научная статья по теме MODIFIED ANALCIME LOADED WITH ZINCON AS A USEFUL MATERIAL FOR THE SEPARATION AND PRECONCENTRATION OF TRACE PALLADIUM AND ITS DETERMINATION BY THIRD DERIVATIVE SPECTROPHOTOMETRY Химия

Текст научной статьи на тему «MODIFIED ANALCIME LOADED WITH ZINCON AS A USEFUL MATERIAL FOR THE SEPARATION AND PRECONCENTRATION OF TRACE PALLADIUM AND ITS DETERMINATION BY THIRD DERIVATIVE SPECTROPHOTOMETRY»

ЖУРНАЛ АНАЛИТИЧЕСКОЙ ХИМИИ, 2007, том 62, № 11, с. 1137-1142

^=ОРИГИНАЛЬНЫЕ СТАТЬИ

УДК 543

MODIFIED ANALCIME LOADED WITH ZINCON AS A USEFUL MATERIAL FOR THE SEPARATION AND PRECONCENTRATION OF TRACE PALLADIUM AND ITS DETERMINATION BY THIRD DERIVATIVE

SPECTROPHOTOMETRY © 2007 r. M. A. Taher, A. Mostafavi, S. Z. Mohammadi Mobarakeh

Department of Chemistry, Shahid Bahonar University Kerman, Iran Received 18.05.2006; in final form 22.09.2006

This work assesses the potential of natural Analcime Zeolite as a sorbent for the preconcentration of palladium. Palladium is quantitatively retained on modified Analcime Zeolite loaded with Zincon by column method in the pH range from 2.5 to 3.5 at a flow rate 1 mL/min. The palladium complex was removed from the column with 5.0 mL of dimethylsulfoxide (DMSO) and determined by third derivative spectrophotometry. The detection limit is 0.03 |g/mL (signal to noise ratio = 3) in the final solution. Since it is possible to retain 0.15 |g of palladium from 600 mL of solution passing through the column, elution with 5.0 mL of DMSO gives a detection limit of 0.25 ng/mL for palladium in the initial aqueous solution. The calibration curve is linear over the range 0.1 to 5.0 |g/mL of palladium(II) in the final solution with a correlation coefficient of 0.9996. Seven replicate determinations of 5.0 |g of palladium in 5.0 mL dimethylsulfoxide gave a mean d3A/dX3 (peak to peak signal between X = 625 and = 654 nm) of 0.64 with a relative standard deviation of 1.2%. The sensitivity of the method (d3A/dX3) is 0.5843 mL/|g of palladium(II) from the slope of the calibration curve. The interference of a large number of anions and cations has been studied and the optimized conditions developed were utilized for the determination of trace palladium in various synthetic and water samples.

Noble metals, particularly palladium, find an extensive use in the electrical industry as contacts in telephone relays and printed circuits, as grids for electronic tubes and electrodes for high quality spark plugs. Palladium affects the environment to an increasing degree as a new pollutant, especially due to the technical use of catalysts containing active palladium metal [1]. Catalytic converters containing palladium group metals (PGMs) have been used for the treatment of pollutants in exhaust gases from motor vehicles. Palladium, rhodium and platinum are the PGMs mostly used for this purpose. While these automotive catalysts enable 90% of the three major gaseous pollutants, namely carbon monoxide, unburned hydrocarbons and nitrogen oxides, to be transformed into harmless products, they present the disadvantage of releasing fine particulate matter or dusts (resulting from the abrasion or deterioration of the catalyst through combined mechanical and thermal effects) containing PGMs into the environment. Therefore, the metals are being accumulated along motorways, in vegetation and on soil surfaces and concerns have arisen with respect to the health risks generated by the possible inhalation of dusts and contamination of food and water [2, 3].

For the determination of traces palladium in environmental samples, highly sensitive and selective techniques including electro-thermal atomic absorption spectrometry (ETAAS) [4], inductively coupled plasma-atomic emission spectrometry (ICP-AES) [5], instru-

mental neutron activation analysis (INAA) [6], inductively coupled plasma-mass spectrometry (ICP-MS) [7-10] and total reflection X-ray fluorescence (TXRF) spectrometry [11] have been used. These techniques need expensive instruments and complicated operation. More simple methods for the determination of palladium have been reported such as spectrophotometric [12-17], extraction-spectrophotometric [18-22], kinet-ic-spectrophotometric [23] and derivative spectrophotometry methods [24-28] but most of them suffer from interferences of transitional metal ions and/or have high limit of detection.

In the present study, natural Analcime Zeolite was used for the solid phase extraction of trace palladium from water and synthetic samples. Zeolites are high crystalline alumino-silicate frameworks comprising [SiO4]4- and [AlO4]5- tetrahedral units. The atoms (Si, Al) are joined by an oxygen bridge. Introduction of an overall negative surface charge requires counter ions e.g. Na+, K+ and Ca2+. Due to the charged nature of the framework, and its ability to form Bronsted acid cites, Zeolites are useful in many applications [29]. Zeolites have been used in purification processes such as gas swetting and air decontamination as well as in separation processes. It is believed that the attitude is attributed to the adsorption of cationic surfactants onto Zeolite surface. The potential of synthetic Zeolites for the enrichment of trace metals has been investigated [30].

In this study, various parameters, i.e. pH, volume of sample, influence of interfering elements and flow rate of sample and solvent have been studied.

EXPERIMENTAL

Apparatus. In the present work, spectra were recorded on a scanning spectrophotometer (Shimdzu, Japan, Model No. 160 A) equipped with a pair of 10 mm path-length quartz cells. A Beckman pH meter was employed for pH measurements. A funnel-tipped glass tube (80 x 10 mm) was used as a column for preconcen-tration. All glass ware and columns were kept overnight in a mixture of concentrated sulfuric and nitric acids (1 : 1) before use.

The instrumental parameters were optimized and the best results were obtained with a scan rate of 300 nm/min, spectral bandwidth of 1 nm, integration time 0.1 s, resolution of spectrophotometer 0.5 nm for the third order derivative mode in the wavelength range 575-675 nm. The ratio spectra of the result were smoothened at AA = 9 nm.

Reagents. All the reagents were of analytical grade and were provided by Merck (Darmstadt, Germany). Approximately 0.5 g palladium (II) chloride was dissolved in a mixture of distilled water:hydrochloric acid 6.0 M (1 + 1) and the volume was made to 100 mL with distilled water in a calibrated flask. Then, this solution was standardized by dimethylglyoxime [Pd(C4H7O2N2)2] and gravimetric methods [31] and it was found that it is 3037 |g/mL. A 2.0 |g/mL solution was prepared with addition of distilled water to standard solution. A 0.05% solution of 2-[1-(2-hydroxy-5-sulforphenyl)-3-phenyl-5-formazano]-benzoic acid monosodium salt (Zincon) in ethanol was prepared. A 0.1% (w/v) solution of tetradecyldimethylbenzyl-ammonium chloride (TDBA-Cl) in water was prepared. Buffer solution of pH 3 was prepared by mixing appropriate ratios of a 0.2 M sodium citrate (2-hydroxy-1,2,3-propanetricar-boxylic acid trisodium salt) and 0.2 M citric acid (2-Hydroxy-1,2,3-propanetricarboxylic acid). Solutions of alkali metal salts (1%) and various metal salts (0.1%) were used for studing the interference of anions and cations, respectively. Natural Analcime Zeolite was collected from Torfeh, Shahr Babak area, Kerman region in Iran [32].

Preparation of Analcime Zeolite. After purification of Analcime [33], Zeolite was sieved to obtain a particle size <150 |m (200 mesh) then, 20 mL of 4 M hydrochloric acid were added to 20 g of Zeolite for removal of the cations present in Analcime. After this, material was washed with 100 mL distilled water. The sorbent in this form was dried at 110°C in an oven and stored in a calcium chloride desiccator until used.

Procedure for the Sorption of Palladium on the Column. 1 g of Analcime Zeolite was added to funnel tipped glass tube and 2.5 mL of TDBA-Cl solution were passed from the column, then, the modified Anal-

cime was saturated with the Zincon reagent by passing 2 mL of a 0.05% Zincon solution in ethanol. Afterwards it was washed with distilled water until reagent excess was eliminated from the column and 2 mL of buffer solution with pH 3 were passed from the column. An aliquot of the solution containing 0.5-25.0 |g of palladium was taken in a 100 mL beaker and 2 mL of buffer with pH 3 were added to it, then, the solution was diluted to 30 mL with distilled water. Then, this solution was passed through the column at a flow rate of 1 mL/min. After passing this solution, the column was washed with 5 mL of distilled water. Palladium adsorbed on the column was eluted with 5.0 mL of DMSO. The third derivative absorption spectra in the range of 500-800 nm were recorded against a blank solution in the same way. The dA/dA3 was measured between A,! = 654 and A2 = 625 nm. A calibration curve was prepared by taking various known amounts of palladium under the conditions given above.

RESULT AND DISCUSSIONS

In order to determine the optimum conditions for the quantitative extraction of palladium, several parameters were assessed.

Spectral Characteristics. The absorption spectrum of the ion associated Pd-Zincon in DMSO against reagent blank prepared under the similar conditions was recorded (Fig. 1). First, second and third derivative spectra are given in Fig. 2. Derivation of the spectra leads to sharper zero order bands and gives higher signal in the resolution spectra. The main instrumental parameters affecting the shape of the derivative spectra are the wavelength, scanning speed, the wavelength increment over which the derivative is obtained (AA), and the response time. These parameters need to be optimized to give a well resolved peak providing good selectivity and higher sensitivity of the determination.

Reaction Conditions. These conditions were established by the use 5.0 |g of palladium. The adsorption of palladium was found to be maximum in the pH range of 2.5-3.5. In subsequent study, the pH was maintained at pH 3. Addition of 1.0-4.0 mL of the buffer (pH 3) did not affect the retention of palladium and the use of 2.0 mL was recommended.

The flow rate of sample solution was varied from 0.5 to 5.0 mL/min. It was found that a flow rate 0.52.0 mL/min did not affect adsorption. A flow rate of 1.0 mL/min was recommended in all exper

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