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Anal. Chem. 1987, 59,924-928
AIDS FOR ANALYTICAL CHEMISTS Direct Determination of Uranium in Water by Cathodic Stripping Voltammetry Constant M. G . van den Berg* and Malcolm Nimmo Department of Oceanography, University of Liverpool, Liverpool L69 3BX, U.K. There are several methods available to determine uranium in water after its precencentration by means of extraction or adsorption techniques. its concentration in water has been determined by spectrophotometry after adsorption on cation exchange resins ( 1 , 2 ) or anion exchange resins (3-5), by neutron activation after chelation and adsorption (6),and by polarography and isotope dilution spectroscopy after extraction ( 5 ) . The predominant oxidation state of uranium in oxygenated water is U(VI), which occurs as the uranyl ion, U02*+,and this is complexed by carbonate ions (7) at neutral, or higher, pH in carbonate bearing waters. Its electrochemical determination is normally based on the reduction wave of U(V1) to U(V), which is located a t -0.4 V in a medium composed of perchloric acid and tartrate (8). U(V1) has been determined voltammetrically after preconcentration on the surface of solid electrodes coated with trioctylphosphine oxide (9, 101, but this method is not sufficiently sensitive for direct analysis of seawater samples. A large increase in sensitivity was obtained by determining uranium by cathodic stripping voltammetry (CSV) preceded by adsorptive collection of surface active metal complexes on the hanging mercury drop electrode (HMDE) (11, 12). When catechol is used as the chelating agent, uranium’s limit of detection is 0.3nM in seawater (12). Measurements in estuarine water samples in our laboratory have since shown that the sensitivity for uranium at low salinities (less than 2) is much less than in seawater and that interference by dissolved iron and organic material under such conditions often is prohibitive. Furthermore, catechol is not stable in the presence of dissolved oxygen, which makes automation of the analytical precedure difficult. A study was therefore undertaken to compare ligands and find one which gives better sensitivity for uranium in freshwater conditions as well as in seawater. The following compounds were tested by CSV and did not give a peak for uranium in seawater a t pH 6.9: trans-1,2-diaminocyclohexane-N,iV,”,”-tetraacetic acid, 4-(2-pyridylazo)resorcinol, gallic acid, benzoin a-oxime, nitroso-R salt, 8-hydroxyquinaldine, and dihydroxyanthraquinone. The following compounds did produce a peak for acid, 3,2-diuranium: 1,2-dihydroxybenzene-3,5-disulfonic hydroxybenzoic acid, salicylaldoxime, l-amino-2-naphthol-4sulfonic acid, and cupferron. Best results (in terms of sensitivity for uranium and absence of interferences) were obtained with 8-hydroxyquinoline (oxine), so the experiments with this compound are presented in this report and form the basis for the proposed CSV procedure.
EXPERIMENTAL SECTION Equipment was the same as before (12, 13). Briefly, a PAR 174 A polargraph with a PAR 303 hanging mercury drop electrode (HMDE),drop size 0.029 cm2,and a PAR 315 electronic magnetic stirrer were used for these experiments. Potentials are given with respect to an Ag/AgCl, saturated KCl, reference electrode. All measurements were performed in a laminar flow hood, or in a “clean-room” supplied with filtered air. The following reagents were prepared: a 0.005M aqueous stock solution of U(V1) was prepared by dissolving spectroscopic grade (99.999%) 1J,08 (Johnson Matthey Chemicals) in a minimum 0003-2700/87/0359-0924$0 1.50/0
amount of concentrated nitric acid (Aristar,BDH) and was diluted with distilled water (double quartz distilled). An aqueous stock solution of 0.1 M oxine was prepared in 0.25 M HC1 (Aristar, BDH) and diluted with distilled water; oxine solutions were found to be stable for at least 6 months. An aqueous pH buffer stock solution was prepared containing 1 M PIPES (piperazine-N,N’-bis(2-ethanesulfonicacid), monosodium salt) and 0.5 M NaOH (Aristar, BDH). An aqueous stock solution of 0.1 M EDTA was prepared in distilled water from its sodium salt and the pH was adjusted to a pH of 7 using NaOH (Aristar). Seawater used for the preliminary experiments had been collected from the Menai Straits, had a salinity of 32, and was stored at room temperature in a 50-L container. The water was filtered through 0.45-pm membrane filters (Oxoid) and 100-mL aliquots were UV irradiated (3 h) by a 1-kw mercury lamp. As a result of storage and irradiation at the original sample pH (pH 8) most of the uranium present in the seawater got lost and different aliquots contained between 1and 4 mM uranium. This loss is prevented by storing and irradiating the sample after acidification to pH2.7. Procedure. A 10-mL sample aliquot is pipetted into the voltammetric cell, and 100 WLof the PIPES pH buffer (final concentration 0.01 M and 20 p L of a 0.01 M oxine solution (final concentration 2 X M) are added. After deaeration of the solution by purging with water-saturated argon (or nitrogen), the potentiostat and the stirrer are switched on, and a new mercury drop is extruded, which signifies the beginning of the adsorption period; the adsorption potential is -0.4 V. Stirring is stopped after 60 s (selection of the adsorption time depends on the dissolved uranium concentration), during 10 s the solution is allowed to become quiescent, and the CSV scan is started the linear scanning mode is used with a scan rate of 50 mV s-l and a sensitivity of 100-200 nA (full scale). The measurement is repeated after a standard addition of uranium to the sample. The peak current-uranium concentration response is linear up to -30 nM U (at a peak current of 80 nA)under the stated conditions; the linear range is extended by reducing the sensitivity, by using shorter adsorption time, or by reducing the stirring rate. Interference by high concentrations of lead (>50 nM) and cadmium (>2 nM) is eliminated by addition of 10 pL of 0.1 M EDTA (final concentration lo4 M EDTA) to the sample prior to the measurement. Interference by surface active organic material is diminished by using a higher oxine concentration M); it can also be eliminated by UV irradiation of the sample after acidification to pH 2.7 f 0.2 with HCl, followed by pH neutralization and buffer addition.
RESULTS AND DISCUSSION Cyclic Voltammetry. At low pH and in a noncomplexing perchlorate solution the U(VI)-U(V) reduction wave is located at -0.175 V (vs. SCE) (14);this reduction is reversible. The reduction wave is shifted in a negative direction with increasing pH and becomes irreversible due to the relatively slow hydroxide complex fromation of U(V1) (12). Cyclic voltammetry preceded by 60 s of adsorption at -0.15 V in the presence of 2 X M oxine and a t a pH of 6.9 shows that the U(V1)-U(V) reduction peak is located at -0.67 V, whereas the broader oxidation peak is located a t -0.30 V (Figure 1). This compares with a cathodic peak potential of -0.58 V in M catechol (12); the difference is due the presence of to the greater stability of the U02*+-oxine complex as compared with the catechol complex. The small peak at -0.35 0 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 6, MARCH 15, 1987
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Figure 3. Effect of the oxine concentration on the CSV peak height of uranium. The solution was composed of seawater, containing 0.01 M PIPES and 20 nM U(V1).
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:: Figure 1. Cyclic voltammetry of 100 nM U(V1) in distilled water. Solution composition was 0.01 M P I E S (pH 6.9) and 2 X lod M oxine. Scan 1 is preceded by 60 s of adsorption at -0.15 V; scan 2 immediately followed scan 1, and scan 3 starts from -0.5 V. Scan rate is 50 mV s-'.
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Figure 4. Effect of the solution pH on the CSV peak height and potential for uranium. Solution composition was seawater, 5 x 1 0 - ~ M oxine, and 20 nM U(V1). Each CSV scan is preceded by 60 s of adsorption of -0.3 V.
Scan rote mv
Figure 2. Effects of the CSV scan rate on the peak height and potential of the U(V1) reduction peak. Each measurement is preceded by 60 s of adsorption at -0.15 V. The solution composttion was 100 nM U(VI), 0.01 M PIPES, and 2 X lo-' M oxine.
V is due to the reduction of Cu(II), which was present at trace levels (5 nM) in the solution. The uranium peaks are reproduced when the scan is repeated immediately upon finishing the return scan (scan 2), as all U(V1) apparently is regenerated at potentials more positive than -0.3 V. The U(VI)reduction peak is much smaller when the negative going scan is repeated (scan 3) after allowing the reversing scan to proceed only until -0.5 V this agrees with the previous finding that the oxidation step occurs a t -0.3 V. The cathodic and anodic uranium peaks are much diminished, when the reverse scan is initiated after stirring the solution briefly while holding the potential a t -0.8 V, due to transport of U(V) away from the electrode. The peak current increases linearly with the scan rate when this is varied from 20 to 200 mV s-l, but the increase is less at 500 mV s-l. The irreversible nature of the reduction step is apparent from the shift in the peak potential
when the scan rate is increased, as shown in Figure 2. Similar results were obtained at a higher concentration of oxine (5 x 10-4MI. Effects of Varying the Oxine Concentration and the Solution pH. The peak enhancement during the preconcentration step of CSV is due to the adsorption of uranyloxine complexes onto the HMDE. The peak height therefore increases with the oxine concentration until all uranium is complexed, as shown in Figure 3. No information is available about 1:l or 1:2 mixtures, only that for 1:3 uranyl-oxine complexes is available in the literature (15);therefore the species distribution can not be calculated. The decrease in M can the peak height at oxine concentrations above 2 X be due to the preferential adsorption of either 1:l or 1:2 complex ions (analogous with the adsorption of UO,-catechol, complexes (12),formation of which would be reduced when higher order complex ions are formed a t higher oxine concentrations. The optimal ligand concentration for analytical purposes is 1-2 X M oxine. The peak height increases with the solution pH until a pH of 6.7-6.9, whereas it diminishes strongly at higher pH values, as shown in Figure 4. This decrease is caused to a large extent by competitive hydroxide formation as well as complexation of UO?+ by carbonate ions, which occurs increasingly at higher
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ANALYTICAL CHEMISTRY, VOL. 59, NO. 6, MARCH 15. 1987
pH. The effect is much less at a higher oxine concentration of 10.:' M, where the peak height continues to increase with the pH over a tested pH range of between 3.9 and 8.2. The peak potential shifts in a negative direction with increasing pH (78inVipH unit at 5 X 10-5 M, and 67 mV/pH unit at 10 ,' M oxine) reflecting the increasing complex stability due to diminished proton competition (Figure 4). The optimal pH for analytical purposes is pH 6.7-6.9, when a lower oxine concentration ( S x LO-' M) is used. Maximum Adsorption Density of the HMDE. Measurement of the reduction current (in coulombs) at a high level of uranium (100 nM) indicated that the maximum adsorbable charge was 20.4 pC c:m-2,equivalent to an adsorbed layer of 2.1 x 10'" mol cm-". When the stirred adsorption time was lengthened from 1to 4 min, the peak was broadened without increasing significantly in height by the formation of a second peak with a peak potential 40 mV more positive than that of the first, suggesting that a second layer was formed, which is adsorbed less strongly than the first (the total adsorbed layer was than 3.5 x mol cm-'). The same effect was produced by doubiing the uranium concentration to 200 nM at a constant adsorption time of 1 rnin, which confirmed that the second peak was indeed due to uranium rather than to complexes of another metal. The area on the HMDE occupied by each adsorbed complex ion in the first layer was about 1.3 nm". which is within 10% of the theoretical area of a 1:2, uranium-oxine, complex, as calculated from the bond lengths. This result suggests that the 1:2 complex adsorbs specifically. The formation of complexes of the type 1:3 by increasing the oxine concentration above 5 x M reduces the solution concentration of the 12 variety, which could explain the diminished peak height at high oxine levels (Figure 3). Adsorption of free and complexed oxine was investigated by measuring the capacitance current by using tensammetry (ac.frequency of' 75 Hz. wave amplitude of 10 mV with 90' out of phase current sampling; each scan after 1 min adsorption at 41.4 V). It was found that although the capacitance decreased by the addition of lo-" M oxine to the solution (seawater), this decrease became much stronger upon adding just 10 nM uranium. This result suggests that complexed rather than free uxine adsorbs preferentially, especially considering the fact that some of the initial oxine adsorption may have been due to the presence of traces of uranium and other metali; in the seawater. Effects of Varying the Adsorption Potential a n d Adsorption Time. The peak potential for the reduction of M oxine and uranium i s at -0.68 1' in the presence of 5 X at a pH of 6.9. 'The CSV peak height is given as a function of the adsorption potential in Figure 5. The peak height dropped when adsorption potentials more negative than -0.4 L7 were applied, and no peak was formed when the CSV scan was preceded by adsorption a t a potential negative to the uranium reductio11 peak. Apparently reduced uranium ions (II(V)l do iiot forni R complex with adsorptive properties in these conditions. 'I'he sensitivity is increased by increasing the adsorption time, as shown in Figure 6. The peak height of 1 nM U in seawater increased linearly with the adsorption time (from stirred solution), whereas at a higher uranium concentration o f 15 nM the increase was nonlinear with adsorption times greater than 1 mixi due t u partial saturation of the surface of the mercury drup. Similar observations were made for vanadium and uranium when using catechol (12, 13). Sensitivity a n d Limit of Detection for Dissolved Uranium. Uranium was determined in freshwater and seawater by using the standard CSV procedure. The peak heiglit~-uraniuincxmceutration relationship was linear up to ahtrtir :io iikl ( I (at ti peak current (.)f 8U nA) when the scans
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Figure 5. Effect of varying the adsorption potential on the CSV peak height for uranium. Solution composition was seawater, 0.01 M PIPES, 5X M oxine, and 25 nM U(V1). Each measurement was preceded by 60 s stirred adsorption at the potentials indicated; CSV scans with adsorption potentials more negative than -0.4 V were started at -0.4 V.
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Figure 6. Effect of increasing the adsorption time on the CSV peak height for uranium. Solution compositions were seawater, 0.01 M PIPES, 2 X M oxine, and (1) 15 nM U(V1) and (2) 1 nM U(V1).
were preceded by 1min of stirred adsorption (Figure 7). The linear range is extended to higher uranium levels by reducing the sensitivity by adsorbing less complex ions on the electrode, i.e. by using a shorter adsorption time, or by adsorbing without stirring. The sensitivity for uranium in a synthetic electrolyte solution of PIPES in distilled water was about 10% greater than in seawater, presumably due to the absence of carbonate ions which compete with the oxine for uranyl ions and the
ANALYTICAL CHEMISTRY, VOL. 59, NO. 6, MARCH 15, 1987
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Potentia1,volt Figure 8. Determination of uranium in seawater by CSV. Solution M oxine. Each scan composition was 0.01 M PIPES and 2 X was preceded by 60 s of adsorption at -0.35 V: scan 1, 9.5 nM U(V1); scan 2, standard addition of 10 nM U(V1). 0 0
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Figure 7. CSV peak helght vs. the dissolved uranlum concentration. Solution composition was seawater, 0.01 M PIPES, and 5 X lo-' M oxine. Each measurement was preceded by 60 s of adsorption at -0.4 V.
major cations which partially saturate oxine in seawater. This similar sensitivity in fresh and seawater is in contrast to the poor sensitivity that was obtained by CSV when using catechol as the chelating compound in freshwater conditions, as its sensitivity was 10-20% of that in seawater. The standard deviation of the measurement of 0.5 nM U in a synthetic electrolyte solution of 0.01 M PIPES and 2 x M oxine was 10% (n = 8). The limit of detection as calculated from 3X the standard deviation waa 0.2 nM U, equal to that obtained from 3X the noise:peak height ratio. This limit of detection can be lowered to 0.02 nM U by increasing the adsorption time to 10 min. This compares with a limit of detection of 0.12 nM (6X higher) after a collection period of 10 min with catechol (12) in seawater. The sensitivity is 5-10 times less in freshwater (salinity < 2) than in seawater when catechol is used, whereas with the oxine the sensitivity remains almost the same (10% higher in freshwater). Interferences. The scans obtained when uranium in seawater is determined by CSV are shown in Figure 8. The uranium concentration of this sample was 9.5 nM as determined by a standard addition of 10 nM U (scan 2) to the sample. The sensitivity was 3.1 nA/nM U. The peak at -0.47 V is due to reduction of adsorbed Cu(I1)-oxine complexes; about 5 nM Cu was present in this sample. Very high levels of copper (>200 nM) can interfere by partially saturating the drop surface and diminishing the uranium sensitivity. This can be overcome by selecting an adsorption potential nearer to and negative of the copper peak at -0.45 or -0.50 V: for this condition the copper-oxine complexes dissociate as the copper is reduced; the copper then diffuses into the mercury drop. The small peak at -0.59 V is due to lead; about 2 nM
P b is present in this sample. The reduction peak potential for cadmium in the presence of oxine is almost identical with, and masked by, that of U(V1); the CSV sensitivity for lead and cadmium is about 0.25 nA/nM with the procedure used for uranium. The lead peak is well separated from that for uranium and even 50 nM P b does not interfere. However addition of 50 nM Cd to the sample increases the uranium peak by the equivalent of 4 nM U. Both cadmium and lead are masked completely by adding lo4 M EDTA to the sample, whereas the CSV peak height for uranium is not affected by M EDTA in fresh or seawater. In addition of up to 3 X seawater the peak height for uranium is diminished by 25%, and that for copper by 50%, when M EDTA is added. Spiking the sample with lo4 M EDTA can therefore be used to eliminate possible interference by cadmium and lead. The peak at -1.02 V (Figure 8) is caused by 10 nM Ni in the sample. Addition of zinc produces a peak at -1.20 V, but this element is masked by lo4 M EDTA. No CSV peaks were apparent upon addition of 100 nM Fe(III), 50 nM V, 25 nM Mn, 50 nM Al, 50 nM Cr, and 100 nM Mo (a Mo-oxine peak is produced at a pH of 3 and can be used for quantitative purposes (16)). An antimony peak (sensitivity 1 nA/nM) became apparent at -0.83 V, when 20 nM Sb(V) was added to the solution. This peak is well separated from that of U(VI), and the antimony concentration in natural waters is normally low (
