Complex formation and liquid-liquid extraction were studied in a system containing cobalt(II), 4-(2-pyridylazo)resorcinol (PAR), 1,4-diphenyl-3-(phenylamino)-1H-1,2,4-triazole (Nitron, Nt), water, and chloroform. The effect of some experimental parameters (pH, shaking time, concentration of PAR, and concentration of Nt) was systematically investigated, and the optimum conditions for cobalt extraction as an ion-association complex, (NtH+)[Co3+(PAR)2], were found. The following key equilibrium constants were calculated: constant of association
Cobalt is a transition metal which plays an essential role in industry and all living organisms. Its main applications are in the production of special steels and alloys, permanent magnets, cutting tools, batteries, catalysts, pigments for enamels and glass, and dryers for oil, paints, and varnishes. In biological systems cobalt acts as an active nutrient and an active center of coenzymes called cobalamines. The most important representative of this class of compounds is vitamin B-12: a key substance, which is normally involved in the metabolism of every cell of the human body, especially affecting DNA synthesis and neurologic function [
4-(2-Pyridylazo)resorcinol (PAR) has been proved to be one of the most important reagents for cobalt separation, preconcentration and determination [
Reagents in the present study, (a) 4-(2-pyridylazo)resorcinol (PAR), (b) 1,4-diphenyl-3-(phenylamino)-1H-1,2,4-triazole (Nitron, Nt).
PAR (96%, Sigma-Aldrich) dissolved in slightly alkalized distilled water, Nitron (≥97%, Fluka), Acetate buffer solution, prepared by mixing of 2 mol Chloroform (additionally distilled). Ultrospec 3300 pro UV/visible spectrophotometer (Amersham Biosciences), equipped with 10 mm path-length cells.
Aliquots of Co(II) solution, PAR solution (up to 1.4 mL), and buffer solution (5 mL; pH ranging from 3.0 to 6.2) were introduced into 250 mL separatory funnels. The resulting solutions were diluted with distilled water to a total volume of 10 mL. Appropriate amounts of Nt solution and chloroform were added in a total volume of 10 mL. Then the funnels were shaken for a fixed time (up to 5.0 min). A portion of the organic extract was filtered through a filter paper (to prevent the opportunity of water droplets transfer) into a cell and the absorbance read against a blank. The blank extraction was performed at the same manner, but in the absence of Co.
The distribution constant
Optimum conditions and analytical characteristics of the Co(II)-PAR-Nt-water-chloroform system.
Optimum conditions | Analytical characteristics |
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Wavelength: 520 nm | Molar absorptivity: |
pH: 5.3 (acetate buffer) | Beer’s law range: |
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Limit of detection: 0.06 |
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Limit of quantification: 0.20 |
Shaking time: 15–20 sec | Sandell’s sensitivity: 0.99 ng cm−2 |
Spectra of the extracted ternary Co-PAR-Nt complex and the blank are shown in Figure
Absorption spectra of the ternary Co-PAR-Nt complex and the blank (PAR-Nt) in chloroform.
Results showed that the optimal pH for the extraction of Co with PAR and Nt is 5.2–5.4 (Figure
Absorbance of Co-PAR-Nt extracts against blank (line 1) and blank against chloroform (line 2) versus pH of the aqueous phase plots.
The effect of PAR and Nt concentrations on the absorbance is shown in Figure
Absorbance of the extracted ternary complex versus concentration of the PAR (curve 1) and Nt (curve 2) plots.
The extraction equilibrium is reached for a short shaking time (about 5 seconds). It was found that a shaking time longer than 1 min can bring about to a slight decrease (5-6%) of the absorbance values. To avoid this disadvantage and to guarantee complete transfer of the complex into organic phase, even under nonoptimum conditions, the authors extracted in their experiments for 15–20 seconds.
The molar PAR-to-Co(II) and Nt-to-Co(II) ratios were determined by the mobile equilibrium method [
Determination of the PAR-to-Co
Determination of the Nt-to-Co molar ratio by the method of Asmus.
Several equilibrium processes should be taken into account for the system of Formation of ion-association complex in the aqueous phase:
Distribution of the complex between the aqueous and the organic phase:
Extraction from water into chloroform:
The equilibrium constants describing these equations and the obtained values are shown in Table
Calculated values of Log
Equilibrium | Equilibrium constant | Value |
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( |
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Log |
Log |
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Log |
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( |
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Log |
( |
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Log |
aCalculated by the mobile equilibrium method [
bCalculated by the Holme-Langmihr method [
cCalculated by the Harvey-Manning method [
dCalculated by the formula
The range of adherence to Beer’s law was studied at the optimum conditions (Table
Spectral characteristics of some extracted in organic solvents Co-PAR complexes*.
Additional reagent(s) | Organic solvent | Molar absorptivity, |
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Ref. |
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Xylomethazoline hydrochloride | Chloroform |
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535 | [ |
Diphenylguanidine | Chloroform |
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520–530 | [ |
Triphenyltetrazolium chloride | Chloroform |
|
515 | [ |
Iodonitrotetrazolium chloride | Chloroform |
|
515 | [ |
Zephiramine | Chloroform |
|
520 | [ |
Tetradecyl-dimethylbenzyl-ammonium chloride + EDTA | Chloroform |
|
520 | [ |
Dicyclohexyl-18-crown-6 | 1,2-dichloroethane |
|
515 | [ |
Tetraphenylarsonium chloride | Chloroform |
|
520 | [ |
Tetraphenylphosphonium chloride | Chloroform |
|
520 | [ |
Nitron | Chloroform |
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520 | This work |
Cobalt(II) forms well chloroform-extractable ternary complex with 4-(2-pyridylazo)resorcinol and Nitron. The complex could be regarded as an ion associate between an intensively colored anion,
The authors would like to thank the Research Fund of the Plovdiv University for its long-time support.