Eco Chemie, Utrecht) and stored in files. For obtaining relevant parameter values (peak height, position and half-width) the mentioned software was used, i.e. linear and polynomial baseline subtraction, peak search and fitting facilities. In such a way the peak position reading with a precision of 0.1 mV was achieved. For the determination of stability constants, a program for non-linear fitting was used [19] as well as Microsoft’s Excel 97 software facility.
3. Results and discussion 3.1. Uranyl–selenite interactions From the polarographic point of view the most important form of selenium is the selenite ion, i.e. oxidation state +4. It gives two reduction signals [3,15] which correspond to the formation and dissolution of the HgSe deposit on the mercury electrode surface. The first process takes place in (or near) the potential range where UO2 2+ transforms to UO2 + . Consequently, overlapping of selenium(+4) and uranyl(+6) reduction signals should be expected. If the selenite concentration is gradually increased while the uranyl(+6) level (5 × 10−5 mol/l), acidity (pH = 3) and ionic strength (0.1 mol/l) are kept constant, the signal corresponding to uranyl reduction changes its shape i.e. looses the symmetry, especially for [SeO3 2− ] ≥ 10−5 mol/l (Fig. 1). Under such conditions two new peaks, which partially overlap with the signal of interest, gradually develop. SW voltammograms recorded in the absence of uranium but under otherwise identical conditions also exhibit these two peaks. An example given in the inset of Fig. 1
R. Djogi´c et al. / Analytica Chimica Acta 404 (2000) 159–166
Fig. 1. The influence of selenite on uranyl(+6) reduction peak in square wave voltammetry. Peak shape corresponding to (in ascending order) selenite concentration of 0, 2 × 10−5 , 7 × 10−5 , and 1 × 10−4 mol/l. Inset: (1) Voltammogram recorded in uranium free selenite solution (1 × 10−4 mol/l) and (2) the difference between the highest composite curve and uranium signal unaffected by selenite. [UO2 2+ ] = 5 × 10−5 mol/l, pH = 3, I = 0.1 mol/l.
shows that they are in qualitative but not quantitative agreement with the difference between composite signal and the uranium(+6) response obtained from selenite free solution. The main difference appears in the form of a small peak near −210 mV especially if the selenite concentration is higher than 10−5 mol/l. At a higher ionic strength (I = 3 mol/l) the increased selenite concentration affects only the cathodic side of uranyl(+6) reduction peak. It is again mainly the consequence of the selenite reduction signal, although the half-peak width (W1/2 = 110 mV) of the response, obtained by subtraction of the independently recorded ‘basic current’ from the composite signal, is significantly smaller than in the case of nondisturbed uranyl(+6) reduction (125 mV). Addition of selenite also affects the uranyl peak potential but in a somewhat erratic way. For I = 0.1 mol/l the pronounced influence appears at very low selenium level (10−8 mol/l) with peak shifts in both directions of up to 8 mV (with respect to the initial position) whereas at a higher selenite concentration (10−5 mol/l)
161
the effect disappears. At I = 3 mol/l such dispersion of values is not obtained although a cathodic shift of even 14 mV appears when the selenium(+4) concentration increases from 5 × 10−7 to 1 × 10−4 mol/l. From the above results it is obvious that selenite can change the shape, height and potential of the uranyl(+6) reduction peak because of its own signal. Complex formation between them cannot be confirmed or rejected without difficulties but it can be expected to occur if the analogy between selenium and sulfur compounds is taken into consideration. The stability constant for UO2 SO3 (log K = 6.7 at I = 0 mol/l) has been published [1] indicating that complexation starts at relatively low concentrations of free, i.e. unprotonated, sulfite but at pH 3 the total ligand concentration should be even 1 × 10−4 mol/l to coordinate 10% of the dissolved uranyl(+6). Taking into account that protonation constants of selenite are higher [1] than the corresponding values of sulfites, whereas higher complexation constant can hardly be expected, pronounced formation of the coordination species should appear only in more concentrated selenite (than sulfite) solutions. In other words, the effects are possible outside the studied concentration range, i.e. under conditions which are highly inappropriate for voltammetric experiments because of the pronounced selenium signals. In the ‘opposite’ type of measurements in which selenite concentration is kept constant at 1 × 10−7 mol/l whereas uranyl level is gradually increased up to 7.2 × 10−5 mol/l, the change in the selenium stripping peak obtained after accumulation during 15 s is given in Fig. 2A. The peak becomes lower whereas some kind of hump or poorly resolved second signal develops on its cathodic side. In similar experiments with copper ions [16], after initial appearance of the hump, the peak corresponding to reduction of HgSe completely disappeared whereas a new peak with different characteristics developed at more negative potentials. This was taken as an indication that copper ions were included in the deposit on the mercury surface and nowadays it is generally accepted [23] that selenium can be accumulated in the form of Cu2 Se. Results obtained with uranyl ions at two different deposition times (15 and 60 s) could not be interpreted in this way because a clearly defined new signal was not obtained. The whole effect, however, could not be taken as a simple peak splitting (which often appears
162
R. Djogi´c et al. / Analytica Chimica Acta 404 (2000) 159–166
during reduction of accumulated HgSe [15] under the influence of different factors). At a higher electrolyte concentration (I = 2.8 mol/l) and longer deposition time (120 s) up to 250% peak height enhancement was obtained in the uranyl(+6) concentration range 0–6 × 10−5 mol/l (Fig. 2B). The appearance of a poorly pronounced, i.e. overlapping, signal on the cathodic side of the selenium stripping peak was obvious again but interpretation of the whole effect remains questionable. The change with respect to the previous results (Fig. 2A) is not a consequence of the prolonged deposition time (120 instead of 15 s). After 30 s of accumulation (at I = 2.8 mol/l) during inspection of the peak characteristics at different frequencies (up to 600 s−1 ) the selenium signal was always higher and wider in the presence than in the absence of uranyl ions (although significant change in the form of the forward and reverse signals could not be observed). Formation of a new deposit which includes UO2 2+ or UO2 + and selenide (produced at the electrode surface) is not to be expected if analogy with sulfide [1] is taken into account.
3.2. Selenite as an impurity
Fig. 2. The influence of uranyl(+6) on selenium stripping peak (A) I = 0.1 mol/l, selenite concentration: 1 × 10−7 mol/l, uranyl concentration (in the descending order): 0.1, 10.5, 22.8, 35.1, 47.4, 59.7 and 72.0 mol/l, accumulation time: 15 s; accumulation potential: −0.5 V; (B) I = 2.8 mol/l, selenite concentration: 3 × 10−7 mol/l, uranyl(+6) concentration (in the ascending order): 0, 1.5, 2.5, 3.5, 4.5, 5.5, 7.5, 9.5, 10.5, 22.8, 35.1, 47.4 and 59.7 mol/l, accumulation time: 120 s. In (A) and (B) the inset gives corresponding dependence of selenium (stripping) peak area (in arbitrary units) on uranyl concentration.
Sodium selenate usually contains some selenite. According to the specification Na2 SeO4 ·10H2 O (analytical reagent by Hopkin & Williams) contains maximally 0.1% of selenite. If so, in a solution containing 1 mol/l of sodium selenate the selenite concentration could be as high as 2 × 10−3 mol/l. In reality, however, it is more than an order of magnitude lower. Newer Na2 SeO4 by Fluka officially does not contain selenite but if the perchlorate solution is titrated by selenate at a constant acidity and fixed ionic strength, pronounced signals corresponding to the reduction of HgSe appear again (Fig. 3). As already mentioned, the presence of selenite could influence the shape and potential of the uranyl(+6) reduction peak. The effect could also be observed in selenate solutions which contain selenite as an impurity. An example is given in Fig. 4. It shows the cathodic shift of uranium reduction signal (as a result of complexation) and the change of its shape in the potential range in which reduction of selenite takes place.
R. Djogi´c et al. / Analytica Chimica Acta 404 (2000) 159–166
163
Fig. 5. Elimination of selenite from selenate solution by prolonged electrolysis on a large mercury electrode (46.6 cm2 ) at potential of (a) −0.1; (b) −0.4 and (c) −0.8 V. Selenate concentration: 0.0333 mol/l, pH = 3. Fig. 3. Signals obtained during titration of a sodium perchlorate solution by selenate in the absence of uranyl ions. I = 3 mol/l, pH = 3, selenate concentrations (in the ascending order): 0.032, 0.080, 0.211, 0.286 and 0.40 mol/l. Inset: signals recorded under the same conditions but presented on a different scale. Selenate concentrations (in the ascending order): 0.60, 0.79 and 1.00 mol/l.
It follows that selenium(+4) could be a problem during investigation of uranyl(+6) complexation with selenium(+6) i.e. selenate ion. Analysis of environmental samples usually includes transformation of selenate to selenite because only the latter can be measured at trace levels. For this purpose UV irradiation [24] or heating of the sample in highly acidic chloride solutions [25,26] are usually used. In the present study it was necessary to induce the opposite reaction or to eliminate the selenite in some other way. Previous experience [15] with cathodic stripping voltammetry and potentiometric electrolysis of selenite inspired us to use a similar procedure here taking into account that selenate ion is practically electroinactive. Results presented in Fig. 5 show the procedure by which the elimination of selenite was achieved using prolonged electrolysis on a large (46.6 cm2 ) mercury electrode. The purified solution did not give any reduction signal in experiments with a short (15 s) accumulation time or without it. Uranyl(+6) signals obtained in the presence of purified and nonpurified selenate, however, were only slightly different as described in the following paragraphs. 3.3. Uranyl–selenate interactions
Fig. 4. Uranyl(+6) reduction peak in the absence (a) and presence (b) of 0.51 mol/l of Na2 SeO4 ‘spiked’ with Na2 SeO3 . I = 3 mol/l, pH = 3, [UO2 2+ ] = 1 × 10−4 mol/l.
Reduction of noncomplexed uranyl(+6) to uranyl (+5) is generally [11] recognized as a reversible process what can easily be confirmed by different
164
R. Djogi´c et al. / Analytica Chimica Acta 404 (2000) 159–166
voltammetric techniques. The measurements should be performed in acidic media because hydrolysis starts at about pH = 3.5 [14,27] being more pronounced at high ionic strengths and/or high uranyl(+6) concentrations. In concentrated acids disproportionation, of uranyl(+5) to uranyl(+6) and uranyl(+4) occurs [11]. Therefore, all experiments were performed at pH = 3.00 ± 0.05 to maximally suppress both types of possible complications. At this acidity, according to the formation constant of HSeO4 − at I = 0 mol/l (K = 1.7 [1]), 95.2% of dissolved selenate should be free. At the applied ionic strengths (0.1 and 3 mol/l) the same constant is expected to be even lower resulting in virtually complete dissociation of hydrogen selenate. Buffers were not used in order to prevent possible side reactions. By gradual increase of selenate concentration the reduction peak of interest shifts cathodically. Reversibility of the process was checked by following of the square wave half-peak width (W1/2 = 122 ± 2 mV) and the form of the forward and reverse signal over the whole ligand concentration range. Measurements were performed at two different ionic strengths (3 and 0.1 mol/l) using two kinds of unpurified chemicals as well as solutions from which selenite was eliminated by electrolysis. One of such series for I = 3 mol/l is presented in Fig. 6. The curve giving 1Ep versus log [SeO4 2− ]
Fig. 6. Dependence of uranyl(+6) peak potential shift on selenate concentration. Solid line corresponds to the shifts expected for the literature [10] values of stability constants (log β 1 = 1.57, log β 2 = 2.42) I = 3 mol/l (maintained with NaClO4 ). For clarity, only some of totally 32 measuring points are given.
seems to indicate the formation of UO2 SeO4 as the only complex species because dEp /dlog [SeO4 2− ] = 59 mV/d.u. Additionally, the overall shift at the highest ligand concentration is significantly lower (40 mV) than calculated from the stability constants given by Lubal and Havel [10]. Numerical treatment of our experimental data indicates that they can be explained by formation of one or two complexes but in the latter case β 2 < β 1 . As pointed out by Bond [28] such cases in which the ultimate constant (β N ) is (significantly) lower than the preceding one (β N−1 ), are problematic because the appearance of a small β N value could result from systematic errors in data such as non-constancy of activity coefficients at a constant ionic strength or other experimental factors. Taking into account that results given in Fig. 6 were obtained using nonpurified selenate solution, we supposed that this could be the reason for not finding UO2 (SeO4 )2 2− (although the treatment of signals included subtraction of basic current before peak potential reading). When a purified ligand solution was used the final conclusion was the same. Moreover, all results obtained in several titration series using solutions of different purity with respect to the interfering selenium(+4) could be included in the same graphical presentation (1Ep versus log [SeO4 2− ]) although some dispersion of experimental points becomes obvious. They indicate formation of only the lowest complex species whose value (log β 1 = 1.58 ± 0.01) is in good agreement with the β 1 value published by Lubal and Havel [10]. At the same time the impression is that in the presence of selenate, uranyl(+6) reduction signal is less sensitive to selenite interferences. The results corresponding to different series of measurements (and different levels of interfering selenite), in which the second constant is also given (its existence being statistically approved in three of five series by the F-test of additional term and the F-test of two variances [29,30]), are presented in Fig. 7. Taking into account that reduction of uranyl(+6) gives uranyl(+5) (but not amalgam) as a product one should keep in mind that the peak potential shift can be used for the determination of stability constants only if the product does not coordinate to the ligand. Otherwise, only the ratio of the corresponding constants could be calculated [31]. Although the assumption that uranyl(+5) is not involved in the complex formation seems reasonable [1] the opposite situation cannot
R. Djogi´c et al. / Analytica Chimica Acta 404 (2000) 159–166
165
4. Concluding remarks From the analytical point of view the uranyl– selenium interactions can be important if they appear as: (i) interferences during selenium or uranium determinations or (ii) basis for development of new methods. Taking into account that all measurements were made at relatively high concentrations of reacting species it stays questionable if any of the effects could be obtained at trace levels which are of practical importance. The only uranyl(+6)–selenium interaction which seems to be evident is the formation of UO2 SeO4 , but UO2 (SeO4 )2 2− could not be confirmed although its formation constant has been published [10]. In this system, in which results are highly dependent on the applied electrolyte concentration some problems should be studied further (effects produced when singly charged perchlorate ion is substituted with doubly charged selenate ion at a constant ionic strength [32], the influence of complexation and/or disproportionation of uranyl(+5) on the voltammetric signal of interest etc.). Fig. 7. Formation constants of uranyl(+6) complexation with selenate. Results based on measurements with solutions containing (1) ≈10−4 (2), (3), (4) 8 × 10−5 mol/l of selenite and (5) purified solutions. For comparison the average values (AV) of those five sets of constant are given together with the values calculated from all experimental points (AP) belonging to all series, whereas (LD) are published [10] values of stability constants.
be excluded. Comparison with the corresponding sulfur species indicates that two uranyl(+6) sulfato complexes were reported [1] at several ionic strengths with β 2 > β 1 . It is interesting, however, that for I = 3 mol/l only the first constant is given. At I = 0.1 mol/l results are different. Peak height increases with increasing selenate concentration (20%) whereas at I = 3 mol/l it decreases for less than 10%. Peak potential shift indicates that the β 1 value at I = 0.1 mol/l is higher (log β 1 = 2.0 ± 0.1) than obtained at I = 3 mol/l, as in the uranyl–sulfate system [1]. Additionally, experimental results can be described by the formation of two complexes with β 2 > β 1 although the higher constant could not be determined precisely because of the short accessible ligand concentration range.
Acknowledgements The financial support of the Ministry of Science and Technology of the Republic of Croatia is gratefully acknowledged. References [1] NIST Critically Selected Stability Constants of Metal Complexes. Version 3.0, US Department of Commerce, Gaithersburg, 1997. [2] Joint Group of Experts on the Scientific Aspects of Marine Pollution, Review on Potentially Harmful Substances-Arsenic, Mercury and Selenium, Reports and Studies No. 28, World Health Organization, Geneva, 1986. [3] G. Jarz˛abek, Z. Kublik, J. Electroanal. Chem. 137 (1982) 247. [4] G. Mattsson, L. Nyholm, Å. Olin, U. Örenmark, Talanta 42 (1995) 817. [5] J. Wang, J. Lu, Anal. Chim. Acta 274 (1993) 219. [6] I. de Gregori, H. Pinochet, M. Potin-Gautier, M. Astruc, Bol. Soc. Chil. Quim. 41 (1996) 383. [7] N. Myung, C. Wei, K. Rajeshwar, Anal. Chem. 64 (1992) 2701. [8] L. Thouin, S. Massaccesi, S. Sanchez, J. Vedel, J. Electroanal. Chem. 374 (1994) 81.
166
R. Djogi´c et al. / Analytica Chimica Acta 404 (2000) 159–166
[9] S. Massaccesi, S. Sanchez, J. Vedel, J. Electroanal. Chem. 412 (1996) 95. [10] P. Lubal, J. Havel, Talanta 44 (1997) 457. [11] T. Mueller, M. Petek, in: A. J. Bard (Ed.), Encyclopedia of Electrochemistry of the Elements, vol. IX, Dekker, New York, 1986, p. 439. [12] R. Djogi´c, M. Branica, Electroanalysis 4 (1992) 151. [13] R. Djogi´c, D. Krznari´c, M. Branica, Electroanalysis 9 (1997) 936. [14] R. Djogi´c, M. Branica, Anal. Chim. Acta 281 (1993) 291. [15] M. Zeli´c, L. Sipos, M. Branica, Croat. Chem. Acta 58 (1985) 43. [16] M. Zeli´c, M. Branica, Electroanalysis 2 (1990) 455. [17] M. Branica, I. Pižeta, G. Branica-Jurkovi´c, M. Zeli´c, Mar. Chem. 28 (1989) 227. [18] M. Zeli´c, M. Branica, Anal. Chim. Acta 268 (1992) 275. [19] D. Pižeta, I. Pižeta, M. Branica in: Z. Haznadar (Ed.), Proceedings of the IXth International Symposium on Computer-aided Design and Computer-aided Manufacturing, Faculty of Electrical Engineering, University of Zagreb, Zagreb, 1987, p. 291.
[20] M. Czauderna, E. Makowska, S. Smolinski, J. Radioanal. Nucl. Chem. 177 (1994) 393. [21] Gmelins Handbuch der Anorganichen Chemie, No. 55, 8th edn., Verlag Chemie, Weinheim, 1936, p. 136. [22] I. Pižeta, D. Omanovi´c, M. Branica, Anal. Chim. Acta, in press. [23] G. Mattsson, L. Nyholm, Å. Olin, J. Electroanal. Chem. 379 (1994) 49. [24] J.D. Burton, W.A. Maher, C.I. Measures, P.J. Statham, Thalas. Jugoslav. 16 (1980) 155. [25] J. Pettersson, Å. Olin, Talanta 38 (1991) 413. [26] K. Pyrzy´nska, Chem. Anal. 40 (1995) 677. [27] G. Meinrath, Radiochim. Acta 77 (1997) 221. [28] A.M. Bond, Coord. Chem. Rev. 6 (1970) 377. [29] P.R. Bevington, Data Reduction and Error Analysis for the Physical Sciences, McGraw-Hill, New York, 1969, p. 200. [30] J.C. Miller, J.N. Miller, Statistics for Analytical Chemistry, Ellis Horwood, Chichester, 1989, p. 60. [31] D.R. Crow, Polarography of Metal Complexes, Academic Press, London 1969. [32] J.O’M. Bockris, A.K.N. Reddy, Modern Electrochemistry, vol. 1, Plenum Press, New York, 1998.