Sara Salatin1, 2, 3*
1Research Center for Pharmaceutical Nanotechnology, Faculty of Pharmacy, Tabriz University of Medical Sciences, Tabriz, Iran
2 Student Research Committee, Tabriz University of Medical Sciences, Tabriz, Iran
3Department of Pharmaceutics, Faculty of Pharmacy, Tabriz University of Medical Sciences, Tabriz, Iran
*Corresponding author:
Sara Salatin, Department of Pharmaceutics, Faculty of Pharmacy, Tabriz University of Medical Sciences, Tabriz, Iran
E-mail: sarasalatin93@gmail.com, Tel: +98 (41) 3339-2585. Fax: +98 (41) 3334-4798
Abstract
The most commonly used methods for the preconcentration of metal ions are expensive or need many expensive instruments. However, spectrophotometric methods have recently attracted considerable attention owing to their rapidity, simplicity, economy, and good separation yields. In this study, a new and extraction simple method were developed for the preconcentration and spectrophotometric determination of Co (II) in aqueous phase using cloud point extraction (CPE). This technique is based on the complexometric reaction of Co (II) with dithizone (DZ) as a chelating agent and sodium dodecyl sulfate (SDS) as an extracting agent at room temperature. The parameters affecting the CPE including pH, NaCl, DZ concentration, and SDS concentration were optimized. Besides, the interference effect of some anions and cations was evaluated. Overall, DZ gave the most desirable property in terms of rapid extraction and separation of Co (II), on all tested conditions. Finally, the method was successfully used for the determination of Co (II) in vitamin B12 ampoule. This study demonstrates that cloud point extraction may offer a promising technique for the preconcentration of metal ions in water samples.
Keywords: Cloud point extraction, Co (II), Dithizone, Sodium dodecyl sulfate, Metal ions
Introduction
Cobalt is a chemical element with symbol Co (II) and an atomic number of 27 which is found in the earth’s crust only in chemically combined form, save for small deposits found in alloys of natural meteoric iron [1]. Because of the unique properties of Co (II), various methods such as anodic stripping voltammetry [2,3], cathodic stripping voltammetry [4], atomic fluorescence spectrometry [5], flame atomic absorption spectrometry [6], solid-phase extraction and graphite furnace atomic absorption spectrometric determination [7], and electrothermal atomization atomic absorption spectrophotometry [8] have been developed for determination of Co (II) ions. However, most of these methods are expensive or need many expensive instruments. Among them, spectrophotometric methods (based on the reaction with a suitable ligand and producing colored complex) have received considerable attention because of their rapidity, simplicity and inexpensive. The 1, 5-diphenylthiocarbazone (dithizone, DZ) can be selected as the chelating agent for the extraction of cobalt. It is a colorimetric reagent for some heavy metal ions such as mercury. Moreover, due to the presence of thiol groups in the structure of DZ, it has a high affinity to form colored stable complexes [9]. The formation of sparingly water-soluble complexes with Co (II) makes DZ as a potentially useful reagent for the spectrometric determination of soft metals [10]. The cloud point extraction (CPE) method has been investigated as an attractive method that reduces the consumption of organic solvent, disposal costs and extraction time and requires a very small amount of relatively harmful surfactant to the environment. Moreover, CPE can be used for the extraction and preconcentration of metal ions after the formation of sparingly water-soluble complexes. Then the complex in the surfactant-rich phase is determined by different spectrometric methods [11, 12]. Nonionic surfactants such as Triton X-100 and Triton X-114, have been widely employed as extracting agents for the extraction of compounds [13]. However, in this state, CPE has to be carried out at high temperatures depend on surfactant kind and this may affect sometimes the stability of produced compounds. But in the present work micelle mediated extraction using an anionic surfactant, it has been developed for preconcentration and simultaneous determination of Co (II) in the aqueous phase. This method is based on the reaction of the Co (II) with DZ and extraction of the produced complex. This CPE strategy can be performed at a low temperature and provides a high recovery and low limit of detection compared to previous methods [14, 15]. Here, the parameters affecting the CPE including pH, NaCl, DZ concentration, and SDS concentration were optimized. Besides, the interference effect of some anions and cations was evaluated. Finally, the method was successfully used for the determination of Co (II) in vitamin B12 ampoule.
Methods and Materials
Materials
All chemicals used were of analytical reagent grade and doubly distilled water was used throughout the experiments. Cobalt solution (500 mg mL-1) was supplied by dissolving appropriate amounts of Co (NO3)2.6H2O in water. Working solutions were obtained by appropriate dilution of the stock solution. A 10-3 molL-1 DZ (Merck) solution was prepared by dissolving 25.6 mg of DZ in 4 moll-1 NaCl (Merck) first and then diluted to the mark with distilled water in a 100 mL volumetric flask. An aqueous 10% (w/v) solution of SDS (Merck) was prepared by dissolving 10 g of SDS in 100 mL hot distilled water. A phosphate buffer of pH=3 was prepared by using phosphoric acid (Merck) and sodium hydroxide (Merck) at appropriate concentrations. A saturated solution of NaCl was provided by dissolving NaCl (Merck) in water.
Instruments
A spectrophotometer Shimadzu mode UV-2550 (Japan) with a 1-cm quartz cell (0.5mL) was used for the recording of UV-Visible absorbance. All spectral measurements were performed using the blank solution as a reference. A centrifuge (4000 rpm, UrumAzma Co, Urmia, Iran) with 15 mL calibrated centrifuge tubes was applied to accelerate the phase separation process. A pH-meter (Metrohm model 713) with a combined glass electrode was used for pH measurements.
Determination of wavelength of maximum absorbance
A solutions containing 10-5 M of DZ, 0.1% (w/v) SDS, 1mL of saturated solution of NaCl, and 0.1 mgL-1 of Co (II) in pH=3, were placed in 15 mL centrifuge tubes. Subsequently, the solution was shaken and left to stand in an ice bath for 15 min before centrifugation. A cloudy solution was formed. Separation of two phases was achieved by centrifugation for 5 min at 3000 rpm. The aqueous phase was easily decanted by simply inverting the tube. The surfactant-rich phase obtained by this procedure was dissolved and diluted to 0.5 mL with acetonitrile and transferred into a 0.5 mL quartz cell. The absorbance spectrum of the blank and its solution was recorded and stored in the range of 400-700 nm (Fig.1).
Evaluation of the effect of pH
Two series of solutions, a series containing 10-5 M DZ, 0.1% w/v SDS, 1 mL saturated NaCl and 0.2 mgL-1 Co(II) , and another one containing no Co(II) from pH=1 to 6 were prepared. After CPE and the formation of the sediment, absorbance values at 510 nm were calculated.
Fig 1. Absorption spectra of: a) DZ, and b) Co (II)-DZ complex. Experimental conditions: 0.1 mgL-1 Co (II), 10-5 moll-1 DZ, 0.1% w/v SDS and 1mL NaCl in pH=3
Evaluation of the effect of SDS concentration
To investigate the effect of SDS concentration on the absorption of the Co (II)-DZ complex, in a series of 15 mL centrifuge tubes, 1mL of 10-4 M DZ solution containing various concentrations of SDS was poured and by adding 1 mL of phosphate buffer, their pH was adjusted to 3. Into a series of tubes, 1 mL of Co (II) solution at a concentration of 2 ppm was added and distilled up to volume 9. Then, to each of the tubes, they added 1 mL of saturated NaCl and, after stirring, the tubes were left for 5 minutes to complete the formation of the complex. The tubes were then placed into an ice bath and, after reaching the cloudy point, they were transferred to the centrifuge apparatus until the sedimentation process was complete. Finally, the precipitates were dissolved with the addition of acetonitrile.
Evaluation of the effect of DZ concentration
In order to investigate the effect of increasing the concentration of DZ on the absorption of the Co (II)-DZ complex, in 15 mL centrifugal tubes, two solutions were prepared from concentrations of 4×10-6 to 1.2 × 10-4 M DZ. Each of these tubes contained 0.06% w/v SDS and phosphate buffer (pH = 3). Into a series of these solutions, 1 mL of Co (II) solution at a concentration of 2 ppm was added, so that the final concentration of Co (II) in the centrifuge tube was 0.2 ppm. All solutions were distilled until the final volume of 9 mL. Finally, about 1 mL of saturated NaCl was added to all tubes, until the final volume of each solution was 10 mL. All of the tubes were transferred to an ice bath to reach the cloudy point and after centrifuging the two organic and aqueous phases were separated. Subsequently, the absorption of samples was determined against a blank solution containing the same concentration of DZ, at 510 nm.
Evaluation of the effect of saturated NaCl
To evaluate the effect of saturated NaCl, in two series of centrifuge tubes of 15 mL, about 1 mL of a solution containing DZ and SDS was poured. Then, 1 mL of phosphate buffer (pH =3) was added. 1 mL of Co (II) solution at a concentration of 2 ppm was added. Finally, by adding different volumes of NaCl and water, the final volume of all the tubes was reached into 10 mL. After precipitation of the complex, absorption spectrum of each sample was determined at a wavelength of 510 nm.
Results and discussion
Evaluation of the effect of pH
Fig. 2. It shows the effect of pH on the adsorption of Co (II)-DZ complexes. As it can be seen, the absorption of Co (II)-DZ complex increases to pH=3, and then remains constant to pH=5 with an acceptable error.
Fig 2. The effect of pH: 10-5 M DZ, 0.1% w/v SDS, 1 mL saturated NaCl and 0.2 mgL-1 Co (II)
Due to the change in the nature of DZ, the absorption of the Co (II)-DZ complex approaches zero at a wavelength of 510 nm. According to the obtained data, the pH range of 3-5 can be selected to measure Co (II). In this research, pH was 3 for Co (II) measurements. Phosphate buffer was used to adjust the pH of the environment. To investigate the interference of this buffer with the Co (II)-DZ complex, different concentrations were prepared and the corrected absorbance at 510 nm was compared. The results of this study exhibited that this buffer in the concentration up to 0.1 M has no disturbance on the absorption of the Co (II)-DZ complex, thereby this buffer was used to adjust the pH of the medium at 3.
Evaluation of the effect of SDS concentration
Fig. 3.The effect of SDS concentration: 10-5 M DZ, 1 mL of phosphate buffer (pH=3), 1 mL saturated NaCl and 0.2 mgL-1 Co (II)
The adsorption of the Co(II)-DZ complex at 510 nm exhibited that increasing the concentration of SDS results in an increase in the complex absorption up to 0.06% of the SDS, then a slight decrease in absorption is observed up to 0.08%, and later the amount of absorbance is approximately constant (Fig.3). At a lower concentration than the Critical Micelle Concentration (CMC), dissolution of DZ occurs slowly. So that lower concentrations of SDS are not capable of dissolving DZ, and no clear solution is obtained at these concentrations.
Evaluation of the effect of DZ concentration
The results of this experiment showed that the absorption was up to a concentration of 6×10-6 M and then remained on a constant platform (Fig.4).
In fact, at concentrations below 4×10-6 M, the amount of DZ is not sufficient to form a Co (II) complex, and the problem of dissolution of DZ occurs at concentrations higher than 1.2 × 10-4. So that 0.06% w/v SDS is not enough to dissolve DZ. For further measurement, a concentration of 10-5 M DZ was used.
Evaluation of the effect of saturated NaCl
The results showed that the absorbance of the solution increased with increasing saturated NaCl to 0.8 mL, and then, with a further increase, the amount of this absorption decreased (Fig.5).
Fig. 4. The effect of DZ concentration: 0.06% w/v SDS, 1 mL of phosphate buffer (pH=3), 1 mL saturated NaCl and 0.2 mgL-1 Co (II)
Fig. 5. The effect of saturated NaCl: 0.06% w/v SDS, 1 mL of phosphate buffer (pH=3), 10-5 M DZ and 0.2 mgl-1 Co (II)
Calibration graph
In order to determine the calibration graph using the optimal conditions obtained in the previous steps (10-5M DZ, 0.06% w/v SDS, phosphate buffer (pH=3) and 0.8 mL saturated NaCl), a blank solution and solutions with different concentrations of Co (II) were poured into 15 mL centrifuge tubes and then placed in an ice bath in order to reach the cloudy point. After centrifuging and separating the aqueous and organic phases from each other, the absorbance spectrum of blank and each solution against acetonitrile was plotted by a UV spectrophotometer (Fig.6).
Fig. 6. Absorption spectra of Co (II)-DZ complex using different concentrations of Co (II) under optimized conditions, b) Calibration graph
Interferences Studies
In this section, the effect of some common ions on the adsorption of the modified Co (II)-DZ complex in the proposed method was investigated. This study was conducted in optimal conditions for the proposed method for 0.5 mgL-1 Co (II). In order to determine the interference level, the concentration of ion interrupted by 1000 times the concentration of Co (II) (in terms of mass) was first tested, and in case of interference, this ratio was gradually reduced to an extent where no disturbance was observed. The results of this study are presented in Table (1). The deviation of more than 5% absorption of solutions containing the intruder agents from the adsorption of solutions containing Co (II) without these agents has been selected as a disturbance threshold.
Table 1. Study of interfering ions.
| ions | |
| SCN–, CH3COO–, S2O32-, Tartrate, Thiourea, Oxalate, Urea, F–, Citrate, Ascorbic acid, Al3+, Cl–, I–, Cr3+, La3+
Cr3+ Mn2+ |
>1000
500
250
|
| AS3+, Pb2+
Fe2+, Fe3+, Sb5+, EDTA
Cu2+, Sb3+, Ni2+, Bi3+, Zn2+, Cd2+, Ag+ |
50
10
1 |
Application of the method to the real sample
Content of 5 vitamin B12 ampoule (the production of the Darupakhsh Company) was injected into a 100 mL beaker and about 4 mL of concentrated nitric acid was added on it and heated on an electric heater to break down the cyanocobalamin complex, and the solution was colorless and transparent. After drying the specimen, it was dissolved with the addition of water, and distilled in a 10 mL volumetric balloon. One ml of this solution was added to the 15 mL centrifuge tubes, which was ready for testing under optimal conditions within them. Finally, the measurement of Co (II) concentration in these samples was done by interpolation with calibration curve drawing. The results of a five-time independent measurement were presented in Table 2.
Table 2. The measurement of CO (II) in vitamin B12 ampoule.
| 5 | 4 | 3 | 2 | 1 | Sample code |
| 0.06 | 0.058 | 0.06 | 0.065 | 0.061 | Corrected absorption |
| 0.196 | 0.190 | 0.196 | 0.21 | 0.2 | Co (II) concentration (ppm) |
| 3.92 | 3.8 | 3.92 | 4.2 | 4 | Co (II) concentration in each ampoule |
By calculating the dilution operation that was performed after digestion of the recent sample, the concentration of cobalt in the current vitamins B12 ampoule was achieved 3.968 mgL-1.
Conclusion
In this research work, DZ was successfully used in a CPE method for preconcentration of Co (II) ion from vitamin B12 ampoule prior to its determination by spectrophotometric technique. This study allowed to develop an efficient, fast, safe, inexpensive and simple procedure for the preconcentration and determination of the trace amounts of Co (II). However, the potential and efficiency of this method can be studied for the quantitation of other elements in a wide variety of biological and pharmaceutical compounds.
Acknowledgment
The results included in this paper were part of the MS student thesis. This study was supported in part by a grant from Uremia University.
Conflict of interests
The author declares that there are no conflicts of interest associated with this work.
References
How to cite this article;
Salatin, S. (2019). Preconcentration and spectrophotometric determination of Co (II) by cloud point extraction. Journal of Advanced Chemical and Pharmaceutical Materials (JACPM), 2(1), 89-94. Retrieved from http://advchempharm.ir/journal/index.php/JACPM/article/view/80
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