Journal of Analytical Chemistry, Vol. 56, No. 2, 2001, pp. 178–181. Translated from Zhurnal Analiticheskoi Khimii, Vol. 56, No. 2, 2001, pp. 201–205. Original Russian Text Copyright © 2001 by Nadzhafova, Lagodzinskaya, Sukhan.

ARTICLES

Test Paper for the Determination of Aluminum in Solution O. Yu. Nadzhafova, S. V. Lagodzinskaya, and V. V. Sukhan Chemistry Department, Shevchenko National University, Kiev, Vladimirskaya ul. 64, Kiev, 252033 Ukraine Received July 14, 1999; in final form, January 25, 2000

Abstract—A test paper for the determination of aluminum(III) was developed on the basis of chromatographic cellulose paper impregnated with Chrome Azurol S. The test paper was applied to monitoring the concentration of aluminum(III) in potable and natural water within the range 0.05–0.6 mg/L using diffuse reflectance spectroscopy or visual detection. Chromaticity measurements were used to construct a visual test scale.

To determine toxic elements in water, screening methods are used, which assure the rapid detection of toxic substances in amounts greater than their maximum permissible concentrations (MPCs) [1–3]. Visual test methods [4, 5] are among the simplest techniques for the rapid monitoring of metal ions in environmental samples at a level of maximum permissible concentrations and lower. Aluminum should be set off from the toxic metal ions, because its elevated concentrations in water cause poisoning of human body, especially the nervous system and brain [6]. Paper chromatography with development by Eriochrome cyanine R [7] or morin [8] is used for the rapid determination of aluminum. However, the detection limit is 2 mg/L, which is ten times higher than the MPC for aluminum in water according to toxicology [2]. Sensor filters with adsorption preconcentration by an ultrafiltration equipment were proposed for the determination of aluminum traces [9]. However, the method requires special equipment and is hardly applicable for in-the-field analysis. Test papers [5, 10–12] were proposed for the visual test determination of aluminum. The most sensitive paper was prepared from filter paper impregnated with aluminon [5]. Interferences from iron(II, III), copper(II), and beryllium(II) are eliminated by previously precipitating these ions with sodium hydroxide. The detection limit for aluminum(III) with the use of this paper is 5 mg/L, which is 20 times higher than its maximum permissible concentration in potable water according to toxicology. Chrome Azurol S is known to be one of the most sensitive photometric reagents for aluminum(III) [13]. Modification of the reagent with cationic or nonionic surfactants substantially improves the spectral characteristics of the complex and reduces the detection limit for aluminum to 1–2 µg/L [13–15]. One may believe that the advantages of the adsorption preconcentration of metal traces and the test determination of the metal in the adsorbent phase can be combined by immobilizing Chrome Azurol S on the surface of a solid matrix, in particular, cellulose.

Therefore, the goal of this paper was to impregnate chromatographic cellulose paper with Chrome Azurol S, to study the properties of this reagent on the paper surface, and to develop a procedure for the rapid test determination of aluminum(III) in water. EXPERIMENTAL We used the following chemically pure reagents: a 0.05% aqueous solution of Chrome Azurol S (Merck); a 0.2% solution of 1,10-phenanthroline (Chemapol); 20% solution of hexamethylenetetramine; 0.1 mg/mL standard solutions of aluminum(III), copper(II), and iron(III); a ≈10–3 M solution of the OP-10 nonionic surfactant; twice-distilled water; and Filtrak FN-16 cellulose chromatographic paper (12 mg/cm2 density, 0.2 mm thick). Diffuse reflectance spectra were recorded by a Specord M-40 spectrophotometer; the acidity was measured by a pH-673 M pH-meter. The test paper was produced by successively treating the chromatographic paper with aqueous solutions of 1,10-phenanthroline (0.2%), Chrome Azurol S (0.05%), and hexamethylenetetramine (20%). The paper was dried for 30 min in a desiccator at 70°C and cut into squares 10 mm on a side. An aliquot portion of the test solution with a given pH was pipetted onto a paper square. After 5-min air drying, the diffuse reflectance of the paper was measured, or its color was compared with a standard scale constructed under similar conditions with the use of standard aluminum solution. To obtain a visual test scale, chromaticity measurements were used [16, 17]. The range and color characteristics of the scale were calculated on the basis of the measured diffuse reflectance spectra for a light source C in the CIE uniform color system (1976) using converted color coordinates (a and b) and luminosity (L) [16]. The color difference in luminosity (∆L) and the total color difference (∆E) were calculated by the following equations: ∆L = L0 – L; ∆E = [(L0 – L)2 + (a – a0)2 + (b – b0)2]1/2, where L, a, b, L0, a0, and b0 are the color coordinates of the sample

1061-9348/01/5602-0178$25.00 © 2001 åAIK “Nauka /Interperiodica”

TEST PAPER FOR THE DETERMINATION OF ALUMINUM IN SOLUTION

in study and a reference sample, respectively. A paper impregnated with Chrome Azurol S was used as a reference sample. To verify the test procedure developed, the photometric determination of aluminum(III) with Chrome Azurol S and OP-10 was used [15].

A(1, 2); R(3) 0.4 0.3

2 3

0.2

RESULTS AND DISCUSSION Figure 1 shows the absorption spectra of Chrome Azurol S in solution with and without OP-10 and the diffuse reflectance spectra of the reagent on the paper surface. One can see that, in the presence of OP-10, the Chrome Azurol S absorption band broadens, and a small bathochromic shift is observed as compared to the aqueous solution of the reagent. The shift is also observed in the diffuse reflectance spectrum of Chrome Azurol S immobilized on a paper surface. This can be attributed to the greater stabilization of the H2R2– form of the reagent (λmax = 480 nm) on the cellulose surface at pH 6. Similarly, this form is more stabilized in a surfactant solution as compared to the HR3– form (λmax = 420 nm), which prevails in an aqueous solution of Chrome Azurol S at this pH [18]. These data can point to the fact that the active groups of cellulose and the surfactant similarly affect the state of the reagent, which agrees with the features of functional groups of cellulose discussed earlier [19]. The obtained results are of practical interest, because they make it possible to improve the contrast and sensitivity of complexation of Chrome Azurol S with metal ions on the adsorbent surface without any additional reagents. Figure 2 shows the absorption spectra of aqueous solutions of Al–Chrome Azurol S complex with and without OP-10 and the diffuse reflectance spectrum of the complex on the cellulose surface. One can see that the absorption spectrum of the complex in the presence of OP-10 and the diffuse reflectance spectrum on the cellulose surface also exhibit bathochromic shifts relative to the absorption spectra of an aqueous solution of the complex. Therefore, the obtained data suggest that the functional groups of the nonionic surfactant and cellulose exert a similar effect on the complexing capacity of the reagent and point to the utility of the cellulose paper impregnated by Chrome Azurol S for the rapid test determination of aluminum(III) traces in water. Best conditions for paper impregnation and aluminum determination. Studying the order of treating the paper by reagent solutions revealed that the best conditions are as follows: first, the chromatographic paper is immersed into a 0.2% solution of 1,10-phenanthroline; next, it is impregnated with a 0.05% Chrome Azurol S solution; and finally, it is treated with a 20% solution of hexamethylenetetramine to provide the best pH (pH = 6) for the complexation of Al(III) with Chrome Azurol S. An increase or a decrease in the concentration of Chrome Azurol S does not improve the JOURNAL OF ANALYTICAL CHEMISTRY

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0.1 0 400

1 450

500

550

600

650

700 λ, nm

Fig. 1. Absorption spectra of Chrome Azurol S in solution (1) without OP-10 and (2) with OP-10 and (3) the diffuse reflectance spectrum of Chrome Azurol S on the surface of cellulose paper (cChrome Azurol S = 4 × 10–5 M for curves 1 and 2; cOP-10 ~ 6 × 10–4 M for curve 2; pH = 6).

A(1, 2); R(3) 1.0 0.8

2 3

1

0.6 0.4 0.2 0 440

490

540

590

640

690 λ, nm

Fig. 2. Absorption spectra of Al–Chrome Azurol S complex in solution (1) without OP-10 and (2) with OP-10 and (3) the diffuse reflectance spectrum of the complex on the surface of cellulose paper (cAl = 1 × 10–5 M, cChrome Azurol S = 4 × 10–5 M for curves 1 and 2; cOP-10 ~ 6 × 10–4 M for curve 2; pH = 6).

sensitivity of aluminum determination, because the color of the blank test changes similarly, thus decreasing the analytical range. The best pH of the test solution is 1–2 as adjusted by HCl. At higher pH, Al(III) is not fully bound by Chrome Azurol S because of hydrolysis. At lower pH, the buffer capacity of a hexamethylenetetramine solution is insufficient for providing the best pH for complexation on the paper surface. Studying how the volume of the test solution applied to the paper affects the color intensity of the spot, we have found that 40 µL is enough for obtaining reproducible results. When the sample volume is more than 60 µL, the spot becomes partially smeared. The best time for keeping the test paper after applying the test solution is 5 min, which is enough for the most intense color to develop.

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–2

–1

interfere with the determination of aluminum with the use of this test paper.

1

2

2

b 0 –2

1

2 3

3

4 a

4

–4 –6 5 6

–8 –10

Fig. 3. The change in the chromaticity coordinates a and b of the cellulose paper impregnated with Chrome Azurol S as a function of Al(III) concentration in solution. cAl = (1) 0, (2) 0.05, (3) 0.1, (4) 0.2, (5) 0.4, and (6) 0.6 mg/L.

The following ions do not interfere with the determina3– tion of 10 µg Al(III): 20-fold amounts of P O 4 , 20002–

fold amounts of S O 4 , 5000-fold amounts of Cl–, 10 000-fold amounts of alkali metals (Na+ and K+), 200-fold amounts of Ca2+, and 2000-fold amounts of Mg2+. Among the heavy metals present in potable and natural water in concentrations comparable with those of aluminum(III), Fe(III) and Cu(II) interfere with the Chrome Azurol S determination of Al(III) at the best pH [13–15]. Their interference was eliminated by 1,10-phenanthroline, which forms stable complexes with Fe(III) and Cu(II) [20]. We showed that impregnating the paper with a 0.2% solution of 1,10-phenanthroline and adding a 0.01% solution of this reagent to the solution in study completely eliminates the interference effects of 0.5 mg/L Fe(III) and 1 mg/L Cu(II), that is, the maximum permissible concentrations of these metals in potable water [2]. Fluoride ions (1 mg/L)

The obtained test paper was used for the determination of aluminum(III) in water by diffuse reflectance spectroscopy and by visual test method. The detection limit for Al(III) by diffuse reflectance spectroscopy is 0.02 mg/L. The calibration plot is linear in the range 0.05–0.6 mg/L, which comprises 0.25–3 MPCs for Al(III) in water. The equation of the calibration plot is 645 R rel = 0.324x(cAl mg/L) + 0.013. To optimize the range of the scale for the visual test determination of aluminum(III), we used chromaticity measurements. With the use of these measurements, the color perceived by a human eye can be described objectively and the range and points of the visual scale can be to optimized [16, 17]. For this purpose, the color of the complex formed on the surface of the test paper was identified by chromaticity coordinates (a, b, L). Our calculations showed that the linearity range for coordinates a and b lies within 0.05–0.6 mg/L of aluminum(III) (Fig. 3). The color difference in luminosity (∆L) is directly proportional to the analyte ion concentration in solution in the same range. The straight-line equation is ∆L = 12.373x(cAl mg/L) + 0.047. The test scale for the visual determination of aluminum(III) was constructed within the linearity range for the chromaticity coordinates a and b using standard Al(III) solutions with concentrations of 0, 0.05, 0.1, 0.2, 0.4, and 0.6 mg/L. The total color difference (∆E) changed in this concentration range with the step ∆E = 5–7 depending on the concentration of the analyte. The detection limit for aluminum by the visual test method is 0.05 mg/L, which is 0.25 MPC for Al(III) in potable water. The test scale is stable for one month. We also used a simulated test scale constructed in CorelDRAW on the calculated (a, b, L) coordinates. 3–

Determination of Al(III) with the test paper (I) in model solution containing 0.5 mg/L Fe(III), 1 mg/L Cu(II), 1 mg/L PO 4 , 2–

–

300 mg/L Cl–, 100 mg/L SO 4 , and 100 mg/L HCO 3 ; (II) in potable water; and (III) in lacustrine water by diffuse reflectance spectroscopy and by visual test method (n = 5, P = 0.95) Al(III), mg/L Type of water

I ″ ″ II ″ ″ III ″

found x ± ∆x added

diffuse reflectance spectroscopy

RSD, %

visual test method

RSD, %

0.1 0.2 0.4 0* 0.1 0.2 0** 0.1

0.12 ± 0.02 0.21 ± 0.03 0.37 ± 0.02 0.19 ± 0.03 0.28 ± 0.03 0.40 ± 0.04 0.14 ± 0.03 0.24 ± 0.03

13 12 5 13 9 8 17 10

0.10 ± 0.025 0.20 ± 0.05 0.40 ± 0.05 0.20 ± 0.05 0.30 ± 0.05 0.40 ± 0.05 0.10 ± 0.03 0.25 ± 0.05

20 20 11 20 13 10 24 16

Found by photometry: * 0.21 ± 0.01 mg/L, ** 0.12 ± 0.01 mg/L. JOURNAL OF ANALYTICAL CHEMISTRY

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Determination of Al(III) in potable and natural water with the use of the test paper. Hydrochloric acid (0.1 mL of 1 M solution) and 0.25 mL of 0.2% aqueous solution of 1,10-phenanthroline are added to 5 mL of water containing 0.05–0.6 mg/L Al(III). Next, 40 µL of the obtained solution is pipetted onto a square of the test paper. After 5 min, the diffuse reflectance of the test paper at λ = 625 nm is measured, and the concentration of aluminum(III) is determined by a calibration plot constructed under similar conditions using a standard solution. In visual test determination, the color of the test paper is compared to the test scale. The results of the determination of aluminum in model solutions, potable, and lacustrine water by diffuse reflectance spectroscopy and by visual test method are given in the table. The concentration of aluminum in potable and lacustrine water was also determined by photometry using the reaction with Chrome Azurol S and OP-10 [15]. One can see that the results obtained are characterized by a reasonable accuracy and precision. The procedure can be used for the determination of the most reactive species of aluminum(III) in water like hydroxo complexes and complexes with inorganic ligands. To determine aluminum bound by organic ligands with many binding sites, fulvic and humic acids, organic compounds in the analyzed sample should be oxidized. If the concentration of humic and fulvic acids in water is 0.5–1 mg/L, boiling the sample with K2S2O8 for 10–15 min is sufficient [13]. Therefore, the obtained test paper can be used for the test determination of aluminum in various waters in the range 0.25–3 MPCs. REFERENCES 1. Maistrenko, V.N., Khamitov, R.Z., and Budnikov, G.K., Ekologo-analiticheskii monitoring superekotoksikantov (Environmental Analytical Monitoring of Supertoxicants), Moscow: Khimiya, 1996. 2. Fomin, G.S., Voda. Kontrol’ khimicheskoi, bakteriologicheskoi i radiatsionnoi bezopasnosti po mezhdunarodnym standartam. Entsiklopedicheskii spravochnik (Water: Chemical, Bacteriological, and Radiation Safety Control According to International Standards. Encyclopedic Reference Book), Moscow: Gosstandart Rossii, 1995.

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3. GOST (State Standard) 2874-82: Potable Water: Methods of Analysis, 1984. 4. Testovye metody analiza vod (Standard Methods of Water Analysis), Kravchenko, M.S. and Osyka, V.F., Eds., Moscow: SEV, 1990. 5. Jungreis, E., Spot Test Analysis. Clinical, Environmental, Forensic, and Geochemical Application, Chemical Analysis: A Series of Monographs on Analytical Chemistry and Its Applications, New York: Wiley–Interscience, 1985, vol. 75. 6. Concepts for Metal Ion Toxicity, Ziegel, X. and Ziegel, A., Eds., New York: Decker, 1986. Translated under the title Nekotorye voprosy toksichnosti ionov metallov, Moscow: Mir, 1993. 7. Lewandowski, A. and Chwiola, A., Zesz. Nauk. Uniw. Poznaniw., 1966, no. 28, p. 15. 8. Ader, D. and Primov, M., Analyt. Chim. Acta, 1964, vol. 31, no. 2, p. 191. 9. Materialy Mendeleevskogo s’’ezda po obshchei i prikladnoi khimii (Proc. Mendeleev Congress on General and Applied Chemistry), St. Petersburg, May 23–25, 1998, vol. 5, p. 319. 10. Japan Patent 18 298, 1964. 11. Cartine Reattive per la Ricera Ionica “Carlo Erba,” Milano, 1970. 12. Merck. Analytical Test Kits, Darmstadt: Merck, 1992– 1993. 13. Pilipenko, A.T., Safronova, V.G., and Falendysh, N.F., Khim. Tekhnol. Vody, 1994, vol. 16, no. 4, p. 344. 14. Savranskii, L.I. and Nadzhafova, O.Yu., Zh. Anal. Khim., 1992, vol. 47, no. 9, p. 1613. 15. Nemodruk, A.A., Arevadze, N.G., and Supatashvili, G.D., Zh. Anal. Khim., 1980, vol. 35, no. 8, p. 1511. 16. Kirillov, E.A., Tsvetovedenie (Chromatics), Moscow: Legprombytizdat, 1987. 17. Morozko, S.A. and Ivanov, V.M., Zh. Anal. Khim., 1997, vol. 52, no. 8, p. 858. 18. Shtykov, S.N. and Sumina, E.G., Zh. Anal. Khim., 1997, vol. 52, no. 7, p. 697. 19. Adsorption from Solution on the Solid/Liquid Interface, Pafitt, G. and Rochester, C., Eds., London: Academic, 1983. Translated under the title Adsorbtsiya iz rastvorov na poverkhnosti tverdykh tel, Moscow: Mir, 1986. 20. Lur’e, Yu.Yu., Spravochnik po analiticheskoi khimii (Handbook of Analytical Chemistry), Moscow: Khimiya, 1989.

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