Complex Formation in a Liquid-Liquid Extraction System Containing Cobalt(II), 4-(2-Pyridylazo)resorcinol, and Nitron

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)[Co(PAR) 2 ], were found. The following key equilibrium constants were calculated: constant of association (Logβ = 4.77 ± 0.06), constant of distribution (LogKD = 1.34 ± 0.01), and constant of extraction (LogKex = 6.11 ± 0.07). Beer’s law was obeyed for Co concentrations up to 1.7μgmL −1 with a molar absorptivity of


Introduction
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 [1].Cobalt deficiency (and hence vitamin B-12 deficiency) can lead to a wide spectrum of hematologic, neuropsychiatric, and cardiovascular disorders.On the other hand, cobalt can be toxic when consumed in excessive quantities [2,3].That is why its content in various samples is monitored, despite the fact that the existing methods for cobalt determination are not enough sensitive or cost effective [4][5][6][7].
(iv) Acetate buffer solution, prepared by mixing of 2 mol L −1 aqueous solutions of CH 3 COOH and NH 4 OH.The resulting pH was checked by HI 83140 pH meter (Italy).

Procedure for
Establishing the Optimum Operating Conditions.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.  under the optimum extraction-spectrophotometric conditions (Table 1).The organic layers were transferred into 25 mL calibrated flasks and the flask for the single extraction was brought to volume with Nt solution.The second stage of the double extraction was performed by adding another 10 mL portion of the Nt solution to the aqueous phase, which remained after the first stage.After shaking, the organic layer was added to the one obtained after the first stage and the volume was brought to the mark with Nt solution.Before the spectrophotometric measurement, the calibrated flasks were shaken for homogenization.

Results and Discussion
3.1.Absorption Spectra.Spectra of the extracted ternary Co-PAR-Nt complex and the blank are shown in Figure 2. A maximum is recorded at 520 nm, where the blank absorbs insignificantly.It is shifted to 10 nm as compared to the maximum of the binary Co-PAR chelate existing in aqueous medium (in the pH interval from 3.5 to 10): 510 nm [9,27,29,31,39].The observed bathochromic effect is small and gives us grounds to suggest the formation of a ternary compound of the ion-association type.

Effect of pH.
Results showed that the optimal pH for the extraction of Co with PAR and Nt is 5.2-5.4(Figure 3).A buffer solution with a concentration of 2 mol L −1 was applied to control pH.The use of 0.25-5 mL of the buffer solution per 10 mL (final aqueous solution) was found to give a constant absorbance.All further experiments were carried out with 5 mL buffer solution with pH = 5.2-5.3.

Effect of Reagents' Concentrations. The effect of PAR and
Nt concentrations on the absorbance is shown in Figure 4.
For up to 1.7 g mL −1 of Co, the use of about 0.5 mL of 2.0 × 10 −3 mol L −1 PAR and 8.5 mL of 3.5 × 10 −4 mol L −1 Nt was found to be sufficient for a complete cobalt extraction.

Effect of Shaking Time.
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 (2)  = 1 + 4.7 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.

Composition of the Complex and Suggested Formula.
The molar PAR-to-Co(II) and Nt-to-Co(II) ratios were determined by the mobile equilibrium method [40] (Figure 5), molar ratio method [43] (see Figure 4), and the  method of Asmus [44] (Figure 6).The results showed that the ternary complex has a composition of 1 : 2 : 1 (Co : PAR : Nt).
Having in mind the obtained molar ratios and several reports, which convincingly demonstrate that the labile Co(II)-PAR complex can be easily oxidized to an inert Co(III)-PAR complex, [Co 3+ (PAR) 2 ] − , by the atmospheric oxygen [13,26,29,32,33,36,45], we suggest the following formula of the extracted ternary species: this formula, PAR is in deprotonated form (PAR 2− ), while Nt is in protonated form (NtH + ).The mentioned formula and the known properties of PAR [45][46][47] and Nt [48] fit well to the observed pH curve of the ternary complex presented in Figure 3: (1) at pH values lower than pH opt PAR is hardly possible to be in its PAR 2− form; (2) at pH values higher than pH opt Nt is hardly possible to be in its NtH + form.The right part of the pH curve has a steeper slope, because the limitation 2 is more strictly.

Equilibrium Constants and Recovery.
Several equilibrium processes should be taken into account for the system of [Co(PAR) 2 ] − , NtH + , water, and chloroform.
(i) Formation of ion-association complex in the aqueous phase: (ii) Distribution of the complex between the aqueous and the organic phase: (iii) Extraction from water into chloroform: The equilibrium constants describing these equations and the obtained values are shown in Table 2.The association constant  was determined by several independent methods: Holme-Langmihr method [41], Harvey-Manning method [42], and mobile equilibrium method [40] (Figure 5, straight line 2).The distribution constant   was calculated from the absorption values obtained after single and double extraction as described above.The extraction constant  ex was calculated by the formula  ex =   × .The recovery factor  was estimated by the dependence % = 100   /(  + 1) and the following value was obtained  = 95.7%.All experiments were performed at room temperature of ∼22 ∘ C and the calculations were carried out at a probability of 95%.

Beer's Law, Molar Absorptivity, and other Analytical
Characteristics.The range of adherence to Beer's law was studied at the optimum conditions (Table 1).The linearity is observed up to 1.7 g mL −1 of Co with a correlation coefficient of 0.9995.The obtained straight line equation is  = 0.973X + 0.014.The molar absorptivity was calculated to be 6.0 × 10 4 L mol −1 cm −1 .This value could compete successfully with the ones obtained for similar PAR-containing complexes (Table 3).The limit of detection (LOD) and limit of quantification (LOQ) were estimated at 3 times and 10 times standard deviation of the intercept divided by the slope.Sandell's sensitivity was calculated as well.The values of the above-mentioned characteristics are included in Table 1.The molar absorptivity of the binary cobalt-PAR complex in aqueous medium is  510 = 5.6 × 10 4 L mol −1 cm −1 [39].

Conclusions
intensively colored anion, [Co(PAR) 2 ] − , in which cobalt is in +3 oxidation state, and a bulky hydrophobic cation (protonated Nitron, NtH + ).The following equilibrium constants and analytical characteristics were determined: constant of extraction, constant of association, constant of distribution, recovery factor, molar absorptivity, Sandell's sensitivity, limit of detection, and limit of quantification.The obtained values show that the studied extraction system in the present work could compete successfully with many similar systems used for cobalt determination.
Distribution Constant.The distribution constant   was found from the ratio   =  1 /( 2 −  1 ), where  1 and  2 are the absorbances (measured against blanks) obtained after a single and double extraction, respectively.The single extraction and the first stage of the double extraction were performed

Table 2 :
Calculated values of Log , Log   , Log  ex , and R%.

Table 3 :
Spectral characteristics of some extracted in organic solvents Co-PAR complexes * .