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value reported here. Only a fraction of the total absorbance change is observed in PAF experiments. For the study of rapid reactions under pseudo-fmt-order conditions (k,= 124OOO s-l) or second-order equal concentration conditions ( k l z = 2 x lo9 M-' s-l), it is necessary that the reagents have large molar absorptivities in order to be able to observe this fractional change. In these studies, molar absorptivities of 4000 and 8000 M-l cm-l, respectively, were sufficient. For fast reactions, the signal is due to both the mixing process and the reaction. I t is not possible to determine the reaction order from the PAF signal itself. A similar limitation exists in relaxation experiments where a first-order decay is observed for nearly all processes. Therefore, an assumed reaction order should be tested a t various concentration ratios. The reaction of Cu(dmp),+ with IrC12- is too fast to study under first-order conditions. This reaction was confirmed as second-order because the secondorder rate constant, 1.8 x 109 M-' s-l, is independent of initial half-life (varied from 200 to 22 gs) and concentration ratio (varied from 1:l to 2;l). Such tests should be applied to all reactions where the order is uncertain. Although there are limitations associated with the PAF method, the present system represents an important advance in the ability to study very rapid irreversible reactions.
ACKNOWLEDGMENT The authors are grateful to C. R. Dennis for the synthesis of K4W(CN)g2Hz0and to G. W. Kramer, W. E. Baitinger, and J. G. Skifstad for helpful discussions. Supplementary Material Available: Physical characteristics of the cells D, E, and F, tables of the individual rate constants, and a schematic of cell D (6 pages). Photocopies of the supplementary material from this paper or microfiche (105 X 148 mm, 24X reduction, negatives) may be obtained from Microforms & Back Issues Office, American Chemical Society, 1155 16th Street,
29 1
NW, Washington, DC 20036. Orders must state whether for photocopy or microfiche and give complete title of article, names of authors, journal issue date, and page numbers. Prepayment, check or money order for $14.50 for photocopy ($16.50 foreign) or $10.00 for microfiche ($11.00 foreign), is required and prices are subject to change. LITERATURE CITED Gibson, Q. H.; Milnes, L. Biochem. J. 1964, 97, 161-171. Owens, G. D.; Margerum, D. W. Anal. Chem. 1960, 5 2 , 91A-97A. Owens, G. D.; Taylor, R. W.; Ridley, T. Y.; Margerum, D. W. Anal. Chem. 1980, 5 7 , 130-137. Jacobs, S.A.; Nemeth, M. T.; Kramer, G. W.; Ridley, T. Y.; Margerum, D. W. Anal. Chem. 1964, 56, 1058-1065. Gerischer, H.; Heim, W. 2. Phys. Cbem. (Munich) 1985, 4 6 , 345-352. Gerischer, H.; Heim, W. Ber. Bunsen-Ges. Phys. Chem. 1967, 71, 1040-1046 .- .- .- . - . LeipoMt, J. G.; Bok, L. D. C.; Cllliers, P. J. 2. Anorg. Allg. Chem. 1974, 407, 350-352. McMillin, D. R.; Buckner, M. T.; Ahn, B. T. Inorg. Chem. 1977, 76, 943-945. . . . . Kirchhoff, J. R.; Gamache, R. E.; Blaskle, M. W.; DelPagglo, A. A,; Lengei, R. K.; McMlliin, D. R. Inorg. Cbem. 1983, 2 2 , 2380-2384. Jorgenson, C. K. Mol. Phys. 1959, 2 , 309. McClain, C. H. Fluid Flow In Pipes; The Industrial Press: New York, 1964; pp 27-32. McClain, C. H. Fluid Flow in Pipes; The Industrial Press: New York, 1964; pp 61-67. Gerischer, H.; Holzwarth, J.; Seifert, D.; Strohmaier, L. Ber. BunsenGes. Phys. Chem. 1972, 6 , 11-16. Vassilator, G.; Toor, H. L. AIChE J. 1965, 7 I , 666-672. Lappin, A. 0.; Youngblood, M. P.; Margerum, D. W. Inorg. Chem. 1960, 19, 407-413. Campion, R. J.; Purdie, N.; Sutln, N. Inorg. Chem. 1964, 3 , 1091-1094. Lin, C. T.; Rorabacher, D. B.; Cayley, G. R.; Margerum, D. W. Inorg. Cbem. 1975, 74, 919-925. Lin, C. T.; Rorabacher, D. B. J. Phys. Chem. 1974, 78, 305. Eigen, M. Angew. Chem., Int. Ed. Engl. 1964, 3 , 1-72. Hiby, J. A. Verf8hrenstechnik 1970, 4 , 538-543.
RECEIVED for review June 10,1986. Accepted September 18, 1986. This investigation was supported by the National Science Foundation, Grant No. CHE-8319935.
Selective Concentration of Cobalt in Seawater by Complexation with Various Ligands and Sorption on Macroporous Resins Kenji Isshiki Department of Chemistry, Faculty of Science, Kyoto University, Kyoto 606, Japan
Eiichiro Nakayama* Instrumental Analytical Research Center, Faculty of Science, Kyoto University, Kyoto 606, J a p a n
Selectlve concentratlon of trace amounts of cobalt In seawater was examlned by use of 14 Ilgandd and 2 XAD reslns. Complexed cobalt was collected afler passage through a small XAD resln packed column. Complexes of coexisting metals were eluted wlth acidic solutbn. Cobalt complex was then eluted wlth a methanol-chloroform mlxture. When Ilgands InsOluMe In water were used, the recovery of cobalt was not quantltatlve because of copreclpltatlon of the complex on the preclpttated Ilgand. The combination of TAR, whkh forms a water-soluble Inert complex wlth cobalt, and XAD-4 resin was preferable for the selective concentratlon of cobalt In seawater. This method was applled to the preconcentratlon of cobalt dissolved In seawater at nanogram-per-liter levels.
INTRODUCTION In recent years, reliable analytical values for the concentration of cobalt dissolved in ocean water have been obtained as well as those for other trace metals. This is mainly due to the development of clean analysis techniques such as clean samplers, clean rooms, and purification methods for reagents. In combination with such techniques, the preconcentration technique is also an important problem for the determination of cobalt, because of its extremely low concentration in seawater. Many methods such as coprecipitation, cocrystallization, solvent extraction, column extraction, and electrolysis have been developed for the preconcentration of trace metals in seawater (I). In such methods, solvent extraction and
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column extraction are most frequently used. The latter has two advantages over the former: (i) a relatively high concentration factor, and (ii) the ability of treating large volume samples in a closed system free from contamination (2). These advantages are essential for the determination of cobalt in seawater. Column concentration techniques, which have been applied to the preconcentration of trace metals including cobalt, are classified into the following three types of methods: (i) the method using a column packed with ligand-immobilized material (3-51, (ii) the method using a column packed with ligand-impregnated sorbent (6-9),(iii) the method collecting the metal complex with a sorbent-packed column after complexing the metal with ligand in aqueous phase (10-16). In methods i and ii, metals are usually eluted from the column by dissociating them from the complexing reagent with mineral acids. In this process, most metals are easily released except for a few metals such as cobalt and chromium. It is well-known that cobalt(I1) complex is easily oxidized with dissolved oxygen or its own ligands in both the aqueow phase and organic phase (17)and that the resulting trivalent complex is too inert to be easily dissociated even with strong acids. Insufficient recoveries of cobalt reported by several workers ( 4 , 5 )are caused by the incomplete elution of such inert cobalt species as well as by the incomplete complexation. On the other hand, in method iii, the trivalent cobalt complex retained in the column can be easily eluted by using organic solvent instead of acidic solution. For such reasons, method iii is suitable for the preconcentration of cobalt in aqueous samples. In the present study, method iii, using various available ligands and XAD resins, was investigated. Attention was focused on the selection of the optimal ligand and adsorbent for efficient and selective concentration of cobalt in seawater.
EXPERIMENTAL SECTION Apparatus. Cobalt was determined with a Nippon Jarrel-Ash AA-8200 atomic absorption spectrometer equipped with a Nippon Jarrel-Ash FLA-100 graphite furnace atomizer (GFAAS). Aqueous samples were manually injected in a graphite furnace with a 20-pL Eppendorf micropipet. Radioactive cobalt was determined with a Packard 1-5110 autogamma scintilation spectrometer. Reagents. The following ligands were tested. Ammonium pyrrolidinecarbodithioate (APDC), dibenzylammonium dibenzyldithiocarbamate (DBADBDC) (18),diphenylthiocarbazone (dithizone), and &hydroxyquinoline (oxine),which are commonly used in preconcentration of metals in seawater, 2-nitroso-lnaphthol (NN), and disodium l-nitroso-2-naphthol-3,6-disulfonate (nitroso R salt), which are commonly used for liquid-liquid extraction or colorimetric determination of cobalt, 1-(2-pyridylazo)-2-naphthol (PAN), 4-(2-pyridylazo)resorcinol(PAR), 242thiazoly1azo)-p-cresol (TAC), 2-(2-thiazolylazo)-5-(dimethylamino)phenol (TAM), l-(2-thiaz~lylazo)-2-naphthol (TAN), and 4-(2-thiazolylazo)resorcinol(TAR), which are heterocyclic azo compounds used for colorimetric determination of metal ions (19, 20), and benzoylacetone (BA) and dibenzoylmethane (DBM), which are common reagents that chelate through two oxygens. Ligand solutions were prepared by dissolving commercial analytical grade reagents in methanol or water. For all ligands, cobalt blanks were negligible compared with added amounts of cobalt in each experiment. Cobalt(I1) standard solution was prepared by dissolving cobalt nitrate ( C O ( N O ~ ) ~ . ~ in H ~0.1 O )M hydrochloric acid. All other chemicals were of analytical reagent grade. They were used after checking the cobalt blank. Artificial seawater was prepared according to Lyman and Fleming’s formula (21)and used after purifying with a Chelex-100 column to remove heavy metal impurities and a XAD-4 column to remove organic impurities. Surface seawater sample was collected at station 1 (29’05’ N, 142’51’ E) in the northwest Pacific during a cruise of R. V. Hakuho Maru (KH-84-3) in the summer of 1984. It was filtered through a 0.4-pm Nuclepore filter immediately after the collection, acidified to pH 2 with hydrochloric acid, and stored in a precleaned polyethylene bottle. All aqueous solutions were prepared with distilled deionized
water further purified by a Millipore Milli-Q water purification system. The resins used were purified XAD-4 and purified XAD-7 (20-50 mesh; Gasukuro Kogyo, Inc., Tokyo, Japan). Column. The XAD resins were gently ground with an acetone-water mixture in a ceramic mortar and sieved with a nylon mesh sifter to obtain 100-200 mesh grains. Then the resins were soaked with methanol-4 M HC1 (1+ 1) solution overnight and washed with water. Next, 200 mg of the resins was slurry-packed in a Teflon column (8 mm i.d.) fitted with porous Teflon filters (pore size, 1pm) and Teflon stopcocks. It was then conditioned with water before use. After completion of each experiment, the columns were rinsed successively with a methanol-nitric acid mixture and water and stored for the next experiment. A column packed only with a porous Teflon filter (pore size, 1 pm) was prepared for the collection of precipitated cobalt complexes. Procedure. The following procedure was employed for each concentration experiment. (1) Add 100 pL of 1.7 X 10“ M cobalt standard solution to 50 mL of artificial seawater in a Pyrex glass container and adjust the pH to the desired value with a small amount of hydrochloric acid or aqueous ammonia. (2) Add 100 pL of 5.0 X M ligand solution to the solution (l),recheck the pH, and then equilibrate the mixture for 10 min. (3) Pass the solution (2) through a column with a peristaltic pump (flow rate, 3.0 mL/min) or a suction pump (flow rate, ca. 30 mL/min). (4) Rinse the column with 10 mL of water. ( 5 ) Elute the adsorbed metal complex by back flushing with 5 mL of a methanol-chloroform mixture (1 + 1). (6) Evaporate the eluate, decompose the organic residue with a small amount of nitric acid and perchloric acid, dissolve the residue in 5 mL of 0.1 M nitric acid, and determine the cobalt with GFAAS. In the acid leaching test of retained cobalt complex, the procedure was the same as that above except for step 5. Step 5 was replaced by the following: (5’) Elute the metal with 10 mL of 1 M hydrochloric acid at a flow rate of 3 mL/min. In the selective concentration of cobalt dissolved in the test solutions, the column was rinsed with 10 mL of 1 M hydrochloric acid at a flow rate of 3 mL/min before rinsing the column with 10 mL of water in the above procedure. Then cobalt and other metals both in the acidic eluate and in the organic eluate were determined with GFAAS. The complex adsorbed on the wall of the container or the precipitate of the ligand and ita complex remaining on the bottom of the container was collected by rinsing the container with a methanol-chloroform mixture (1 + 1). pH dependence was examined mainly to obtain information concerning the complex formation, and thus the recovery of cobalt was calculated from the sum of the amount of cobalt eluted with the organic eluent and that remaining in the container because the latter portion was expected to be collected in a column if it is introduced into the column. The percentage of cobalt trapped by a filter was measured at a cobalt concentration of both 3.4 X lo-* M and 3.4 X 10-l’M. In the latter experient, cobalt solution labeled with 57C0was used. Analysis of seawater was carried out according to the following procedure: (1”)Prepare 0.5 L of seawater in a precleaned Pyrex Erlenmeyer flask and adjust the pH to 8.0 with aqueous ammonia. (2”) Add 1 mL of 5.0 X M TAR solution and equilibrate it for 10 min. The procedure of concentration and elution is the same as that of the selective concentration of cobalt given above. (6”) Evaporate the eluate, decompose the organic residue with a small amount of nitric acid and perchloric acid, dissolve the residue in 0.5 mL of 0.1 M nitric acid (concentration factor was IOOO), and determine the concentration of cobalt against an aqueous calibration curve with GFAAS. The analytical blank was determined from the mixture of all the reagents added to the sample which was treated according t o step 6”.
RESULTS AND DISCUSSION Amberlite XAD resins have been widely used as an adsorbent because of their high affinity for organic materials
ANALYTICAL CHEMISTRY, VOL. 59, NO. 2, JANUARY 1987
Table I. Percent Recovery of Cobalt Spikes from Artificial SeawateP XAD-4 ligand APDC BA DBADBDC DBM dithizone NN
nitroso R salt nitroso R + Zephiramine oxine
PAN PAR TAC TAM TAN TAR
l b
86 f 6 2 f 1 80 f 4 3 f 3 77 f 9 93 f 4
