Ion-Associated Complex of the Anionic Chelate of Germanium ( IV ) with Nitro Derivative of the Catechol and the Cation of Monotetrazolium Salt

The formation of an ion-associated complex between of the anionic chelate of germanium(IV) with 4-nitrocatechol (4NC) and the cation of 3-(2-naphtyl)-2,5-diphenyl-2H-tetrazolium chloride (TV) was investigated by spectrophotometry. The optimum conditions for the chelate formation and the extraction of the complex in the liquid-liquid extraction system Ge(IV)‒4-NC‒TV‒H2O‒CHCl3 were established. The validity of Beer’s law was checked and some analytical characteristics were calculated. The association process in the aqueous phase and the extraction equilibria were studied and quantitatively characterized by the following key constants: association constant, distribution constant, extraction constant and recovery factor. The molar ratios of the components in the ion-associated complex were determined by independent methods. Based on this, a reaction scheme, a general formula and a structure of the ion-associated complex were suggested.


Introduction
2][3][4][5][6][7][8][9] The colored anionic chelates of Ge(IV) form ion-associated complexes with bulky organic cations, like tetradecyl(trihexyl)phosphonium, methyltrioctylammonium, cetylpyridinium, cetyltrimethylammonium, tetraphenylammonium, tetraphenylarsonium. [10][11][12][13][14][15][16] The structure and the properties of the tetrazolium salts determine their ability to form ion-associated complexes. 17,18The 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 with chelates of metals in aqueous phase without protonation, as opposed to the amines.The preparation and the application of ion-associated complexes of anionic chelates of metals with various natural organic and inorganic ligands containing N-or O-donor atoms and with the participation of mono-and ditetrazolium cations is a special scientific research field of chemistry of the coordination compounds.Tetrazolium salts are used as reagents for the preparation of various ion-associated complexes of metals, e.g.0][21][22][23][24][25][26] The liquid-liquid extraction, as a part of the chemistry of the solutions and the coordination compounds, is applied to study the processes of complex formation and the extraction equilibria.][29][30][31][32] The aim of this research was to study spectrophotometrically the formation of the ion-associated complex between the anionic chelate of germanium(IV) with 4-nitrocatechol (4-NC) and the cation of of 3-(2-naph-tyl)-2,5-diphenyl-2H-tetrazolium chloride (TV) in the liquid-liquid system Ge(IV)-4-NC-TV-H 2 O-CHCl 3 as well as to evaluate the possible applications of the system for determination of traces of germanium(IV) in alloys, biological, medical and pharmaceutical samples.
The acidity of the aqueous medium was set using a buffer solution prepared by mixing 2.0 mol L -1 aqueous solutions of CH 3 COOH and NH 4 OH.
The organic solvent CHCl 3 was additionally distilled.
The pH was checked by HI 83140 pH meter (Romania).A Camspec M508 spectrophotometer (United Kingdom), equipped with 10 mm path length cells, was employed for measurement of the absorbance.

Procedure for Establishment of the Optimum Extraction-Spectrophotometric Conditions
Aliquots of Ge(IV), 4-NC, TV and buffer (pH = 3.5-4.5)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 measured against a blank sample. 20,23

3. Procedure for Determination of the Distribution Constant
In order to determine the distribution constant K D , it is necessary to measure the light absorbances А 1 and А 3, which are the light absorbance after a single extraction in chloroform under the optimum conditions for complex formation (Table 1, column 1) and after a triple extraction performed under the same conditions, respectively.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 А 1 is performed against a blank sample, pre-pared 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 an extraction is performed for the third time.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. 22

1. 1. Absorption Spectra
The colored anionic chelate of germanium(IV)-4-NC can be efficiently extracted in chloroform in the presence of the bulky hydrophobic organic cations of monotetrazolium salt (TV).The absorption spectrum of the extract of the ion-associated complex, formed between the anionic chelate of Ge(IV) with 4-NC and the cation of monotetrazoliun salt in CHCl 3 , is characterized by an absorption maximum in the visible range (λ max = 420 nm) (Figure 1).

1. 2. Effect the Acidity of the Aqueous Phase and the Shaking Time
The acidity of the aqueous phase has a substantial effect on the extraction of the anionic chelate Ge(IV)-4-NC into the organic phase.The maximum and constant extraction of the ion-associated complex is achieved in Stojnova and Lekova: Ion-Associated Complex of the Anionic Chelate ... the pH range from 3.5 to 4.5 (Figure 2).Acetate buffer solution with pH = 4.0 was used in all further experiments.The carried-out experiments, showed that the extraction equilibrium is achieved for shaking time of not less than 60 s.The longer shaking time did not affect the absorbance.The further experiments were performed with shaking time 2 min.tion in the aqueous phase (C Ge(IV) , μg mL -1 ) and the absorbance of the ion-association complex in the organic phase after extraction was studied using regression analysis under the optimum conditions for complex formation.The equation of a straight line was found to be Y = 0.3933 X + 0.0020 with a correlation coefficient squared 0.9995.Under the optimum conditions for complex formation, the linearity is observed for concentrations up to 4.07 μg cm -3 Ge(IV).Further analytical characteristics, such as apparent molar absorptivity εʹ, 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 Ion-Associated Complex
The molar ratios of the ion-associated complex were determined by three independent methods: the mobile equilibrium method, the straight-line method of Asmus and the method of continuous variations. 33he mobile equilibrium method and the straightline method of Asmus were applied to prove the molar ratios Ge(IV):4-NC and Ge(IV):TV.The values of the correlation coefficient squared R 2 , determined by the straightline method of Asmus are presented in Table 2.

1. Effect of Reagents' Concentrations
The concentrations of the reagents are the most important factor, influencing the extraction equilibrium.The chelate formation of Ge(IV)-4-NC requires a 15.0-fold excess of 4-NC (≥3.0 × 10 -4 mol L -1 ).For a maximum association and extraction the amount of TV should not be lower than a 4.5-fold excess (≥9.0 × 10 -5 mol L -1 ).

2. Beer's Law, Apparent Molar Absorptivity and other Analytical Characteristics
The range of obedience to the Beer's law, i.e. the linear relationship between the germanium(IV) concentra-Table 1. Optimum extraction-spectrophotometric conditions and analytical characteristics of the system Ge(IV)-4-NC-TV-H 2 O-CHCl 3
On the basis of the results it can be concluded that Ge(IV), 4-NC and TV interact in molar ratio 1:3:2.The application of the method of continuous variations confirmed the molar ratio Ge(IV):TV = 1:2 (Figure 5). 33

4. Reaction Scheme and Suggested General Formula
Ions containing germanate [Ge(OH) 6 ] 2-are already described in the literature although in dilute aqueous solutions the major determined ions appear to be [GeO(OH) 3 ] -, [GeO 2 (OH) 2 ] 2-and {[Ge(OH) 4 ] 8 (OH) 3 } 3-. 1 The performed experiments showed that the complex formation and the extraction of the ion-associated complex have occurred in the dilute solutions and the molar ratio established by the independent methods mentioned above was Ge(IV):4-NC:TV = 1:3:2.Therefore, the complex formation of anionic chelate Ge(IV)-4-NC can be given by equation ( 1): Having in mind the molar ratio indicated above and the reaction of chelate formation of Ge(IV)-4-NC, it can be suggested that the formation of the ion-associated complex in the aqueous phase, its distribution between the aqueous and the organic phases and its extraction in chloroform can be given by the following equations (2-4).

5. Equilibrium Constants, True Molar Absorptivity, Recovery Factor and Structure of the Ion-Associated Complex
The association process in aqueous phase and the extraction equilibria were investigated and quantitatively characterized with respect to the following key constants: distribution constant K D , association constant β, extraction constant K ex and recovery factor R%.
The distribution constant K D was determined by equation (5), where A 1 and A 3 are the absorbances (measured against blanks) obtained after a single and triple extraction, respectively.
The recovery factor was determined from the equation (6): The extraction constant K ex was calculated by two independent methods: (i) from the equation log K ex = log K D + log β (7) (ii) by the method of Likussar-Boltz. 34) The association constant β was determined by two independent methods: Komar-Tolmachev method and Holme-Langmyhr method and their values are given in Table 3, column 2. 33,34 The association constant β was calculated by the method of Komar-Tolmachev from equation (8).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 = 2), ε -the true molar absorptivity.The true molar absorptivity ε was determined by the method of Komar-Tolmachev (Figure 6) from the equation of a straight line Y = 0.4182 X + 3.3895 (ε = 1 / (b × 10 −5 ) and its value is given in Table 1, column 2. 33 (ii) The method of Likussar-Boltz 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 1:2 (equation 9): 35 log where K is the total concentration of the reagents (К = The values of the equilibrium constants and the recovery factor, describing quantitatively the equilibrium in the aqueous phase and the extraction of the ion-associated complex in the organic phase are presented in Table 3. The results obtained by the independent methods are statistically similar and confirm the proposed scheme of the process of formation of the ion-associated complex in the aqueous phase, its distribution between the aqueous and the organic phases and its extraction in chloroform.Based on this, the proposed structure of the ion-associated complex is represented in Figure 7.The bulky organic molecule of TV determined the extractability of the ion-associated complex in organic phase.The optimum conditions for the association in the aqueous phase and for the extraction of the ion-associated complex Ge(IV)-4-NC-TV 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 (εʹ), the true molar absorptivity (ε), the limit of detection (LOD), the limit of quantification (LOQ) and the Sandell's sensitivity (SS).From the analytical characteristics of the extraction system Ge(IV)-4-NC-TV-H 2 O-CHCl 3 , it can be concluded that the ion-associated complex formed between the anionic chelate of Ge(IV)-4-NC and the monotetrazolium cation could allow determinations of Ge(IV) in various samples with a high sensitivity.The molar ratio of the components, determined by independent methods, showed that the ion-associated complex could be represented with the general formula (TV) 2 {Ge[O 2 C 6 H 3 (-NO 2 )] 3 }.A corresponding reaction scheme of the complex formation and a structure of the ion-associated complex were also suggested.

Table 2 .
Values of the correlation coefficient squared (R 2 ), corresponding to various molar ratios of Ge(IV):4-NC and Ge(IV):TV (n and m, respectively)