ЖУРНАЛ АНАЛИТИЧЕСКОЙ ХИМИИ, 2014, том 69, № 2, с. 157-162
ОРИГИНАЛЬНЫЕ СТАТЬИ
УДК 543
A NEW SENSITIVE OPTICAL BULK TEST-SYSTEM FOR THALLIUM BASED ON PYRIDYLAZO RESORCINOL © 2014 Ali A. Ensafi, Masoud Fouladgar
Department of Chemistry, Isfahan University of Technology Isfahan 84156-83111, Iran Received 14.03.2011; in final form 16.04.2012
A color changeable optode for thallium(III) ions in aqueous solutions was prepared by physical inclusion of a of 4-(2-pyridylazo)-resorcinol into a plasticized PVC film. The increasing in the absorbance of the optode at 524 nm is proportional to thallium(III) concentration. Different parameters effecting on the sensitivity such as sample parameters and composition of the membrane were optimized. The response times of the prepared test-system are found to be 230, 210, and 180 s for 4.8 x 10-6, 4.8 x 10-5, and 4.8 x 10-4 M Tl(III), respectively. The analytical performance of the optode was evaluated, obtaining a linear concentration range of two decades of concentration, 3.1 x 10-6 - 4.7 x 10-4 M Tl(III), with a limit of detection of 1.8 x 10-6 M Tl(III). Selectivity of the optode is also studied. Application of the optode to the determination of Tl(III) in some aqueous samples yields good results.
Keywords: optical test-system, thallium determination, resorcinol derivative, spectrphotometry.
DOI: 10.7868/S0044450214020029
Recent decades have shown an increase in the development of optical chemical sensors as viable alternatives to other types of sensors. Such optical chemical sensors being termed 'optodes' or 'optrodes.' The great activity seen in the field of ion-sensitive optical devices and their application to trace analysis of heavy metal ions have given rise to several sensing schemes, new indicator dyes and highly diversified methods of immobilization. Both the method of immobilization and the class of matrix exert a significant effect on the performance of ion-sensitive layers [1]. Different methods are used to determine metal ions. These include atomic absorption spectroscopy, inductively coupled plasma spectroscopy, and mass spectrometry, and others. They generally require expensive instrumentation and/or complex sample preparation. Optical chemical sensors have been used for the determination of cations and anions because they require simple instrumentation and are cheaper to employ than some other analytical methods available. Optical chemical sensors (optodes) form a category of chemical sensors that require only a spectrophotometer or a spectroflurorimeter to use. These types of sensors usually work based on an interaction between a suitable ligand and ions that cause changes in the optical properties. In the direct method, interaction (usually com-plexation) of an analyte and a suitable ligand yields an optical signal [2]. In the indirect method, when the charged analyte (cation as an example) penetrates into the bulk of the optode to interact with the ligand, an-
other ionophore (chromoionophore, usually H+-indi-cator) releases protons and transforms to the basic form giving rise to an optical signal [3—5]. For preparation of a suitable optode, several polymeric compounds have been used for the immobilization of ion-ophore components such as plasticized—PVC [6], nafion [7, 8], and plasticized cellulose triacetate [9]. Because of the good diffusion ability of hydrophobic PVC membranes, plasticized PVC is the most commonly used polymer for the preparation of optodes.
Thallium in one of the most toxic metal cations. The biological mechanisms of Tl(III) ions in the body include ligand formation with proteins and sulfhydryl groups, inhibition of cellular respiration, interaction with riboflavin and riboflavin—based cofactors, and disruption of calcium homeostasis [10]. Therefore, a simple and selective method is required for its determination. Various methods have been used for the determination of thallium ions. These methods include solid phase extraction combined with spectroscopy [11], voltammetry [12], kinetic spectrophotometric method [13] electrothermal atomic absorption spectrometry [14] and inductively coupled plasma mass spectrometry [15]. Although some of these methods have good sensitivity for the determination of thallium ions, they require sample preparation and for expensive instruments while they may not have enough selectivity for application for real samples. The characteristic of some of these methods are shown in Table 1.
320 370 420 470 520 Wavelength, nm
570
620
Fig. 1. Absorption spectrum of the membrane (a) after equilibrium in a buffer solution (pH 4.0), and (b) in a solution containing 8.0 x 10-5 M Tl(III).
Several optodes are available for the determination of heavy metals, but their use has rarely been reported for the determination of thallium [16]. In this paper, a new bulk optical chemical test-system is introduced. This optode works based on the formation of a complex between thallium(III) and 4-(2-pyridylazo)-re-sorcinol (PAR) that causes the color of the optode to change from yellow to red—brown. This absorbance peak is used for the quantitative measurement of Tl(III). The method is fast, selective, and simple, especially for field analysis.
EXPERIMENTAL
Apparatus. UV-Vis spectra were measured by a double beam spectrophotometer, Jasco Model V-750, using 1.0 cm quartz cells.
A pH-meter, Metrohm Model 827 pH Lab, equipped with combined glass—Ag/AgCl electrodes (Model 6.0228.010) was used to determine pH levels of the solutions.
Reagents and chemicals. Polyvinyl chloride (PVC) high molecular mass, sodium tetraphenylbo-rate (NaTPB), and tributylphosphate (TBP) were purchased from Fluka. Tetrahydrofuran (THF), 4-(2-py-ridylazo)-resorcinol, and Tl(NQ3)3 • 3H2O were purchased from Merck. In the preparation of solutions, doubly distilled water was used throughout.
Tl(III) stock solution (0.010 M) was prepared by dissolving 0.4535 g Tl(NO3)3 • 3H2O (98%) in water and diluted to 100 mL in a standard flask. More diluted sample solutions were prepared daily by appropriate dilution of the stock solution with water.
Universal buffer was prepared by mixing phosphoric acid (0.05 M), acetic acid (0.05 M), and boric acid (0.05 M) in NaOH (0.1 M).
Preparation of the membrane. A slide glass plate cut into 9 x 50 mm pieces was used as a support matrix of the test-system film. A mixture of 40.0 mg PVC,
80.0 mg TBP, 1.0 mg NaTPB and 3.0 mg PAR was dissolved in 1.0 mL of THF. The mixture was shaken to achieve a homogenous solution. Then, the optode membrane was prepared using this solution with the help of a spin—on device [17] (50 ^L solution, 500 rpm rotation frequency) on the glass plates. The THF was allowed to evaporate for 2 h and the membrane was incorporated into the spectrophotometer cell.
Measurement of absorbance. The membrane was placed in the universal buffer (pH 4.0) for 70s to reach equilibrium. Then, the sensing film was placed into a quartz cell, which was subsequently mounted in the spectrophotometer. The cell in the reference path of the spectrophotometer consisted of the membrane film without the ligand (PAR). The sample containing an appropriate concentration of Tl(III) ions was injected into the cell using a microsyringe to measure the absorbance change.
Sample preparation. For sampling of water, polyethylene bottles were cleaned with 1 : 1 HNO3, conditioned over 2 h and finally rinsed with water. Tap water was sampled from our laboratory in the Department of Chemistry at Isfahan University of Technology, Isfahan. Wastewater was obtained from Mobarake Steel Co. (Isfahan, Iran).
Dolomite (BCS 368) standard sample as a certified reference material was also used to check the accuracy of the test-system. Exactly, 1.00 g of the sample and suitable amout of Tl(NO3)3 • 3H2O were added to an Erlenmeyer flask, to which 10 mL of 6 M HNO3 plus 10 mL of 6.0 M HCl were added. The mixture was heated (70—80°C) for 20 min to dissolve the sample. After cooling the residue to room temperature, the flask content was dissolved in 25 mL of water and the mixture was filtered through a 0.45 ^m Milli-pore filter. Then, the supernatant liquor was diluted to 50 mL with water in a 50-mL volumetric flask. The thallium content was measured according to the recommended procedure.
RESULT AND DISCUSSION
Principle of operation. PAR is a complexing agent, which makes a color complex with thallium(III) ions. Formation of the complex between PAR and thalli-um(III) causes a new peak in the absorption spectrum at 524 nm (Fig. 1). The response of an optode may be defined as the ratio of concentration of the uncom-plexed ligand ([C]) to the total amount of the ligand present in the membrane ([CT]), i.e. a = [C]/[CT]. Then, the value of a can be calculated by absorbance measurements at ^max of the complexed ligand (here 524 nm) as a = (AC—A)/(AC—AL), where AC is the absorbance of the membrane for complete complexation (i.e. a = 0), Al is the absorbance value of the membrane for the free ligand (i.e. a = 1), and A is absorbance measured at any time during the analysis [2, 16].
A NEW SENSITIVE OPTICAL BULK TEST-SYSTEM FOR THALLIUM BASED
159
0.35 0.30 0.25 0.20 0.15
<D О
g 0.10
£ 0.05
10
1.0 0.9 0.8 0.7 ö 0.6 0.5 ^ 0.4 0.3 0.2 0.1 0
1
2.44 x 10-5 M Tl(III)
2.62 x 10-4 M Tl(III)
4.88 x 10-4 M Tl(III)
pH
345 Amount of PAR, mg
Fig. 2. Effect of pH of the test solution on the response of the optode in the presence of 8.0 x 10-5 M Tl(III) at 524 nm.
Fig. 3. Effect of the amount of PAR on the response of the test-system.
6
2
7
0
2
4
6
8
In the direct response mechanism of bulk optodes, the cation ions penetrate into the bulk of the optode to react with the ionophore. The bulk of the optodes usually includes a polymeric membrane such as PVC, a plasticizer that affects permeability of the membrane, ionophore, and, sometimes, lipophilic anionic sites. It was found that in the absence of lipophilic anionic sites in the bulk of the mem
Для дальнейшего прочтения статьи необходимо приобрести полный текст. Статьи высылаются в формате PDF на указанную при оплате почту. Время доставки составляет менее 10 минут. Стоимость одной статьи — 150 рублей.