ЖУРНАЛ АНАЛИТИЧЕСКОЙ ХИМИИ, 2007, том 62, № 2, с. 138-141
^=ОРИГИНАЛЬНЫЕ СТАТЬИ
УДК 543
TERNARY COMPLEX OF MOLYBDENUM(VI) WITH 4-NITROCATECHOL AND TETRAZOLIUM BLUE CHLORIDE AND ITS APPLICATION TO EXTRACTION-SPECTROPHOTOMETRIC ANALYSIS OF FERROUS METALLURGY PRODUCTS
© 2007 r. A. N. Dimitrov*, V. D. Lekova*, K. B. Gavazov* and B. S. Boyanov**
*Department of General and Inorganic Chemistry, Plovdiv University **Department of Chemical Technology, Plovdiv University 24 Tsar Assen St., 4000 Plovdiv, Bulgaria Received 06.12.2005; in final form 31.05.2006
Abstract—The formation of a new ternary complex of molybdenum(VI) with 4-nitrocatechol (NC) and tetra-zolium blue chloride (BTC), which is well extractable from water into dichloroethane, is reported. The optimum extraction conditions (pH, concentracion of the reagents, extraction time) and composition of the complex (Mo:NC:BTC = 1:2:1) were found. The spectrophotometry parameters of the extract were determined as well. Beer's law is obeyed for concentrations of Mo ranging from 0.2 to 6.7 |g/mL with the molar absorptivity £445 = 2.38 x 104 L mol1 cm1. A precise, sensitive and simple method for determination of Mo in steels and ferromolybdenum was developed.
It is known that Mo(VI) forms with o-polyphenols and organic bases ternary complexes which are easily extracted into organic solvents. The obtained yellow, red or violet extracts could be used for extraction-spec-trophotometric determination of Mo [1-5]. Our studies on ternary ion-association complexes of various metal ions with o-polyphenols and tetrazolium salts [6-14] showed their applicability to chemical analysis. In a previous paper [l4] we investigated the extraction systems: Mo(VI)-2,3-dihydroxynaphthalene-thiazolyl blue-water-chloroform and Mo(Vl)-4-nitrocatechol (NC)-thi-azolyl blue-water-chloroform and pointed out some advantages of the system containing NC. The present paper aims at studying the ternary complex of Mo(VI) with NC and another tetrazolium salt: 3,3'-[3,3'-dime-toxy(1,1'-biphenyl)-4,4'-diyl]-fe[2,5-diphenyl-2H-tet-razolium] dichloride (C40H32N8O2Q2, tetrazolium blue chloride, BTC) with a view to its application to extrac-tion-spectrophotometric determination of Mo. BTC is well known ditetrazolium salt used for determination of steroids and other reducing organic compounds [4] but no information is available in the literature about its participation in ion-association complexes.
EXPERIMENTAL
Reagents and apparatuses. A standard Mo(VI) solution with a concentration of 1 x 10-2 M was prepared by dissolving of Na2MoO4 ■ 2H2O in distilled water. The aqueous solutions of NC and BTC were with concentrations of 1 x 10-2 M and 1 x 10-3 M, respectively. The other reagents were sulphuric acid, chloroform, dichloroethane and solutions of diverse ions and re-
agents. All used reagents were of analytical grade from Fluka.
A Specol-11 spectrophotometer (Germany) and a LAMBDA-15 Perkin-Elmer UV-VIS spectrophotometer (USA) were employed for reading the absorbance. The pH measurements were made with a TM-5 pH-meter (Germany) with a combined glass electrode.
General procedure. The necessary amounts of Mo(VI), NC, BTC and H2SO4 to adjust the pH of the aqueous medium were placed in separatory funnels or extraction test tubes, diluted up to 5 mL with distilled water and extracted with equal volume of organic solvent. The samples were extracted and after the phase separation the organic layer was filtered through a paper filter into a cell. The absorbance was measured against a blank prepared in the same way.
Fig. 1. Absorbance spectra: 1 - ternary complex against dichloroethane; 2 - blank against dichloroethane.
TERNARY COMPLEX OF MOLYBDENUM(VI) Table 1. Optimum extraction-spectrophotometric conditions
Optimum conditions Mo(VI)-NC-BTC
Absorbance maximum 445 nm
Volume of the aqueous phase 5 mL
Volume of the organic phase 5 mL
Extraction time 2 min
Organic solvent dichloroethane
pH of the aqueous phase 1.8-4.0
Concentration of reagents NC: 5.0-fold excess BTC: 1.8-fold excess
Beer's law 0.2-6.7 |g/mL
Molar absorptivity (2.38 ± 0.02) x 104 L mol-1cm-1
Determination limit 0.050 absorption units 0.2 |g/mL Mo
bathochromically with 5 nm in comparison with the one reported in [13, 14]. The optimum extraction-spec-trophotometric conditions are shown in table 1. The system Mo(VI)-NC-BTC is notable with the small excesses of the reagent needed. Another advantage of this system in comparison with the Mo(VI)-NC-TV [13] and Mo(VI)-NC-MTT [14] systems is the broader pH-range for maximum extraction.
The molar ratios Mo(VI) to NC and Mo(VI) to BTC were determined by employing a set of widely used spectrophotometric methods: the equilibrium shift method [15] and the method of continuous variations [15]. The results as well as the previous investigations [13.14] give us grounds to assume that a 1:2:1 (Mo:NC:BTC) complex is formed according to the scheme:
RESULTS AND DISCUSSION
Preliminary studies showed that the ternary Mo(VI)-NC-BTC complex could be extracted with chloroform, dichloroethane, benzene, toluene, tetrachloromethane, etc. We used dichloroethane for further investigations because of its lowest toxicity and good extraction ability. The absorbance in this solvent is stable for at least an hour. The spectra of the ternary ion-association complex and the blank are presented in fig. 1. The absorption maxima lie at 235 nm and 445 nm. The maximum in a visible region is more convenient. It is shifted
(1)
(2)
2C6H5O2NO2 + MoO4- — — [(C6H3O2NO2)2MoOJ2- + 2H2O [(C6H3O2NO2)2MoOJ2- + BT2+ ^ ^ (BTMQ^NO^MoOJ
The extraction constant (Kex) was calculated by the method of Likussar and Boltz [16] with two values of the sum Cmg + Cbtc
Kex1 = 2.6 x 106,
Log Kexl = 5.43 (cmo + Cbtc = 1 x 10-4 M);
KeX2 = 2.7 x 106,
Log KeX2 = 5.44 (cmo + Cbtc = 4 x 10-5 M).
Table 2. Effect of foreign ions and reagents on the determination of 20 |g Mo in 5 mL of dichloroethane
Foreign ion mg Mo found, % Foreign ion mg Mo found, %
Ascorbinate Al3+ 0.5 0.2 interferes 102.9 I- Fe3+ 0.05 0.5 interferes interferes
Br- 0.5 interferes HP o\ 0.05 interferes
CH3COOH-Citrate Complexone IV Cd2+ 0.5 0.5 0.4 5.0 98.0 interferes 101.2 102.8 Mg2+ Mn2+ Mn(VII) Ni2+ 0.05 0.05 0.05 2.0 98.0 interferes interferes 102.3
C2 o^- 0.2 98.5 N O3 0.5 interferes
Co2+ 1.0 101.3 p o3- 0.05 interferes
Cr3+ Cr(VI) Cu2+ F- EDTA 0.5 5.0 7.0 0.5 1.0 interferes 98.8 98.8 interferes 100.5 SCN- Tartrate V(V) W(VI) Zn2+ 0.05 0.05 0.05 0.05 5.0 interferes interferes interferes interferes 102.7
140
DIMITROV и др.
Table 3. Results from analysis of steels and ferromolybde-num (n = 10, P = 95%)
Reference standard (Steel Certified Obtained by the
and ferromolybdenum) value, % proposed method, %
Steel A* 1.34 1.33 ± 0.02
Steel B** 0.25 0.25 ± 0.01
Ferromolybdenum*** 53.2 52.8 ± 0.8
* Steel A (mass %): C (0.386), Mn (7.38), Si (0.66), P (0.012), S (0.005), Cr (12.33), V(1.24), Ni (6.96), Cu (0.101), Ti (0.036), Nb (0.51), N (0.044), Mo (1.34); ** Steel B (mass %): C (0.255), Mn (0.64), S (0.009), P (0.013), Si (0.36), Cr (1.03), Mo (0.25); *** Ferromolybdenum (mass %): S (0.12), Si (0.12), Mo (53.2).
The effect of various ions and reagents on the extraction of Mo(VI) with NC and BTC was studied under the optimum extraction conditions. The results are presented in table 2. As interfering effect we accepted a ±3 % deviation from the absorbance of the complex in absence of a foreign ion. Some cations interfere seriously on the extraction of molybdenum and must be separated or masked (if present). Our previous investigations [14] showed that Fe3+, Al3+, Cr3+, Nb(V), Ti(IV), Ge(IV) and Mn2+ could be precipitated with OH- at pH 11 and V(V) could be coprecipitated in iron-containing samples. Unprecipitated amounts of Fe3+, Al3+ and Cr3+ could be masked with Na-EDTA.
The studied extraction system Mo(VI) - NC - BTC -water - dichloroethane has some better characteristics in comparison with other similar systems [5, 9, 13, 14, 17, 18] and has an application value.
Determination of molybdenum in steels and ferromolybdenum. Dissolve a known weight (0.5-1 g) sample in 20 mL of H2SO4 (1:1). Add 2 mL of concentrated HNO3 to dissolve the carbides. Heat to white fumes of SO3 for removal the excess of HNO3. Neutralize with 30% NaOH until a stable precipitate appears. Dissolve the latter with a minimum amount of H2SO4 (1:10) and after cooling with running water add 10 mL of a 25% solution of ferroammonium sulfate. Allow to stay for 2 min. Transfer the solution into a 500-mL volumetric flask containing 60 mL of hot 15% solution of NaOH. Shake up the flask and after cooling dilute it up to the mark with distilled water. Filter the content through a double filter paper. Place an aliquot (3.033.5 ^g Mo) of the filtrate into a separation funnel and adjust the pH to 1.8-4.0 (with H2SO4). Add 0.2 mL of 6 x 10-3 M NC, 1.0 mL of 1.0 x 10-3 M BTC, 1 mL of 0.1% Na-EDTA and bring up the aqueous phase volume to 5 mL with distilled water. Add 5 mL of dichlo-roethane and shake up for 2 min. After the separation of the phases, transfer a portion of the organic layer through a filter paper into a 1.0 cm cell and measure the
absorbance at 445 nm against a blank. Determine the amount of Mo from a calibration graph.
The described method of molybdenum determination in products from ferrous metallurgy gives satisfactory results (Table 3) and could successfully compete with known and practiced methods [1, 13, 14, 19-23].
The formation of a ternary ion-association complex of Mo(VI) with 4-nitrocatechol (NC) and tetrazolium blue chloride (BTC) was proved by the present study. The bulkiness of the cationic part of the complex guarantees its poor solubility in water and good extraction ability. The anionic part in its turn ensures a high sensitivity of determination. The extraction system Mo(VI)-NC-BTC-water-dichloroethane has some better characteristics in comparison with other similar s
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