Study on the Complex Equilibria of Molybdenum ( VI ) with 3 , 5-Dinitrocatechol and Ditetrazolium Salt

The complex formed between an anionic chelate of Mo(VI)–3,5-dinitrocatechol (3,5-DNC) with the cation of 3,3'-(3,3'dimethoxy-4,4'-biphenylene)bis(2,5-diphenyl-2H-tetrazolium chloride) (Blue Tetrazolium Chloride, BTC) in the liquid-liquid extraction system Mo(VI)–3,5-DNC–BTC–H2O–CHCl3 was studied. The optimum conditions for the complex formation and extraction of the ion-associated complex were established by spectrophotometry. The molar ratio of the reagents was determined by independent methods. The validity of Beer’s law was checked and some analytical characteristics were calculated. The association process in aqueous phase and the extraction equilibria were investigated and quantitatively characterized. The following key constants of the processes were calculated: association constant, distribution constant, extraction constant and recovery factor. Based on this, a reaction scheme, a general formula and a structure of the complex were suggested.


Introduction
The molybdenum is the only second row transition metal essential from biochemical point of view.It stimulates the synthesis of nucleic acids and proteins.In the biological systems, the molybdenum forms complexes with the carboxylic or hydroxide groups of tyrosine and serine.The most important utilization of the molybdenum atom in the living organisms is as a metal hetero-atom at the active site in certain enzymes, e.g.][3][4] The molybdenum occurs in various oxidation states, coordination numbers, geometries and its chemistry is among the most complex of the transition elements. 1,5,6[9][10][11][12][13][14][15][16] Molybdenum(VI) gives colored chelates with aromatic compounds, containing two or more hydroxyl groups in o-position relative to each other.The colored anionic chelates of molybdenum(VI) form ion-associated complexes with bulky organic cations, like methyltrioctylammonium, cetylpyridinium, cetyltrimethylammonium, tetraphenylammonium. 7,[17][18][19][20] The structure and properties of tetrazolium salts determine their ability to form ion-associated complexes. 21The bulky hydrophobic organic substituents in the molecules of the tetrazolium salts increase the extractability of the ion-associated complexes.The presence of a quaternary nitrogen atom in the molecules of the tetrazolium salts determines the ability to form ionic associates in aqueous phase without protonation, as opposed to the amines.Tetrazolium salts are used as reagents for the preparation of various ion-associated complexes of metals, e.g.8][29][30][31] The liquid-liquid extraction is a part of the chemistry of the solutions and the coordination compounds.It is applied to study the processes of complex formation and the extraction equilibria.This present work aims to study the extraction equilibria for complex formation between the anionic chelate of Mo(VI)-3,5-dinitrocatehol (3,5-DNC) and the cation of 3,3'-(3,3'-Dimethoxy-4,4'-biphenylene)bis(2,5-diphenyl-2H-tetrazolium chloride) (Blue Tetrazolium Chloride, BTC) by spectrophotometry.
A Camspec M508 spectrophotometer (UK), equipped with 10 mm path length cells, was employed for reading the absorbance values.

Procedure for Establishment of the Optimum Conditions for Complex Formation
Aliquots of the solution of Mo(VI), BTC and H 2 SO 4 were introduced into 250 cm 3 separatory funnels.The resulting solutions were diluted with distilled water to a total volume of 10 cm 3 .A required volume of a chloroform solution of 3,5-DNC was added and the organic phase was adjusted to a volume of 10 cm 3 with chloroform.The funnels were shaken for a fixed time (up to 180 s).A portion of the organic extract was filtered through a filter paper into a cell and the absorbance was read against a blank.The blank extraction was performed in the same manner in the absence of molybdenum.

3. Procedure for Determination of the Distribution Constant
The distribution constant (K D ) was determined by the equation (1), where A 1 and A 3 are the absorbances (measured against blanks) obtained after a single and triple extraction, respectively.
The single extraction and the first stage of the triple extraction were performed under the optimum conditions for complex formation (Table 1, column 1).The organic layers were transferred into 25 cm 3 calibrated flasks and the flask from the single extraction was brought to volume with chloroform.The second stage of the triple extraction was performed by adding 7 cm 3 of chloroform to the aqueous phase that remained after the first stage.After extraction, the obtained extract was added to that of the first stage of the triple extraction.The third stage of the triple extraction was performed in the same manner as for the second stage and the extract was added to those the first two stages.The volume of the flask was brought to the mark with chloroform.The calibrated flasks were shaken before the spectrophotometric measurements. 32

1. Absorption Spectra, Effect of Acidity of the Aqueous Phase and Shaking Time
The colored anionic chelate of molybdenum (VI)-3,5-DNC was extracted in chloroform in the presence of the bulky hydrophobic ditetrazolium cation.The absorption spectrum of the extract of the ion-associated complex Mo(VI)-3,5-DNC-BTC in CHCl 3 is characterized by an absorption maximum in the visible range (λ max = 405 nm, Figure 1).
The acidity of the aqueous phase has a substantial effect on the extraction equilibrium.The maximum and constant extraction of the ion-associated complex is achieved in strongly acidic solution of (0.2-1.6) mol L -1 H 2 SO 4 .The further experiments were performed with 0.4 mol L -1 H 2 SO 4 .The carried out experiment showed that the extraction equilibrium cannot be achieved within less than 30 s.The prolonged shaking does not have an impact on the absorbance.The next experiments were performed for 1 min.
The optimum extraction-spectrophotometric conditions for the chelate formation and the extraction of the ion-associated complex Mo(VI)-3,5-DNC-BTC are summarized in Table 1, column 1.

Beer's Law, Apparent Molar Absorptivity and other Analytical Characteristics
For establishment of the range of adherence to Beer's law, i.e. the linear relationship between the molybdenum concentration in the aqueous phase (C Mo(VI) , μg cm -3 ) and the absorbance of the ion-association complex in the organic phase after extraction regression analysis under the optimum conditions for complex formation was used.The equation of a straight line was found to be Y = 0.1547 X + 0.0123 with a correlation coefficient squared 0.9969.Under the optimum conditions for complex formation, the linearity is observed for concentrations up to 7.48 μg cm -3 Mo(VI).Further analytical characteristics, e.g.apparent molar absorptivity ε´, Sandell's sensitivity, limit of detection and limit of quantification, are shown in Table 1, column 2.

4. Molar Ratios of the Complex, Reaction Scheme and Suggested General Formula
The straight-line method of Asmus, the mobile equilibrium method and the method of continuous variations were applied to prove the molar ratios Mo(VI):3,5-DNC and Mo(VI):BTC. 33The results from the application of the independent methods are shown in Figure 2, Figure 3 and Figure 4, respectively.

Optimum conditions Analytical characteristic Absorption maximum (λ max ) 405 nm
Apparent molar absorptivity (ε´) (1.551 ± 0.078) × 10 4 L mol -1 cm -1 Volume of the aqueous phase 10 cm 3 True molar absorptivity (ε) (1.609 ± 0.062) × 10 4 L mol -1 cm -1 Volume of the organic phase 10 cm 3 Sandell's sensitivity (SS) 6.19 ng cm -2 Concentration of H 2 SO 4 in the aqueous phase 4.0 × 10 -1 mol L -1 Adherence to Beer's law up to 7.48 μg cm -3 Shaking time (τ) 1 min Relative standard deviation (RSD) 2.50% Concentration of 3,5-DNC 2.0 × 10 -4 mol L -1 Limit of detection (LOD) 0.46 μg cm -3 Concentration of BTC 2.4 × 10 -4 mol L -1 Limit of quantification (LOQ) 1.52 μg cm -3 Based on the performed studies, it could be concluded that Mo(VI), 3,5-DNC and BTC interact in molar ratio 1:2:1.The carried out experiments showed that the complex formation and the extraction of the ion-associated complex occurred in strongly acidic solution.Under these conditions, the complex formation of anionic chelate Mo(VI)-3,5-DNC is given by the equation ( 2): Having in mind the molar ratio indicated above and the reaction of chelate formation of Mo(VI)-3,5-DNC, it can be suggested that the formation of the ion-associate 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 (3-5).

5. Extraction Equilibria, True Molar Absorptivity, Recovery Factor and Structure of the Complex
The association process in aqueous phase and the extraction equilibria were investigated and quantitatively  The association constant β was determined by two independent methods: Komar-Tolmachev method and Holme-Langmyhr method. 33,34The true molar absorptivity ε was determined by the method of Komar-Tolmachev (Figure 5).
The association constant β was calculated by the equation ( 6): 33 where l is the cuvette thickness (l = 1 cm); n -the molar ratio between the components independently determined (e.g. by the mobile equilibrium method, the straight-line method of Asmus or the method of continuous variations) (n = 1), ε -the true molar absorptivity.The distribution constant K D was determined from the equation ( 1) and the recovery factor -from equation ( 7) The extraction constant K ex was calculated by two independent methods: where β was determined by the method of Komar-Tolmachev.
(ii) the method of Likussar-Boltz 35 The method uses the data from the method of continuous variations (Figure 5).The extraction constant K ex was calculated by the equation of Likussar-Boltz for molar ratio 1:1 ((equation ( 9)): where The values of the equilibrium constants and the recovery factor are presented in Table 2.The analysis of the results obtained showed that sufficiently stable ion-associated complex was formed in the aqueous phase and it was quantitatively extracted into the organic phase with high sensitivity.
The results obtained by the independent methods are statistically dissimilar and confirm the proposed scheme of the process of complex formation of the ion-associate in the aqueous phase, its distribution between the aqueous and the organic phase and its extraction in chloroform.

Equilibrium constant and recovery factor Value
Equilibrium (equation 3) -Association constant β log β = (5.02± 0.90)  Based on this, the proposed structure of the ion-associated complex is represented in Figure 6.

Conclusion
The solvent extraction of an ion-associated complex formed between the anionic chelate of Mo(VI)-3,5-Dinitrocatechol (3,5-DNC) with the cation of the ditetrazolium salt, Blue Tetrazolium Chloride (BTC) was studied by spectrophotometry.The processes of the chelate formation and extraction of the ion-associated complex Mo(VI)-3,5-DNC-BTC into chloroform were investigated.The optimum conditions for the association in aqueous phase and extraction of the ion-associated complex were established.The equilibrium constants and analytical characteristics needed for the quantitative assessment of the extraction equilibrium were calculated, i.e. the association constant (β), the distribution constant (K D ), the extraction constant (K ex ), the recovery factor (R), the apparent molar absorptivity (ε´), the true molar absorptivity (ε), the limit of detection (LOD), the limit of quantification (LOQ) and the Sandell's sensitivity (SS).The molar ratio of reagents determined by independent methods showed that the ion-associated chelate complex of Mo(VI)-3,5-DNC with BTC could be represented by the general formula (BTC){MoO 2 [O 2 C 6 H 2 (NO 2 ) 2 ] 2 }.A reaction scheme and a structure of the complex were suggested.

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