Journal of Analytical Chemistry, Vol. 58, No. 10, 2003, pp. 990–994. Translated from Zhurnal Analiticheskoi Khimii, Vol. 58, No. 10, 2003, pp. 1104–1108. Original Russian Text Copyright © 2003 by Gur’eva, Savvin.
ARTICLES
A Test Method for the Determination of Mercury(I, II) on a Polymer Surface R. F. Gur’eva and S. B. Savvin Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, ul. Kosygina 19, Moscow, 119091 Russia Received June 11, 2001; in final form, October 23, 2002
Abstract—It was found that mercury(I, II) reacts with 5-chloro-2-hydroxy-3-[(tetrahydro-2,4-dithioxo-1,3-thiazin-5-yl)azo]benzenesulphonic acid (tyrodine) in weakly acid solutions in the presence of acetates, formates, and some organic solvents (ethanol and acetone) to give an intensely colored insoluble compound (λmax = 600 nm, ε > 3 × 104). The composition of the solution affected the sensitivity of the color reaction between mercury and tyrodine. A test procedure was developed for the determination of mercury(I, II) on the surface of a polycaproamide membrane with a detection limit of <50 ng in the adsorbent zone. The procedure involves the quantitative and selective adsorption of the mercury complex of tyrodine on the support surface from 0.5 M acetate solutions of pH 3–4 in the dynamic mode; the sample volume was 5–25 mL. Mercury was then determined by the change in the support color using a color scale or by the change in diffuse reflectance at 600 nm. The adsorption of the mercury complex was accompanied by an abrupt change in the color of the support surface from pale crimson (reagent) to stable blue-violet. The test procedure was used in the analysis of an industrial sample of complex composition with a mercury concentration of lower than 0.05%. The determination error (relative standard deviation) was 20%.
Azo compounds synthesized from rhodanine and thiopropiorhodanine at the Vernadsky Institute of Geochemistry and Analytical Chemistry, Russian Academy of Sciences, were found to be promising reagents for determining not only noble metals, but also toxic heavy metals, including organic and inorganic mercury species [1–10]. 4[(3-Amino-4-oxo-2-thioxo5-thiazolidinyl)azo]phenol (p-phenolazo-3-aminorhodanine or FATAR) was proposed for the selective photometric determination of mercury(II) in concentrations higher than its MPC in different materials, including natural water and wastes from lead and zinc-producing plants [5, 6]. Rapid visual tests were proposed for the determination of inorganic and organic (methyl, propyl, and phenyl) mercury species using adsorption on plates coated with a thin adsorbent layer (Sorbfil silica gel immobilized with silica sol and Silufol silica gel immobilized with starch on glass or foil) and color reactions with sulfochlorophenolazorhodanine (5-chloro-2-hydroxy3-[(4-oxo-2-thioxo-5-thiazolidinyl)azo]benzenesulphonic acid, SCFAR), p-phenolazo-3-aminorhodanine, and benzeneazobenzeneazorhodanine (4-(2-thioxo-4oxo-5-thiazolidinyl)azo]benzene, BABAR) [7]. The detection limits in the adsorbent zone were 0.3–0.4 µg for inorganic mercury and phenyl mercury and 0.5 µg for methyl and propyl mercury. The advantages of azorhodanines as developers compared to, e.g., dithizone, which is applied for similar purposes, are in their selec-
tivity, the higher color stability of the resulting element complexes, and no necessity in using toxic organic solvents. p-Phenolazo-3-aminorhodanine immobilized on a polyacrylonitrile fiber filled with a finely dispersed KU-2 cation exchanger was proposed for the rapid determination of mercury(II) in natural and waste waters in the concentration range (0.25–2.5) × 10−6 M (RSD = 3–6%) [8]. The detection limit was 0.02 µg/mL. The sensitivity of determining mercury using the immobilized reagent exceeded that attained for the reaction in solution by more than two orders of magnitude. The restricted sample volume because of the weak retention of the colored mercury complex on the support is a disadvantage of the procedure with the use of an immobilizing agent. Analysis in the dynamic mode is also complicated in this case. Mercury(I) gives no color reaction with immobilized p-phenolazo-3-aminorhodanine [8]. This work deals with the further study of the application of rhodanine derivatives in the analytical chemistry of mercury, namely, of the use of tyrodine [5chloro-2-hydroxy-3-(tetrahydro-2,4-dithioxo-1,3-thiazin-5-ylazo)benzenesulphonic acid], a six-membered analogue of sulfochlorophenolazorhodanine(II), for the dynamic adsorption preconcentration and rapid test determination of mercury(I) and mercury(II) or total mercury on the surface of a polymer support (polycaproamide membrane).
1061-9348/03/5810-0990$25.00 © 2003 åAIK “Nauka /Interperiodica”
A TEST METHOD FOR THE DETERMINATION OF MERCURY(I, II)
EXPERIMENTAL
A
The stock solutions of inorganic mercury(I) and mercury(II) species were prepared by the dilution of exact portions of Hg2(NO3)2 and metallic mercury, respectively, in nitric acid. The working solutions of mercury(I) and mercury(II) in 0.1 M HNO3 with mercury concentrations of 1–10 µg/mL were prepared by the dilution of the stock solutions. Tyrodine (85% of the major substance in the preparation) was obtained by the procedure described in [2]; 0.01–0.005% aqueous solutions of the reagent were used. A polycaproamide membrane, made of a microporous polymer material 0.1 mm thick with 0.45-µm pores (NPO Polimersintez, Vladimir), was used as the adsorbent. It was a white film with a smooth surface and contained no foreign impurities. The adsorption activity of the support was achieved at pH 1–5 [3, 4, 9–11]. The adsorbent was used as colored or uncolored disks 1 cm in diameter and 2.7 mg in weight. Tyrodine was immobilized on the support by immersing a support disk in a beaker with 20–30 mL of a 1 × 10−5 M tyrodine solution (pH 2–3) for 30 min. The disk was then removed from the solution, washed with water, dried between sheets of filter paper, and stored in a tightly stoppered dry flask. The weight of tyrodine immobilized on the support surface was ~1% of the disk weight. Apparatus. The absorption spectra of tyrodine and its mercury(I, II) complexes were recorded with a Specord UV VIS spectrophotometer. The required pH was adjusted using a pH-121 pH-meter–millivoltmeter. Experimental procedure. The adsorption of mercury on the support surface was studied by two methods, using tyrodine immobilized on the adsorbent or the mercury complex of tyrodine preliminary synthesized in solution. In the former case, colored support disks were placed in a flask (or a beaker) containing 5–25 mL of the test solution for 1–2 h at room temperature. In the latter case, the color reaction of mercury(I, II) with tyrodine was first performed in solution, and the resulting complex was adsorbed on the support surface without pretreatment. For this purpose, the support disk was slightly wetted with water, placed onto a glass filter, and fixed between the ground walls of a collector for test solutions and a glass filter. The solution of the mercury complex of tyrodine (5–25 mL) was passed at a rate of 5–10 mL/min and pressure of 10–20 mm Hg. The adsorption of tyrodine and its mercury(I, II) complex resulted in contrasting changes in the color of the supports disk. The disk was removed, placed between two filter papers and blotted carefully. After few minutes, the color change was estimated visually by comparison with a color scale or from the change in diffuse reflectance at 600 nm. The quantitative adsorption of the reagent and the mercury complex (95–98%) was achieved in the concentration range between 5 × 10−8 and 5 × 10−6 M from 5–25 mL of the solution during JOURNAL OF ANALYTICAL CHEMISTRY
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0.8
1
5 0.4
4 3 2
520
600
700 λ, nm
Fig. 1. Absorption spectra of (1) tyrodine and (2–5) its mercury complex (differential curves) at pH 3.5 [(3–5) in the presence of (3) 1, (4) 3, and (5) 9 mL of 2 M CH3COONa]; reagent concentration, 1 × 10−4 M; mercury concentration, 1.2 × 10−5 M.
several minutes, with the uniform coloration of the support surface. The concentration factor was 103–104. The recovery of the complex was controlled by the absence of the coloration of the filtrate or the surface of an additional disk through which the filtrate was passed. RESULTS AND DISCUSSION Depending on the reaction conditions, mercury(I) and mercury(II) react with tyrodine to form insoluble colored compounds differing in their spectral parameters. An orange compound (λmax = 470 nm, ε = 10 × 103) is formed in acid solutions (0.05–0.2 M HNO3, H2SO4), and a blue-violet compound (λmax = 600 nm, ε = 15 × 103) is formed at pH 3–5. The reactions proceed in the presence of excess reagent at room temperature. The initial form of inorganic mercury has no effect on the optical parameters of the resulting tyrodine complexes. It was found that the sensitivity of the color reaction significantly (more than twice) increases when the reaction performed at pH 3–5 in the presence of more than 10% of organic solvents (ethanol or acetone) or of acetates and formates (>0.2 M) (Fig. 1). Depending on the composition of the solution, the difference in the sensitivity of the color reactions of mercury(I, II) with tyrodine may be due to several factors: changes in the state of the reagent (as was found for the tyrodine–silver reaction in different acid solutions and in the presence of organic solvents [12]) and mercury; the formation of a mixed-ligand complex (it was noted earlier that tyrodine forms mixed-ligand complexes with silver in the presence of some macro-
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GUR’EVA, SAVVIN A 4
3
5 2 1
450
500
600 λ, nm
Fig. 2. Absorption spectra of tyrodine in (1–3) water and (4, 5) a water–acetone (1 : 1) solution; pH: (1, 4) 2.2, (2) 3.7, (3, 5) 4.1.
cyclic compounds with extremely high molar absorptivities [13, 14]); changes in the stoichiometric ligandto-mercury ratios; and solvation processes. Taking into account the similarity of the changes in the color of mercury complexes of tyrodine in the presence of acetates, formates, and organic solvents, we attempted to consider this fact in terms of the reagent behavior in different media. It was shown previously that, in the presence of organic solvents (acetone, ethanol, propanol, and acetic or formic acid), tyrodine is converted to a structure with λmax = 470 nm, which was not found in the pure form in aqueous solutions (Fig. 2) [12]. The study of the acid ionization of tyrodine by potentiometric titration in water and water–ethanol (1 : 1) solutions showed that the structure of this reagent corresponds to its neutral state or to that with the dissociated sulfo group. The resolution of the absorption spectra of tyrodine in weakly acid aqueous solutions into components allowed us to identify two reagent structures with λmax of 470 and 550 nm. Thus, organic solvents in acid and weakly acid solutions shift the tautomeric equilibrium of tyrodine toward the formation of a structure with λmax = 470 nm. A correlation was found between the yield of the silver–tyrodine complex and the yield of this structure, which in turn depends on the concentration of organic acids and solvents. A similar situation probably occurs in reactions of mercury with tyrodine in the presence of organic additives. The low solubility of mercury(I, II) complexes of tyrodine restricts the application of tyrodine for the photometric determination of mercury in solutions. At the same time, the use of tyrodine in the test method for
determining inorganic mercury(I) and mercury(II) or total mercury directly on the support surface seems well-founded. Along with an intense color and a low solubility of mercury–tyrodine complexes, it is important that the conditions of their formation (pH, temperature, and solvent) coincide with the conditions under which the polymer support exhibits adsorption activity. Mercury complexes of tyrodine are adsorbed from aqueous solutions in the presence of small amounts of organic solvents (e.g., in 0.2–1.0 M acetate solutions). A polycaproamide membrane was taken as the support because of its high adsorption capacity for the reagent and complex compared to other supports, e.g., to ion exchangers [8, 9]. Therefore, the analytical range of mercury can be extended. The reagent and complex are strongly retained on the membrane, and adsorption can be performed in the dynamic mode to ensure rapid determination. Finally, the adsorbent is nontoxic, thermally stable, non-explosive, and easy to store: adsorbent films or disks are kept in closed polyethylene flasks or bags; in practice, the adsorbent needs no additional preparation [9–11]. We studied the potentialities of two methods for adsorbing mercury from the test solutions on the adsorbent surface. The dynamic sorption of mercury as a tyrodine complex preliminarily obtained in solution was preferred because of the rapidity of determination. This was mainly due to the higher rate of formation of the mercury–tyrodine complex in solution compared to that in the adsorbent phase and the high rate of complex sorption on the support surface. The adsorption of the mercury complex is accompanied by an abrupt change in the support color from pale crimson (reagent) to intense blue-violet. The color is stable for several months. The colors of tyrodine and its mercury complex in the solid phase and solution are similar under the same conditions. Mercury complexes are selectively adsorbed from solutions (pH 3.5, 0.2 M CH3COONa, 5 ng/mL Hg) containing excess copper, vanadium, zinc, nickel, cobalt, rare earths, molybdenum (100-fold), cadmium (10-fold), 3– lead (5-fold), P O 4 , and Cl–. In the case of tyrodine immobilized on the adsorbent surface, the color reaction with mercury develops in time and, after an hour, the yield of the colored complex is no more than 60–70%. Taking into account the adsorbent properties, we used 0.2–0.5 M acetate solutions of pH 3–4 for the development of a test method for determining mercury; mercury was adsorbed under dynamic conditions as a tyrodine complex; the complex was first prepared in solution. The analytic characteristics of the test procedure for determining mercury(I, II) with tyrodine by a color scale or by a change in diffuse reflectance at 600 nm are given in the table, as well as those of the procedure for determining individual mercury species with rhodanine derivatives. The simple test procedure developed in this
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Analytical characteristics of procedures for determining inorganic and organic mercury with rhodanine and tyrodine derivatives in solution and in the solid phase Analyte
Method
Reagent
Adsorbent –
Hg(II)
Photometry
FATAR
Hg(II), phenyl mercury Phenyl mercury Methyl mercury, Cd, Hg(II), Cu(II)
Visual drop
FATAR Sorbfil
Visual drop BABAR Chromatography– BABAR photometry A 0.03% reagent in ethanol as an eluant Phenyl mercury, Chromatography– FATAR Cd, Hg, Cu photometry A 0.03% reagent in ethanol as an eluant Propyl phenyl Chromatography– SCFAR mercury, Cu photometry Hexane–ethanol (4 : 1) as an eluant Methyl mercury, Chromatography– FATAR Cd, Hg, Cu photometry Hexane–ethanol (2.5 : 1) as an eluant Methyl propyl phe- Chromatography– SCFAR nyl mercury, Hg photometry A 0.03% SCFAR solution as an eluant, pH 9 Hg(II) Sorption–photometry FATAR
Hg(I), Hg(II)
Sorbfil Sorbfil
Determination (detection) conditions
Analytical range, source
7–15 M H3PO4 7–10 M H2SO4 A 0.03% reagent solution in 2–4 M H3PO4 A 0.03% reagent solution, pH 7 By complex colors; Rf : methyl mercury, 0.9; Cd, 0.45; Hg and Cu, 0.0
1–10 µg/mL (V = 25 mL) [5, 6]
Silufol
by complex colors; Rf : phenyl mercury, 0.8; Cd, 1.0; Hg and Cu, 0.0
Sorbfil
0.03% SCFAR, pH 9; Rf : methyl mercury, 0.55; propyl mercury, 0.85; phenyl mercury and Cu, 0.0
Sorbfil
0.03% FATAR solution; Rf : methyl mercury, 0.98; Cd, 0.75; Hg and Cu, 0.7
Sorbfil
By complex colors; Rf : methyl mercury, 0.9; propyl mercury, 0.45; phenyl mercury and Hg, 0.0
KU-2 cation pH 1 (H2SO4) exchanger pH 1.3 (H3PO4)
Sorption–photometry Tyrodine Polycaproa- pH 3–4 (test method) mide membrane
10–1000 ng in the adsorbent zone (V = 0.01–0.04 mL) [7]
0.25–2.5 µM, 0.02 µg/mL (V = 10–100 mL) [8] 0.2–10 µM, <5 ng/mL (V = 10–25 mL)*
* This work.
work can determine both individual inorganic mercury(I, II) species and their sum with equal sensitivities; in some cases, it exceeds the procedures for determining mercury with rhodanine derivatives in sensitivity and speed. The procedure developed was used for testing complex industrial samples containing lower than 0.05% mercury (RSD = 20%). The accuracy of the procedure was evaluated using the standard addition method. Analysis of industrial samples. A 25-mg portion was placed in a 25-mL glass beaker, and 4 mL of conc. HNO3 was carefully added dropwise; the mixture was stirred for several minutes, heated to 80°C for 15–20 min until complete decoloration, and cooled. The solution was filtered through a grass filter and washed; the filtrates were combined, transferred in a 25-mL flask, and diluted with water. A series of solutions was prepared as follows. Portions of 0 to 0.4 mL (with a step of JOURNAL OF ANALYTICAL CHEMISTRY
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0.05 mL) were placed in 10-mL graduated test tubes and treated with 0.4 to 0 mL (with a step of 0.05 mL) of 0.04 M NHO3, respectively; next, several milliliters of water and 2 mL of 1 M sodium acetate (pH 3.5) were added. The mixtures were shaken, treated with 0.2 mL of a 0.01% tyrodine solution, and diluted with water. The resulting solutions were passed through membrane disks (1 cm in diameter) at a rate of 10 mL/min under a pressure of 10–20 mm Hg; the color of the disk surface changed from pink to blue-violet. The colored disks were placed onto filter paper and blotted carefully. After several minutes, the concentration of mercury in a sample was determined by comparison with a color scale, which was prepared from a standard solution containing 1 µg/mL mercury. The standard solution of mercury was pretreated with 4 mL of conc. HNO3, heated to 80°C for 20 min, and then processed as the test solution.
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7. Volynets, M.P., Gur’eva, R.F., and Dubrova, T.V., Zh. Anal. Khim., 1991, vol. 4, no. 8, p. 1595. 8. Savvin, S.B., Trutneva, L.M., Shvoeva, O.P., and Efendieva, K.A., Zh. Anal. Khim., 1991, vol. 46, no. 4, p. 709. 9. Gur’eva, R.F. and Savvin, S.B., Zh. Anal. Khim., 1997, vol. 52, no. 3, p. 247. 10. Gur’eva, R.F. and Savvin, S.B., Zh. Anal. Khim., 2000, vol. 55, no. 3, p. 280. 11. Mikhailova, A.V., Cand. Sci. (Chem.) Dissertation, Moscow: Vernadsky Inst. Geochem. Anal. Chem., Russ. Acad. Sci., 1998. 12. Savvin, S.B., Gur’eva, R.F., and Trutneva, L.M., Zh. Anal. Khim., 1979, vol. 34, no. 8, p. 1493. 13. Morosanova, E.I., Zolotov, Yu.A., Kuz’min, N.M., et al., Zh. Anal. Khim., 1987, vol. 42, no. 3, p. 456. 14. Morosanova, E.I., Kosyreva, O.A., Kuz’min, N.M., et al., Zh. Anal. Khim., 1988, vol. 43, no. 9, p. 1614.
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