Complex Formation in a Liquid-Liquid Extraction System Containing Co ( II ) , 4-( 2-Thiazolylazo ) resorcinol and Monotetrazolium Salt

The ion-associated complex formed between the anionic chelate of Co(II)–4-(2-thiazolylazo)resorcinol (TAR) and the monotetrazolium cation of 2-(4-iodophenyl)-3-(4-nitrophenyl)-5-phenyl-2H-tetrazolium chloride (INT) in the liquid-liquid extraction system Co(II)–TAR–INT–H2O–CHCl3 was studied by the spectrophotometric method. The optimum extraction conditions of Co(II) were established and the extraction equilibria were investigated. The equilibrium constants, the recovery factor and some analytical characteristics were calculated. The validity of Beer’s law was checked. The molar ratio of the components in the ternary ion-associated complex Co(II)–TAR–INT was determined and the general formula of the complex was suggested. The effect of various foreign ions and reagents on the process of complex formation in the liquid-liquid extraction system was studied.


Introduction
The cobalt refers to the group of elements essential from biochemical point of view.For example, one of compounds containing cobalt complexes is vitamin B 12 which participates in the synthesis of the hemoglobin and affects the protein and lipid metabolism.2][3][4] Cobalt is a typical transition metal participating in complex formation.1][12][13][14][15][16][17][18][19] Tetrazolium salts are used as reagents for the extraction-spectrophotometric determination of metals, e.g.1][22] The preparation and application of ion-associated complexes of anionic chelates of metals with various natural organic and inorganic ligands with N-and Ocontaining donor atoms and with the participation of mono and ditetrazolium cations is a special scientific research field of the chemistry of the coordination compounds.It is up-to-date topic, not only as a theoretical background for the preparation of novel ion-associated complexes, but mainly due to the possibility for their application in the analytical chemistry for determination of various metals in natural, industrial, pharmaceutical and biological samples, addressing in such a way a number of ecological issues.4][25][26] The extraction equilibria were characterized quantitatively (association constant, distribution constant, extraction constant and recovery factor) and the analytical characteristics (molar absorptivity, Sandell's sensitivity, Divarova et al.: Complex Formation in a Liquid-Liquid Extraction ... adherence to Beer's law, limit of detection, limit of quantification) were calculated.The present work is a part of a thorough and comprehensive study on the ion-associated complexes of cobalt(II) with azo derivatives of resorcinol and mono-or ditetrazolium salts.

Procedure for Establishment of the Optimum Extraction-Spectrophotometric Conditions
Aliquots of Co(II), TAR, INT and buffer (pH = 3.0-6.5)solutions were filled into separatory funnels.The resulting solutions were diluted with distilled water to a total volume of 10 mL.Then 10 mL of chloroform was added and the funnels were shaken.A portion of the organic extract was filtered through a filter paper into a cell and the absorbance was read against a blank sample. 25,26

3. Procedure for Determination of the Distribution Constant
In order to determine the distribution constant K D , it is necessary to measure the light absorbance A 1 and A 3, which are respectively the light absorbance after a single extraction in chloroform under optimum operating conditions and after a triple extraction performed under the same conditions.After that the distribution constant K D can be calculated according to the ratio K D = A 1 / (A 3 -A 1 ).Single extraction: the single extraction is conducted with 10 mL of chloroform.After the separation of the two phases, the organic extract is transferred into a 25 mL calibrated flask which is brought to volume with chloroform.The measurement of the light absorbance A 1 is done against a blank sample, prepared under the same conditions.Triple extraction: the first stage of the triple extraction is performed with 10 mL of chloroform and the extract is transferred into a 25 mL calibrated flask.During the second stage of the extraction, 8 mL of chloroform are added to the aqueous phase remaining after the first stage.The organic layer is added to that from the first stage.For the third stage of extraction, 7 mL of chloroform are added to the aqueous phase remaining after the second stage and for the third time an extraction is performed.The organic layer is transferred to the previous two.The calibrated flask is brought to volume with chloroform.The measurement of A 3 is performed against a blank sample prepared in the same way.

1. Optimum Extraction-Spectrophotometric Conditions
The absorption spectrum of the extract of the studied ternary ion-associated complex Co-TAR-INT in CHCl 3 was characterized by an absorption maximum in the visible range (λ max = 520 nm) (Figure 1).The acidity of the aqueous phase is the most important factor influencing the extraction of the anionic chelate Co(II)-TAR into the organic phase as an ion-associated complex.The maximum and constant extraction of the ion-associated complex was achieved in the pH range from 4.5 to 5.5 (Figure 2).Aceta- The optimum experimental conditions for the extraction of the ion-associated complex are summarized in Table 1 column 1.

2. Beer's Law and Analytical Characteristics
The range of adherence to Beer's law, i.e. the linear relationship between the cobalt concentration in the aqueous phase and the absorbance of the ion-association complex in the organic phase after extraction was studied us-ing regression analysis under the optimum conditions for complex formation.The equation of a straight line was found to be Y = 0.8474 X + 0.0207 with a correlation coefficient squared 0.9978.Further analytical characteristics, e.g.apparent molar absoptivity ε -, adherence to Beer's law, Sandell's sensitivity, limit of detection and limit of quantification, are shown in Table 1, column 2.

Molar Ratios of the Complex
The mobile equilibrium method and the straight-line method of Asmus were applied to prove the molar ratios Co-TAR and Co-INT. 27The results of application of the two independent methods are shown in Figure 3 and Figure 4, respectively.On the basis of the results it can be concluded that Co(II), TAR and INT interact in molar ratio 1:2:1.

4. Reaction Scheme, Suggested General Formula, Extraction Equilibria and True Molar Absorptivity
In the pH range from 1.5 to 6.5, the reagent TAR is presented in a molecular form (H 2 R).The deprotonation of TAR (HR -) starts at pH = 4.0, while a complete deprotonation (R 2 -) is achieved in the alkaline range (pH > 11). 28The performed experiments showed that the extraction of the ternary ion-associated complex Co(II)-TAR-INT took place in the pH range 4.5-5.5.Under these conditions, an equilibrium between the molecular form of TAR (H 2 R) and the monoprotonated form (HR -) exists in the solution.Hence, the complex formation of anionic chelate Co(II)-TAR is given by the equation (1): Having in mind the indicated above molar ratio and the reaction of chelate formation of Co(II)-TAR, it can be suggested that the formation of ion-association complex in the aqueous phase, its distribution between the aqueous and the organic phase and its extraction in chloroform can be given by the following equations (2-4): Therefore, the ion-associated complex chelate of Co(II)-TAR with INT can be represented by the general formula (INT)[Co(HR)R].The association process in aqueous phase and the extraction process were investigated and quantitatively characterized.The constants needed for the quantitative assessment of the equilibria were calculated as follows: (i) the association constant β and the true molar absorptivity ε by the method of Komar -Tolmachev (Figure 5).27 The value of the true molar absorptivity ε is shown in Table 1 column   (ii) the distribution constant K D and the recovery factor R%: where A 1 is the absorption for a single extraction, A 3 -the absorption for a triple extraction; (iii) the extraction constant K ex : The values of the equilibrium constants and the recovery factor are presented in Table 2.They indicate that the ternary ion-associated complex of the chelate Co(II)-TAR with INT is characterized by sufficiently high stability and good extraction.Based on this, it can be proposed that the ion-associated complex of Co-TAR-INT can be successfully used for determination of cobalt in alloys, biological, medical and pharmaceutical objects.For this purpose the effect of various foreign ions and reagents on the process of complex formation was studied.

5. Effect of Foreign Ions and Reagents on the Complex Formation
The effect of various foreign ions and reagents on the process of ion-association and complex formation between the anionic chelate Co(II)-TAR and monotetrazolium cation was studied under optimum extraction conditions (Table 1, column 1).The concentration of Co(II) in the presence of the foreign ion was determined from the sequence of Beer's law. 27For an interfering effect a deviation of ±3% from the absorbance of the complex in the absence of foreign ions or reagents was accepted.The results are presented in Table 3.
From them it can be concluded that most of the cations studied do not interfere.The ions of Al(III), Cu(II), Zn(II), Ni(II), Fe(II), Fe(III), Cr(III), V(IV), V(V), SCN -and C 4 H 4 O 6 2-in concentrations, comparable with that of Co(II), also do not hinder the extraction of Co(II) as an ternary associated complex with TAR and INT.However, by the application of the method for different objects, in many cases these concentrations are much higher.For this reason they have to be masked or removed from the extraction system.Fe(II) is readily oxidized and the influence of the Fe(III), Cr(III) and Al(III) is removed after pre-precipitation with NH 3 .The separation of Co(II) from Cd(II) and Zn(II) is based upon the different sustainability of their dithizonates to acids. 30Upon extraction with dithizone (pH = 8), dithizonates of Cd(II), Zn(II) and Co(II) pass in the chloroform extract.Upon subsequent pre-extraction with a solution of hydrochloric acid (pH = 2-3), the dithizonates of Cd(II) and Zn(II) are destroyed. 31The organic layer is analyzed for Co(II) based on above-mentioned procedure.The interference of Cd(II) is removed by a pre-extraction with dithizone. 30The extraction equilibrium is hindered by Ni(II) and V(V).

Conclusion
The complex formation of anionic chelate of cobalt(II)-4-(2-thiazolylazo)resorcinol with the monotetra-   by the extraction-spectrophotometry method.In the presence of a monotetrazolium cation, the colored anionic chelate complex Co(II)-TAR forms well soluble in chloroform ion-associated complex.The optimum conditions for the formation and solvent extraction of the ion-associated chelate complex of Co(II) were established.The effect of various foreign ions and reagents on the process of complex formation was studied.The molar ratio of the components, determined by independent methods, shows that the ion-association complex could be represented with the general formula (INT) + [Co(HR)R] -.From the values of the equilibrium constants, the recovery factor and the analytical characteristics of the ion-associated complex Co-TAR-INT it can be concluded that the availability of hydrophilic substitutes in the phenyl radicals of the tetrazolium salt INT stabilizes the ion-associate in the aqueous phase.The bulky organic molecule of INT determined the extractability of the ion-associated complex in organic phase.

Table 1 .
Optimum extraction-spectrophotometric conditions and analytical characteristics of the system Co(II)-TAR-INT-H 2 O-CHCl 3

Table 3 .
Effect of foreign ions and reagents on the complex formation of the ion-associate Co(II)-TAR-INT for extraction in the presence of 5 μg Co(II)

Table 2 .
Values of the equilibrium constants and the recovery factor