УДК 543


*Phase Separation Laboratory, Department of Chemistry, University of Zanjan 45195-313, Zanjan, Iran 1E-mail: nasser_zn@yahoo.com **Department of Chemistry, Faculty of Science, University of Mazandran Camma, Mazandaran, Iran

Received 21.08.2012; in final form 17.09.2013

A novel homogeneous liquid-liquid microextraction technique based on use of ionic liquids (ILs), termed in situ solvent formation microextraction (ISFME) is developed for separation/preconcentration of Co(II) ions. In this method, small amount of sodium hexafluorophosphate (NaPF6, as an ion-pairing agent) was added to the sample solution containing very small amount of 1-hexyl-3-methylimidazolium tetrafluorobo-rate ([Hmim][BF4], as hydrophilic IL). A cloudy solution was formed as a result of formation of fine droplets of1-hexyl-3-methylimidazolium hexafluorophosphate [Hmim][PF6]. After centrifuging, the fine droplets of the extractant phase settled to the bottom of the conical-bottom glass centrifuge tube. ISFME is a simple and rapid method for extraction and preconcentration of cobalt ions from water samples that can be applied for the sample solutions containing very high concentrations of salt. Furthermore, this technique is much safer in comparison with the organic solvent extraction. Reliability of the introduced methodology was evaluated by analyzing water reference material. ISFME was successfully applied to determining cobalt(II) in real water samples. Schiff base ligand, 3,3'-(1E,1E')-(propane-1,2-diylbis(azan-1-yl-1-ylidene)bis(methan-1-yl-1-ylidene)bis(4-bromophenol) (L) was chosen as a complexing agent. Analysis was carried out using atomic absorption spectrometry. Type and amount of IL, pH and the other parameters were optimized. Under the optimum conditions, the limit of detection (LOD) was 0.06 ng/mL and the relative standard deviation (RSD) was 1.8% for 10 ng/mL cobalt.

Keywords: homogeneous liquid—liquid microextraction, Schiff base, ionic liquids, cobalt.

DOI: 10.7868/S0044450214120093

Cobalt has both beneficial and harmful effects on human health. Cobalt is widely distributed in the environment and it is essential for good health in humans since it is a component of the vitamin B12. The sources of cobalt in the environment are both natural and anthropogenic. Trace amounts of this metal occur in rocks, dust, soil, sediments, water, plants, animal tissues and fluids, and they are mobilized in volcanic eruptions, forest fires and biogenic emissions. Cobalt can be found in tea, coffee, fruits, vegetables, seafood and tobacco. The man-made sources are the by-products of burning coal and oil, industrial processes, vehicular exhausts and sewage sludge. The metal and its compounds are used in steel and alloys (magnetic and stainless steel, orthopedic implants), in metallurgy, electroplating, nuclear technology, fertilizers, medicine (as vitamin B12; radiation source), as a drier for paint, a foam stabilizer in beer brewing, in the porcelain enameling and colored pigments. Cobalt has been used as a treatment for anemia. However, harmful

health effects can occur when too much metal is taken into the body, such as pulmonary diseases (asthma, pneumoconiosis), skin allergies and effects on the cardiovascular and hematological systems and thyroid gland. The International Agency for Research on Cancer has determined that cobalt is possibly carcinogenic to humans based on animal data [1, 2].

Many methods for the determination of trace metals by atomic spectrometry techniques are still commonly carried out using a preconcentration step prior to their detection. Analytical chemists continue to search for sample preparation procedures that are faster, easier, safer, and less expensive to perform yet provide accurate and precise data with reasonable detection limits [3]. An indispensable requirement these days is the employment of novel environmentally friendly technologies and procedures in all fields of human activity. Chemistry in general and analytical chemistry in particular, is no exception. One possible application of the known principles of green chemistry

in analytical chemistry lies in replacing toxic reagents and in minimizing the amount and toxicity of wastes. To this end, a variety of miniaturized sample pretreatment techniques have been developed in recent years [4]. The continuous quest for novel sample preparation methods (especially miniaturized sample pretreatment) has led to development of new miniaturized sample pretreatment techniques such as solidphase microextraction (SPME), stir bar sorptive extraction (SBSE), single drop microextraction (SDME), hollow fiber—liquid phase microextraction (HF—LPME), dispersive liquid—liquid microextraction (DLLME), solid phase extraction (SPE), cloud point extraction (CPE), membrane extraction and cold-induced aggregation microextraction (CIAME). Main advantages of the mentioned techniques are their high speed and negligible volume of solvent used. However, in the presence of high content of salt their performance decreases significantly [5—12].

Among the different miniaturized sample preparation methods, a new mode of homogeneous liquidliquid microextraction (HLLME) based on ILs termed in situ solvent formation microextraction was developed. In ISFME, there is no interface between aqueous and extractant phases. During the formation of fine droplets of the extractant phase, the extractant molecules collect the hydrophobic species, and the extraction process is complete after formation of the droplets. As a result, mass transfer from aqueous phase into separated phase has no significant effect on the extraction step. In the presence ofhigh content of salt, the solubility of ILs increases and phase separation cannot occur. However, according to the common ion effect, this solubility of ILs decreases in the presence of a common ion. Consequently, the volume of the ex-tractant phase does not alter under such circumstances, which can otherwise alter the extraction and concentration efficiency. Because of high density of ILs, even in the saturated solutions (40%, w/v) the fine droplets of extractant phase can settle. Due to very low solubility of water in the hydrophobic ILs, residual salinity from the matrix is negligible. Rather than the other techniques that used for preconcentration of heavy metal ions, ISFME is faster and simpler and is applicable for solutions containing higher concentration of salts [13-15].

By definition, ionic liquids are known as solvents consisting of entirely ionic species and do have melting points close to or below room temperature. Ionic liquids are salts in which one or both the ions are large, and the cation has a low degree of symmetry. ILs are becoming increasingly interesting fluids for application in soft-matter materials systems from electrochemistry to energetic materials, and are also studied as potential solvents in separation processes. Properties, including low melting points, wide liquid ranges, and negligible vapor-pressure, have encouraged re-

searchers to explore the uses of ILs to replace volatile organic solvents.

This paper is aimed at developing a microextraction technique against very high content of salt. For evaluating the performance of ISFME, cobalt was selected as a test analyte and determined in water samples using atomic absorption spectrometry.


Apparatus. A HERMLE centrifuge equipped with a swing out rotor (4-place, 6000 rpm, Cat. No. Z 360) was obtained from Hettich (Kirchlengern, Germany). A Metrohm digital pH-meter (model 692, Herisau, Switzerland), equipped with a glass-combination electrode was used for pH adjustment. The measurements of cobalt ions were performed with a Varian specterAA220 flame atomic absorption spectrometer equipped with a hollow cathode lamp and a deuterium background corrector. The hollow cathode lamp of cobalt was operated at 4 mA, using the wavelength at 240.7 nm. All measurements were carried out in peak area mode (measurement time of 3 s).

Materials. All reagents used in this work were of analytical grade ofMerck (Darmstadt, Germany). Sodium hexafluorophosphate (NaPF6) purchased from Acros (Geel, Belgium). All aqueous solutions were prepared in double-distilled water. Working standard solutions were obtained by appropriate stepwise dilution of the stock standard solution (1000 mg/L). Working solutions of IL [Hmim][BF4] 0.5 mg/^L were prepared in ethanol. A solution of 80 mg/mL NaPF6 was prepared by dissolving an appropriate amount of NaPF6 in doubly distilled water. A solution of chelating agent 3,3'-(1E,1E')-(propane-1,2-diylbis(azan-1-yl-1-ylidene)bis(methan-1 -yl-1 -ylidene)bis(4-bromophe-nol) (Scheme) that synthesized according to the literature [16, 17] was prepared by dissolving an appropriate amount of this complexing agent in ethanol.





ISFME procedure. A 5 mL solution containing cobalt was adjusted to pH with an appropriate buffer solution, 2 x

10-2 M L, and 25 mg [Hmim][BF4] was transferred to 10 mL screw-cap conical-bottom glass centrifuge tube. After shaking, 0.5 mL NaPF6 (80 mg/mL) was added to the solution, and a cloudy solution was

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0.35 -

g 0.30 - !

1 °.25 -J 0.20 -

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Fig. 1. Effect of pH on the absorbance of cobalt. Conditions: 10 ng/mL Co, 2 x 10-2 M L, 25 mg [Hmim][BF4], 80 mg NaPF6, 100 p,L ethanol.

formed. Then, the mixture was centrifuged for 5 min at 5000 rpm. As a result, the fine droplets of IL settled at the bottom of the centrifuge tube. Aqueous phase was removed simply by inverting the tubes. Subsequently, IL-phase

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