A cloud point extraction (CPE) procedure which was developed for the separation and preconcentration of trace amounts of cobalt is combined with flame atomic absorption spectrometry (FAAS) to determine trace amounts of cobalt in water and food samples. The procedure is based on the formation of the hydrophobic complex between Co(II) and 4-methoxy-2-sulfo-benzenediazoaminoazo-benzene (MOSDAA) followed by its extraction into a Triton X-114 surfactant-rich phase. The parameters such as pH of sample, concentrations of MOSDAA and Triton X-114, equilibrium temperature, and equilibrium time, which affect both complexation and extraction, are optimized. Under the selected optimum conditions, the preconcentration of 10.0 mL, 0.1
Cobalt is an essential trace element in human body. Being a component of vitamin B12 (cyanocobalamin), it plays an important role in the production of the blood red cells and the prevention of pernicious anemia [
In the aspect of determining trace amounts of cobalt in different matrixes, many analytical techniques such as flame atomic absorption spectrometry (FAAS) [
Recently, cloud point extraction (CPE) has become an attractive area for the separation and preconcentration of trace metal ions [
All chemicals used in the work were of analytical reagent grade. Deionized water
The structure of MOSDAA.
A Shimadzu AA-6800 atomic absorption spectrometer equipped with a cobalt hollow cathode lamp (Beijing Shuguangming Electronic Lighting Instrument Co., Ltd.) and an air-acetylene flame atomizer; was used in all determinations. The instrumental parameters were adjusted according to the manufacturer’s recommendations. The working conditions are as follows: wavelength (240.7 nm), lamp current (4.0 mA), spectral bandwidth (0.2 nm), height of observation (5.0 mm), flow rate of air (6.0 L min−1), and flow rate of acetylene (1.6 L min−1). An 80-2 centrifuge (Jintan Ronghua Instrument Manufacture Co., Ltd.) was used to accelerate the phase-separation process. A pHS-3C pH-meter (Shanghai Dapu Instruments Co., Ltd.) furnished with a combined glass-saturated calomel electrode was used for pH measurements.
A cloud point experiment has been carried out according to the following procedure. Aliquots of 15 mL of the working solution containing Co(II) ion (2.0
The real samples include water and millet samples. Water samples were taken from Shili river of Datong city. They were filtered into a clean flask. Millet samples (2.50 g) were porphyrized and dried. They were put into the dry beaker, and then 30.0 mL hydrogen peroxide was added. After 10 h of the reaction, the mixture was placed on the heating furnace to dissolve at a low temperature for about 3 h. When the resultant solution was heated to dryness, 5.0 mL HNO3, was added. After the solution was dry again, proper amount of water was added. The solution was heated for a while, and then it was cooled at room temperature. The solution was diluted to 20 mL. The blank solution was also prepared.
To achieve the best performance for the cloud point extraction procedure, the effects including pH, concentrations of ligand and surfactant, temperature and time of equilibration on the analytical signal, dilution condition, centrifuge time and rates, and interfering ions were evaluated and optimized.
The formation of metallic complex and its chemical stability are the two important factors involved in the separation and preconcentration of metal ions by cloud point extraction (CPE). They need to present sufficient hydrophobicity to be extracted into the small volume of the surfactant-rich phase. The pH plays a critical role on metallic complex formation and subsequent extraction and has been a significant parameter for CPE. Thus, extraction yield depends on the pH at which complex formation is investigated. A set of similar experiments was carried out in the pH range of 4.0–10.0. The solutions were buffered by mixtures of sodium tetraborate 10-hydrate and acid boric or acetic acid and sodium acetate. As it can be seen in Figure
Effect of pH on the analytical signal of cobalt. Conditions: 2.0
In addition, it is noted that the absorbance values are large and stable when the amount of buffer solution (pH 9.0) is in the range of 1.0–2.0 mL (see Figure
Effect of the amount of buffer solution on the absorbance of Co(II). Conditions: 2.0
After the formation of sparingly water-soluble complexes, the CPE is used for the preconcentration of metal ions. Its efficiency strongly depends on the hydrophobicity of the ligand and the complex formed in the miceller medium. Therefore, the effect of the amount of MOSDAA on the analytical responses was subsequently studied. Therefore, a set of similar experiments under the conditions of 2.0
Effect of the amount of MOSDAA on the absorbance of Co(II). Conditions: 2.0
Triton X-114 is one of the nonionic surfactant extensively used in CPE [
Effect of the amount of Triton X-114 on the absorbance of Co(II). Conditions: 2.0
Two important factors in cloud point extraction are equilibration temperature and incubation time. It is known that when CPE is conducted using equilibration temperatures that are well above the cloud point temperature of the surfactant, the greatest analyte preconcentration factors will be obtained [
Effect of equilibration temperature on the absorbance of Co(II). Conditions: equilibration time of 20 min, centrifugation for 5 min at 1500 rpm.
Effect of equilibration time on the absorbance of Co(II). Equilibration temperature of 80°C, centrifugation for 5 min at 1500 rpm.
It should be noted that after cloud point preconcentration, the surfactant-rich phase obtained becomes very viscous owing to the Triton X-114 that it contains. In order to facilitate the sample introduction in the FAAS nebulizer, it is necessary to decrease the viscosity of the surfactant-rich phase to facilitate the subsequent handling and introduction into the atomizer. So it is usually necessary for the addition of a diluting solution in the surfactant-rich phase to obtain a clear and homogenous solution of low viscosity compatible with the requirements of flame and plasma nebulizer. Hence, a solution of 0.1 mol L−1 HNO3 was selected as the diluting agent. Various amounts of 0.1 mol L−1 HNO3 were added to decrease the viscosity of the surfactant-rich phase. The results indicate that the largest absorbance is obtained when 0.5 mL of 0.1 mol L−1 HNO3 is added and then it was diluted to 2.0 mL by the secondary distilled water (see Figure
Effect of the amount of HNO3 on the absorbance of Co(II).
It is very necessary to preconcentrate trace amounts of Co(II) ion with high efficiency in a short time. Therefore, on the basis of the optimum conditions so far obtained, the effect of the centrifuge time and rates was studied. The results suggest that centrifugation for 5 min at 1500 rpm and cooling for 10 min in an ice-bath lead to the highest recovery and sensitivity for Co(II) ion.
The effects of foreign ions on the extraction of 2 ug/mL Co(II) were tested. The tolerated amounts of each ion were the concentration values tested that caused error less than ±5% of the recovery alteration. The tolerable concentration ratios of foreign ions within the ranges are summarized in Table
Effects of the foreign ions on the recoveries of the Co(II) ion (
Interfering ions | Interfering ion/analyte fold ratio |
Recovery/% |
---|---|---|
K(I) | 1000 | 101.4 |
Na(I) | 1000 | 99.3 |
Mg(II) | 1000 | 103.8 |
Ca(II) | 1000 | 96.2 |
Mn(II) | 200 | 98.7 |
Pb(II) | 200 | 100.6 |
Cu(II) | 20 | 97.6 |
Zn(II) | 20 | 95.6 |
Ba(II) | 100 | 96.0 |
Ni(II) | 100 | 95.2 |
Fe(III) | 10 | 96.3 |
A calibration curve of absorbance versus concentration was constructed by collecting the analytical signals of different volumes of Co(II) standard solutions submitted to the method proposed. Under the optimum experimental conditions, the calibration curve for Co(II) ion is linear from 0.002 to 1.2
Characteristics performance of the presented CPE method.
Parameter | Optimum value |
---|---|
Equation for the calibration curve, |
|
Correlation coefficient ( |
0.9984 |
Linear range ( |
0.002–1.2 |
RSD (%, |
2.78 |
Limit of detection (ng mL−1) ( |
0.47 |
Enrichment factor | 19 |
Extraction coefficient | ~1 |
Phase volume ratio | 0.02 |
Enrichment factor: ratio between Co concentration in the surfactant-rich phase and in the original solution.
Extraction coefficient: ratio between Co quantity in the surfactant-rich phase after the cloud point and in the original solution.
Phase volume ratio: ratio between the final volume of the surfactant-rich phase and the aqueous phase.
A comparison of the represented method with other reported cloud point extraction methods is given in Table
In order to validate the methodology, the proposed method was applied to the determination of Co(II) ion concentration in water and millet samples. The accuracy was checked by spiking the samples with different concentrations of Co(II) ion. The results are shown in Table
Determination of cobalt in water and millet samples (
Sample | Added/ |
*Found value/ |
Recovery/% |
---|---|---|---|
Millet | 0 |
|
|
0.05 |
|
100.0 | |
0.1 |
|
100.0 | |
1.0 |
|
98.0 | |
| |||
River water | 0 |
|
|
0.1 |
|
100.0 | |
0.2 |
|
95.0 | |
1.0 |
|
96.0 |
*
Procedure using cloud point extraction prior to cobalt determination by FAAS.
Reagent | Surfactant | Sample volume/mL | EFa | LODb/( |
Reference |
---|---|---|---|---|---|
Ammonium pyrrolidine dithiocarbamate | Triton X-114 | 10 | 20 | 5 | [ |
1-(2-Thiazolylazo)-2-naphthol | Triton X-114 | 50 | 57 | 0.24 | [ |
1-(2-Pyridylazo)-2-naphthol | Triton X-114 | 10 | 115 | 0.38 | [ |
2-(5-Bromo-2-pyridylazo)-5-diethylaminophenol | Triton X-100 | 12.5 | 28.5 | 1.06 | [ |
1-nitroso-2-naphthol | PONPE 7.5 | 10 | 27 | 1.22 | [ |
2-[2′-(6-Methyl-benzothiazolylazo)]-4-bromophenol | Triton X-114 | 10 | 28 | 0.9 | [ |
Methyl-2-pyridylketone oxime | Triton X-114 | 15 | 67 | 2.1 | [ |
2-[(2-Mercaptophenylimino)methl]phenol | Triton X-114 | 25 | 97 | 0.21 | [ |
2-guanidinobenzimidazole | Triton X-114 | 50 | 13 | 7.8 | [ |
1-Phenylthiosemicarbazide | Triton X-114 | 50 | 25 | 1 | [ |
4-Methoxy-2-sulfo-benzenediazoaminoazo-benzene | Triton X-114 | 15 | 19 | 0.47 | This work |
aEnhancement factor, blimit of detection.
Cloud point extraction is a simple, inexpensive, sensitive, and rapid method in preconcentration and separation of trace metal. TritonX-114 is used as cloud point extractant because it has the low cloud point temperature and high density, and, more importantly, it is very cheap. In the pH 9.0 buffer system of sodium tetraborate 10-hydrate and acid boric, Co(II) and MOSDAA can form stable complexation. After extraction preconcentration by neutral surface active agents Triton X-114, the element Co can be measured to the level of ng mL−1, which is very satisfactory.