ЖУРНАЛ АНАЛИТИЧЕСКОЙ ХИМИИ, 2010, том 65, № 1, с. 25-28
ОРИГИНАЛЬНЫЕ СТАТЬИ =
УДК 543
EXTRACTION EQUILIBRIA AND SPECTROPHOTOMETRIC DETERMINATION OF VANADIUM(V) WITH 4-NITROCATECHOL AND THE ION-PAIR REAGENT THIAZOLYL BLUE TETRAZOLIUM © 2010 P. V. Racheva, K. B. Gavazov, V. D. Lekova, A. N. Dimitrov
Department of General and Inorganic Chemistry, Plovdiv University "Paisii Khilendarski" 24 Tsar Assen St., 4000 Plovdiv, Bulgaria Received 13.08.2008; in final form 27.04.2009
The formation and liquid-liquid extraction of a yellow ternary complex of vanadium(V) with 4-nitrocatechol (NC) and the ion-pair reagent Thiazolyl Blue Tetrazolium {3-(4,5-dimethyl-2-thiazol)-2,5-diphenyltetrazolium bromide, MTT} with 1 : 2 : 3 stoichiometry (V : NC : MTT) were studied. The optimum extraction conditions (pH, concentration of the reagents, extraction time), spectrophotometry parameters of the extract and key constants (extraction constant, association constant, distribution constant) were determined. Beer's law was obeyed for concentrations ranging from 0.12 to 1.2 ^g/mL of vanadium(V) with a molar absorptivity of s = 3.13 x 104 L/mol cm at lmax = 400 nm. The effect of diverse ions was studied and extraction-spectrophotometric procedures for determination of vanadium in catalysts and steels were proposed.
Nitro derivatives of catechol (NDC) are suitable ligands for the formation of stable complexes [1—6] and well-known analytical reagents [7] used for extraction-spectrophotometric [8—16], flotation-spectrophotomet-ric [17—19] and amperometric [20] determination of a number of metals. The presence of nitro substituent(s) in the aromatic ring ofthese compounds enhances the acidity of the catechol function [21] and increases the com-plexing and chromogenic power [3, 8, 11] with respect to catechol and other catechol derivatives. That is why the complexes of NDC with some metal ions are perspective in view of its analytical application.
It is known that the chromophore and extraction properties of the complexes of vanadium(IV,V) with NDC could be improved in the presence of cationic reagents [8, 10, 11, 22—24]. A promising ion-pair reagent for the extraction-spectrophotometric determination of metal species incorporated in yellow anionic chelates is 3-(4,5-dimethyl-2-thiazol)-2,5-diphenyltetrazolium bromide (Thiazolyl Blue Tetrazolium, MTT) [25]. MTT+ absorbs light in the same spectral range as the vanadium— NDC chelates, and one can expect promotion of the sensitivity ofdetermination in comparison with the sensitivity achieved with other reagents of the same class [25].
In the present paper we have studied the vanadi-um(V) - 4-nitrocatechol (NC)-MTT-water - chloroform system and developed extraction-spectropho-tometric procedures for the determination of vanadium in catalysts of oxidation of SO2 and steels.
EXPERIMENTAL
Reagents and instruments. A standard vanadium(V) solution with a concentration of 2.0 x 10-4 M was pre-
pared by dissolving of NH4VO3 (VEB Laborchemie Apolda, purissimum) in distilled water. NC (Fluka, pro analysis) and MTT (Loba Feinchemie AG, pro analysis) aqueous solutions were prepared with concentration of 2.0 x 10-3 M. The other reagents were 8-hydroxyquino-line, Complexone IV (CDTA), buffer solutions, solutions ofdiverse ions, mineral acids and chloroform (redistilled). A Specol-11 spectrophotometer (Carl Zeiss, Germany) equipped with 1.0 cm-in-width cells was employed for reading the absorbance. The pH measurements were made with a HI 83140 pH meter (Italy) with a combined plastic electrode.
Procedure for establishing the optimum operating conditions. Aliquots of V(V) solution, acetate buffer solution (prepared by mixing of appropriate volumes of 0.1 M aqueous solutions of CH3COOH and CH3COONa), MTT solution and NC solution were introduced into 100 mL separatory funnels. The resulting solutions were diluted with distilled water to a total volume of10 mL. Then 10-mL of organic solvent were added and the funnels were shaken for several minutes. A portion of the organic layer was filtered through a filter paper into a cell and the absorbance was read against a blank.
Procedure for determination of vanadium in catalysts.
500 mg of the powdered catalyst sample was placed in a 100-mL beaker, 25 mL of H2SO4 (1 : 1) were added and the content was heated gently for about 30-40 min [26, 27]. The resulting mixture was diluted to 100 mL and filtered through a filter paper at medium speed. The precipitate of the silicic acid was carefully washed. The filtrate and liquid fraction obtained after washing were transferred into a 1 L volumetric flask. Any V(IV) was carefully oxidized to V(V) with 0.02 M KMnO4 until a steady (for
26 RACHEVA h gp.
Table 1. Optimal conditions and characteristics of the extraction-spectrophotometric determination of V(V) with NC and MTT
Optimal conditions Analytical characteristics
Wavelength — lmax = 400 nm smax = (3.13 ± 0.02) x 104 L/mol cm
pH 4.4—4.8 (acetate buffer) Sandell's sensitivity —1.63 x 10-3 ^g/cm2
CNC (aqueous phase) — 1.6 x 10-4 M Beer's law range — up to 1.2 p.g/mL V(V)
CMTT (aqueous phase) — 1.6 x 10-4 M Correlation coefficient — R = +0.9996
Volume of the aqueous phase — 10 mL Limit of detection - 0.035 ^g/mL V(V)
Volume of the organic phase — 10 mL Limit of quantification — 0.12 ^g/mL V(V)
Organic solvent — chloroform Absorbance of the blank — 0.230
Extraction time — 2 min Standard deviation of the blank — 0.013
a few minutes) pink color appeared. Then the solution was diluted with water to 1 L [27].
An aliquot (1.2—12 ^g of V) of the obtained solution was placed in a 100-mL separatory funnel. pH was adjusted to 4—5 by adding dropwise 3% ammonia solution. Then, 3 mL of the acetate buffer solution (pH 4.66; CH3COOH to CH3COONa v/v ratio = 55 : 45), 0.8 mL ofthe 2.0 x 10-3 M MTT solution, and 0.8 mL ofthe 2.0 x x 10-3 M NC solution were added. After diluting the aqueous phase with water to 10 mL, 10 mL of chloroform were added. The content was shaken for 2 min. The organic layer was filtered through filter paper, and the absor-bance was measured at X = 400 nm against a blank. The vanadium content was read from a calibration plot.
Procedure for determination of vanadium in steels.
500—1000 mg of the steel sample was dissolved in 3 mL of H3PO4 (p = 1.7) and 40 mL ofa mixture ofHCl (p = 1.19) and HNO3 (p = 1.4) in a ratio of 3 : 1, as described in [11, 28]. Nitric acid was removed by treatment with two portions (10 mL each) of H2SO4 (1 : 1) until white fumes of SO3 appeared. After cooling, 80—100 mL of water were added and the mixture was heated again to dissolve the salts. Then, the insoluble acids were separated by filtration (medium fast filtering paper) and the residue on the filter was carefully rinsed. The filtrate and the washing were transferred into a 500-mL volumetric flask and the solution was diluted to volume with water.
A portion of the final solution (25—250 ^g V) was placed into a 100-mL separatory funnel and 0.02 M KMnO4 was added dropwise under continuous stirring until a steady (for a few minutes) pink color appeared; 3 mL of 2 x 10-3 M NH4F were then added and pH was adjusted to 2.8 with NH4OH. The mixture was extracted with two 10 mL portions of 0.5% solution of 8-hydrox-yquinoline in chloroform for two minutes. The chloroform extracts were mixed together and vanadium was back extracted with two 20-mL portions of buffer with pH 9.4 [11, 29] for 5 min. After separation the aqueous phases were filtered through a filter impregnated with water and mixed in a 50-mL flask. The solution was acidified with H2SO4 (1:4) and the volume was brought to 50 mL with water. A portion of this solution was analyzed according to the procedure described above.
RESULTS AND DISCUSSION
In slightly acidic medium (acetate buffer) vanadi-um(V) forms with NC and MTT ternary complexes easily extractable from water into chloroform. The composition of these complexes was established to be 1 : 2 : 2, 1 : 1 : 1 and 1 : 2 : 3 (V: NC : MTT) according to the equilibrium shift method [30]. In the first complex, the oxidation state of vanadium is most probably + 4, that is in agreement with the concept that NC, as a ortho-polyphenolic compound is able to reduce V(V) to V(IV) in acidic solutions [8, 11, 31]. This complex is formed only at deficiency of MTT with respect to NC. In the presence of a sufficient amount of the tetrazolium salt, V(V) saves its initial oxidation state [25, 32] and 1 : 1 : 1 (at deficiency of NC) or 1 : 2 : 3 (V : NC : MTT) complexes are formed. Under the optimum extraction-spec-trophotometric conditions (Table 1), the dominating vanadium species was established to be V(V) : NC : INT = 1 : 2 : 3 with a suggested formula (MTT)3 [Vo2(NC)2] and an equation of formation: 3MTT+ + [Vo2(NC)2]3— — (MTT)3[VO2(NC)2]. The association constant p characterizing this equilibrium was determined by using the method of Komar-Tolmatchev [30]. This method allows both the association constant p and the true molar absorptivity s ofthe complex {lg p = 17.6 ± 0.6; s = (3.13 ± ± 0.04) x 104 L/mol cm} to be calculated. The apparent molar absorbtivity s' determined by using Beer's law (Table 1) agrees very well with those obtained by the method of Komar-Tolmatchev, that is an indication that at the optimum operating conditions the complexes with composition 1 : 2 : 2 (suggested formula (MTT)2[VO(NC)2]) and 1 : 1 : 1 (suggested formula (MTT)[VO2(NC)]) could be ignored.
The distribution constant KD, characterizing the distribution of the complex between the phases {(MTT)3[VO2(NC)2]}aq — {(MTT)3][VO2(NC)2]}0rg was evaluated by comparing the absorbance for a single extraction (A{) to that for triple extraction (A3) in
equal volumes Kd = [V]org/[V]aq=A1/A -A1); lgKd = = 1.62 ± 0.01}. Recovery factor R = (97.6 ± 0.1)% was calculated according to the formula R% = KD x x 100/(KD + 1).
EXTRACTION EQUILIBRIA AND SPECTROPHOTOMETRIC DETERMINATION
27
Table 2. Influence of the
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