ЖУРНАЛ АНАЛИТИЧЕСКОЙ ХИМИИ, 2014, том 69, № 6, с. 563-568
ОРИГИНАЛЬНЫЕ СТАТЬИ
УДК 543
TEMPERATURE-INDUCED AGGREGATION IONIC LIQUID DISPERSIVE LIQUID-LIQUID MICROEXTRACTION METHOD FOR SEPARATION TRACE AMOUNT OF COBALT ION © 2014 Mohammad Mirzaei, Najme Amirtaimoury
Department of Chemistry, Shahid Bahonar University of Kerman Kerman, 76169 Iran Received 27.02.2012; in final form 27.03.2013
A simple and rapid temperature induced aggregation micro extraction method based on ionic liquid for extraction and preconcentration of cobalt ions from water samples was proposed. In this investigation small amounts of1-hexyl-3-methylimidazolium hexafluorophosphate [Hmim][PF6] and 1-hexyl-3-methylimida-zolium ¿is-(trifluoromethylsulfonyl)imide [Hmim][Tf2N] (as extractant solvents) were added in a sample solution containing Triton X-114 (as an anti-sticking agent). After centrifuging the cooled solution, analysis was carried out by flame atomic absorption spectrophotometry. Some effective parameters have been optimized. Under the optimum conditions, detection limit of the method was 0.44 ng/mL and the relative standard deviation (RSD) for 100.0 ng/mL cobalt was ±2.3% (n = 7).The calibration curve was linear in the range of 3.0—200.0 ng/mL cobalt and enrichment factor was obtained as 26.5. The applicability of the technique was evaluated by the determination of trace amounts of cobalt in several water samples and synthetic alloys.
Keywords: liquid phase micro extraction, cobalt, ionic liquid, temperature-induced aggregation, flame atomic absorption.
DOI: 10.7868/S0044450214060115
Cobalt is an essential micronutrient for humans, animals and plants for a range of metabolic process or some biochemical metalloenzyme reactions [1, 2]. Cobalt acts as the central atom of vitamin B12, which is widely responsible for the production of red blood cells and the prevention of pernicious anemia [3]. Toxico-logical effects oflarge amounts of cobalt include vasodilatation, flushing and cardiomyopathy in humans and animals [4]. Although cobalt is not considered to be as toxic as most of the heavy metals, it is an equally harmful element. The preconcentration and determination of cobalt(II) have been studied by various extraction methods such as: liquid—liquid extraction (LLE) [5], solid phase extraction (SPE) [6, 7], cloud point extraction (CPE) [8—10], and dispersive liquid—liquid microextraction (DLLME) [11]. Some disadvantages are involved in LLE method: it is time-consuming, tedious, and uses large amounts of organic solvents, which are expensive and toxic. Several SPE precon-centration procedures for cobalt have been reported using various sorbents. Unfortunately, the amounts of used eluents in these methods are significant. Despite many benefits of using CPE, in high content of salt the background is increased since the enrichment phase contains a little aqueous sample and a very high con-
tent of salt, the density of sample becomes equal or even higher than that of micelles, so they cannot be settled. DLLME method is simple, fast and inexpensive. However, the amount of disperser solvent used is relatively high, so for less hydrophobic species it is possible that recoveries decrease proportionately. Room-temperature ionic liquids (ILs) are being recently considered as replacement solvents in the sample preparation, due to their unique chemical and physical properties such as negligible vapor pressure, non-flammability, good extractability for various organic compounds and metal ions as a neutral or charged complexes, as well as tunable viscosity and miscibility with water and organic solvents [12, 13]. ILs' non-volatility effectively eliminates a major pathway for environmental release and contamination. However, this property is distinct from toxicity. ILs' aquatic toxicity is as severe as or higher so than that of many current solvents [14]. Balancing VOC reductions against waterway spills (via waste ponds/streams, etc.) requires further research. ILs' substituent diversity simplifies the process of identifying compounds that meet safety requirements [15].
In this method, a very small amount of hydropho-bic IL as an extractant solvent is dissolved in the sample solution containing Triton X-114. (Triton X-114
prevents IL sticking onto the surface of the centrifuge tube wall, so it is named "anti-sticking agent"). After centrifuging, fine droplets of extractant phase, dissolved in an adequate solvent and the analysis was carried out. Considering the above reports, the aim of the present work is to investigate the performance oftemperature-in-duced aggregation microextraction (TIAME) with the determination of cobalt in water samples using flame atomic absorption spectrometry (FAAS).
EXPRIMENTAL
Instruments. The flame atomic absorption spectrometer model SpecrAA 220 was purchased from Varian (Australia). A cobalt hollow cathode lamp (GBC, Australia), operated at a current of 10 mA and a wavelength of 240.7 nm with slit width of0.2 nm. Acetylene flow, 1.5 L/min, and air flow as oxidant, 3.5 L/min, and the centrifuge (1EC Model HN-S) were used; pH values were measured with a pH-meter (Metrohm Model 827, Switzerland) supplied with a glass-combined electrode.
Reagents and material. All reagents which were used are of analytical grades. Distilled water was used throughout the experiment. 2-(5-Bromo-2-pyridyla-zo)-5-(diethyl amino)phenol (5-Br-PADAP), acetone, ethanol and all salts used were obtained from Merck (Darmstadt, Germany). 1-Hexyl-3-meth-ylimidazolium hexafluorophosphate [Hmim][PF6] and 1-hexyl-3-methylimidazolium bis-(trifluorome-thylsulfonyl)imide [Hmim][Tf2N] were synthesized. Triton X-100 and Triton X-114 were purchased from Fluka (Buchs, Switzerland). A stock solution of co-balt(II) (1000.0 mg/L) was prepared by dissolving an appropriate amount of Co(NO3)2 • 6H2O and working standard solutions were obtained by appropriate stepwise dilution of the stock standard solutions. Conical bottom centrifuge tubes were cleaned before use by soaking in 10% nitric acid solution for at least 24 h and then rinsing thoroughly with distilled water. A 2.78 x 10-4 M 5-Br-PADAP solution was prepared by dissolving an appropriate amount in ethanol. The viscosity of ILs is high and their handling is difficult, so working solutions ([Hmim][PF6], 0.8 mg/^L, and [Hmim][Tf2N], 0.5 mg/^L) were prepared in acetone. A buffer solution (pH 7.5, 0.1 M) was prepared by mixing of sodium dihydrogen phosphate (0.1 M) and disodium hydrogen phosphate (0.1 M).
Preparation of ionic liquids. [Hmim][PF6] and [Hmim][Tf2N] were synthesized according to the procedure approved in the literature [16,17]. Briefly, 1-hexyl-3-methylimidazolium chloride [Hmim][Cl] was prepared by adding equal molar amounts of 1-chlorohex-ane and 1-methylimidazole to a round-bottom flask fitted with a reflux condenser for 24—72 h at 70°C with stirring until the formation of a golden viscous liquid. The viscous liquid was cooled and washed thoroughly several times with ethyl acetate in a separator funnel.
The lower liquid portion [Hmim][Cl] was slowly added to potassium hexafluorophosphate or lithium bis-(trifluoromethylsolfonyl)imide (in a 1 : 1.1 molar ratio), and stirring were continued at room temperature for 30 min, finally this mixture was washed with water. The washed IL was heated under vacuum at 70°C to remove the solvent. The proton and carbon NMR spectra were recorded in DMSO solvent. The data for 1-hexyl-3-methylimidazolium hexafluorophosphate [H NMR : 8 9.08 (1H, s, CH), 7.75 (1H, d, CH), 7.69 (1H, s, CH), 4.14 (2H, t, CH2), 3.84 (3H, s, CH3), 1.77 (2H, m, CH2), 1.26 (6H, m, 3CH2), 0.86 (3H, t, CH3) and C NMR : 8 137.33 (C2), 124.44 (C4), 123.08 (C5), 49.65, 36.55, 31.39, 30.17, 25.98, 22.71, 14.63 (CH3)] confirm the structure of the synthesized ionic liquids.
General procedure.A sample or standard solution containing Co(II), 5-Br-PADAP (1.5 x 10-5 M), Triton X-114 (0.043%, w/v), sodium nitrate (0.4%, w/v), disodium hydrogen phosphate/sodium dihydrogen phosphate buffer (pH 7.5, 0.1 M), [Hmim][PF6] (64 mg) and [Hmim][Tf2N] (5 mg) was poured in a 10 mL conical-bottom glass centrifuge tube. The tube was kept in a thermostated bath at 35°C for 4 min. After shaking, it was placed in an ice-water bath for 10 min, and a cloudy solution was formed. The mixture was centri-fuged for 8 min at 2500 rpm. As a result, the fine droplets of IL settled down at the bottom of the centrifuge tube. Bulk aqueous phase was removed simply by inverting the tubes. Afterwards, the IL-phase (about 8 ^L) was dissolved in 350 ^L of 96% ethanol. The absorbance of cobalt was measured at 240.7 nm wavelength by flame atomic absorption.
RESULTS AND DISCUSSSION
It is necessary to investigate the effect of all the parameters that can probably influence the extraction performance. In this methodology these parameters are the kind and amount of IL and anti-sticking agent, ligand concentration, pH, salt concentration, temperature and centrifuging conditions that were studied and optimized in order to achieve a high recovery and enrichment factor. In all optimization steps concentration of cobalt was 100 ng/mL.
Selection of ionic liquid. We focused on the ILs containing imidazolium cation. Imidazolium ILs containing PF6- as anion are hydrophobic, relatively inexpensive and liquid in the experimental conditions, so they are suitable for LLE. [Hmim][PF6] was chosen as the extractant among [Bmim][PF6], [Hmim][PF6] and [Omim][PF6] according to such physicochemical properties as density, viscosity and water solubility [18].
Selection of anti-sticking agent. After centrifuga-tion it was observed that some of the IL-phase sticks to the wall of the centrifuge tube. In order to overcome this problem, a non-ionic surfactant was added to the
TEMPERATURE-INDUCED AGGREGATION IONIC LIQUID
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