Determination of Cobalt in Seawater by Catalytic Adsorptive Cathodic

The potential of the cobalt peak shifted about 10 mV in the cathodic direction ...... metals dynamic in a highly stratified estuary: The Krka River es...
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Anal. Chem. 1997, 69, 874-881

Determination of Cobalt in Seawater by Catalytic Adsorptive Cathodic Stripping Voltammetry Marisol Vega† and Constant M. G. van den Berg*

Oceanography Laboratories, University of Liverpool, Liverpool L69 3BX, U.K.

A new procedure for the direct determination of picomolar levels of cobalt in seawater is presented. Cathodic stripping voltammetry is preceded by adsorptive accumulation of the cobalt-nioxime (cyclohexane-1,2-dione dioxime) complex from seawater containing 6 µM nioxime and 80 mM ammonia at pH 9.1, onto a hanging mercury drop electrode, followed by reduction of the adsorbed species. The reduction current is catalytically enhanced by the presence of 0.5 M nitrite. Optimized conditions for cobalt include a 30 s adsorption period at -0.7 V and a voltammetric scan using differential pulse modulation. According to the proposed reaction mechanism, dissolved Co(II) is oxidized to Co(III) upon addition of nioxime and high concentrations of ammonia and nitrite; a mixed Co(III)-ammonia-nitrite complex is adsorbed on the electrode surface; the Co(III) is reduced to Co(II) (complexed by nioxime) during the voltammetric scan, followed by its chemical reoxidation by the nitrite, initiating a catalytically enhanced current. A detection limit of 3 pM cobalt (at an adsorption period of 60 s) enables the detection of this metal in uncontaminated seawater using a very short adsorption time. UV digestion of seawater is essential, as part of the cobalt may occur strongly complexed by organic matter and rendered nonlabile. The method was applied successfully to the determination of the distribution of cobalt in the water column of the Mediterranean.

Dissolved cobalt occurs in seawater at concentrations ranging from 0.01 to 0.2 nM;3,4 calculation of the inorganic complexation of cobalt using an ion-pairing model and stability constants valid for seawater8 shows that it is weakly complexed by inorganic ligands, the predominant inorganic species being Co2+ and its chloride complexes. There is some evidence that cobalt in seawater occurs strongly complexed by organic ligands.9,10 The available data on cobalt distribution in seawater3,4,11-13 show surface minima, a maximum within the upper thermocline as a result of atmospheric input, and depletion at depth due to its removal from seawater, probably in association with MnO2.3,14,15 Analytical procedures for the determination of cobalt in seawater generally use graphite furnace atomic absorption spectrometry (GFAAS) after a preconcentration step involving solvent extraction, coprecipitation, or ion-exchange on Chelex-100 resin.3,12,16,17 These techniques have difficulties achieving the sensitivity required for the determination of the low levels of cobalt in seawater and include a high risk of sample contamination or loss of analyte during the several sample preparation steps involved. Adsorptive cathodic stripping voltammetry (CSV) has an advantage over GFAAS in that the metal preconcentration is performed in situ, hence reducing analysis time and risk of contamination if the metal can be determined without additional sample treatment; additional advantages are low cost of instrumentation and maintenance and the possibility to use adapted instrumentation for on-line and shipboard monitoring. Fundamentals and applications of CSV can be found elsewhere.18-20

Knowledge of the concentration and distribution of trace metals in seawater allows a better understanding of their biogeochemical behavior and cycling. Comparatively little is known of the distribution and marine chemistry of cobalt in seawater due to analytical problems associated with its very low concentration. Because cobalt is an essential element in biological compounds like vitamin B12 and some metalloproteins,1,2 the low concentration of this metal in seawater points to the possible role of cobalt as a biolimiting nutrient.3,4 The discharge of various cobalt radionuclides from nuclear installations to coastal waters and their accumulation by marine organisms5-7 has also increased the interest in the fate of this element. † Present address: Departamento de Quı´mica Analı´tica, Universidad de Valladolid, 47005 Valladolid, Spain. (1) Bowen, H. J. M. Trace elements in biochemistry; Academic Press: London, 1966. (2) Kendrick, M. J.; May, M. T.; Plishka, M. J.; Robinson, K. D. Metals in biological systems; Ellis Horwood: Chichester, 1992; pp 71-79. (3) Knauer, G. A.; Martin, J. H.; Gordon, R. M. Nature 1982, 297, 49-51. (4) Bruland, K. W. Trace elements in sea-water. In Chemical Oceanography; Riley, J. P., Chester, R., Eds.; Academic Press: London, 1983; Vol. 8, Chapter 45.

(5) Fukai, R.; Murray, C. N. Environmental behaviour of radiocobalt and radiosilver released from nuclear power stations into aquatic systems. In Environmental behaviour of radionuclides released in the nuclear industry; IAEA: Vienna, 1973; pp 217-242. (6) Carvalho, F. P. Health Phys. 1986, 53, 73-81. (7) Nolan, C. V.; Fowler S. W.; Teyssie, J. L. Mar. Ecol. Prog. Ser. 1992, 88, 105-116. (8) Turner, D. R.; Whitfield, M.; Dickson, A. G. Geochim. Cosmochim. Acta 1981, 45, 855-881. (9) Zhang, H.; Van den Berg, C. M. G.; Wollast, R. Mar. Chem. 1990, 28, 285300. (10) Donat, J. R.; Bruland, K. W. Anal. Chem. 1988, 60, 240-244. (11) Huyng Ngoc, L.; Whitehead, N. E. Oceanol. Acta 1986, 9, 433-438. (12) Jickells, T. D.; Burton, J. D. Mar. Chem. 1988, 23, 131-144. (13) Zhang, H.; Wollast, R. Distributions of dissolved cobalt and nickel in the Rhone and the Gulf of Lions. In Water Pollution Research report 20, EROS 2000, 2nd Workshop on the Northwest Mediterranean Sea; Martin, J. M., Barth, H., Eds.; Blanes: Spain, 1990; pp 397-414. (14) Murray, J. W. Geochim. Cosmochim. Acta 1975, 39, 635-647. (15) Balistrieri L. S.; Murray J. W. Geochim. Cosmochim. Acta 1986, 50, 22352243. (16) Danielsson, L. G. Mar. Chem. 1980, 8, 199-215. (17) Bruland, K. W.; Franks, R. P.; Knauer, G. A.; Martin, J. H. Anal. Chim. Acta 1979, 105, 233-245. (18) Wang, J. Stripping analysis: Principles, instrumentation and applications; Verlag Chemie: Deerfield Beach, FL, 1985. (19) van den Berg, C. M. G. Anal. Chim. Acta 1991, 250, 265-276.

874 Analytical Chemistry, Vol. 69, No. 5, March 1, 1997

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© 1997 American Chemical Society

The preconcentration step used in CSV involves adsorption of a complex with a specific chelating agent on a hanging mercury drop electrode (HMDE). Several ligands can be used to adsorb cobalt,20 good sensitivity being achieved with dioximes such as dimethylglyoxime (butane-2,3-dione dioxime, DMG)21,22 and nioxime (cyclohexane-1,2-dione dioxime).10 Although low levels of cobalt in seawater can thus be detected with CSV, the sensitivity is only barely sufficient to detect picomolar levels of cobalt in uncontaminated deep seawater, and long deposition times (10-15 min) are required. The reduction current can be enhanced greatly if the reduction product of a voltammetric scan is chemically reoxidized in the presence of an oxidant on a time scale that is fast relative to the scan rate or the pulse rate: this happens if the reduction product catalyzes its oxidation. Inclusion of a catalytic effect has been demonstrated to increase greatly the sensitivity of CSV, allowing the direct determination of picomolar levels of trace metals (titanium,23 iron,24 chromium,25,26 platinum,27 and vanadium28) in seawater using very short deposition times. The reduction of nitrite is known to be catalyzed by cobalt-dioxime complexes in ammonia buffer solutions.29 Low levels of cobalt in electrolytes can be determined by catalytic CSV using DMG,30 nioxime,31 or diphenylglyoxime32 as ligands. During preliminary work, several ligands were evaluated with the aim to determine picomolar levels of cobalt in seawater using CSV. Best results were obtained using nioxime; mixed cobalt complexes with ammonium and nioxime were found to greatly enhance the sensitivity by catalyzing the reduction of nitrite. The procedure to determine picomolar levels of cobalt in seawater, and to purify the reagents, is presented here. A likely electrode mechanism is discussed, and the method is applied to cobalt in the water column of the Mediterranean. Low levels of nickel can be determined simultaneously using the conditions optimized for cobalt. EXPERIMENTAL SECTION Instrumentation and Reagents. The voltammetric experiments were carried out with an Autolab PSTAT10 voltammeter (Eco Chemie) connected to a Metrohm 663VA hanging mercury drop electrode (HMDE). The reference electrode was Ag/ saturated AgCl/3 M KCl, and the counter electrode was a platinum rod. Solutions in the voltammetric cell were stirred by a rotating Teflon rod. The potentiostat was controlled by a PC-compatible computer (286 Intel processor) using a compiled Basic program (GPES 3.2 from Eco Chemie). The mercury was triple-distilled (20) Paneli, M. G.; Voulgaropoulos, A. Electroanalysis 1993, 5, 355-373. (21) Pihlar, B.; Valenta, P.; Nu ¨ rnberg, H. W. Fresenius’ Z. Anal. Chem. 1981, 307, 337-346. (22) Zhang, H.; Wollast, R.; Vire, J. C.; Patriarche, G. J. Analyst 1989, 114, 15971602. (23) Yokoi, K.; Van den Berg, C. M. G. Anal. Chim. Acta 1991, 245, 167-176. (24) Yokoi, K.; Van den Berg, C. M. G. Electroanalysis 1992, 4, 65-69. (25) Boussemart, M.; Van den Berg, C. M. G.; Ghaddaf, M. Anal. Chim. Acta 1992, 262, 103-115. (26) Golimowski, J.; Valenta, P.; Nu ¨rnberg, H. W. Fresenius’ Z. Anal. Chem. 1985, 322, 315. (27) Van den Berg, C. M. G.; Jacinto, G. S. Anal. Chim. Acta 1988, 211, 129139. (28) Vega, M.; Van den Berg, C. M. G. Anal. Chim. Acta 1994, 293, 19-28. (29) Bobrowski, A. Anal. Chem. 1989, 61, 2178-2184. (30) Bobrowski, A.; Bond, A. M. Electroanalysis 1992, 4, 975-979. (31) Bobrowski, A. Anal. Lett. 1990, 23, 1487-1503. (32) Godlewska, B.; Golimowski, J.; Hulanicki, A.; van den Berg, C. M. G. Analyst 1994, 120, 143-147.

quality, and the largest drop size of the HMDE was selected. pH values were determined with an EDT Instruments RE357 pH meter and an EDT E8081 combined glass pH electrode calibrated with buffer standards of pH 7.02, 4.0, and 9.22. A stock solution of 10-4 M cobalt(II) was prepared by dilution of BDH Spectrosol standard solution (1 mg/mL) and acidified to pH 2 with distilled HCl. The required cobalt standard was prepared daily by dilution of the stock solution. An aqueous stock solution of 0.1 M nioxime (1,2-cyclohexanedione dioxime) was prepared by dissolving the appropriate amount in 0.2 M sodium hydroxide (BDH, Aristar grade). A solution of 2 mM nioxime was prepared weekly by dilution of the stock solution. A 4 M ammonia buffer solution was obtained by addition of HCl to 4 M NH3 to obtain a buffer of pH 9.1. A 5 M stock solution of nitrite was prepared by dissolution of the sodium salt (BDH Analar grade) in 500 g of solution. A high level of cobalt in this solution was initially responsible for high blanks; cobalt was removed by electrolysis in an ESA Reagent Cleaning System Model 2014PM with a pool of mercury as cathode, a Ag/AgCl/0.1 M NaCl reference electrode, and an anode consisting of a platinum wire immersed in 3 M sodium nitrite solution separated from the external solution by a frit. The 5 M nitrite solution was placed in the electrolysis device and deaerated for 20 min by bubbling nitrogen through the stirred solution; the bubbling rate was then reduced, and the electrodes were connected to the potentiostat. A potential of -1.35 V was set to the working electrode for a period of 24 h. The purified solution (which was 10-fold diluted into seawater) contributed 4 pM cobalt to the reagent blank. Purified 6.5 M ammonia and 10.3 M hydrochloric acid solutions were obtained by subboiling distillation of the respective BDH Analar reagents. Stock solutions and standards were kept at 4 °C when not utilized. Water used for dilution of the reagents and for rinsing of sample containers and the voltammetric cell was purified by reverse osmosis (Milli-RO) and deionisation (MilliQ). Sample bottles were cleaned by soaking in 50% HCl for a week, rinsed with Milli-Q water, and soaked in 2 M HNO3 for another week, finally rinsed with Milli-Q water, filled with Milli-Q water acidified at pH 2 (by addition of purified HCl), wrapped in polyethylene bags, and stored until required. Sampling and Analytical Procedures. Samples were collected during a cruise with RRS Discovery (No. 203) in July 1993 at two sampling stations at 38°41.79′ N, 4°40.63′ E (station D9) and 36°40.50′ N, 12°19.22′ E (station D14) in the Mediterranean using Teflon-coated Go-Flo bottles. The samples were immediately filtered under nitrogen pressure through Nuclepore filters (pore size 0.45 µm) into polyethylene bottles and acidified to pH 2 by addition of 500 µL of 10 M HCl to each 500 mL sample. Seawater used for the optimization experiments originated from the Atlantic Ocean and contained low concentrations of trace metals. This seawater was UV-digested for 3 h at natural pH using a 1 kW mercury vapor UV lamp. The water was further purified by passing the UV-digested sample through a column packed with ∼3 mL of Chelex-100 resin in the sodium form at a flow rate of 1 mL/min to produce “clean” seawater. The efficiency of the resin in removing cobalt was 95%. This seawater sample was used to test the reagent blank and in optimization experiments where indicated. Analytical Chemistry, Vol. 69, No. 5, March 1, 1997

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Procedure To Determine Cobalt in Seawater Samples. About 25 mL of each acidified sample was UV-digested for 3 h in Teflon capped silica tubes. Aliquots of 10 mL of this seawater were pipetted into the voltammetric cell, and the pH was adjusted to 9.1 by adding 15 µL of 6.5 M ammonia and 200 µL of 4 M ammonia buffer. Then, 20 µL of 2 mM nioxime and 1 mL of 5 M NaNO2 were added, giving final concentrations of 4 µM nioxime and 0.5 M NaNO2. Dissolved oxygen was removed by purging with water-saturated nitrogen for 6 min. A new mercury drop was extruded, and the deposition potential was set to -0.7 V for a period of 30 s, while the solution was stirred at 2500 rpm. The stirrer was then stopped and the potential set to -1.0 V for a period of 10 s, whereafter the potential was scanned toward more negative potentials using differential pulse modulation. Scan parameters were as follow: modulation time, 10 ms; interval time, 0.1 s; pulse amplitude, 50 mV; potential step, 2.5 mV (scan rate, 25 mV s-1). A peak corresponding with the reduction of cobalt appeared at -1.12 V, and its height (nA) was used as a measure of the reduction current. A second peak at -0.93 V, caused by the reduction of adsorbed nickel nioximate, was obtained if the scans were initiated at -0.7 V rather than at -1.0 V, but this peak deteriorated the definition of the cobalt peak at low cobalt concentrations. Each scan was repeated three times, and the measurement was repeated with two cobalt standard additions to calibrate the sensitivity. RESULTS AND DISCUSSION Comparison of Different Ligands. The influence of a number of chemical variables on the sensitivity of the CSV technique toward cobalt in seawater was investigated in UVirradiated “clean” seawater. Milli-Q water was not used for this purpose as chloride can form ternary complexes with cobalt,33 possibly changing the sensitivity of the method from that in seawater. Preliminary experiments were carried out comparing different pH buffers (HEPES at pH 7.8, borate at pH 8.6, and ammonium chloride/ammonia at pH 9.2), showing that the cobalt sensitivity was very poor and the catalytic effect was absent unless high concentrations of ammonia were present; ammonia buffers were, therefore, used for further experiments. Three dioxime ligands (nioxime, DMG, and diphenylglyoxime) were compared to evaluate their sensitivity for cobalt. Seawater containing 0.4 M nitrite and 40 mM ammonia buffer (pH 9.2) was spiked with cobalt(II), and the ligand concentrations were varied to determine the optimal ligand concentration for maximum sensitivity; voltammograms were recorded from -0.8 to -1.4 V, preceded by 60 s adsorption at -0.7 V. Using the optimized condition for each ligand, greatest sensitivity was obtained using nioxime (Table 1), which had more than twice the peak height for cobalt as for diphenylglyoxime and 4 times that for DMG. The optimum ligand concentrations, the sensitivities achieved, and the peak potential of cobalt for each ligand tested are reported in Table 1. Nioxime was selected as complexing ligand because it had the highest sensitivity. The peak potential for cobalt in the presence of diphenylglyoxime was slightly more positive and, therefore, further removed from the hydrogen wave, but a drawback of this ligand was (in addition to the lower sensitivity for cobalt) that it caused a comparatively high background current. Effects of Varying the Solution Composition on the CSV Sensitivity for Cobalt Using Nioxime. Chemical parameters (33) Cotton, F. A.; Wilkinson, G. Advances inorganic chemistry; Wiley: New York, 1988; pp 724-741.

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Table 1. Comparison of Optimized Conditions and Sensitivity for the Determination of Cobalt in Seawater by Catalytic CSV Using Various Ligandsa ligand nioxime DMG diphenylglyoxime

optimum concn sensitivity peak (mol L-1) (nA nM-1 min-1) potential (V) 6 × 10-6 3 × 10-4 2 × 10-5

270 69 116

-1.130 -1.157 -1.037

a The complexes were adsorbed for 1 min at -0.7 V on an HMDE from seawater containing 0.4 M nitrite and 40 mM ammonia buffer (pH 9.2).

Figure 1. Effect of varying the nioxime concentration (A), pH (B), ammonia buffer concentration (C), and nitrite concentration (D) on the CSV sensitivity for 0.2 nM cobalt in seawater. Metal nioximates were accumulated at -0.7 V for 30 s in the presence of 4 µM nioxime, 80 mM ammonia buffer (pH 9.1), and 0.5 M nitrite, unless indicated otherwise.

affecting the CSV sensitivity were varied while the reduction current of the cobalt nioximate complex was monitored after 30 s adsorption at -0.7 V and 10 s equilibration at -0.8 V; the solution was pH 9.1 seawater containing 4 µM nioxime, 80 mM NH4+/ NH3, 0.5 M NO2-, and 0.2 nM cobalt unless indicated otherwise. The results are summarized in Figure 1. Nioxime Concentration. Variation of the nioxime concentration showed (Figure 1A) that the peak current for cobalt increased beyond 15 µM nioxime, but the increase was strongest up to 4 µM, diminishing at higher nioxime concentrations. Increasing the nioxime concentration also caused the appearance of a broad reduction peak at ∼-1.0 V at enhanced nioxime concentrations, presumably due to reduction of the ligand. An optimum nioxime concentration of 4 µM was selected for further experiments. The potential of the cobalt peak shifted about 10 mV in the cathodic direction when the nioxime concentration was increased from 1 to 4 µM, remaining constant above 4 µM: it is likely that adsorption stabilization masked an expected peak shift due to complex stabilization with the higher ligand concentrations tested. pH. The effect of varying the pH on the CSV response in seawater was investigated in the presence of ammonia (10, 40, and 80 mM as NH4Cl), as preliminary tests indicated that its presence improved the peak height; the pH of seawater containing

added NH4Cl (initial pH 8.2) was increased gradually to 10.2 by adding NaOH. The peak height for cobalt was strongly affected by the pH, with a maximum at pH 9.1, declining at higher and lower pH values (Figure 1B); comparable results were obtained at all ammonia concentrations tested. At pH < 8.7, the reduction peak of nioxime increased greatly, interfering with the cobalt peak, whereas at pH > 10 the peak of cobalt disappeared progressively, probably as a result of hydrolysis of the metal ion in competition with the electroactive complex. The peak potential for cobalt was found to shift toward more negative potentials with increasing pH, with a slope of 45 mV/pH unit, as a result of complex stabilization due to decreased proton competition. Ammonia Concentration. The concentration of the ammonia buffer was varied at a constant pH of 9.1 to test whether the ammonia was involved in mixed complexes with cobalt. The cobalt peak height increased almost 3-fold when the ammonia concentration was increased from 10 to 100 mM (Figure 1C). The broad peak for free nioxime at -1 V decreased strongly with the increasing ammonia buffer concentration, causing improved (lower) background currents and better resolved peaks. No further improvement was observed in the background current for ammonia concentrations above 80 mM. Independent of the actual reaction mechanism, the improvement of the CSV sensitivity for cobalt by enhanced ammonia concentrations was a useful finding, and an ammonia buffer concentration of 0.08 M was used for the optimized analytical procedure. Although Co(II) is the usual oxidation state in seawater, Co(III) is stabilized in the analytical conditions used here (see below), and Co(II) is readily oxidized by the added nitrite. The electron configuration of Co(III) allows the formation of hexacoordinated complexes by binding ligands in octahedral positions.33 In the presence of nioxime, ammonia, and nitrite, two molecules of bidentate nioxime are bonded in a planar structure, and molecules of ammonia and nitrite are linked to cobalt in axial positions. The ammonia increasingly replaces chloride and nitrite in the cobalt complex when the ammonia concentration is increased: apparently, these mixed ammonia-HL-Co(III) complexes adsorb better on the mercury surface, leading to higher currents. The potential of the cobalt peak shifted to more positive potentials with increasing ammonia concentration, suggesting that the ammonia substitution in the cobalt-nioxime-ammonia complex facilitated the catalytic current. Nitrite Concentration. Variation of the oxidant (nitrite) concentration caused the peak height for cobalt to increase with the nitrite concentration (Figure 1C); the rate of increase in the cobalt peak height was linear up to ∼0.5 M nitrite, leveling off at higher concentrations. An increase in the reduction current at a reduction potential of ∼-1.0 V, corresponding with that for free nioxime, was also observed, which suggests that it is involved in the catalytic cycle similar to the cobalt complexes. For the case of catalytic currents, the reduction current should be proportional with the square root of the oxidant concentration. The following relationship,34

ipc ) f([NO2-]1/2) ip where ipc is the catalyzed current and ip is the current in the (34) Galus, Z. Fundamentals of electrochemical analysis; Ellis Horwood: Chichester, 1976; pp 311-330.

Figure 2. Cyclic voltammetry of UV-digested seawater containing 40 nM nickel and 20 nM cobalt in the presence of 6 µM nioxime and 80 mM ammonia buffer (pH 9.1), preceded by 60 s of adsorption at -0.7 V: (a) no nitrite added, (b) with 0.01 M nitrite, and (c) with 0.02 M nitrite.

absence of nitrite, was used to verify whether the nitrite reduction was catalyzed by cobalt. This relationship was linear for cobalt over the range of nitrite concentration tested, demonstrating the catalytic nature of the electrode process of this element. The cobalt peak was found to shift toward more negative potentials as the nitrite concentration increased, with a slope of 31 mV/ decade of nitrite concentration, as a result of ammonia substitution by nitrite causing the formation of more stable cobalt complexes. Cyclic Voltammetry (CV). CV at enhanced concentrations of cobalt (20 nM) in seawater showed that addition of 0.01 M nitrite enhanced the cobalt peak height by a factor of 6.9, and shifted the cobalt peak 10 mV toward more positive potentials: these effects are in accordance with a catalytic reaction. Addition of nitrite up to 0.07 M caused the cobalt peak to increase by a factor of 8.3 (compared to 0.01 M nitrite), without further modifying the peak potential. The sensitivity was further increased at higher concentrations of nitrite, now with a corresponding negative shift of the cobalt peak potential. The increased peak current in the presence of nitrite at the peak potential corresponding with that for the cobalt-nioxime complex is in accordance with a catalytic current due to the nitrite reduction initiated by the electrochemical reduction of the cobalt-nioxime complex; the negative peak shift at enhanced nitrite concentrations indicates that nitrite substitutes for ammonia in the coordination sphere of cobalt and causes the reduction of the ternary complex to become more difficult. A second cobalt peak appeared at -1.23 V (Figure 2) at the high cobalt concentration employed in this CV experiment, which was not apparent at low cobalt concentrations. This peak can be ascribed to the formation of a second cobalt-nioxime complex of a different stoichiometry: a 1:2 (metal:ligand) complex is formed at low cobalt concentrations ( s-DC > LS > SW, in agreement with the electrode process (irreversible nature of the catalytic current of the cobalt-nioxime-nitrite system). DP was most suitable for CSV of cobalt nioximates, leading to higher sensitivity, better peak resolution, and lower background current. The DP modulation was selected for further investigations using the following optimized conditions: pulse amplitude, 50 mV; modulation time, 10 ms; interval time, 100 ms; and step height, 2.5 mV (effective scan rate, 25 mV/s). (36) Nicholson, R. S.; Shain, I. Anal. Chem. 1964, 36, 706-723.

878 Analytical Chemistry, Vol. 69, No. 5, March 1, 1997

Figure 4. Effect of varying the adsorption potential (A) using 30 s adsorption and adsorption time (B) at -0.7 V on the CSV sensitivity for 0.2 nM cobalt in UV-irradiated seawater containing 4 µM nioxime, 80 mM ammonia (pH 9.1), and 0.5 M nitrite.

Effect of Varying the Adsorption Potential and Adsorption Time. The adsorption potential was varied using a constant adsorption time of 30 s; each CSV scan was initiated after 10 s of equilibration (without stirring) at -0.8 V. The adsorption of the cobalt complex was greatest at adsorption potentials between -0.5 and -1.0 V (Figure 4A): adsorption was negligible at potentials more negative than -1.0 V due to complex reduction during the adsorption step, while the absence of adsorption at potentials more positive than -0.5 V suggests that the adsorbing complex is positively charged, as then the positive charge on the mercury electrode (which has a zero point of charge at ∼-0.5 V in chloride electrolytes) would repel a positively charged complex (the residual peak height was ∼20 nA as a consequence of complex adsorption during the 10 s equilibration period at -0.8 V). An optimum deposition potential of -0.7 V was selected as it produced maximum sensitivity for cobalt with comparatively small interference by free nioxime. Increasing the adsorption time up to 8 min showed that the peak height for cobalt reached a maximum after 3 min deposition, diminishing at longer periods, possibly due to competitive adsorption of free nioxime (Figure 4B). An adsorption time of 30 s was selected for cobalt determinations in seawater, combining high sensitivity with short analysis times and minimum interference from free nioxime. Reaction Mechanism. The EDTA competition experiments (below) show that the stability of the cobalt-nioxime complexes is very high, causing all cobalt to be fully complexed by the lowest tested nioxime levels. Nevertheless, the peak height for cobalt continued to increase gradually with increasing nioxime concentrations between 1 and 15 µM (Figure 1A). It is, therefore, likely that the cobalt peak height depends on the concentration of a higher order complex species, probably 2:1 (ligand:metal) because of likely steric hindrance of complex adsorption of higher order species.

Figure 5. Effect of surface-active substances (Triton X-100, dodecylbenzenesulfonic acid and hyamine-1622) on DPCSV of 0.3 nM cobalt. Solution composition as in Figure 1.

as these complexes could be of similar stability as the nickel complexes. It is not clear whether the reduced cobalt remains adsorbed at the electrode surface: no anodic current was observed during the returning cyclic voltammetry scans (suggesting desorption of Co(II) nioxime), but this would have been readily masked by the catalytic effect; Co(II) nioxime complexes are known to adsorb on the mercury surface at more positive potentials,10 but it is likely (in view of the catalytic effect) that desorption of the reduced species occurs at -1.0 V, followed by diffusion into the double layer, where it is reoxidized to Co(III) by nitrite and readsorbed. Therefore, the following electrode mechanisms is proposed: Cobalt in solution: CoII + 2nioxime + NH3 + NO2–

There are now several possibilities for the reaction mechanism at the electrode initiated by reduction of either cobalt or nitrite. A possibility is that complexed nitrite is reduced, released, and replaced by other nitrite in a catalytic cycle.29 We wonder why in this case the reduction potential of the catalytic reaction coincides with that of cobalt. The same argument is valid for reduction of complexed nioxime. Because the catalyzed current rises out of the reduction peak for cobalt it is more likely that the cobalt, is directly involved in the catalytic cycle and initiates it. Possibilities are then that Co(III) is reduced to Co(II) or to Co(0), Co(II) is reduced to Co(0), or complexed nioxime or nitrite is reduced. In the absence of nitrite (no catalysis), the cobalt peak behaves similarly to that for nickel,21,22 which is a reduction of the divalent metal to the elemental state. However, catalytic reactions tend to be centered on a one-electron reduction step, as the kinetics of a two-electron step would be too slow; participation of cobalt with a catalytic cycle is unlikely once it is reduced to the elemental state and amalgamated. This mechanism is supported by recent spectroelectrochemical experiments, showing that Co(III) complexes participate with catalytic reactions.37 The standard reduction potential of the redox couple Co(III)/ Co(II) is 1.8 V,38 and hence Co(III) is not stable as it oxidizes water to oxygen. However, in the presence of ammonia, the standard potential of the couple CoIII(NH3)6/CoII(NH3)6 is decreased to 0.1 V due to the higher stability of the Co(III) complexes; a further stabilization of Co(III) may well be caused by the nioxime. The conditional standard reduction potential for the redox couple NO2-/NO at pH 9.1 is ∼0.44 V: nitrite can, therefore, easily oxidize the Co(II) to Co(III), and it is likely that this is the predominant oxidation state of cobalt after the addition of nitrite, nioxime, and ammonia. Co(III) forms very stable ternary or quaternary complexes with donor ligands containing atoms of nitrogen, coordinated in an octahedral configuration.33 In the presence of dioximes, nitrite, and ammonia, the central cation coordinates two dioxime molecules in the planar positions and in the axial positions binds the nitrite and/or ammonia ligands (the actual stoichiometry depends on the relative concentrations of the ligands). We propose a mechanism in which the Co(III) is reduced first to Co(II) and subsequently to Co(0) during the voltammetric scan. The generated Co(II) probably remains complexed by nioxime, (37) Postlethwaite, T. A.; Hutchison, J. E.; Hathcock, K. W.; Murray, R. W. Langmuir 1995, 11, 4109-4116. (38) Milazzo, G.; Caroli, S. Tables of standard electrode potentials (Project of the IUPAC Electrochemistry Commission; ISBN 0 471 99534 7); John Wiley & Sons: Chichester, 1978; p 337.

CoIII(nioxime)2NO2NH3sol + e–

Adsorption step on the electrode surface: CoIII(nioxime)2NO2NH3sol

Eacc = –0.7 V

CoIII(nioxime)2NO2NH3ads

During the voltammetric scan: CoIII(nioxime)2NO2NH3ads + e–

E < –1.0 V

CoII(nioximered)diffusion layer

catalytic cycle NO2–

Interferences. Surface-active substances occurring in natural waters are known to lower the sensitivity of CSV due to competitive adsorption on the electrode surface. The extent of this kind of interference was tested using Triton X-100 (a nonionic surfactant), hyamine-1622 (cationic), and dodecylbenzenesulfonic acid (DBS, an anionic surfactant) as model compounds for natural organic surfactants in seawater. The effect of these substances on the CSV sensitivity for cobalt is shown in Figure 5. The peak of 0.3 nM cobalt in UV-digested seawater was diminished by ∼50% in the presence of 1 ppm of the nonionic (Triton X-100) and cationic (hyamine) surfactants. A similar suppression was achieved with 3 ppm DBS (anionic surfactant). It is likely that the lower interference by the anionic surfactant is due to the negative surface charge of the mercury electrode at the adsorption potential at -0.7 V. The CSV sensitivity for cobalt was increased by 12% in Atlantic seawater by UV digestion, corresponding to a concentration of interfering surfactants equivalent with 0.2-0.4 ppm Triton X-100. The UV digestion is a convenient sample treatment to remove this interference, as well as that of natural complexing ligands, as it does not require the addition of reagents and sample handling is minimal. Dissolved organic matter present in seawater can also interfere with the determination of trace metals by CSV due to competition with the added ligand for the metal. Naturally occurring organic matter is present in open seawater at very low concentrations but forms very stable complexes with cobalt.9,39 The competitive effect of complexing organic ligands on the cobalt peak was modeled by addition of EDTA to a sample of UV-digested and purified seawater and is shown in Figure 6. The nioxime-ammonianitrite complexes with cobalt are very stable, as there was no decrease in the metal peaks up to 0.01 M EDTA; the peak heights were suppressed by ∼50% in the presence of 0.05 M EDTA. (39) Van den Berg, C. M. G.; Nimmo, M. Sci. Total Environ. 1987, 60, 185195.

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Figure 6. Dependence of the relative height (ip/ip,o) of the peak for nanomolar cobalt on the concentration of competitive complexing organic ligands (modeled by EDTA). Experimental conditions as in Figure 1. Table 2. Influence of UV Digestion on the Concentration and Recovery of Cobalt in Seawater Originating from the Atlantic Ocean Determined by Catalytic CSV (n Is the Number of Repeats) sample pretreatment

detected cobalt (nM)

recovery (% added)

Original Sample untreated 0.033 ( 0.003 (n ) 9, RSD ) 9.1%) UV digested at pH 8 0.068 ( 0.003 (n ) 10, RSD ) 4.4%) UV digested at pH 2 0.072 ( 0.003 (n ) 9, RSD ) 4.2%) +0.5 nM Cobalt untreated 0.503 ( 0.012 (n ) 7, RSD ) 2.4%) UV digested at pH 8 0.573 ( 0.023 (n ) 6, RSD ) 4.0%) UV digested at pH 2 0.557 ( 0.010 (n ) 7, RSD ) 1.8%)

94 101 97

The high stability of the nioxime complexes would suggest that natural complexes would dissociate and release all cobalt. However, comparative determination of the concentrations of cobalt in the Atlantic seawater (which had been stored at its natural pH, so it is possible that the concentration and composition of organic matter had changed from that originally present) showed that cobalt was partially masked, being released by UV digestion: the labile cobalt concentration (labile being that available to complexation by the added nioxime-nitrite-ammonia mixture) was more than doubled (from 33 to 72 pM; Table 2) as a result of the UV digestion (at pH 2). This result suggests that cobalt is partially very strongly bound by either dissolved or colloidal organic matter, consistent with other measurements in seawater from oceanic and estuarine conditions,9,10,39 but in contradiction to a recent experiment using CSV with diphenylglyoxime as electroactive ligand.40 Losses of metal due to adsorption on the silica walls are normally expected if the samples are UV-digested at the natural pH (near 8) and metal ions are released from the organically complexed state. We therefore normally acidify seawater prior to the UV digestion. However, repeated comparative UV digests at natural pH (pH 8) and pH 2 revealed only a very small (insignificant as the 2σ standard deviations overlap) difference (Table 2) between the cobalt concentration in the acidified (72 pM) and natural pH (68 pM) aliquots. Repeated comparative UV digests of a sample aliquot to which 0.5 nM Co had been added showed again no loss, with recoveries of 101% (pH 8) and 97% (40) Golimowski, J.; Tykarska, A.; Melian, J. A. H.; Pena, J. P. Chem. Anal. (Warsaw) 1995, 40, 201-206.

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(pH 2). It is not possible to predict whether the cobalt would have been rendered as Co(II) or Co(III) as a result of the UV digestion, as either oxidizing or reducing conditions can prevail at the end of the digestion period; the absence of losses suggests that the mineralized cobalt occurred as Co(II), as Co(III) would have been more reactive and would probably have adsorbed on the silica tube wall. This finding contradicts a previous finding that cobalt was removed from solution as a result of oxidation to Co(III) by the UV digestion, therefore requiring reduction of the UV-digested seawater with NaBH4 to recover the cobalt.10 Possibly, the UV digestion in that work was less extensive and did not exhaust all dissolved oxygen, so the solution remained oxidizing. A significant fraction (6%) of the added cobalt was CSV nonlabile without UV digestion (recovery 94%; Table 2), suggesting that the concentration of complexing organic ligands in the seawater sample was higher than the cobalt concentration and some of the cobalt added was additionally complexed. This aspect requires additional careful investigations as the difference was small; organic complexation of cobalt, and its possible impact on the redox state of this element, are of importance to its availability to microorganisms which require it for the carbonic anhydrase enzyme. To avoid complications in this study, samples were UVdigested at pH 2 for 3 h prior to the CSV determination of cobalt using the proposed method. Interference by Other Metals. Possible interference by other metals with the CSV determination of cobalt was investigated by metal additions to purified seawater containing 0.2 nM Co using the optimized method. Little interference is to be expected, as nioxime has high selectivity for cobalt, but it is known to form complexes with nickel, copper, iron, and palladium.41,42 The cobalt peak height decreased slightly (2% at 5 nM and 7% at 10 nM nickel) with nickel additions at an adsorption time of 30 s. This interference was stronger at longer adsorption times (approximately doubled at a 60 s adsorption time, for instance). The peak potentials of cobalt and nickel are well separated by ∼0.2 V, so it is not difficult to measure both peaks at normal seawater concentrations. Interference of nickel with the determination of cobalt is eliminated by initiating the scans from -1.0 V (with a 10 s equilibration period), causing nickel reduction and free nioxime desorption prior to the scan. Copper additions showed that a peak for this element was produced at -0.32 V (sensitivity, 0.2 nA/nM). Additions of 50 and 100 nM copper diminished the cobalt peak by 4 and 7%, respectively. Additions of 100 and 200 nM Pd(II) reduced the cobalt peak by 6 and 7%. Addition of up to 20 nM Fe(III) to the solution did not affect the cobalt peak, but higher iron concentrations of 50 and 100 nM Fe enhanced the cobalt peak by 3 and 4%, respectively. The relevance of these effects to normal seawater is negligible since usual concentrations of iron, copper, and palladium are up to 1 nM, 4 nM, and subpicomolar, respectively, and no interference was observed when seawater samples were analyzed. Potential interferences of metals Al(III), As(III), Cd(II), Cr(VI), Ga(III), In(III), Mn(II), Mo(VI), Pb(II), Sn(II), Ti(IV), U(VI), V(V), and Zr(IV) were investigated by addition of up 200 nM of each element to a solution containing 0.2 nM Co. No effect was observed in each case. (41) Martell A. E.; Smith, R. M. Critical stability constants; Plenum Press: New York, 1977; Vol. 3, p 307. (42) IUPAC Chemical Data Series. Stability constants of metal-ion complexes: Part B; Organic ligands; Pergamon Press: Exeter, 1979; p 450.

Reagent Purification. Pure reagents were used to minimize the cobalt blank level. The contribution from the subboiling distilled ammonia and hydrochloric acid used to make the ammonia buffer was found to be negligible compared to that from the nitrite solution, of which a final concentration of 0.5 M had to be added. It was found that the stock nitrite solution could be purified conveniently by electrolysis using a mercury pool electrode (as described in the Experimental Section). The contribution to the reagent blank from the nitrite was found to drop in an exponential fashion from 53 to 4 pM over a period of ∼24 h; a correction for these levels was made in the sample analyses. A low nioxime concentration (4 µM) was used for the analyses, and this was used without purification. Linear Range, Sensitivity, Detection Limit, Accuracy, and Reproducibility of the Method. The linear range for cobalt was evaluated at three deposition times (15, 30, and 60 s) by increasing the concentration of cobalt in UV-digested and purified seawater. The peak height increased linearly with the cobalt concentration over the entire concentration range tested (up to 3 nM cobalt; about 10-100-fold higher than cobalt concentrations normally occurring in seawater). The sensitivities were 225 (15 s deposition), 330 (30 s), and 520 nA/nM (for a deposition time of 60 s). The cobalt concentration in purified seawater was repeatedly determined using the optimized conditions to establish the limit of detection of this method at an adsorption time of 60 s. We estimated the residual cobalt concentration at 4 ( 1 pM (n ) 11, RSD ) 25%). The detection limit calculated from 3σ (σ being the standard deviation of the measurement) was 3 pM. This detection limit can be reduced further by increasing the deposition time. The accuracy of the method for cobalt was verified by replicate analyses of certified seawater (NASS-243 ). This sample is supplied acidified to pH 1.65 with nitric acid and is certified to contain 68 ( 17 pM cobalt. Aliquots of this reference material were UVirradiated, and the cobalt content was quantified in the optimum conditions using a deposition time of 30 s. We found a cobalt concentration of 79 ( 7 pM (n ) 9, RSD ) 8.9%), within but at the high end of the comparatively large standard deviation of the certified value (68 ( 17 pM). Interestingly, this result is in excellent agreement with that (81 ( 7 pM) found previously by CSV after UV digestion,22 suggesting that CSV with UV digestion reaches part of the cobalt not detected by other techniques. Repeated voltammograms for 0.2 nM cobalt in seawater after 30 s deposition produced average peak heights of 75.3 ( 1.2 nA (RSD ) 1.6%), demonstrating the high reproducibility of the electrode process. The analytical precision of the method was estimated from the reproducibility of nine determinations of cobalt in UV-digested Atlantic seawater, giving an average cobalt concentration of 72 ( 3 pM, yielding a precision of (4.2% RSD. Determination of Cobalt in Samples from the Water Column of the Mediterranean. Samples from the water column of the Western Mediterranean were collected during a cruise with RRS Discovery (July 1993).44 Between two and four aliquots of 30 mL of each sample were UV-digested for 3 h to destroy organic ligands and surfactants interfering with the cobalt determination, and the concentration of dissolved cobalt was determined by CSV using a 30 s adsorption time. The distribution of dissolved cobalt (43) Seawater reference material for trace metals, NASS-2; National Research Council Canada, Ottawa, ON, Canada. (44) Achterberg, E. P.; van den Berg, C. M. G. Deep-Sea Res., in press.

Figure 7. Vertical distribution of dissolved cobalt at two stations (D9 and D14) in the Mediterranean Sea. Measurements after UV digestion and using the optimized conditions with a 30 s adsorption period.

in the water column (Figure 7) is in general agreement with other data for this region11,13 and with its known oceanographic properties. The cobalt distribution is in line with expectation for atmospheric inputs at the surface with subsequent mixing with deep water of a lower cobalt concentration: the concentration diminishes from 130 pM at the surface to ∼50 pM in the deep waters below ∼500 m. The scavenged element-type shape of the profile is similar to that in the Pacific,3 but the deep water concentration in the Mediterranean is considerably higher than that in the Pacific, where it drops to ∼20 pM, in accordance with the greater residence time of the water. The apparent good quality of the data (Figure 7) confirms that this new voltammetric method is a good alternative to existing more laborious or less sensitive techniques. CONCLUSIONS Our experiments show that low picomolar levels of cobalt can be determined in seawater by catalytic CSV after 30 s of adsorptive deposition. The experiments show that greatest sensitivity is obtained when a mixed complex of nioxime, nitrite, and ammonia is adsorbed; the reduction current is catalytically enhanced by the reduction of nitrite, which reoxidizes the Co(II), which is produced from Co(III) during the voltammetric scan. Further experiments are required to assess the potential of this method to determine the chemical speciation of cobalt in seawater. ACKNOWLEDGMENT M.V. is grateful for the hospitality of the Oceanography Laboratories and financial assistance from the University of Valladolid toward part of the travel costs of this collaboration. The authors also thank anonymous referees and A. Bobrowski (University of Mining and Metallurgy, Krakow, Poland) and R. Pardo (University of Valladolid, Spain) for discussions and comments on this work.

Received for review March 4, 1996. Accepted December 2, 1996.X AC960214S X

Abstract published in Advance ACS Abstracts, January 15, 1997.

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