Spectrophotometric Determination of Trace Arsenic in Water Samples

A new spectrophotometric method was developed for the determination of low ppb levels of arsenic in water. We found that Ethyl Violet with molybdateâˆ...
0 downloads 0 Views 320KB Size
Download PDF
Anal. Chem. 2006, 78, 7682-7688

Spectrophotometric Determination of Trace Arsenic in Water Samples Using a Nanoparticle of Ethyl Violet with a Molybdate-Iodine Tetrachloride Complex as a Probe for Molybdoarsenate Keisuke Morita and Emiko Kaneko*

Department of Materials Processing, Graduate School of Engineering, Tohoku University, Aoba 6-6-11-1018, Sendai 980-8579, Japan

A new spectrophotometric method was developed for the determination of low ppb levels of arsenic in water. We found that Ethyl Violet with molybdate-iodine tetrachloride complex forms nanoparticles under acidic conditions, which provide a sensitive probe for molybdoarsenate. The nanoparticles form stable particles with a diameter micrometers in size in the presence of heteropolyacid, and the resulting particles give a purple color to the apparently homogeneous solution, the intensity of which depends on the arsenic concentration. The nanoparticle itself is unstable due to conversion of the dye to a colorless carbinol species under acidic conditions without heteropolyacid. Although triphenylmethane dyes have been the subject of a number of investigations, there do not appear to be any reports on the dye particles for trace determination. The calibration curve is linear up to 20 µg L-1 arsenic, and the detection limit is 0.5 µg L-1 (6.6 × 10-9 mol L-1). The coefficient of variation for spectrophotometry at 10 µg L-1 is 5.8% (n ) 8). Furthermore, it is possible to detect concentrations as low as 1 µg L-1 arsenic visually using this method. The interferences from phosphorus and silica were eliminated using an anion exchange column and sodium fluoride as a masking agent, respectively. The proposed method has been successfully applied to water samples in abandoned mine water, groundwater, and river water. There was good agreement between the results obtained by the proposed method and those by hydride generation atomic absorption spectrometry. Since this method is specific for As(V), it is applicable to the speciation of arsenic oxidation states. Our method has enormous practical potential for simple and field detection of arsenic, requiring no complex apparatus or skilled laboratory support. The presence of arsenic in drinking water has reached calamitous proportions in many parts of the world.1,2 There are numerous reports in the literature based on past and ongoing * To whom correspondence should be addressed: (e-mail) [email protected]; (tel) +81-22-264-2384; (fax) +81-22-264-2384. (1) Dasgupta, P. K., Ed. Talanta 2002, 58, 1-235. (2) Chakraborti, D.; Hussam, A.; Alauddin, M., Eds. J. Environ. Sci. Health 2003, A38, 1-305.

7682 Analytical Chemistry, Vol. 78, No. 22, November 15, 2006

experience in various countries in Asia and South America concerning the higher risks of skin, bladder, lung, liver, and kidney cancer that result from continued consumption of elevated levels of arsenic in drinking water.3-8 Consumption of even low levels of arsenic over a long period can cause a multitude of diseases.9 The World Health Organization’s (WHO) maximum permissible contaminant level of arsenic in drinking water is 10 µg L-1, and in some countries where the arsenic-contamination problem has become widespread and alarming, an interim working limit of 50 µg L-1 has been set. Arsenic is a ubiquitous element, and its presence in soil is due to both natural and anthropogenic sources. Soil erosion and leaching lead to arsenic dissolution in the aquatic compartment. This element occurs in natural waters mainly in inorganic trivalent (arsenite, As(III)) and pentavalent (arsenate, As(V)) oxidation states.10 Arsenic is usually removed by classical (coagulationflocculation, adsorption on activated carbon) or specific (lime softening, membrane process, ion exchange) water treatment processes. A simple and effective method to remove As(V) is precipitation with Fe(III) salts. However, this process fails when As(III) has to be removed because of its poor ability to bind to Fe(III).11 In order to carry out these treatment processes, small plants are required for the analytical methods so that optimization and efficiency control is possible. When combined with concerns over the limits of conventional field kits,12 it is clear there remains (3) Chakraborti, D.; Mandal, B. K.; Dhar, R. K.; Biswas, B.; Samanta, G.; Saha, K. C. Ind. J. Environ. Prot. 1999, 19, 565-575. (4) Chakraborti, D.; Rahman, M. M.; Raul, K.; Chowdhyry, U. K.; Sengupta, M. K.; Lodh, D.; Chanda, C. R.; Saha, K. C.; Mukherjee, S. C. Talanta 2002, 58, 3-22. (5) Charterjee, A.; Das, D.; Mandal, B. K.; Chowdhury, T. R.; Samanta, G.; Chakraborti, D. Analyst 1995, 120, 643-650. (6) Mandal, B. K.; Chowdhury, T. R.; Samanta, G.; Basu, G. K.; Chowdhury, P. P.; Chanda, C. R.; Lodh, D.; Kuran, N. K.; Dhar, R. K.; Tamili, D. K. Curr. Sci. 1996, 70, 976-986. (7) Mazumder, D. N. G. Curr. Sci. 1997, 72, 114-117. (8) Dhar, R. K.; Biswas, B. K.; Samanta, G.; Mandal, B. K.; Chakraborti, D.; Roy, S.; Jafar, A.; Islam, A.; Ara, G.; Kabir, S.; Khan, A. W.; Ahmed, A. S.; Hadi, A. S. Curr. Sci. 1997, 73, 48-59. (9) Karim, M. M. Water Res. 2000, 34, 304-310. (10) Smedley, P. L.; Kinniburgh, D. G. Appl. Geochem. 2002, 17, 517-568. (11) Borho, M.; Wilderer, P. Water Sci. Technol. 1996, 34, 25-35. (12) Rahman, M. M.; Mukherjee, D.; Sengupta, M. K.; Chowdhury, U. K.; Lodh, D.; Chanda, C. R.; Roy, S.; Selim, M. D.; Quamruzzaman, Q.; Milton, A. H.; Shahidullah, S. M.; Rahman, M. D. T.; Chakraborti, D. Environ. Sci. Technol. 2002, 36, 5385-5394. 10.1021/ac061074h CCC: $33.50

© 2006 American Chemical Society Published on Web 10/14/2006

an urgent need for a simple, easy-to-handle, and inexpensive method to analyze arsenic with sufficient sensitivity. Although there are a number of analytical methods to detect arsenic, none of them are suited to on-site analysis. Hydride generation atomic absorption spectrometry (HG-AAS) is the most commonly used method but requires comparatively expensive equipment and skillful operators. Other methods such as stripping voltammetry, neutron activation analysis, and X-ray fluorescence require expensive, heavy instrumentation which render them unsuitable for use in the field. Although modified Gutzeit methods have been used as field tests for more than 120 years since Gutzeit developed his test in 1879,13 a significant drawback is that the necessary test paper is impregnated with a mercury compound. Despite many attempts over the past decades, up until now there have been no reports of a simple, inexpensive, yet highly sensitive method for the field determination of arsenic. The molybdenum blue colorimetric method has been the focus of a number of authors for arsenic measurement. In earlier work, Rupasinghe et al.14 transferred gaseous AsH3 to a triiodide receptor and followed with a molybdenum blue reaction to attain an detection limit of 15 µg L-1. Dasgupta et al.15 described a lightemitting, diode-based photometric method for the differential determination of ppb levels of As(III) and As(V) in the presence of ppm levels of phosphate. Recently, Toda el al.16 reported on a technique in which they used hydride generation to form arsine and then used an in-line preconcentration step in an alkaline KMnO4 receiver, and finally followed with molybdenum blue colorimetric determination for a highly sensitive measurement of total inorganic arsenic and arsenite. Since Itaya and Ui developed a spectrophotometric method for the determination of phosphate using Malachite Green (MG+),17 this dye has been extensively studied with a variety of methods being explored for the determination of phosphate, silicate and arsenic. Wu and Liu described a spectrophotometric method for arsenic in aqueous solution, based on the formation of an ion-association complex between MG+ and arsenoantimonomolybdenum.18 This work was extended to a spectrophotometric determination of arsenic using Ethyl Violet.19 Matsubara et al. reported on the collection of aggregates on a membrane filter in their work on the determination of phosphate and arsenate using MG+.20 Motomizu et al. showed that heteropolyacid can be extracted with Ethyl Violet (EV+) into a mixed solvent of cyclohexane-4-methylpentan-2-one (1:3 v/v).21 In our previous work, we reported a spectrophotometric method for the determination of arsenic in water samples using EV+ based on the particle formation of EV+-molybdoarsenate.22 The detection limit of the spectrophotometric method is 4 µg L-1 (13) Gutzeit, H. Pharmaz. Zeitung 1879, 24, 263-265. (14) Rupasinghe, T.; Cardwell, T. J.; Cattrall, R. W.; Castro, M. D.; Kolev, S. D. Anal. Chim. Acta 2004, 445, 229-238. (15) Dasgupta, P. K.; Huang, H.; Zhang, G.; Cobb, G. P. Talanta 2002, 58, 153164. (16) Toda, K.; Ohba, T.; Takaki, M.; Karthikeyan, S.; Hirata, S.; Dasgupta, P. K. Anal. Chem. 2005, 77, 4765-4773. (17) Itaya, K.; Ui, M. Clin. Chim. Acta 1966, 14, 361-366. (18) Wu, Q. F.; Liu, P. F. Talanta 1983, 30, 275-276. (19) Zhao, X.; Xu, S.; Yuan, X. Huanjing Huaxue 1987, 6, 70-75. (20) Matsubara, C.; Yamamoto, Y.; Takamura, K. Analyst 1987, 112, 12571260. (21) Motomizu, S.; Wakimoto, T.; Toei, K. Bunseki Kagaku 1982, 31, 717-721. (22) Morita, K.; Kaneko, E. Anal. Sci. 2006, 22, 1085-1089.

and is 10 µg L-1 by visual colorimetry. In the present work, a new sensitive method for the determination of low ppb levels arsenic in water has been developed using nanoparticles of Ethyl Violet with an isopolymolybdate-iodine tetrachloride complex, which provides a sensitive probe for molybdoarsenate. There are numerous investigations on the ion association between triphenylmethane dyes and heteropolyacid. There do not, however, appear to be any reports on dye particles for trace determination. The method reported in this paper is a marked improvement in various regards to the earlier works based on ion association previously mentioned. This method will be useful as a simple, highly sensitive, and cost-effective measurement of arsenic, which has long been in great demand. EXPERIMENTAL SECTION Apparatus. Spectrophotometric measurement was made using a V-530 spectrophotometer (Jasco Co. Ltd., Tokyo, Japan), and temperature control was employed using an EHC-573 air-cooled homoiothermal cell holder (Jasco Co. Ltd.). For field determination, a TNP-10 spectrophotometer (DKK-TOA Co. Ltd., Tokyo, Japan) was used. Particle size distribution measurement was made using a Dynamic light scattering particle size analyzer LB-550 (Horiba Co. Ltd., Kyoto, Japan). Elemental analysis of the particle was carried out with a JSM-5310LV scanning electron microscope (SEM; JEOL Ltd., Tokyo, Japan) equipped with a EX-23000BU energy dispersion X-ray spectrometer (EDS; JEOL Ltd.). Molybdenum-95 Fourier transform NMR spectra were obtained at 26.05 MHz on a JNM-ECX400 spectrometer (JEOL Ltd.) with 10-mm sample tubes. Several water samples were also analyzed by AA6500S hydride generation-atomic absorption spectrometry (Shimadzu Co. Ltd., Kyoto, Japan). Reagents. Arsenic solution (1 mg L-1) was prepared by diluting analytical grade standard solution of 1000 mg L-1 As (H3AsO3, Kanto Chemical Co., Inc., Tokyo, Japan) in 0.01 mol L-1 hydrochloric acid. Ammonium molybdate solution (3.3 × 10-2 mol L-1) was prepared by dissolving the reagent (Wako Pure Chemical Industries, Ltd., Osaka, Japan) in distilled and deionized water. Iodine tetrachloride solution (1.3 × 10-1 mol L-1) was prepared by dissolving iodine trichloride (Kanto Chemical Co., Inc.) in 0.1 mol L-1 hydrochloric acid. Ethyl Violet solution (1.3 × 10-3 mol L-1) was prepared by dissolving the dye (Wako Pure Chemical Industries, Ltd.) in distilled and deionized water. All other reagents used were of analytical grade. A weak base anion-exchange resin, Dowex Marathon WBA (The Dow Chemical Co., Ltd., West Midlands, UK) with a mean particle size 525 ( 50 µm was used to remove phosphate in water samples. After 8 g of the resin was dry-packed in a polypropylene column (12 × 150 mm), the resin was conditioned with 10 mL of 1 mol L-1 nitric acid, 50 mL of 1 mol L-1 sodium hydroxide, and 100 mL of 1 mol L-1 sodium chloride. For visual detection, 350 µL of white polystyrene multiwell plate module (FluoroNunc Modules C8, Nunc, Roskild, Denmark) was employed. Procedure. To a 50-mL sample solution, 0.5 mL of 1 mol L-1 hydrochloric acid and 1 mL of 0.01 mol L-1 EDTA as a masking agent for iron(III) were added. Then, the mixture was passed through an ion-exchange resin column in order to eliminate phosphate. The natural flow rate was 10 mL min-1 when the sample solution was loaded. The 30-mL sample volume necessary Analytical Chemistry, Vol. 78, No. 22, November 15, 2006

7683

Figure 1. Absorption spectra of (a) EV+ under neutral pH, (b) HEV2+ and H2EV3+ under acidic conditions, and (c) degradation of (b) after 30 min. Hydrochloric acid, 0.34 mol L-1 for (b) and (c); Ethyl Violet, 2.2 × 10-5 mol L-1; temperature, 25 °C.

to replace the solution in the column was disposed of, and an aliquot of 2.5 mL of the fresh effluent was put in a flask. To mask silica, 6 µL of 1 mol L-1 sodium fluoride was added to the solution. To the sample solution pretreated as described above, the following solutions were added: 40 µL of 0.05 mol L-1 potassium iodate, 170 µL of 6 mol L-1 hydrochloric acid, 20 µL of 1.3 × 10-1 mol L-1 iodine tetrachloride, 220 µL of 3.3 × 10-2 mol L-1 ammonium molybdate, and 50 µL of 1.3 × 10-3 mol L-1 Ethyl Violet. After letting it stand for 30 min, absorbance at 550 nm was measured. A visual determination was also carried out by comparison with a standard series using 350-µL white wells, which were chosen as the primer because of their suitability for clear detection. In a separate experiment, particle size distribution measurements were taken, and an elemental analysis of the particles was carried out to investigate the mechanism of the coloration. For SEM-EDS, a 1-L mixture containing hydrochloric acid, ICl4-, ammonium molybdate, and EV was filtered through a mixed cellulose ester membrane filter with a pore size of 3 µm. The particles collected on the membrane filter were dried in an oven at 70 °C for 12 h and then analyzed by SEM-EDS. For kinetic studies on the characteristics of particles, the timeabsorbance curves were measured at 298 K to determine the rate constants, and also at 5 K intervals in the 293-313 K range to determine the activation energy. RESULTS AND DISCUSSION Absorption Spectra of Ethyl Violet. The absorption spectra of Ethyl Violet are shown in Figure 1. EV+ in aqueous solution has an absorption maximum at 596 nm (, 94 000)21 in neutral pH regions (Figure 1 curve a). Under acidic conditions, protonation leads to a spectral shift from 596 nm to the maximum wavelengths at 638 (, 75 000) and 428 nm (, 20 000), which are attributed to HEV2+, and 434 nm (, 31 000), H2EV3+.23 The acid dissociation constants of Ethyl Violet (eq 1) were reported by Cigen as pKa1 ) 1.66, pKa2 ) 3.68.24 The protonated dyes are gradually converted to two colorless carbinol types due to the hydration reaction (Figure 1 curve b f curve c). In this study, (23) Hagiwara, T.; Motomizu, S. Bull. Chem. Soc. Jpn. 1994, 67, 390-397. (24) Cigen, R. Acta Chem. Scand. 1958, 12, 1456-1475.

7684

Analytical Chemistry, Vol. 78, No. 22, November 15, 2006

Figure 2. 95Mo NMR spectra. (A) Mo, 2.4 × 10-3 mol L-1; HCl, 0.34 mol L-1; (B) Mo, 2.4 × 10-3 mol L-1; O, -34 ppm; HCl, 0.34 mol L-1; ICl4-, 8.7 × 10-4 mol L-1; temperature, 25 °C.

the rate constants k1 and k2 of the first-order reactions shown in eqs 2 and 3 were determined as 1.3 × 10-3 and 1.5 × 10-3 from the spectrophotometric data at 25 °C. pKa1

pKa2

H

H

H2EV3+ 9 8 HEV2+ 9 8 EV+ + + k1

8 H2EVOH2+ HEV2+ 9 HO 2

k2

H2EV3+ 9 8 H3EVOH3+ HO 2

(1) (2) (3)

Formation of Probe Particle. Under acidic conditions, EV+ forms nanoparticles in the presence of molybdic acid ions and iodine tetrachloride (ICl4-), providing a sensitive probe described below. In aqueous solution in the neutral pH region, molybdenum ion (MoO42-) is easily protonated and then shows a strong tendency to form isopolymolybdate ions by oxygen bridging and the release of water molecules. The structures of isopolymolybdate in aqueous solution have been investigated extensively. It is well recognized that molybdates, depending on the pH and molybdenum concentrations, form in excess of 20 isopolymolybdates.25-27 An earlier work by Coddington and Taylor showed that the component of dimeric molybdenum cation is [Mo2O5(H2O)6]2+ in 2 mol L-1 hydrochloric acid solution with 0.5 mol L-1 molybdate ion based on NMR and Raman spectra data.28 The chemical shift of -56 ppm in this study (The 95Mo NMR spectrum A in Figure 2) is similar to the value (-63 ppm) Coddington and Taylor reported and indicates that the isopolymolybdate species in the initial solution under analytical conditions is [Mo2O5(H2O)6]2+. Nanoparticles were formed when EV+ was added after the addition of MoO42- to ICl4- under acidic conditions. However, the nanoparticles were not formed when ICl4- was added to acidic (25) Pope, M. T. Heteropoly and Isopoly Oxometalates; Springer: Berlin, 1983. (26) Michael, T. P. Prog. Inorg. Chem. 1991, 39, 182-257. (27) Cruywagen, J. J. Adv. Inorg. Chem. 1999, 49, 127-182. (28) Coddington, J.; Taylor, M. J. Chem. Soc., Dalton Trans. 1990, 41-47.

Table 1. Comparison of Rate Constant (k) and Activation Energy (Ea)

degradation of the probe particle carbinol reaction of HEV2+ carbinol reaction of H2EV3+

rate constant ka (s-)

activation energy Eab (kJ/mol)

(1.34 ( 0.1) × 10-3

60.1 ( 0.4

(1.74 ( 0.1) × 10-3

61.4 ( 0.1

10-3

64.6 ( 0.1

(1.57 ( 0.1) ×

a 298 K. b Calculated from Arrhenius equation at 293-313 K with intervals of 5 K.

Figure 3. Elemental analysis of the particles using SEM-EDS. b, Mo peak; O, I peak; 2, Cl peak. Filter, mixed cellulose ester membrane filter (pore size, 3 µm); sample volume, 1L; hydrochloric acid, 0.34 mol L-1; iodine tetrachloride, 8.7 × 10-4 mol L-1; molybdate, 2.4 × 10-3 mol L-1; Ethyl Violet, 2.2 × 10-5 mol L-1.

molybdate solution prior to the addition of EV+. This indicates that the molybdenum species, [Mo2O5(H2O)6]2+, formed under acidic conditions is not an essential component of the probe particles. When MoO42- was added to ICl4- under acidic conditions, the 95Mo NMR spectrum showed chemical shifts of -56 and -34 ppm (see spectrum B in Figure 2). The -34 ppm shift is an unknown peak. Although it is well known that molybdenum forms various halogenated compounds (e.g., MoOF4, MoOCl4, MoO2Br2, MoO2Cl2), there are no reports indicating that molybdenum reacts with ICl4-. In this study, it was found that when the molybdenum-ICl4compound forms nanoparticles with EV+, the resulting purple solution appears to be homogeneous, and these nanoparticles provide a sensitive probe for heteropolymolybdate. To determine the probe particle composition, an elemental analysis using SEMEDS was carried out in the manner described in the Experimental Section. Figure 3 shows that the Mo:I:Cl stoichiometry is 1:1:2 in the probe particles. Thus, the molybdenum-ICl4- compound is expressed as [MoOnICl2]3-2n. In a separate experiment, the stoichiometry of EV+ and the new compound [MoOnICl2]3-2n was determined by gravimetric analysis and spectrophotometry. The particles were collected on a membrane filter in the same manner as described above for the SEM-EDS analysis. The weight of the solid was 6.1 mg (RSD ) 9.4%, n ) 5). The EV content in the probe particles was determined using the dissolution of the membrane filter with the particles in methyl cellosolve followed by spectrophotometric measurement. The EV content of the probe particles constituted 4.0 mg of the 6.1 mg. The weights of 4.0 mg of EV and 2.1 mg of [MoOnICl2]3-2n indicate that the molar ratio was 1:1, resulting in a charge (3 - 2n) of -1. Thus, n is determined as 2. Consequently, the probe particle composition was found to be [EV-MoO2ICl2] (eq 4).

as described in the discussion below. An interpretation of this phenomenon is given based on rate constant and activation energy calculated by the Arrhenius equation. The degradation of particles is a first-order reaction. The values for probe particles showed good agreement with those for carbinol reactions of HEV2+ and H2EV3+ (Table 1). Consequently, the mechanism of the degradation is attributed to the conversion of Ethyl Violet to the colorless carbinol species under acidic conditions (eq 5). H+

{EV-MoO2ICl2}agg 9 8 -MoO2ICl2

HEV

2+

+ H2O

+ H2EV3+ 98 HEVOH2+ + H2EVOH3+ (5)

Color Development for Arsenic. In the presence of arsenic, the probe particles provide a stable absorbance for molybdoarsenate (Figure 4A, spectrum a). The time curve of the absorbance of 10 µg L-1 arsenic at 550 nm is shown in Figure 4B together with that for the reagent blank. Taking blank values into consideration, a sitting time of 30 min was chosen for the measurement of arsenic. Figure 5 shows the particle size distributions measured using a dynamic light scattering particle size analyzer. In the case of the reagent blank, the particles were 140 nm in size on average at 1 min, and they disappeared after 15 min (Figure 5A). As described above, in the absence of molybdoarsenate, the absorbance arising from the probe particle is eliminated due to its conversion to a colorless carbinol species. On the contrary, in the presence of molybdoarsenate, the particles gradually grow to 1300 nm in size on average (Figure 5B). In our previous work, we showed that the molar ratio of EV+-molybdoarsenate is 3:1 by gravimetric analysis.20 Consequently, the nanoparticles, {EVMoO2ICl2}agg, form the stable microparticles with EV+-molybdoarsenate (eq 6), giving an apparently homogeneous purple color to the solution depending on arsenic concentration.

{EV-MoO2ICl2}agg + EV3-AsMo12O40 h {[EV-MoO2ICl2}agg [EV3-AsMo12O40]} (6)

EV+ + [MoO2ICl2]- f EV-MoO2ICl2 f {EV-MoO2ICl2}agg (4)

The probe particle itself is unstable under acidic conditions in the absence of heteropolyacid. As a result, the reagent blank is eliminated, and arsenic can be clearly detected at low ppb levels,

Optimization of Analytical Conditions. The analytical procedure was optimized with respect to concentrations of potassium iodate, hydrochloric acid, iodine tetrachloride, molybdic acid, and EV. Potassium iodate (1.7 × 10-3 mol L-1) was used to oxidize trivalent arsenic so that it became pentavalent, forming a heteropolyacid. A constant response was obtained with hydrochloric Analytical Chemistry, Vol. 78, No. 22, November 15, 2006

7685

Figure 4. Color development for arsenic (panel A, absorption spectra; panel B, time curve of absorbance). (b) Blank; (a) arsenic, 10 µg L-1 (1.3 × 10-7 mol L-1); potassium iodate, 1.7 × 10-3 mol L-1; hydrochloric acid, 0.34 mol L-1; iodine tetrachloride, 8.7 × 10-4 mol L-1; molybdate, 2.4 × 10-3 mol L-1; Ethyl Violet, 2.2 × 10-5 mol L-1; temperature, 25 °C.

acid concentrations in the range 0.3-0.4 mol L-1. Thus, the hydrochloric acid concentration of 0.34 mol L-1 was adopted. The absorption of heteropolyacid (molybdenum yellow) measured at 326 nm reached its peak almost instantaneously, and the spectrum remained unchanged for 60 min at least. Therefore, oxidation (As(III) f As(V)) and heteropolyacid formation is considered to be extremely rapid under analytical conditions. A constant response was obtained with iodine tetrachloride concentrations in the range (0.8-0.9) × 10-5 mol L-1, molybdate concentrations in the range (2-3) × 10-3 mol L-1, and EV concentrations in the range (2-3) × 10-5 mol L-1. To obtain a constant and maximum absorbance, 8.7 × 10-4 mol L-1 iodine tetrachloride, 2.44 × 10-3 mol L-1 molybdate, and 2.2 × 10-5 mol L-1 EV were employed. Calibration. Using a V-530 spectrophotometer, the calibration curve was found to be linear over the concentration range up to 20 µg L-1. The detection limit defined as 3σ/m is 0.5 µg L-1 (6.6 × 10-9 mol L-1), where σ is the standard deviation of five measurements of the reagent blank, and m, the slope of the calibration graph. The between-batch coefficient of variation for eight replicate analyses of 10 µg L-1 was 5.8%. When a 350-µL polystyrene white well was used for visual colorimetry as a cell, the visual detection limit was 1 µg L-1. Concentrations of arsenic in the range 0-20 µg L-1 can be visually determined (Figure 6). Interference. The effect of foreign ions on the spectrophotometric determination of arsenic is summarized in Table 2. Iron(III) is masked with EDTA as a masking agent. It is well known that phosphate and silica also form heteropolymolybdate under acidic conditions, resulting in a significant positive error on arsenic determination. When a sample containing 50 mg L-1 phosphate was passed through a column of the anion-exchange resin (Dowex 7686 Analytical Chemistry, Vol. 78, No. 22, November 15, 2006

Figure 5. Particle size distribution for 10 µg L-1 (1.3 × 10-7 mol L-1) arsenic (B) and the blank (A). Apparatus, dynamic light scattering particle size analyzer LB-550 (Horiba); aging time, every 5 min; the other conditions are the same as those in Figure 4.

Figure 6. Color series for arsenic. Vessel, 350-µL white polystyrene multiwell plate module; the conditions are the same as those in Figure 4.

Marathon WBA), phosphate was completely eliminated as shown in Figure 7. On the contrary, both trivalent and pentavalent arsenic were quantitatively recovered in the effluent at pH 2-7 (Figure 7). These data show that the anion-exchange resin makes it possible to completely separate phosphate from arsenic. Silica is tolerated up to 25 mg L-1 in the presence of sodium fluoride (0.02 mol L-1) as a masking agent. No other ions had any significant effect.

Table 2. Effect of Foreign Ions on the Determination of Arsenica ion

added amt (µg L-1)

molar ratio (foreign ion/As)

as found (µg L-1)

recovery (%)

Na+ K+ Ca2+ Mg2+ Cu2+ Mn2+ Zn2+ Al3+ Fe3+ b ClNO3SO42PO43PO43- c SiO22SiO22- d

2300 3900 400 200 100 100 100 80 50 6200 6200 960 0.08 50 0.5 25

750 000 750 000 75 000 61 000 12 000 14 000 11 000 22 000 6 500 750 000 750 000 75 000 20 12 000 130 6 600

10.2 10.5 10.4 10.1 9.6 10.1 10.4 10.5 10.1 10.2 10.1 10.4 14.3 9.8 15.9 10.4

102 105 104 101 96 101 104 105 101 102 101 104 143 98 159 104

Table 3. Analytical Results of Water Samples sample tap waterb groundwaterb

As added (µg L-1)

As found (µg L-1)

recovery (%)

10 5 10 5

9.8 4.8 10.2 5.2 26.0

98 96 102 104

water in abandoned mine 1c water in abandoned mine 2c river water 1c river water 2c river water 3c river water 4c river water 5c

HG-AASa (µg L-1)

21.2

3.2

2.6

31.1 16.5 9.7 4.3 2.0

29.4 15.1 7.9 2.3 0.9

a Hydride generation atomic absorption spectrometry. b Arsenic was not detected in the tap waters or groundwaters. c With 2-fold dilution.

a As taken, 10 µg L-1. b Masked using EDTA. c Pretreated using an anion-exchange resin. d Masked using sodium fluoride.

Figure 7. pH effect on adsorption of phosphate and arsenic with anion-exchange column. 4, 50 mg L-1 phosphate; O, 10 µg L-1 arsenite; b, 10 µg L-1 arsenate; column size, 12 × 150 mm; anionexchange resin, Dowex Marathon WBA; resin amount, 8 g; flow rate, 10 mL min-1; sample volume, 50 mL.

Application to Water Samples. Our newly established method was applied to the determination of arsenic in spiked tap water and groundwater, water in abandoned mines, and river water samples (Table 3). The analytical recovery of arsenic added to tap water and groundwater ranged between 96 and 104%. There was good agreement between the results by the proposed method and those by HG-AAS. It is well known that arsenic in the form of arsenite (As(III)) is significantly more toxic than arsenate (As(V)). For this reason, the speciation of arsenic oxidation states is imperative. Gong et al. comprehensively reviewed the different arsenic speciation techniques that have been reported in the past decade.29 Although there are highly developed instrumental techniques available, including CE, LS-ICPMS, and LC-HG-AAS, none of the methods can be easily implemented in field determination. None of the numerous reports on the preservation of arsenic species have yet to become established practice, and the difficulties of implement(29) Gong, Z.; Lu, X.; Ma, M.; Watt, C.; Le, X. Talanta 2002, 58, 77-96.

Figure 8. Sampling locations on the Amemasu River (41.59°N, 140.46°E). The abandoned mine is at A, and B-D are downstream. Table 4. Analytical Results of River Water Samples total As As(V) As(III)d As(V) As(III) HG-AAS samplea (µg L-1) (µg L-1) (µg L-1) total As total As (µg L-1) Ab Bb Cc Dc b

251.1 79.8 30.4 18.5

172.7 61.4 27.0 16.9

78.3 18.4 3.4 1.6

68.8 76.9 88.7 91.3

31.2 23.1 11.3 8.7

266.0 73.6 26.4 15.7

a A-D correspond to the sampling locations (A-D) in Figure 8. With 10-fold dilution. c With 2-fold dilution. d Calculated value.

ing any of the instruments in the field has yet to be overcome. The high tech methods available simply do not lend themselves to field work. A technique for the on-site separation of As(III) and As(V) immediately after sample collection is needed. The proposed method was applied to the speciation of As(V) and As(III) in real samples taken from Amemasu River (41.59°N, 140.46°E). The sampling locations are shown in Figure 8. The pithead of an abandoned mine can be found at point A of the Amemasu River. In our method, potassium iodate, a oxidizing agent for As(III), is used to detect total amount of As(V) + As(III). Without the use of an oxidizing agent, the absorbance corresponds to the concentration of As(V) alone. The potable Analytical Chemistry, Vol. 78, No. 22, November 15, 2006

7687

spectrophotometer described in the Apparatus section was used for the field determination. Table 4 shows the analytical results of total As and As(V) by this method and calculated value of As(III) and HG-AAS data (total As). These data indicate that oxidation of As(III) to As(V) gradually takes place as the river flows downstream. The proposed method reported here enables a highly sensitive and selective arsenic measurement even though it is composed of simple and easy operations, requiring neither complex apparatus nor skilled laboratory support.

7688

Analytical Chemistry, Vol. 78, No. 22, November 15, 2006

ACKNOWLEDGMENT The authors acknowledge Mr. Tagiru Ogino of Geological Survey of Hokkaido, Ms. Yoko Shinozaki of Horiba Co. Ltd., and Mr. Junichi Isoe of Tohoku University, for their technical advice and useful discussion on this article.

Received for review June 13, 2006. Accepted September 12, 2006. AC061074H