484
Anal. Chem. 1987, 59, 484-486
(22)Severs. L. W.; Mekher, R. G.; Kocsis, M. J. Am. Ind. Hyg. Assoc. J . 1978, 39(4),321-326. (23)Gradiski, D.; Bonnet, P.; RaouA, G.; Magadur, J. L. Arch. Mal. Prof. Med. Trav. Secur. SOC.1978,39, 249-257. (24) Taylor, D. G.;Kupel, R. E.; Bryant, J. M. Documentstion of the NIOSH vaihtion Tests; NO. s 139,s 142,s 144,s 147,s 150; US. ~ 0 ernment Printing Office: Washington, DC, 1977.
(25) Taylor. D. 0. NIOW Menuel of Analytcal Methods; No., 221; US. Government Printing Office: Washington, DC, 1977.
~
-RECEIVED for review June
25,1986. AccepM October 7,1986. This is Publication No. 1285 from the INRS.
Comparison of Anion-Exchange Methods for Preconcentration of Trace Aluminum Corrado Sarzanini, Edoardo Mentasti,* Valerio Porta, and Maria Carla Gennaro Department of Analytical Chemistry, University of Torino, Via P. Giuria, 5, 10125 Torino, Italy
Alumhum( I I I ) traces have been preconcentrated through formam of anfonlc compkxee whkh are r e t a w by the anlon exchanger. The ligand was Pyrocatcrchol Vldet (PV) whose structure contains a sutronato group (not Involved In AI( I1 I ) coordlnatlon) which Is rerpondMe for the metal uptake by the resin. Two d#tcHent prooedues were hves@atd and compared. One Involves the fonnatlon of ACPV complex(es) followed by elutlon through a column contahdng the anlon exchanger. The second procecbe lnvotves the kadlng of the PV llgand on the redn and subsequent ehdlon of the sample. The results obtalned wHh the two dlfferent procedures have been compared wHh reference to percent recovery yield as a functh of pH and effect of foreign compommts (eurfactants, Inorgsnk: SelQ such as phosphates and chkrkleq organlc compombsuch as hunlc ack!8, and NTA). The fint procedure was superb and hae been UNlZedfor the analysis of synthetic samples contalnlng AI at the parts-perbllllon level and for drlnklng water.
Ion exchangers have been used for the preconcentration of trace elements, speciation studies in complex mixtures, removal of interfering components from analyzed solutions, and separations of ions (1-4). Resins, both cationic or anionic, ion-exchange papers (5) or membranes (6) have been shown to be suitable for collecting and separating trace quantities of ions (7,8). Aromatic complexing agents containing sulfonic acid groups are particularly useful for the separation of metal ions on anion-exchange resins (9-14). Two methods are compared here employing Pyrocatechol Violet in preconcentration techniques for removal of aluminum traces. This chelating agent gives highly stable complexes with aluminum and shows, owing to its very large molecular size, an increase of affinity with a macroporous anion exchanger. The use of Pyrocatechol Violet for metal-matrix separations and preconcentration is based mainly on two methods (i) complex formation coupled with an anion-exchange resin and (ii) column chelation procedure. In the first case the solutions of the metal are added of the chelating agent and fluxed through the exchanger at the proper pH. In the latter the samples are eluted through the macroporous anion-exchange resin previously loaded with an appropriate amount of ligand. Synthetic and natural samples have been investigated in the presence of interfering agents. The final determination of the preconcentrated element was accomplished by dc argon
plasma emission spectrometry.
EXPERIMENTAL SECTION Apparatus and Materials. All laboratory glassware and polyethylene and polypropylene equipment were cleaned in 6 M nitric acid and repeatedly rinsed with high-purity water (HPW). Thermostated (water-jacketed)borosilicate glaas columns 8 mm i.d. and 30 cm high) were employed and a slurry of 0.5-1.0 g of resin was supported. A rotary vacuum pump with a bypass flowmeter ensured a 1-5 mL/min constant flow of samples through the column. A peristaltic pump equipped with a variable-speed control ensured reduced flows, generally within 0.5-1.0 mL/min. Metal concentration measurements were performed by dc argon plasma emission spectrometry (Spectraspan IV, SMI, Andover, MA) at X 396.15 nm where the detection limit for Al is about 5-10 ng/mL. Two-point calibration was used. The pH measurements were performed with a Orion 811 pH meter equipped with a combined glass calomel electrode. Adjustable Eppendorf pipets were used to prepare the solutions. Analytical grade Bio Rad AG MP 1 macroporous anion-exchange resin, 1W200 mesh, was used in chloride form. Pyrocatechol Violet (3,3f,4f-trihydro~chsone-2"-sulfonic acid, PV, Merck) was an analytical grade reagent. A standard aluminum (Merck) stock solution (loo0mg/L metal concentration) was diluted to the desired concentrations. Other reagents were of analytical grade and all solutions were prepared by using deionized water further purified by a Milli-Q water purification system (Millipore, Bedford, Ma). Procedures. Precomplexation Anion Exchange (PAEJ. The borosilicate column filled with 0.5-1.0 g of AG MP 1 resin was rinsed and preconditioned with HPW to the proper pH. Solutions (100 mL and lo00 mL) each containing 10.0 pg of Al(II1) were added to 2.0 and 7.0mL of 0.05 M Pyrocatechol Violet, respectively, and were brought to the desired pH. The samples were fluxed through the column, which was then washed with HPW after the elution. The metal was recovered by acidic elution (10.0 mL). Blanks were periodically run and the results of all experiments were unaffected by flow rate in the employed range, 1.0-5.0 mL/min. Chelating Agent Loaded Resin (CALR). The chelating agent loaded resin was prepared by running 4.0 mL of 0.05 M Pyrocatechol Violet solution through the column packed with 1.0 g of AG MP1 resin, chloride form. The loaded resin bed was washed with HPW at the pH to be used in the next experiment. The samples were fluxed at 1.0 mL/min and the metal was recovered as in the PAE method. In no case did release of chelating agent from the resin occur. Metal Recouery as Function of p H . The metal recovery as a function of pH was evaluated for both the procedures. In the PAE procedure 100.0-mL samples at 1.0 mg/L metal concentration were used. Solutions, added to 2.0 mL of 0.05 M
0003-2700/87/0359-0484$01.50/00 1987 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 59, NO. 3, FEBRUARY 1, 1987 485 Table I. Comparison of the PAE and CALR Methods in Percent Al(II1) Recovery as a Function of pH (samples, 100.0 mL; Al(III), 1.0 qg/mL)
Table 11. Effect of Interfering Agents on Percent Al(II1) Recovery (0.1 wg/mL solution; pH 7 (PAE method) and pH 9 (CALR method))
% Al(II1) recovery'
a
PH
PAE
2 3 4 5 6 7 8 9
3.8 f 1.0 34.2 f 1.8 81.4 f 0.6 92.1 f 0.5 93.5 f 1.3 100.0 f 0.9 100.2 f 0.7 99.4 f 0.5
CALR
interferent type' CTAB
15.6 24.4 74.3 95.7
f 2.7 f 1.3
f 1.1 f 0.8
5.0 20.0 50.0
98.8 f 1.0
Poly
5.0 20.0 50.0
99.6 f 0.5 100.8 f 1.8 101.2 f 0.6
SDS
5.0 20.0 50.0
NTA
C C
94.4 f 0.7 90.5 f 0.7 87.2 f 0.9 94.5 f 0.9 94.9 f 0.4 93.4 f 0.1
99.1 f 1.1 98.8 f 0.4 98.4 f 0.1
94.8 f 0.9 96.2 f 1.0 96.9 f 1.0
275.0 2750.0
99.8 f 0.6 99.3 f 2.9
95.7 f 0.4 95.8 f 0.4
100.0 200.0 500.0
99.8 f 0.4 100.2 f 0.7 100.4 f 1.0
96.3 f 0.7 94.0 f 1.3 94.3 f 0.6
HU
5.0 15.0 30.0
99.4 f 0.4 99.1 f 1.9 94.6 f 2.2
HF
0.5 4.5 9.0
99.4 f 0.4 99.1 f 1.9 94.6 f 2.2
NaCl
0.1 M 0.5 M 1.0 M
Mean and relative standard deviations for three measurements.
PV and brought to the desired pH, were fluxed through the anion-exchange resin preconditioned at the same pH. The aluminum recovery was obtained eluting the fixed metal with 1.0 mL of 1.0 N HCl followed by 9.0 mL of 0.1 N HC1. The efficiency of the CALR procedure was also tested in the pH range 6-9 (see Discussion). The 100.0-mL samples (1.0 pg/mL Al(II1) at the desired pH) were fluxed through the loaded resin preconditioned at the same pH. Preliminary results suggested an optimum flow rate of 1.0 mL/min. The column was rinsed with HPW and the fixed aluminum was eluted in the same way as for the PAE procedure. Experiments were carried out at higher pH, after filtration on 0.45-pm Millipore filters to ensure the absence of precipitate. Results for the PAE and CALR methods are shown in Table I. Interfering Agents. Interferences of several common surfactants: cetyltrimethylammoniumbromide (CTAB), sodium dodecyl hydrogen sulfate (SDS),and poly(ethy1ene glycol) (POLY, mol w t 6000) at 5, 20, and 50 mg/L concentrations were tested. Potentially interfering salts (including 100, 200, and 500 mg/L K2HP04,0.1,0.5, and 1.0 M NaC1) as well as the effect of competing ligands (0.273 and 2.730 g/L nitrilotriacetic acid (NTA) and 5, 15, and 30 mg/L humic acid (HU)) were evaluated. The experiments were carried out with 100.0-mL solutions containing the above interfering agents and brought to pH 7.0 and 9.0 for the PAE and CALR procedures, respectively. The results are shown in Table 11. Preconcentration. The methods were also evaluated at parts-per-billionlevels. Synthetic samples (1000 mL) containing Al(II1) spikes (10.0 pg) were enriched using the described procedures at the optimum pH for the metal recovery. For three independent measurements the spikes found were 100.0 1.7% and 92.5 f 0.9%, respectively, with the PAE and CALR procedures.
3'% Al(II1) recoveryb PAE CALR
concn, Mg/mL
K2HPOd
99.9 f 0.7 100.4 f 1.2 97.3 f 0.1
94.2 f 0.1 91.2 f 0.5
a Code: CTAB, cetyltrimethylammoniumbromide; POLY, poly(ethylene glycol); SDS, sodium dodecyl hydrogen sulfate; NTA, nitrilotriacetic acid; HU, humic acid. Mean and standard deviations for three measurements. In these conditions, separation of insoluble products occurs.
RESULTS AND DISCUSSION In preliminary experiments Pyrocatechol Violet was found to be bound almost quantitatively on the macroporous anion-exchange resin. The retention of the chelating agent is b a d on both ion exchange (main) and molecular adsorption (partial). Since it gives complexes with aluminum at 1:1,1:2, and 1:3 metal-to-ligand molar ratio, a total recovery of the metal can be expected by operating according to the procedures outlined above. An optimization of kinetic conditions was made before the experimental evaluations, with respect to the species distribution as a function of pH. An iterative fast-converging computer program enabled the data to be presented as both final equilibrium concentrations of each species and the percent distribution of a particular metal between various species as a function of pH (15). The method used published stability constants (16, 17) together with analytically determined concentrations of components. The first step of the computation was made by varying the concentration of PV with prefixed aluminum concentrations (10-1000 parts per billion). The aim of this computation was to obtain the op-
Flgure 1. Aluminum-Pyrocathecol Violet (PV) complexes distribution and AI(II1) percent recovery as a function of pH by PAE (Exp 1) and CALR (Exp 2) methods.
timum concentration of ligand at which the sulfonated complexes, involved in ionic exchange, are better distributed. The resulting concentrations were employed in experimental evaluations of recovery and preconcentration of aluminum traces. Figure 1shows the computed distributions, as a function of pH, of free and complexed A1 (111)at the optimized concentration of PV together with the experimental data of metal recovery. The experimental uptake for the PAE method shows some discrepancy from the computed complex species and this is more pronounced for the CALR method. From Figure 1 it
486
ANALYTICAL CHEMISTRY, VOL. 59, NO. 3, FEBRUARY 1, 1987
Table 111. Measurements of Al(II1) Samples at KO pg/mL Concentration Originally or after Preconcentration in the Presence of Interferents type
CTAB
5.0
POLY SDS
50.0
KZHPOd NaCl a
@g/mL
50.0 100.0 200.0 500.0 0.5 M
dc plasma measurementsa direct PAE method 98.2 f 0.1 105.0 f 3.0 110.0 f 7.0 116.2 f 6.1 114.4 f 5.5 121.3 f 7.1 286.0 f 2.3
98.8 f 1.0 101.2 & 0.6 98.4 f 0.1 99.8 f 0.4 100.2 f 0.7 100.4 f 1.0 100.4 f 1.2
Mean and relative standard deviations for three measurements. ~~~~
~
seems that the 1:l complex is not retained by the column and the total recovery is achieved only for the 1:3 complex species. This could be explained by considering that when the 1:l complex is formed (in the PAE method), a residual positive charge on the molecule is present so that the repulsive driving forces affect the complex retention on the anion resin. On the contrary the total absence of the ligand in the solution emerging from the column shows that the reduced recovery of the metal may be attributed ta a modified apparent stability constant of the complex when it is transferred from the solution to the solid phase. The reduced values of stability constants for complexes of grafted ligands are confirmed in the CALR method where the metal recovery behavior (Figure 1) shows a shift to much higher pH values with respect to the PAE method. In addition, for the CALR method, the adsorption of a PV molecule may occur either through its sulfonato groups or through its hydroxyl groups, and in this second case the ability to chelate metal ions could be reduced. A series of experiments were performed to identify the effect of Al(OH), and Al(0H); (18) species on the chelation reaction. Al(II1) solutions without chelating agent were eluted through the anion-exchange column, at different pH values, to verify whether mechanical or physical retention was shown with respect to the aluminum hydroxylated species. These experiments showed that the resin retains the metal only at higher pH values where the Al(OH)*- species is present. So, one can conclude that in the CALR method the metal recovery from pH 6 up to pH 9 is due only to complex formation. To evaluate the applicability of the methods to natural systems and the effectiveness in the removal of matrix effects in the instrumental determinations, a series of experiments in the presence of interfering agents was performed by both methods. The results, listed in Table 11, show the validity of the ligand particularly for the PAE method. It must be pointed out that the results are unaffected by the presence of chelating agents able to compete with PV in the complex formation at 1/1or 1/10 ligand-interferent molar ratio (see NTA). The adsorption of the complex by the exchanger
allowed the preconcentration to occur at unusually high ionic strength (1.0 M NaC1). Table I11 shows, in addition, the optimization of metal ion measurements by comparing the results obtained with direct determinations and with the described methods. Drinking Water. Both proposed methods show a good applicability in the presence of interfering agents and the PAE method, which gave the best results in synthetic mixtures, was applied to natural systems. The aluminum content of drinking water in the city of Turin was tested by the standard addition method at parts per billion Al(II1) concentrations and all the samples were preconcentrated. Five hundred milliliter samples of drinking water were spiked with 0.0,25.0,37.5, and 50.0 gg of Al(III), the samples were treated according to the PAE method and the analytical results gave a regression data line (Y = 0.096X + 3.75, r = 0.996) which gives the Al(II1) content of 39.1 gg for 500-mL samples, 78.2 ppb, while the direct determination gave 68.1 f 102 ppb. In conclusion, the method, also applied to the preconcentration of Pb(II), Bi(III), Fe(III), and Cu(I1) (19),may be therefore used in the preconcentration of metal ion traces for real samples. Registry No. PV, 115-41-3; HzO, 7732-18-5; Al, 7429-90-5.
LITERATURE CITED Figura, P.; Mc Duffie. B. Anal. Chem. 1980, 52, 1433-1439. Strelow, F. W. E. Anal. Chem. 1084, 56, 1053-1056. Koide, M.; Lee, D. S.; Straiiard, M. 0.Anal. Chem. 1984, 56, 1956- 1959. Riley, J. P.; Taylor, D. Anal. Chim. Acta 1088, 4 0 , 479-485. Goldbach, K.; Lieser, K. H. Fresenius' Z. Anal. Chem. 1982, 311, 183-186. James, M. Analyst (London) 1973, 98, 274-288. Kiriyama. T.; Kuroda, R. Talanta, 1984. 3 1 , 472-474. Nakayama, M.; Itoh, K.: Chikuma, M.; Tanaka, H. Talanta 1984, 3 1 , 260-274. Sarzanini, C.; Mentasti, E.; Gennaro, M. C.; Baiocchi, C. Ann. Chim. (Rome) 1083, 73, 385-396. Sarzanini, C.; Marengo, E.; Gennaro, M. C.; Baiocchi, C.; Mentasti. E. Chemisfty for ProtecHon of the Environment; Elsevier: New York, 1984, pp 381-385. Sarzanini, C.; Mentasti, E.; Gennaro, M. C.: Marengo, E. Anal. Chem. 1985, 57, 1960-1963. Lee, K. S.; Lee, W.; Lee, D. W. Anal. Chem. 1978, 50, 255-258. Brajter, K.; Dabel-Zlotorzynska, E. Talanta 1080, 27, 19-24. Brajter, K.; Olbrych-Sieszynska, E. Talanta 1983, 30, 355-358. Sammartano, S., Analytical Chemistry Institute, University of Messina, Messlna, Italy, personal communication. Sillen, L. G.; Marteil, A. E. Stabiltty Constants of Metal Ion Complexes, Spec. Pubi. 17 and 25; The Chemical Society: London, 1964 and 1971. Perrin, D. D. Stability Constants of Metal Ion Complexes, Part 8;Pergamon Press: Oxford, 1979. Sarzanini, C.; Gennaro, M. C.; Porta, V.; Mentasti, E., submitted for publication in Anal. Chim . Acta . Sarzanini, C.; Gennaro, M. C.; Mentasti, E.; Porta, V. V I Congress of Analytical Chemistry Divislon of the Italian Chemlcal Society, Bari, Sept 1985; Sarzanini, C.; Mentasti, E.; Gennaro, M. C.; Porta, V. Talanta , in Dress.
RECEIVED for review June 4,1986. Accepted September 30, 1986.
