Anal. Chem. 1905, 57, 1941-1943 (22) Wetkinson, J. S. Anal. Chem. 1966, 38, 92. (23) Blotcky, A. J.; Hobson, D.; Leffler, J. A.; Rack, E. p. Anal. them. 1976, 48, 1084.
RECEIVED for review March 8,1985. Accepted April 22,1985.
1941
This research was supported by the U S . Department of Energy, Division of Chemical Sciences, Fundamental Interaction Branch, under Contact DE-FG02-MER13231and a University of Nebraska Research Council NIH Biomedical Research Support Grant No. RR-07055.
Cobalt Preconcentration on a Nitroso-R Salt Functional Resin and Elution with Titanium( I I I) Renato Stella,* M. T. Ganzerli Valentini,* and Luigino MaggiO
Dipartimento di Chimica GeneraleO e Centro di Radiochimica ed Analisi per Attivazione del CNR, Universitci di Pavia, Viale Taramelli 12,27100 Pavia, Italy
The anion exchange resin Dowex 1x8, converted to the nitroso-R salt form, was used for adsorbing cobalt from large freshwater samples. Strongly acid titanium( 111) chloride M solution was found very effective at 60 O C as a new eluant and yielded complete recovery with a preconcentratlonfactor of 100. Subsequent atomic absorption spectrometry determinatlon of cobalt In the eluate was possible with no interference from titanium, reduced organics, and Iron, copper, and nickel which partially might be fixed onto the resin. The suggested procedure allows a reproducibility of 5-10 % for samples with cobait concentrations in the range of 0.01-1 Mg L-1.
Trace elements are usually present in freshwater a t microgram per liter levels. Their direct analysis is highly desirable as it involves minimum sample handling and pretreatment but it is limited to a few elements. Especially, cobalt, whose concentration in natural waters is generally below 1 pg L-l, unfortunately requires preconcentration; its measurement in freshwater is important to cope with the increasing demand for control of nuclear plant discharges. Several batch techniques have been used for cobalt preconcentration including coprecipitation (1,2)and cocrystallization (3),which yield high preconcentration factors but are tedious and time-consuming. Flow methods are simpler and convenient: usually they involve the use of ion exchangers (4),also in the form of membranes (5),or chelating exchangers loaded onto different inert supports (6-10). In some cases the adsorption of complexes on ion exchange resins was reported (11). A highly specific reagent for cobalt complexation, sodium l-nitroso-2-naphthol-3,6-disulfonate, also called nitroso-R salt (NRS), is widely used (2,10, 11). Moreover it has the advantage of bearing ion exchangeable sulfonato groups. In this work NRS was used to develop a simple method for preconcentrating cobalt from natural waters, based on its adsorption on an ion exchange resin previously converted to a functional NRS form. The development of these functional resins is due to Tanaka et al. (12)and to Lee et al. (13). Cobalt may be recovered either by removing the chelate from the resin bed or by displacing the metal ion from chelating groups; we found that, whichever the mechanism involved, the elution is the most critical step in the whole 0003-2700/85/0357-1941$01.50/0
procedure. Preliminary experiments showed in fact that, unlike that claimed by other authors (lo),any strong mineral acid was required in a quite large volume for a complete recovery and no substantial improvement was obtained by using any of the common oxidizing or complexing agents (HzOz,Clz, NH3, CNS-, EDTA). Reduction of nitroso group -NO with TiC13 in HC1 to the amino group -NH2 was much more effective and ensured a concentration factor of 100. Subsequent atomic absorption spectrometry measurements of cobalt was found unaffected by the presence of titanium and leached organic, mostly l-amino-2-naphthol-3,6-disulfonic acid. The preconcentration procedure was particularly devised for freshwater samples which usually bear cobalt in the concentration range 0.01-1 pg L-l. EXPERIMENTAL SECTION Reagents. Titanium trichloride TiCl,, from Merk Co., did not require purification and was added to Suprapure HCl (C. Erba) to reach a 4 M final HCl concentration. Sodium hydrogen carbonate, nitroso-R salt, and other chemicals were reagent grade. Terdistilled water was used in each experiment. Radiotracer “To (t,,,,= 5.3 years) was prepared by irradiating 10 mg of cobalt(I1) nitrate in a TRIGA MARK I1 reactor at the University of Pavia at a thermal neutron flux of 8 X 10I2 n cm-2 s-l for 60 h; the irradiated Co salt was dissolved and repeatedly diluted with distilled water and then with filtered river water to reach a Co concentration twice the natural Co level, while still giving a count rate of 10000 cpm mL-’. Anion exchange resin Dowex 1X8,100-200 mesh, was washed with 1M HCl and 1M NaOH and terdistilled water prior to use. Apparatus. A peristaltic pump (Millipore Co.) was used to feed the column, the latter being equipped with a water jacket to allow operation in thermostated conditions. Column dimensions were 0.8 cm i.d., 9 cm height. Radiotracer (60Co)was y counted by a NaI (Tl) well type crystal coupled to a multichannel analyzer. A Perkin-Elmer 2380 atomic absorption apparatus equipped with HGA 74 graphite furnace was used for Co determination. Resin Preparation. The functional chelating resin was prepared by supending 30 g of purified Dowex 1x8 resin, 1c0-200 mesh, in 100 mL of a solution containing 0.3 g of NRS. After the mixture was shaken for 1h, the resin was filtered on a glass filter under gentle suction and dried in a vacuum desiccator; NRS loading resulted in 20 rmol per g of resin. Procedure. Large Po River water samples were submitted, soon after sampling, to a multistep filtration using Millipore membrane packets of decreasingporosity, 8-0.45 rm, to minimize filter clogging. Filtered water aliquots (5 L) were treated with Na2C03and NaHCO, to adjust pH to 8, and fluxed through a water jacketed 0 1985 American Chemical Society
1942
ANALYTICAL CHEMISTRY, VOL. 57, NO. 9, AUGUST 1985
Table I. Cobalt Uptake (%) by NRS Functional Resin in Different Experimental Setsa total resin amt, g 4
6
8
10
12 PH
Figure 1. pH effect on the efficiency of cobalt uptake by NRS functional resin (4 g of Dowex 1X8 loaded with 80 pmol of NRS; sample, 1 L of Po River water).
chromatographic column (0.8 cm i.d.; 9 cm length) filled with 4 g of functional resin; flux was kept constant at 40 mL m i d with a peristaltic pump. Cobalt elution was performed with 50 mL of a M Tic& solution, strongly acid by HCl(4 M); the temperature of the water in the jacket was raised to 60 "C. Preliminarytests carried out at different water temperatures had shown that only for water temperatures higher than 50 "C was reduction of the (NO) groups completed, and thereafter cobalt elution could be carried out quantitatively. The eluate was submitted to cobalt determinationby atomic absorption spectrometry. Measurementswere carried out at 240.7 nm wavelength; aliquots of 20 pL were atomized in pyrolytically coated graphite tubes. Char and atomization temperatureswere 1400 "C and 2500 "C, respectively.
RESULTS AND DISCUSSION Cobalt preconcentration is mainly based on two different steps whose sequence may be changed according to different operating conditions (field or laboratory analysis): chelate formation between cobalt and NRS or ligand adsorption onto anionic resin through sulfonato groups (after or before metal chelate formation). Two procedures were correspondingly devised. The first one (procedure A) involved the following steps: To 1L of filtered water, 1mL of @Cotracer solution was added. After addition of Na2C03,pH was adjusted to 7.8-8 with 1g of NaHCOs, and 30 mg of nitrous-R salt was added. After 2 h the water was fluxed through a column packed with 4 g of Dowex 1 x 8 resin (100-200 mesh). Complex uptake was monitored as a function of the flux rate and it was found quantitative for values below 15 mL min-l. The whole procedure may be quickened by reversing the sequence of complexation and fixation steps (procedure B). It is very likely that Co complex formed in this procedure does not correspond to the ligand to metal ratio 3:1, which is the highest ontainable in solution. Due to the immobilization of chelating groups in the supporting resin, it is very unlikely that three of them be at one time available to fill up the coordination of one Co(I1) ion. An unsaturated coordination makes, on the other hand, the metal more easily removable from the column in the subsequent elution step. The use of the radiotracer (@Co)was helpful both during the setup of the separation procedure and for possible application of the method to the determination of 6oco in natural waters. Preliminary tests were carried out to verify that the added tracer had achieved a species distribution comparable to the native cobalt. Radiotracer was added to filtered river water samples whose cobalt content had been previously measured. Aliquots (1 L) were sampled at regular time intervals (1 h) and submitted to ultrafiltration and preconcentration as described; cobalt and @Cowere then measured in each concentrated fraction. Results show that after 2 h no difference is detectable in the distribution of cobalt and @Co. A search for optimum preconcentration and elution conditions was therefore undertaken, leading to the procedure outlined in the Experimental Section. All experiments were
NRS loading, pmol g-* resin
% Co uptake from
a water vol of 1000
3000
5000
mL
mL
mL
1.0
100
80
60
40
2.0 4.0
100 100
95
IO
4.0 4.0 4.0
50
100 100
100 100
50 100 100
100
100
100
95
75
60
20 10
"Po River filtered water: DH 8; flux = 40 mL m i d
I\
A
1
250
350
Flgure 2. (A) UV spectra of NRS (a) and of the corresponding amine (b) produced by reduction with Ti(II1). (B) UV spectra of the eluate, after outflowing 10 mL (a), 20 mL (b), and 30 mL (c).
run with Po River water filtered through membranes of 0.45 pm porosity and radiotraced with e°Co. Uptake efficiency as a function of pH was examined and its trend, reported in Figure 1,shows maximum values in the pH range 7.5-8.5. In order to obtain high preconcentration factors, resin amount and loaded NRS should be kept as low as possible. Data reported in Table I show that 4 g of anionic resin, loaded with 80 pmol of NRS, is adequate to meet the desired conditions even at a flux of 40 mL min-l. The loaded NRS amount is in large excess with respect the cobalt amount contained in 5-L river water samples. On the other hand the setup method is not supposed to be applied to larger water volumes because of quite long time intervals in the column operation. Therefore no investigations were carried out to define the column loading capacity. Experiments carried out with increasing cobalt concentration indicate that 100% cobalt uptake can be obtained up to a cobalt total amount of 30 pmol. Titanium trichloride solution in 4 M HC1 is known (14,15) to be a very specific and stoichiometricreagent in the reaction R-NO + 4TiC13 4HC1- R-NH2 + 4TiC14 + H2O
+
In the present case R- represents the disulfonaphthol tied to the anionic resin. The produced naphthol amine is partially eluted but at a slower rate with respect to the released COW) ions: its presence was ascertained by comparing UV spectra of the eluate with those obtained from NRS dissolved in water before and after reacting with TiC13, as shown in Figure 2. Cobalt is not expected to be tied up by naphthol amine, either present in the eluate or still fixed onto the resin, due to the competing role played by highly concentrated H+ions; on the other hand cobalt chloro complexes are not formed as long
ANALYTICAL CHEMISTRY, VOL. 57, NO. 9,AUGUST 1985
1943
~~
Table 11. Cobalt Recovery from NRS Functional Resin as a Function of Concentration and Volume of Eluent Ti(II1)"
Table 111. Cobalt Found in Filtered Po River Water and in the Ultrafiltrate Function of loJ NMWL
% Co(I1) recovered
[TitIIUl, m o l L-'
20 mL
0.12
100 98 90
0.03 0.1 0.005
0.001
amt of Co, ng mL-'
in vol o f Ti(II1)
30 mL
65 10
"Resin, 4 g loaded w i t h 80 pmol of mi&; temperature, 60 "C.
100 98 80 30
50 mL
subsample no.
in water filtered through membranes
in the ultrafiltrate fraction of lo9 NMWL
0.13
0.08
0.15
0.07
100
1 2 3
0.13
0.09 0.08 0.07
90
40
NRS; Ti(II1) flux, 1.5 mL
as C1- concentration does not exceed 4 M. Under the described conditions iron uptake ranged from 60 to 100%with a poor reproducibility; the method therefore cannot be used for iron determination. In order to minimize possible matrix effects in the subsequent cobalt measurement by atomic absorption spectrometry, it was also imperative to keep the amount of eluting Ti(II1) as low as possible. The lowest Ti(II1) concentration and volume necessary for complete recovery were determined through radiotracer experiments and are reported in Table 11. Figures in Table I1 are expected to be rather independent from total cobalt adsorbed onto the column as they are proportional to the reduction yield of -NO groups regardless if the latter are tied or not to cobalt ions; this hypothesis was proved by spiking natural water with Co(II) ions just to double natural level and repeating the set of experiments. Figures reported in Table I1 were confirmed within *5%. Preliminary experiments were carried out by the analytical technique (atomic absorption spectrometry) to verify that no interference would be produced by Ti, mostly Ti(IV), and the organic matter (NRS and corresponding amine), whose concentrations are much higher than that of cobalt. Solutions of Ti(III), oxidized to Ti(1V) by bubbling an air stream, gave low absorbances at the Co analytical line (270.4 nm) in the concentration range to 10-1 M. Several measurements repeated at wavelengths somewhat lower and higher than 240.7 nm confirmed the occurrence of a small interference due to Ti and organic matter on the adsorption measurements; Co absorbance in a 10 ng mL-l Co solution was enhanced up to 5% by the presence of a 10-1 M Ti solution. Among possible interfering ions only Fe(III), Cu(II), and Ni(I1) were considered: they may be partially fixed by the functional resin. No interference in Co analysis was observed for Fe(II1) concentration lower than 5 ng mL-' and Cu(1I) and Ni(II) concentrations lower than 1 ng mL-l. These concentrations are quite above the usual metal content in natural freshwaters and therefore the presence of Fe(III), Cu(II), and Ni(I1) does not affect cobalt assay.
4 5
0.14 0.12
Procedural blanks were carried out by using 5 L of deionized bidistilled water. An average procedural blank of 1 ng mL-l Co, in the final eluate, was obtained when working in conventional laboratory conditions. When all operations were performed in a laminar-flow cleafi hood, which minimizes airborne contamination, a 50% cut of the blank was observed. Five independent cobalt determinations were performed on a single large sample of filtered Po River water and also on an ultrafiltered fraction containing particles of lo3NMWL (nominal molecular weight limit). Results reported in Table I11 are corrected for blank contribution. Their precision is in the range 8-10%. The proposed method is, therefore, adequate to meet the sensitivity and reproducibility requirements necessary for cobalt assay in natural freshwaters. ACKNOWLEDGMENT The authors wish to thank S. Meloni for helpful discussion and a review of the manuscript. Registry No. TiC13,7705-07-9;HzO, 7732-18-5;Co, 7440-48-4. LITERATURE CITED Thompson, G.; Laevastu, T. J. Mar. Res. 1960, 18, 189-194. Forster, W.; Zeltlln, H. Anal. Chim. Acta 1966, 3 4 , 211-224. Young, 0.; Smith, D. G.; Laugllle, W. M. J. Fish Res. Board Can. 1959, 16 (7),7-12. Klrlyama, T.; Kuroda, R. Fresenius' 2. Anal. Chem. 1977, 228.
354-356. James, H. Analyst (London) 1973, 9 8 , 274-288. Braun, T.; Farag, A. 8. Anal. Chim. Acfa 1975, 76, 107-112. Braun, T.; Farag, A. 8.; Maloney, M. P. Anal. Chim. Acta 1977. 9 3 ,
I91-201. Braun, T.; Abbas, M. N. Anal. Chim. Acfa 1980. 719, 113-119. Kubo, M.; Yano, T.; Kobayaskl. H.; Ueno, K. Talanfa 1977, 2 4 ,
519-521. Przeszlakowskl, S.; Kocjan, R. Chromafographia 1982, 15 (1 I),
717-722.
Dean, J. A., Anal. Ch8m. 1051, 2 3 , 1096-1097. Tanaka, H.; Chikuma, M.; Nakayama, M. "Analytlcal Techniques In Environmental Chemistry"; AlbaigC. J., Ed.; Pergamon Press: Oxford, 1982;Vol. 2 pp 381-388. Lee, K. S.;Lee, W.; Lee, D. W. Anal. ch8m. 1978, 50, 255-258. Knecht, E.; Hlbbert, E. Ber. Dfsch. Ch8m. Ges. 1907, 4 0 ,
3819-3827. Dachselt, E. 2.Anal. them. 1926, 68, 404-410.
RECEIVED for review November 20,1984. Resubmitted April 17, 1985. Accepted April 17, 1985.
