Optimization of a Flow Injection System with

ANALYTICAL SCIENCES JULY 2008, VOL. 24 2008 © The Japan Society for Analytical Chemistry

877

Optimization of a Flow Injection System with Amperometric Detection for Arsenic Determination Aleksandar LOLIC,† Snezana NIKOLIC, and Jelena MUTIC Faculty of Chemistry, University of Belgrade, Studentski trg 16, P. O. Box 158, 11000 Belgrade, Serbia

A selective and sensitive analytical procedure for rapid arsenic determination by gas-diffusion flow injection analysis with amperometric detection was developed. The method is based on the arsenite reduction by NaBH4. Derived arsine diffuses through a PTF membrane into the acceptor flow stream and is amperometrically determined on a platinum working electrode. The limit of detection (3s) at room temperature was 5 mg/dm3 of As(III). The relative standard deviation for a 1 mg/dm3 As(III) standard was 1.96% for six repetitive injections. Arsenic(V) was determined after its prereduction with potassium iodide. Arsenic determination was not interferred with by 1 mg/dm3 Sb(III), 5 mg/dm3 Sn(II), 10 mg/dm3 Se(IV), 1 mg/dm3 As(V), 1 mg/dm3 hydrasine, 1 mg/dm3 Fe(II) or 0.5 mg/dm3 Fe(III) solution. The throughput of this method was 60 analyses per hour. This method was successfully applied to arsenic determination in some power plant waste water samples. (Received October 25, 2007; Accepted January 28, 2008; Published July 10, 2008)

Introduction Arsenic exists in several chemical forms in the nature, both organic and inorganic. Its toxicity depends on the chemical form, inorganic arsenic(III) being the most toxic of the watersoluble species. Arsenic is a ubiquitous trace element originating from natural and anthropogenic sources. Hence it is important to develop a rapid, selective and sensitive method for the arsenic determination and speciation. A number of analytical methods are used for the arsenic determination and speciation in environmental samples. The speciation of arsenic is mostly based on the determination of total arsenic after As(V) prereduction and determination of As(III) alone. Some of the methods used are flow injection hydride generation atomic absorption spectrometry (FI-HG-AAS),1–3 HG-AAS,4–6 sequential injection hydride generation atomic fluorescence spectrometry (SIA-HG-AFS),7 flow injection inductively coupled mass spectrometry (FI-ICP-MS),8,9 electrochemical methods,10–12 pervaporation-flow injection determination with photometric detection,13 spectrophotometrical,14–16 colorimetric17,18 and FIA with chemiluminiscence detection.19 Most methods are based on a hydride generation in an acidic media; according to the detection methods, they vary in the detection limits, as well as solution and sample consumption and the expense of the analysis itself. Farrell et al. have reported the determination of arsenic by hydride generation gas diffusion flow injection analysis with electrochemical detection.20 Their electrochemical detector was equipped with the gold electrode, their acceptor solution was 0.01 mol/dm3 H2SO4 and their reagent was 0.5% NaBH4. Their limit of detection of was 10 mg/dm.3 An indirect gas-diffusion FIA method has been developed for the selective and sensitive determination of tetrahydroborate.21 To whom correspondence should be addressed. E-mail: [email protected] †

The precision of the method was better than a relative standard deviation of 2.1% at 60 mmol/dm3 levels and better than 0.5% at 0.1 mmol/dm3, with a throughput of 60 samples per hour. In this research a gas-diffusion FIA method was developed and applied for arsenic determination. The method is based on arsine generation by NaBH4. The formed arsine diffuses through the PTFE (polytetrafluoroethylene) membrane and is quantified amperometrically. Our amperometrical detector was equipped with a platinum working electrode; by using iodine/iodide acceptor solution we achieved low limit of detection (5 mg/dm3). FIA experiments were confirmed by cyclic voltammetry. This optimized system was applied for arsenic determination in some power plant waste water samples and the results were compared with those obtained by the HG-AAS technique.

Experimental Reagents and chemicals All the chemicals used were of analytical reagent grade. Deionized water, distilled over potasium permanganate, was used throughout. A stock arsenic solution was prepared weekly. Sodium arsenite (Carlo Erba, Italy) was dissolved in 6 mol/dm3 HCl and then diluted, yielding appropriate concentration. Each stock solution concentration was tested bromatometrically. Sodium borhydride was used as the reduction agent (Riedel de-Haen, Germany). A stock solution was prepared daily in 0.1% sodium hydroxide (Lachema, Czech Republic). Stock iodine solution was prepared weekly by dissolving iodine (Zorka, Serbia), refined by sublimation in potassium iodide (Carlo Erba). Arsenic(V) solution (Carlo Erba), Sb(III), Sn(II) (Fluka, Germany) and Se(IV) (Schering Kahlbaum, Germany) were prepared in 6 mol/dm3 hydrochloric acid, and were diluted to the appropriate concentrations. Appropriate volumes of waste water samples were acidified

878

Fig. 1 Schematic presentation of the FIA apparatus used for amperometric determination of arsenic content. (C) Carrier; (R) reagent; (A) acceptor flow stream; (P) peristaltic pump; (V) injection valve; (MC) mixing coil; (GDU) gas-diffusion unit; (FC) electrochemical flow through cell; (PO) potentiostat; (RE) writer; (W) waste. Numbers present flow rates in cm3/min.

with hydrochloric acid (0.1 mol/dm3); the samples were set on an ultrasonic bath for 30 min and then injected into the flow system. Instruments and apparatus Figure 1 presents the FIA manifold used for the determination of As(III). Three peristaltic pumps were used. One pump was a Model HPB 5400 (Iskra, Slovenia), the second one was a Model MS Reglo (Ismatec, Switzerland) with flow rate control unit and the third pump was a Model Mini S-840 (Ismatec). The injection valve was a Model 5020 (Rheodyne, USA), equipped with a 0.2-cm3 sample loop. The gas-diffusion unit was manufactured after the model provided by Shenyang Film Projector Factory, China. All connections were made with 0.5 mm i.d. tubing. Due to oxide film formation on its surface, the platinum electrode was occasionally cleaned by polishing with fine alumina pasta. Working potential was regulated by polarogaph MA 5450, Iskra, Slovenia. The obtained FIA signals were recorded on the Servograph Model 61 (Radiometer, Denmark) writer. Temperature was regulated with a thermostat, Model Messgerate-Werk Lauda, Germany. Cyclic voltammetric experiments were registered on a Metrohm 797 VA Computance instrument (Herisau, Switzerland). The triple electrode system consisted of: working electrode, rotating platinum disk (RDE); reference electrode, Ag/AgCl in potassium chloride (3 mol/dm3); and auxiliary platinum electrode. The PC software provided controls for the measurement, recording the measuring data and evaluating it.

Results and Discussion In order to achieve optimal conditions for the arsenic determination we investigated the effects of a few parameters. The first effect to be investigated was the optimal potential. Arsenic standard was injected at several potential values in the range –0.10 to +0.20 V versus Ag/AgCl reference electrode. The concentration of the standard was 1 mg/dm3 in 0.1 mol/dm3 HCl. Reagents were 0.2% NaBH4 in 0.1% NaOH, the carrier was 0.1 mol/dm3 HCl, the acceptor solution was 1 mg/dm3 I2 in 0.05 mol/dm3 KI. From the obtained hydrodynamic voltammogram (Fig. 2) the highest peak was reached at +0.10 V. Figure 3 presents obtained cyclic voltammograms that gave us another proof for choosing the potential for further experiments. The next effect to be investigated was the carrier solution. Several solutions were chosen as carriers: sulfuric, perchloric

ANALYTICAL SCIENCES JULY 2008, VOL. 24

Fig. 2 Hydrodynamic voltammogram for a 0.200-cm3 injection of a 1 mg/dm3 arsenic(III) standard.

and hydrochloric acid. Sulfuric acid was dismissed as it gave a signal for blank injections, probably because of its impurities. When perchloric acid was used, a signal split occurred. Since the highest and the proper signals were obtained when hydrochloric acid was injected, it was chosen to be the carrier. Further experiments with hydrochloric acid as the carrier showed that 0.1 mol/dm3 HCl gave the best results (Fig. 4). Examination of NaBH4 effect as a reagent showed that 0.1% NaBH4 in 0.1% NaOH gave the highest signals (Fig. 5). Using less concentrated reagent we avoided H2 production, which may cross the membrane with arsine and thus cause greater dispersion in the streams, which leads to decrease of the peak height signal. One of the important effects was the mixing coil volume. When arsenic standard was injected with different coils (30, 60 and 130 cm in length) and a tube (10 cm in length), peak currents obtained were 1.22, 0.89, 0.74 and 0.54 mA, respectively. Hence for further experiments, the shortest coil (30 cm) was used. The acceptor stream solution components were investigated by injecting arsenic standard solution with several acceptor solutions. The examined solutions were sulfuric acid, potassium hexacyanoferrate(III) and iodine dissolved in potassium iodide. When sulfuric acid was used as the acceptor, there was no signal for the arsenic standard solution. When a ferro salt was used a signal for the blank injection was noticed. The proper signals were obtained for iodine in potassium iodide solution (0.5 and 1 mg/dm3 I2 in 0.05 mol/dm3 KI). To prove why iodine/iodide solution was used as the acceptor we recorded cyclic voltammograms (Fig. 3). The first voltammogram was recorded for the acceptor solution alone (1); when 1 mg/dm3 As(III) standard was injected, voltammograms 2 – 5 were recorded. The obtained voltammograms clearly show why the iodine/iodide solution was chosen as the acceptor solution (Fig. 3a) compared to 1 mg/dm3 potassium hexacyanoferrate(III) in 0.01 mol/dm3 NaCl (Fig. 3b) and 0.01 mol/dm3 H2SO4 (Fig. 3c). At a potential of +0.100 V, there is a signal for iodine/iodide reduction (Fig. 3a) which decreases when As(III) is added. Effects of stream directions and rates were also investigated. In an optimized system, the opposite direction gave a peak current of 0.21 mA, whereas for the parallel direction the peak current was 0.18 mA for 1 mg/dm3 As(III). With the flow rate increase from 0.5 to 0.9 cm3/min, identical peak currents were obtained for the opposite direction. In this case compensation of two effects takes place: the slower donor rate increases the


879

5 ¥ 10–6A

Fig. 4

Effect of hydrochloric acid concentration as a carrier.

Fig. 5

Effect of tetrahydroborate as a reagent.

5 ¥ 10–6A

5 ¥ 10–6A

Fig. 3 Cyclic voltammograms obtained at 1 mm Pt electrode at u = 100 mV/s. a) Cyclic voltammograms of 1 mg/dm3 I2 in 0.05 mol/dm3 KI, 0.1% NaBH4 in 0.1% NaOH, 0.1 mol/dm3 HCl (1) and with 1 mg/dm3 As(III) injection (2 – 5); b) 1 mg/dm3 potassium hexacyanoferrate(III) in 0.01 mol/dm3 NaCl, 0.1% NaBH4 in 0.1% NaOH, 0.1 mol/dm3 HCl (1), and with 1 mg/dm3 As(III) injection (2 – 5); c) 0.01 mol/dm3 H2SO4, 0.1% NaBH4 in 0.1% NaOH, 0.1 mol/dm3 HCl (1), and with 1 mg/dm3 As(III) injection (2 – 5).

AsH3 amount that passes through PTFE membrane and at the same time the higher acceptor rate increases the base current caused by iodine reduction, causing significant peak current change with As(III) injection. For further experiments, a higher acceptor flow rate was used (0.9 cm3/min) because it enables better throughput (60 samples per hour). The donor flow rate was kept constant at 1.1 cm3/min. Temperature effects were studied by injecting the same As(III) standard (1 mg/dm3), while varying the temperature in the interval 20 – 50˚C. It was observed that there was no signal change with temperature increase, which can be explained as due to the high As(III) reduction rate, its volatility and its diffusion through the PTFE membrane. Possible interferants of this determination were Se, Sn and Sb, since they form volatile hydrides as well as some other

wastewater components such as hydrasine, Fe(II) and Fe(III). We injected solutions containing various ratios of interferants and As(III), whose concentration was kept constant (1 mg/dm3). Results showed that 10 mg/dm3 Se(IV) and 5 mg/dm3 Sn(II) did not interfere with the determination, neither did 1 mg/dm3 Sb(III) and 1 mg/dm3 As(V). Hydrasine of the same concentration as As(III) did not interfere, with the As(III) signal, nor did Fe(II), whereas Fe(III) did increase the signal for 50%; but when we injected 0.5 mg/dm3 Fe(III) solution, no effect was noticed. Our proposed method could be used for speciation of As(III) and As(V). A set of experiments was performed. Standard solutions containing only As(III) or As(V) or their mixtures were injected; each solution was treated with potassium iodide for As(V) prereduction. A 1-cm3 volume of 0.02 mol/dm3 potassium iodide was added to 100.00 cm3 of working solution containing As(III) 30 min prior to its injection. The results obtained are presented in Table 1. It is important to highlight that there was no significant interferring from potassium iodide and iodine excess. The limit of detection of this flow-injection method was 5 mg/dm3 of arsenic (3s), which corresponds to 100 pg of As(III) (the sample loop volume was 0.2 cm3). The reproducibility of the analytical system was investigated by six repetitive injections of the arsenic standard. The relative standard deviation for a 1 mg/dm3 As(III) standard was 1.96%. Linearity was investigated by triple injection of standard As(III)

880


Table 1 Determination of As(III) and As(V) in standard solutions As(III) added/ mg dm–3

As(V) added/ mg dm–3

Found/ mg dm–3 ± RSDa

Recovery, %

10 5 0

0 5 10

9.8 ± 0.2 9.9 ± 0.2 9.8 ± 0.2

98 99 98

a. Results given are obtained as averages of three measurements ± RSD.

The optimized system had good reproducibility (RSD for a 1 mg/dm3 As(III) standard was 1.96% for six repetitive injections), was sensitive (limit of detection (3s) was 5 mg/dm3, which corresponds to 100 pg of As(III), the sample loop volume was 0.2 cm3) and was fast with a throughput of 60 samples per hour. The method was succesfully applied for the arsenic determination in the waste water samples from the power plant “Kolubara” near Lazarevac, Serbia; the accuracy was confirmed by HG-AAS technique.

References Table 2 samples

Results of arsenic determination in waste water

Waste water sample 1 2 3 4 5

Arsenic(III) found/mg dm–3 FIAa

HG-AASa

0.191 ± 0.004 0.136 ± 0.003 0.058 ± 0.001 0.287 ± 0.006 0.119 ± 0.002

0.195 ± 0.008 0.130 ± 0.006 0.060 ± 0.003 0.280 ± 0.012 0.120 ± 0.005

a. Results given are obtained as averages of three measurements ± RSD.

solutions; it spans over two ranges, 0.1 – 1.0 mg/dm3 and 1.0 – 10.0 mg/dm3; the corresponding correlation coefficients were r = 0.9944 and 0.9996. Our optimized system was applied to arsenic determination in the waste water samples from the power plant “Kolubara”, Lazarevac, Serbia. Table 2 represents the obtained results, which were compared to those obtained by the reference technique HG-AAS. The reference results were obtained on a atomic absorption spectrometer, Perkin-Elmer 2380, equipped with hydride generator unit MHS-20. The determinations were carried out at 193.7 nm, slit 0.7 and lamp current of 10 mA.

Conclusion A rapid, indirect gas-diffusion flow injection method with amperometric detector has been developed for selective arsenic determination. Several physical and chemical parameters were investigated. Their optimal conditions were: the potential of the working Pt electrode (+0.10 V vs. Ag/AgCl); the hydrochloric acid as the carrier (0.1 mol/dm3); the acceptor solution 0.5 and 1 mg/dm3 I2 in 0.05 mol/dm3 KI; the reagent solution 0.1% NaBH4 in 0.1% NaOH; both parallel and opposite direction of acceptor and carrier streams; and room temperature.

1. N. M. M. Coelho, A. C. da Silva, and C. M. da Silva, Anal. Chim. Acta, 2002, 460, 227. 2. A. Shraim, B. Chiswell, and H. Olszowy, Talanta, 1999, 50, 1109. 3. J. Y. Cabon and N. Cabon, Anal. Chim. Acta, 2000, 418, 19. 4. C. J. Hsieh, C. H. Yen, and M. S. Kuo, Anal. Sci., 1999, 15, 669. 5. P. K. Petrov, I. Serafimovski, T. Stafilov, and D. L. Tsalev, Talanta, 2006, 69, 1112. 6. L. Zhang, Y. Morita, K. Yoshikawa, and A. Isozaki, Anal. Sci., 2007, 23, 365. 7. N. V. Semenova, F. M. Bauzá de Mirabó, R. Forteza, and V. Cerdá, Anal. Chim. Acta, 2000, 412, 169. 8. L. F. R. Machado, A. O. Jacintho, A. A. Menegário, E. A. G. Zagatto, and M. F. Giné, J. Anal. At. Spectrom., 1998, 13, 1343. 9. X. P. Yan, R. Kerrich, and M. J. Hendry, Anal. Chem., 1998, 70, 4736. 10. P. Salaun, B. Planner-Friedrich, and C. M. G. van den Berg, Anal. Chim. Acta, 2007, 585, 312. 11. Y. Song and G. M. Swain, Anal. Chim. Acta, 2007, 593, 7. 12. M. S. S. Pereira, E. Winter, J. R. Guimarães, S. Rath, and A. H. Fostier, Environ. Chem. Lett., 2007, 5, 137. 13. T. Rupasinghe, T. J. Cardwell, R. W. Cattrall, M. D. Luque de Castro, and S. D. Kolev, Anal. Chim. Acta, 2001, 445, 229. 14. H. D. Revanasiddappa, B. P. Dayananda, and T. N. K. Kumar, Environ. Chem. Lett., 2007, 5, 151. 15. A. Baghel, B. Singh, P. Pandey, and K. Sehkar, Anal. Sci., 2007, 23, 135. 16. M. Hashemi and P. Modasser, Talanta, 2007, 73, 166. 17. K. Morita and E. Kaneko, Anal. Sci., 2006, 22, 1085. 18. M. Rahman, Y. Seike, and M. Okumura, Anal. Sci., 2006, 22, 475. 19. S. Satienperakul, T. J. Cardwell, S. D. Kolev, C. E. Lenehan, and N. W. Barnett, Anal. Chim. Acta, 2007, 554, 25. 20. J. R. Farrell, P. J. Iles, and Y. J. Yuan, Anal. Chim. Acta, 1996, 334, 193. 21. S. D. Nikolic, E. B. Milosavljevic, J. L. Hendrix, and J. H. Nelson, Talanta, 1993, 40, 1283.