Room-temperature liquid phosphorimetry of the aluminium-ferron chelate in micellar media

Analytica Chimica Acta, 212 (1988) 235-243 Elsevier Science Publishers B.V., Amsterdam -

235 Printed in The Netherlands

ROOM-TEMPERATURE LIQUID PHOSPHORIMETRY OF THE ALUMINIUM-FERRON CHELATE IN MICELLAR MEDIA Determination of Aluminium”

M.R. FERNANDEZ

DE LA CAMPA, M.E. DIAZ GARCIA and ALFRED0

SANZ-MEDEL*

Department of Analytical and Physical Chemistry, University of Oviedo, Ovzedo (Spain) (Received 11th February 1988)

SUMMARY Aluminium reacts with 7-iodo-8-quinolinol-5-sulphonic acid (Ferron) in cetyltrimethylammonium bromide (CTAB) micelles to form a highly phosphorescent complex at room temperature. Suitable experimental conditions and the phosphorescent characteristics of the complex are described. Comparison with the results obtained for the phosphorescent niobium-Ferron complex in CTAB micelles helps to elucidate the mechanism of this type of phosphorescence. For aluminium the detection limit is 5.4 ng ml-‘; the relative standard deviation is 4.5% for 20 pg Al. The method is applied to aluminium determination in waters and dialysis fluids.

Conventional phosphorimetry at 77 K has rarely been used for the determination of inorganic ions because of the problems associated with cryogenic techniques, which have also deterred fundamental studies on the chemistry of phosphorescent metal complexes at such low temperatures. The metal chelates investigated by conventional phosphorimetry include those with porphyrins [l-3], chlorophylls [ 4,5] and, to a lesser extent, &quinolinol [ 6,7] and benzoylmethane [ 61. For analytical purposes, the problems derived from cryogenic conditions can be circumvented by the use of liquid solutions of “ordered media”, e.g., micelles or cyclodextrins [ 8,9], which are able to provide effective triplet-state protection from ubiquitous quenchers in solution at room temperature. Particularly, micelle-stabilized room temperature phosphorescence (MS-RTP) has recently seen extensive exploitation in determinations of many organic compounds of significance in clinical and environmental chemistry [lo], but MSRTP has hardly been applied to trace metal determinations; only the determination of niobium has been reported so far [ 111. The unavoidable deoxygenation of micellar solutions can easily be accomplished by chemical means, which greatly simplifies the technique of MS-RTP [ 121. “This work was presented in part at the Second International Symposium on Quantitative minescence Spectrometry in Biomedical Sciences, Gent, Belgium, May 1987.

0003-2670/88/$03.50

0 1988 Elsevier Science Publishers B.V.

Lu-

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In this paper, the technique is extended by evaluating its analytical usefulness for aluminium and applying it to the determination of aluminium in clinical samples. Clinically recognized signs of aluminium toxicity in renal failure patients undergoing long-term dialysis include anaemia, osteomalacic osteodystrophy and cardiotoxicity [ 131. Toxicologically deleterious effects of aluminium have also been recognized in aquatic environments [ 14,151. Here, the analytical characteristics of the MS-RTP reaction of aluminium with 7-iodo8-quinolinol-5-sulphonic acid (Ferron) in a solution of cetyltrimethylammonium bromide (CTAB ) are reported. Some reasons for the difference in behaviour of aluminium compared to the phosphorescent niobium complex [ 111 are offered. The suitability of the method for the sensitive and selective determination of aluminium in real samples is demonstrated. EXPERIMENTAL

Chemicals and apparatus The aluminium stock solution (1000 pg ml-‘) was prepared by dissolving pure foil in (1 + 1) hydrochloric acid. Standard aluminium solutions were prepared by appropriate dilution with the (1 + 1) acid. The CTAB solution (1% w/v) was prepared by dissolving the surfactant powder in water by gentle warming. The other surfactants assessed, Brij-35 (10% w/v) and sodium lauryl sulphate (SLS) (2% w/v) were prepared in a similar way. Ferron solution (1 x lop3 M) was prepared in water. All reagents used were of analytical-reagent grade and distilled/deionized water was used throughout. Phosphorescence measurements were made with a Perkin-Elmer LS-5 fluorescence spectrometer which has a xenon pulsed excitation source (10 ps half-width, 50 Hz), and was equipped with a Perkin-Elmer 3600 data station. The triplet lifetimes were obtained by using the “Obey-Decay” application program. The pH measurements were made by using a WTW pH meter, Model 139 (calibrated against Radiometer buffers). The temperature was maintained at 22 ? 2’ C by a circulating water bath. General procedure for phosphorescence measurements Transfer to a lo-ml volumetric flask an aliquot of the aluminium solution containing up to 5 pg of aluminium, and add 0.2 ml of 0.4 M sodium hydroxide (to neutralize most of the acidity), 2 ml of 1 M acetic acid/sodium acetate buffer (pH 5.5), 1 ml of 1 x 10e3 M Ferron, 0.2 ml of 0.5 M sodium sulphite and 5 ml of 0.2 M CTAB. After dilution to the mark with water, mix and let stand for about 15 min. Measure the phosphorescence at 586 nm with excitation at 386 nm. The “delay” time used was typically 0.06 ms and the “gate” time 2 ms; instrument slits were set at 10 nm throughout this study.

237

RESULTS

AND DISCUSSION

Spectral characterization of the complex Aluminium reacts with 8quinolinol and its derivatives in different micellar media to yield highly fluorescent complexes. The analytical characteristics and the mechanisms of surfactant action have been described [ 161. Among all the complexes assessed, it was observed that only the complex formed with Ferron displayed MS-RTP. In the emission spectra of the aluminium-Ferron complex in a CTAB micellar medium (Fig. 1) , a delayed fluorescence band was observed at 490 nm at short delay times (0.01-0.03 ms) but it disappeared at longer delay times ( 2 0.04 ms). The phosphorescence band observed at 586 nm remained. The maximum excitation wavelength was always 386 nm. The optimum pH for phosphorescence emission was 5-6 (Fig. 2); a value of 5.5 was selected for further studies. Influence of surfactant concentration During a systematic survey of micellar media, it was found that the charge sign of the micelle relative to the negative charge on the aluminium chelate was particularly significant: zwitterionic > cationic > non-ionic surfactants allowed the production of MS-RTP, but anionic micelles were not useful. Elec-

Fig. 1. Photoluminescence spectra of the aluminium-Ferron complex at pH 4.8 in CTAB micelles at different “delay” times: (0 ) 0.03 ms; (0 ) 0.04 ms; (-) 0.06 ms. Conditions: gate time 2 ms; scale X 1; 3.7 X 10m5 M Al, 1 X 10m4 M Ferron, 0.1 M CTAB, 9 X 1O-3 M Na$lO,; A,, = 386 nm; i,, = 566 nm. Spectra corrected for source and detector.

238

n50

q 2

4

6

8

PH

Fig. 2. pH effect on the phosphorescence of the aluminium-Ferron other conditions as in Fig. 1.

complex. Delay time 0.06 ms;

Fig. 3. Influence of CTAB concentration: (0 ) 1.85 X 10e5 M Al, 7.1 X 10m5 M Ferron; (0 ) 3.70 X lop5 M Al, 1.42 X 10d4 M Ferron. Conditions: lo-’ M NaxSOs; a,,=386 nm, a,,=586 nm; gate time 2 ms, delay time 0.06 ms. The arrow indicates the critical micelle concentration (1.3 x 10e3 M).

trostatic repulsions between the micellar surface and the negatively charged complex (through the -SO, group) probably prevent association in this last case. Although the use of the zwitterionic micelles resulted in a slightly greater phosphorescence the cationic surfactant CTAB was selected for the rest of the studies because it is easily available, inexpensive and its physical parameters (critical micelle concentration, cloud point, Kraft point, etc.) are well documented [ 171. The surfactant concentration is one of the most important factors that must be controlled in order to observe RTP in solution. The experimental results plotted in Fig. 3 clearly demonstrate that only at surfactant concentrations well above the critical micelle concentration for CTAB (1.3 x 10e3 M) [ 181 was the aluminium complex in the triplet state effectively protected from dissolved quenchers. The chemical composition of the surfactant had a marked effect on the development of MS-RTP. Alkylpyridinium polar head groups caused a decrease in the phosphorescence, while alkylammonium groups did not. These effects correlate well with findings in micelle-enhanced metal chelate fluorescence [ 191, and therefore the quenching properties of the pyridinium ring might act in a similar way on both luminescence emissions. Oxygen exclusion A sodium sulphite solution was used as oxygen scavenger [ 121, a concentration about 10m2 M efficiently removed oxygen from the micellar solution. Lower

239

sulphite concentrations were ineffective in that the residual oxygen quenched phosphorescence, while higher sulphite concentrations (above 2 x 10v2 M) decreased the emission (probably by the salt acting as a static quencher in solution ) . It was surprising that, when sulphite was used as the chemical oxygen scavenger for developing the MS-RTP of the niobium-Ferron complex, preirradiation with sunlight was necessary to obtain a high and stable phosphorescent signal [ 111. This light effect was not observed for the aluminium complex. Kinetic studies [20,21] demonstrated that the sulphite-oxygen reaction is trace-metal catalyzed. Because the niobium stock solutions were prepared with tartaric acid to keep the metal in solution, the excess of tartaric acid could complex those metal traces present in solution, thus hindering their catalytic effect on the sulphite-oxygen reaction. The p-rosaniline spectrophotometric method [ 221 was used to determine the sulphite-oxygen reaction rate. The results (Fig. 4) show that tartaric acid markedly decreases the rate. In the presence of aluminium, the slope of the plot is slightly greater than in its absence (Fig. 4)) which indicates a catalytic action of the metal ion on the deoxygenation reaction. The reaction between oxygen and sulphite in the presence of niobium and tartaric acid must proceed by some thermal or photo-induced (or both) pathway, explaining the effect of exposure to sunlight [ 111.

Fig. 4.Rateof the sulphite-oxygen reaction: (_k) aqueous solution; (0 ) 3.7 x 10m5 M Al; (0 ) 0.1% (w/v) tartaric acid; (A) 1.O7X1O-6 M Nb(V), 0.1% tartaric acid. In all cases, 9X10m3 M NazSO, and Labs= 560 nm. Fig. 5. Influence of Ferron concentration for 1.85 X 10e5 M aluminium at different delay times: ( X )0.05 me; (0 ) 0.2 ms. Gate time 0.5 ms; other conditions as for Fig. 1.

240

Heavy atom effect The effect of “heavy atoms” to facilitate population of the triplet state is well known. In the present system it is possible that the heavy atoms were provided by the bromide ions of the surfactant. However, use of the chloride form of the surfactant showed no change of the phosphorescence intensity of the complex. Moreover, non-ionic surfactants unable to provide heavy atoms were also tested. The same phosphorescence emission could still be measured. These results suggest that the internal heavy atom (iodine ) in the Ferron molecule seems to be responsible for promoting the intersystem crossing in the aluminium complex. It is worth noting that, unexpectedly, these results do not parallel those obtained with niobium [ 111. The niobium-Ferron-CTAB system did not produce detectable phosphorescence unless an additional heavy atom, e.g., as bromoform, was added. As niobium is a transition metal, one would have expected a higher degree of intersystem crossing because the ligand field splits the d-levels of the metal and those of adequate symmetry could combine with ligand orbitals to produce additional states and thus additional couplings. Similar deviations from expected behaviour have been reported for the low-temperature phosphorescence of some porphyrin complexes [ 21. The discrepancies were ascribed to other factors such as metal electronegativity, electronic configuration and metal ion size and its effect on geometrical distortion of the complexes, producing “internal quenching” of the triplet state. Influence of other factors Because the use of too large a reagent concentration could result in an inner filter effect on the phosphorescence, the optimum dye concentration was estimated. The results (Fig. 5) showed that a 5-fold molar excess of dye over aluminium gives maximum phosphorescence. Application of the continuous variations and mole ratio methods to these phosphorescence measurements at 586 nm demonstrated that the analytically useful complex has a metal ligand ratio of 1:3. These results are in good agreement with those obtained spectrophotometrically for the aluminium-Ferron complex in CTAB micelles by Goto et al. [23]. Complete phosphorescence development at room temperature took place in 10 min and remained constant for at least 12 h in stoppered containers. The emission increased as the temperature was diminished, until surfactant crystallization occurred at about 10°C. Therefore, a constant temperature of 22 +-2 ’ C was fixed for all experiments reported here. The order in which the reagents are mixed proved to be unimportant; however, in order to avoid premature sulphite oxidation, it is desirable to add this reagent last. The ionic strength, studied by addition of increasing amounts of sodium chloride (up to 0.7 M), had no influence on the phosphorescence of the aluminium complex. The influence of common anions and about thirty cations on the phospho-

241 TABLE I Maximum tolerance limit for various ions in the determination of 10 pg of aluminium” Added species Li,K,Mg,Ca,Sr,Ba As(II1) Sb(II1) Bi Sn(IV) Pb Cr(II1)

Aluminium/ion ratio (w/w) 1:lOO 1:lOO 1:l 1:l 1:lOO 1:lOO 1:l

RPI”

Added species

Aluminium/ion ratio (w/w)

RPI”

100.0 100.0 106.0 99.0 93.5 98.4 112.4 99.0

Cr(V1) Mo(V1) W(V1) Mn(I1) Ag Cd,Hg(II) Cu(II)b,Znb

1:l 1:l 1:lOO 1:lOO 1:lOO 1:lOO 1:lOO

98.0 100.0 101.5 112.7 103.4 100.0 100.0

“Relative phosphorescence intensity (A,,= 386nm, i,, = 586 nm). bA lOO-fold amount of EDTA overcame the interference.

rescence intensity of the aluminium complex was examined. Titanium(IV), V(V),Nb(V),Ta(V),Fe(II),Fe(III),Co(II) andNi(I1) interferedatalllevels tested. Nitrate, sulphate, phosphate, fluoride, iodide, silicate, borate, tartrate, oxalate and EDTA did not interfere even in a lOO-fold amount relative to 1 ,ug ml-’ aluminium. Citrate interfered at 3 lOO-fold excess. The effects of various metal species are summarized in Table 1, where the tolerance levels are given. A slight excess of Ferron was used in order to minimize reagent consumption in these experiments. Because aluminium is an impurity in the various salts, slight increases in phosphorescence were obtained when high concentrations of other ions were tested. The niobium chelate phosphorescence was more sensitive to foreign quenchers [ 113 than that of the aluminium chelate. Analytical figures of merit The calibration graph obtained under the optimized conditions had a linear range up to 500 ng ml-’ aluminium. The resulting linear least-squares fit has a regression coefficient of 0.998 (n= 11). The detection limit using the 20s criterion (on being the standard deviation of the blank) was found to be 5.4 ng ml-‘. The relative standard deviation for measurements of the phosphorescence intensity of ten replicates each containing 20 ng ml-’ aluminium was 4.5%. The triplet lifetime of the complex was found to be 0.182 2 0.006 ms at 25’ C measured for ten replicates of 10 lug of aluminium. Determination of aluminium in waters and haemodialysis fluids Tap waters from different urban areas and haemodialysis fluids were analyzed for aluminium by the proposed RTP method. Tap water samples were

242 TABLE 2 Phosphorimetric determination of aluminium in tap waters (T.W. ) and haemodialysis fluids (H.F.) Sample

T.W.l T.W.2 T.W.3 T.W.4 H.F.l H.F.2

Al found” (ng ml- ’ ) GF/AASb

Phosphorimetry

182.0 f 3.6 164.6 + 0.4 69.9 + 0.3 20.1+ 1.4 181.2 + 2.8 156.4 + 5.3

176.3 + 1.3 162.4 f 2.3 68.9 f 1.0 19.5kO.8 187.4 f 4.6 158.8 f 1.8

“Mean + standard deviation of three separate analyses. bGraphite-furnace spectrometry.

atomic absorption

collected in polyethylene bottles and acidified with concentrated nitric acid (ca. 1.5 ml 1-l). In order to allow maximum sample aliquots (8 ml of tap water or l-2 ml of haemodialysis fluids), most reagents were mixed together prior to use in order to obtain a “concentrated reagent solution” (0.035 g of Ferron and 34 g of CTAB dissolved in 500 ml of the 1 M acetate buffer, pH 5.5), and 1.75 ml of this solution was used in the general procedure. To avoid premature oxidation, sulphite was not added to the concentrated reagent solution but was introduced to the sample immediately before dilution to the mark. All sample analyses were done in triplicate and the values were corrected for the blank. The results, shown in Table 2, agree satisfactorily with those obtained by graphite-furnace atomic absorption spectrometry. The authors are pleased to acknowledge the assistance of J. Perez Parajon for the atomic absorption analyses and P.L. Martinez Garcia for the kinetic measurements. They also thank the Comision Asesora Cientifica y TQcnica (CAICYT) for financial support.

REFERENCES 1 2 3 4 5 6 7 8

R.S. Becker and J. Allison, J. Phys. Chem., 67 (1963) 2662. R.S. Becker and J. Allison, J. Phys. Chem., 67 (1963) 2669. R.S. Becker and J. Allison, J. Phys. Chem., 67 (1963) 2675. I. Singh and R.S. Becker, J. Am. Chem. Sot., 82 (1970) 2082. R. Livingston, W. Watson and J. McArdIe, J. Am. Chem. Sot., 71 (1949) 1542. G. Crosby, R. Whan and R. Alire, J. Chem. Phys., 34 (1961) 743. W.E. Ohnesorge and L.B. Rogers, Spectrochim. Acta, 14 (1959) 27. L.J. Cline Love, M. Skrilec and J.G. Habarta, Anal. Chem., 52 (1980) 754.

243 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23

S. Scypinsky and L.J. Cline Love, Anal. Chem., 56 (1984) 322. L.J. Cline Love and R. Weinberger, Spectrochim. Acta, Part B, 38 (1983) 1421. A. Sanz-Med&l, P.L. Martinez Garcia and M.E. Diaz Garcia, Anal. Chem., 59 (1987) 774. M.E. Diaz Garcia and A. Sanz-Medel, Anal. Chem., 58 (1986) 1436. P. Galle, Recherche, 61 (1986) 856. T.M. Florence, Trends Anal. Chem., 2 (1983) 162. J.W. Robinson and P.M. Deano, Int. Lab., 16 (1986) 14. A. Sanz-Medel, R. Fernindez de la Campa and J.I. Garcia Alonso, Analyst, 112 (1987) 493. J.H. Fendler, in Membrane Mimetic Chemistry, Wiley, New York, 1982. L.J. Cline Love, J.G. Habarta and J.G. Dorsey, Anal. Chem., 56 (1984) 1132A. A. Sanz-Medel, J.I. Garcia Alonso and E. Blanc0 Gonzhlez, Anal. Chem., 57 (1985) 1681. A. Huss, Jr., P. Kong Lim and C.A. Eckert, J. Am. Chem. Sot., 100 (1978) 6252. D.B. Hobson, P.J. Richardson, P.J. Robinson, E.A. Hewitt and I. Smith, J. Chem. Sot., Faraday Trans. I, 82 (1986) 869. P.W. West and G.C. Gaeke, Anal. Chem., 28 (1956) 1816. K. Goto, S. Taguchi, K. Miyabe and K. Haruyama, Talanta, 29 (1982) 569.