LIQUID-LIQUID EXTRACTION-SPECTROPHOTOMETRIC DETERMINATION OF MOLYBDENUM USING o-HYDROXYTHIOPHENOLS

3003 | P a g e N o v e m b e r 0 5 , 2 0 1 4 LIQUID-LIQUID EXTRACTION-SPECTROPHOTOMETRIC DETERMINATION OF MOLYBDENUM USING o-HYDROXYTHIOPHENOLS Ali Z. Zalov, Kiril B. Gavazov* Department of Analytical Chemistry, Azerbaijan State Pedagogical University, 34 Gadzhibekova St., Baku 1000, Azerbaijan zalov1966@mai.ru Department of General and Inorganic Chemistry, University of Plovdiv “Paisii Hilendarskii”, 24 Tsar Assen Str., 4000 Plovdiv, Bulgaria * Corresponding Author: kgavazov@abv.bg ABSTRACT


INTRODUCTION
Molybdenum has been recognized as an essential trace element for plants, animals and humans [1][2][3]. In plants, it has a key function in the fixation of the atmospheric nitrogen, while in humans, it is important for the activity of the enzymes xantine oxidase, sulphite oxidase, and aldehyde oxidase. It is known that molybdenum has a beneficial effect on patients with sulphite sensitivity and asthma. It has also been stated that it reduces the incidence of dental caries. On the other hand, molybdenum can be toxic at high concentrations [2][3][4]. In mammals, it increases the lability of blood pressure, disturbs the cholesterol transport and causes bone deformation. Effects of acute molybdenum toxicity include diarrhoea, anaemia and gout. Chronic occupational exposure has been associated with weakness, fatigue, lack of appetite, anorexia, joint pain and tremor [3].
Despite its relative rarity in the Earth's crust (estimated abundance in the range 0.05-40 mg kg -1 with a mean value of 1.5 mg kg -1 ) [5,6], geochemical anomalies leading to molybdenum deficiencies in plants are not common [7,8] and are mostly of concern for leguminous crops. Molybdenum fertilization is often based on visual deficiency symptoms and/or history of crop rotation [8]. However, in order to assess the need for fertilization and the molybdenum dosage it is preferable to use analytical methods [5,6].

Reagents and apparatus
A stock solution of Mo(VI) (1 dm 3 ) was prepared by dissolving 1.8402 g of (NH4)6Mo7O24•4H2O (Sigma-Aldrich, 99.98%) in distilled water. The solution was standardized gravimetrically. Working solutions (0.1 mg cm -3 ) were prepared daily by appropriate dilution of the stock solution.
HTPDs were synthesized according to the procedure [30]; their purity was verified by paper chromatography and melting point determination. AAs were products of Sigma-Aldrich (98-99% purity). Chloroform solutions (0.01 mol dm -3 ) of HTPDs and AAs were used.
A masking solution (1 dm 3 ), containing 75 g of citric acid and 150 g of ascorbic acid, was prepared weekly and stored in a refrigerator. A second masking solution was prepared from KI (w=20%).
To create the optimum acidity, 0.1 mol dm -3 solutions of HCl or NaOH were used.
The absorbance of the extracts was measured using a KFK-2 photocolorimeter (USSR) and a Camspec M508 spectrophotometer (UK), equipped with 5 and 10 mm path-length cells. pH of aqueous phase was measured using an I-120.2 potentiometer with a glass electrode. Muffle furnace was used for dissolution of the samples.

Procedure for determining the optimum conditions
Aliquots of Mo(VI) solution, HTPD solution (up to 2.5 cm 3 ) and AA solution (up to 2.5 cm 3 ) were transferred in a 50 cm 3 calibrated tube with ground-glass stopper. pH of the aqueous phase was adjusted in the interval 3.5-7.9 by adding a small amount of HCl or NaOH solution. The volume of the aqueous phase was increased with water to 20 cm 3 and the volume of the organic phase was set to be 5 cm 3 . The tube was closed with the stopper and shaken for a fixed time (up to 15 min). After separation of the layers, a portion of the organic extract was transferred into a cell and the absorbance was read against organic solvent or simultaneously prepared blank sample.

Procedure for molybdenum determination
An aliquot containing molybdenum (no more than 90 μg when HTPD=HCTP or 85 μg when HTPD=HBTP) was placed in a calibrated tube. Chloroform solutions of HTPD (0.7 cm 3 ) and An (0.7 cm 3 ) were added and the organic phase was adjusted to 5 cm 3 with chloroform. The volume and pH of the aqueous phase were adjusted to 20 cm 3 and 5.3-5.8, respectively. The tube was closed with a stopper and after 10 minutes of shaking a portion of the organic extract was N o v e m b e r 0 5 , 2 0 1 4 transferred through a filter paper into a cell. The absorbance was read at max (535 nm when HTPD=HCTP or 530 nm when HTPD=HBTP) against a simultaneously prepared blank sample. The molybdenum content was found from a calibration graph.

Soil sample preparation for analysis
A soil sample (15 g) collected from depths of 155 cm was subjected to available molybdenum extraction procedure [31,32] with an oxalate buffer of pH 3.3. Then the procedure [31] was followed: An aliquot of the obtained soil extract (50-100 cm 3 ) was placed in a quartz beaker and evaporated to dryness on a hot plate. For dehydration of the residue and partial sublimation of the oxalates, the beaker was left on the plate for 30 more minutes. The beaker was transferred in a cold muffle furnace. The temperature was raised to 500 °C and held for 1 hour. After cooling, 2 cm 3 of perchloric acid were added for complete destruction of the organic matter. The content was heated to near dryness on a hot plate and the beaker was placed again in the muffle furnace. The temperature was raised to 500 °C and held for 15-20 minutes. The residue was cooled and then dissolved in 25 cm 3 of 14% hydrochloric acid under heating. Masking solutions were added (4 cm 3 of the citric acidascorbic acid solution and 2 cm 3 of the KI solution) and the resulting solution was filtered into a 100-cm 3 separatory funnel. Aliquots of thus obtained filtrate were used to determine the molybdenum content.

Plant sample preparation for analysis
10 g of the powdered plant material (seeds of pea) were soaked in 50 cm 3 of 96% ethanol for 24 h. The sample was dried and carefully carbonized in a quartz beaker on a hot plate. The beaker was placed into a cold muffle furnace and its temperature was slowly increased (at a rate of 100 °C per hour) up to 450°C. The sample was dry-ashed for 10-15 h. The obtained gray ash was cooled and moistened with a few drops of nitric acid (1:1). Then the ash was heated to dryness on a water bath and placed into the muffle furnace. The temperature was raised to 300°C and held for 30 minutes. This cycle was repeated several times until white ash was obtained. Then, several drops of bidistilled water and 2 cm 3 of perchloric acid were added. The beaker was heated on a hot plate until smoking ceased and transferred in a cold muffle furnace. The temperature was raised to 500 °C and held for 15 minutes. After cooling, 25 cm 3 of 14% hydrochloric acid were added and the beaker was kept in a boiling water bath for 10-20 min. The obtained solution was quantitatively transferred to a volumetric flask of 50 cm 3 [31]. Masking solutions were added (4 cm 3 of the citric acidascorbic acid solution and 2 cm 3 of the KI solution) and the flask was filled to the mark with bidistilled water.

Studies on the oxidation state of molybdenum
It is known that HTPDs have reducing properties in acidic medium [26,28,33]. Previous investigations with Mo(VI)-HCTP and Mo(V)-HCTP [26] suggested that only Mo(V) forms stable complexes with this reagent. To elucidate the oxidation state of molybdenum in the presence of other HTPDs (HBTP and HITP), we conducted two series of experiments. In the first series we used Mo(VI), while in the second series we used Mo(V) obtained by addition of a supplementary reducing agent (SnCI2 or KI). The comparison of the obtained spectra showed that max Mo(VI)-TPHHD=max Mo(V)-TPHHD. This fact can be regarded as an indication [26,34] that Mo(VI) is reduced to Mo(V) by the reagent itself during the process of complex formation.

Charge of the complexes
The charge of the Mo(V)-HTPD binary complexes was determined by electromigration and ion-exchange. Experiments on electromigration in a U-shaped tube and sorption on anion exchanger EDE-10P demonstrated the anionic nature of the complexes. Under the experimental conditions, these red binary complexes were insoluble in nonpolar organic solvents. However, when AAs were introduced the formation of electroneutral chloroform extractable ternary complexes was observed.

Choice of organic solvent
The following organic solvents were tested for the extraction of the complexes: chloroform, dichloroethane, carbon tetrachloride, benzene, toluene, diethyl ether and n-butanol. Chloroform was found to be the most effective in terms of degree of extraction (R%) and rapid equilibration. As can be seen from Table 1, chloroform provides R98.4%. At that, the nature of AA does not appreciably affect the extraction. Figure 1 represents the influence of pH on the absorbance of the Mo(V)-HTPD-An extracts. The optimum pH ranges are wide enough to ensure stable and reproducible results without using buffer solutions. The course of all pH curves supports the assumption that only one complex is formed in each of the extraction systems. The optimum pH intervals are listed in Table 1. At higher pH values, the efficiency of the extraction is impaired, which relates to the lower degree of AA protonation. At lower pH values, the extraction is also impaired most probably due to decrease of the concentration of the anionic HTPDs forms. N o v e m b e r 0 5 , 2 0 1 4

Stoichiometry of the ternary complexes
The molar ratios of the components of the ternary complexes were established by the equilibrium shift method [35] and the method of Asmus [36].

Effect of foreign ions and masking reagents
The influence of foreign ions and reagents on the extraction-spectrophotometric determination of molybdenum with HCTP+An and HBTP+An was examined in details. It was found that great excesses of alkali, alkali earth, and rare earth cations do not interfere with determination. The same was valid for anions, such as F -, Cl -, Br -, SO3 . Serious interferences were observed from small amounts (less than 200 μg) of Fe(III), Ti(IV), Cu(II), Nb(V), Ta(V), Hg(II) and W(VI). Тhe interfering influence of these ions can be eliminated by using various masking agents ( Table 2). For the sake of convenience, we used only two masking solutions in our further analytical work: 1) ascorbic acid + citric acid solution; and 2) KI solution.  Table 3 summarizes the calibration characteristics obtained with HCTP+An and HBTP+An. The procedures with these reagents were applied to the determination of molybdenum in samples of soil and pea. The accuracy of the results was checked by two independent methods. The results are listed in Table 4.

Table 3. Analytical characteristics for the Mo-HCTP-An and Mo-HBTP-An extraction-chromogenic systems
Parameter

Mo-HCTP-An Mo-HBTP-An
Linear calibration range / g cm

Some linear relationships involving the spectral characteristics of the complexes
The nature of the substituents and the molecular mass of the associating ions are important factors in the chemistry of ionassociation [20,38]. Linear relationships have been described between the molecular mass of a particular class of cationic ion-association reagents [20,38] and some parameters, such as constant of association [38], temperature of decomposition [39], constant of extraction [40], and molar absorptivity [40].
The HTPDs used in this work provided an excellent opportunity to examine the influence of the halogen substituent (Cl, Br, I) on the spectral characteristics of the ternary complexes (molar absorptivity max and absorption maximum max). The results showed that straight lines can be obtained by plotting the atomic mass of the halogen atom (AHal) vs. max ( Figure  2) or max ( Figure 3). The experimental data plotted in coordinates max -Hal and  -Hal (where Hal is the Pauling electronegativity of the halogen atom) also fit linear equations (Table 5) (the squared correlation coefficients R 2 are in the range of 0.9347-1.000). The differences in the slopes (ai) of the straight-lines max=a1AHal+b1 and max=a2Hal+b2 can be attributed to the different nature of the AAs used. However, the slopes were relatively constant for the straight lines max=a3AHal+b3 and max=a4Hal+b4: a3=-0.00600.0005 and a4=1.080.09. This fact can be used for simple prediction of molar absorptivities of still uninvestigated ternary complexes (e.g. Mo-HTPD-AA complexes where AA is 2,3-xylidine, 2,6xylidine or 3,5-xylidine). For this purpose, only one of the ternary complexes of a given AA (Mo-HCTP-AA, Mo-HBTP-AA or Mo-HITP-AA) must be experimentally examined. Table 5 shows that the straight-line equations with participation of max are characterised by relatively constant ordinate cuts bi (b1=5344 and b2=48512). This information can be used for predicting the absorption maxima of new ternary complexes after a single experimental step as outlined above.   Table 5.  Table 5 CONCLUSIONS Molybdenum(VI) forms well chloroform-extractable ternary complexes with HTPDs and AAs which can be used for liquidliquid extraction-spectrophotometric determination of molybdenum. The complexes have a composition of 1:2:2 (Mo:HTPD:AA) and can be regarded as ion associates between doubly charged anionic chelates of Mo(V) and protonated AA species: (AAH + )2[MoO(OH)(HTPD)2]. The collected information about the spectral characteristics (absorption maximum max and molar absorptivity max) of the complexes makes it possible to conclude that linear relationships exist between some fundamental properties of the halogen substituent in the HTPD (atomic mass AHal and electronegativity Hal) and max or max: (i) The higher the atomic mass, the lower max and max; (ii) The higher the electronegativity Hal, the higher the max and max.The established constancy of the slopes or ordinate cuts of the obtained straight-line equations can facilitate the prediction of the mentioned spectral characteristics of new complexes of the same class.