EXTRACTIVE SPECTROPHOTOMETRIC STUDIES OF 2,3,4-TRIHYDROXY PHENYLETHYLIDINE BENZOIC ACID HYDRAZIDE WITH MOLYBDENUM (VI)

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S. Paul Douglas et. al. / International Journal of Engineering Science and Technology Vol. 2(9), 2010, 4655-4658

EXTRACTIVE

SPECTROPHOTOMETRIC STUDIES

OF 2,3,4-TRIHYDROXY

PHENYLETHYLIDINE BENZOIC ACID

HYDRAZIDE WITH MOLYBDENUM

(VI)

Dr. S. Paul Douglas*,

Department of Engineering Chemistry, A. U. College of Engineering, Andhra University, Visakhapatnam–530003. AP. India

Prof. P. Nageswara Rao

Department of Chemistry, National Institute of Technology, Warangal – 506 004. AP. India

Abstract:

2,3,4-Trihydroxy Phenylethylidine Benzoic Acid Hydrazide (TPBH) as a photometric reagent for the extractive spectrophotometric determination of Molybdenum(VI) is presented in this paper. The reagent TPBH gave instantaneous and stable yellow colour with Molybdenum(VI) in the acidic pH range. The colour reaction in detail has been explored and the possibility of photometric determination of the micro amounts of molybdenum(VI) is established with necessary conditions. The system obeys Beer’s law in the range in the concentration range of 0.4-10 g/ml at 385 nm The optimal concentration range is calculated from Ringbom plots are found to be 3-8 g/ml at 385 nm. The stoichiometry of the complex is established as 1:1 (M:L) by Job’s method of continuous variation and confirmed by mole ratio method. The stability constant, the standard deviation and the coefficient of variations are presented. The results of the prescribed procedure applied for the determination of the micro amounts of Mo(VI) in standard steel samples are presented.

Keywords: Extractive Spectrophotometric Determination, 2,3,4-Trihydroxy Phenylethylidine Benzoic Acid Hydrazide(TPBH), Molybdenum(VI), Methyl isobutyl ketone, Beer’s Law, Molar Absorptivity, Sandell’s Sensitivity, Ringbom Plot, Stability Constant, Determination of Micro Amounts of Molybdenum(VI) in Standard Steel Samples.

1. Introduction

Molybdenum is found in earth’s crust in abundance of 10-4%. It is generally present as molybdates. The amount of molybdenum in soils and plants has been found to be a critical factor in recent years. The metal is an important alloying element and is present as minor constituent in many industrially important materials. Even small amounts cause tremendous increase in hardness and strength. The increasing use of the metal necessitates development of rapid and sensitive methods for the determination of minute quantities of the metal. Colorimetric and Atomic Emission or Atomic Absorption methods are most commonly used for the determination of Mo(VI). However, colorimetric methods are generally preferred, as they involve less expensive instrumentation and afford better sensitivity when appropriate chromogenic reagents and solvent extraction pre-concentration steps are employed.

Most of the extractive spectrophotometric methods developed for Mo(VI) are based on reactions with suitable colour producing reagents such as 2-(2’-furyl)-3-hydroxychromone in [1], quinolin-8-ol[2], thiocyanate and pyrogallol in MIBK[3], 2-hydroxy-3-methoxy benzaldehyde thiosemicarbazone[4]. However, most of the existing methods suffer from limitations such as longer periods of time for phase separations, weak stability of coloured complexes and interferences from metal ions like tungsten, tin, antimony and anions and complexing agents like thiocyanate.

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Aroyl hydrazones are extensively used in the detection, determination and isolation of metal ions as they are highly selective and form inert complexes with metal ions. Phenyl hydrazine was used as a reagent for the spectrophotometric determination of molybdenum in air and tungsten [5]. Gallacetophenone oxime, Gallacetophenone phenylhydrazone were employed as reagents for spectrophotometric and gravimetric determination of some transition metals by Adinarayana Reddy [6], Raja Reddy [7] employed Gallacetophenone isonicotinoyl hydrazone as a reagent for molybdenum. Resacetophenone benzoic hydrazone was employed as analytical reagent for Mo (VI) by Kesava Rao et al [8]. Das R [9] used 2’,4’-dihydroxy acetophenone benozoyl hydrazone as a reagent for selective and rapid micro determination of molybdenum, Cinnamaldehyde-4-hydroxy benzoylhydrazone for direct and derivative spectrophotometric determination of Molybdenum (VI) in presence of micellar medium in food stuffs, pharmaceutical samples and in alloys [10].

During the course of investigations of the analytical applicability of 2,3,4-Trihydroxy Phenylethylidine Benzoic Acid Hydrazide (TPBH) (Gallacetophenone benzoyl hydrazone)[11,12] the reagent produced colouration with various transition metals. When a solution of sodium molybdate was treated with an ethanolic solution of the ligand, it was observed that the resulting solution had shown a yellow colouration, which was not present, both in the metal and the ligand solutions. It was confirmed by the observation of a shift in the absorption maximum of the complex to that of metal and ligand in the absorption spectra. The formation of the complex was studied in acidic and basic conditions, which revealed that the TPBH reacts with the metal in all pH conditions and forms a complex. It was observed that the complex was stable for a much longer period in organic phase when compared to the aqueous phase.

In the present paper, extractive spectrophotometric determination of molybdenum(VI) with 2,3,4-Trihydroxy Phenylethylidine Benzoic Acid Hydrazide is presented.

2. Experimental

Electrochemically and spectrally pure water obtained from triple distillation of deionised feeder water was used for conducting analytical operations. Ethanol and Methyl isobutyl ketone (MIBK) were purified following established methods [13] and used.

All the chemicals were of Analytical grade quality. The inorganic salt solutions of various metal ions in higher concentrations were prepared by dissolving appropriate salts in requisite quantities in triple distilled water to give a solution containing ~1mg per ml and few drops of a suitable acid were added before dilution wherever necessary to prevent hydrolysis.

A number of buffer solutions of constant ionic strength of 0.1M covering a wide pH range were prepared. First generation stock solutions were prepared [14], appropriately combined in different combinations and used for spectral studies.

A 110-3 M stock solution of Mo(VI) was prepared by dissolving 0.0242g of Na2MoO4.2H2O in 100 ml of

distilled water and solutions of lower concentrations were prepared from the stock solution by dilution. The ligand TPBH [11] was synthesised, recrystalised, dried and used for preparing a 110-3 M stock solution in ethanol and also in MIBK and further diluted with the respective solvent wherever necessary. Solutions of ions for interference studies were prepared by dissolving the amount of each compound needed to give 10 mg ml-1 of the ion concerned. All the solutions were stable for several weeks.

Synthetic alloy samples:

0.5 g sample of alloy was digested in 15 ml of aquaregia by warming and the solution was evaporated to dryness. The residue was dissolved in 10 ml of diluted HCl and the resulting solution concentrated to 5 ml, diluted to 50 ml with distilled water, filtered and made up to the mark in a 100 ml volumetric flask. The sample solution was appropriately diluted to obtain the concentration in the required range. Suitable aliquots were taken and analysed for molybdenum using the proposed procedure. Citrate solution was added to mask iron when necessary.

Recommended procedure:

An aliquot of the solution containing 0.1-20g of Mo(VI) solution was taken in a 10 ml volumetric flask, to this 1.0 ml of 0.1M HCl and the total volume of the solution made to 10.0 ml with distilled water. The solution is taken into a 50 ml separating funnel and 10.0 ml of 110-4 M reagent in MIBK was added and shaken vigorously for 4 min. The organic layer was separated, dried over anhydrous sodium sulphate and absorbance was measured at 385 nm against the reagent blank.

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For absorbance measurements and other spectrophotometric studies a Shimadzu UV-160A UV-Visible-NIR spectrophotometer with 1cm quartz cells in the wavelength range of 1100-200nm equipped with a Shimadzu Model 200-91027 six colour X-Y plotter was used.

An Orion Expandable ion analyzer EA-940 equipped with 91-01 Ag/AgCl, glass pH electrode was used for pH measurements.

3. Results and Discussion Absorption Spectrum

The absorption spectra of 2,3,4-Trihydroxy Phenylethylidine Benzoic Acid Hydrazide (TPBH), Mo(VI) ion and the TPBH-Mo(VI) complex in ethanolic solution and in MIBK were recorded from 200-800 nm against reagent blank. From the spectra, the absorption maximum ( max) of the complex is observed at 385 nm, which is identical for both aqueous and organic phases of the complex, at which, the absorbance of the metal ion and the ligand are negligible or nil. The Hence for the spectrophotometric study, all the measurements were carried out at 385nm.

Effect of solvents and equilibration time:

The absorbance value and the percentage of extraction of Mo(VI) decreased in the order MIBK (100%) > benzene (91.2%) > chloroform (76.2%) > carbon tetrachloride (74.5%) > amyl alcohol (63.6%). It was observed that the extraction was quantitative in 2-3 minutes of equilibration. Hence to get reproducible results 4 minutes of equilibration was carried out for all extractions.

Effect of pH:

The optimum pH range in which the metal complex shows maximum and constant absorbance and the extraction was quantitative at a wavelength of 385 nm against identically prepared reagent blanks was 1.0-4.0. The extraction was incomplete below and above this pH region and there was no extraction above pH 6.0. Hence, for all further extractions the pH was maintained at 2.0 by using 0.1 M HCl.

Effect of Reagent concentration and stability of the complex:

The minimum amount of reagent to acquire maximum colour intensity with a given amount of Mo(VI) ion was found to be a minimum of ten fold excess of the reagent and the absorbance of the complex is uniform even up to a fifty fold excess and after that there is a slight downward deviation. The absorbance of the complex Mo(VI) –TPBH in MIBK is found to be stable for 96 hours.

Applicability of Beer’s Law: A linear plot was obtained when the measured absorbance values are plotted against the amount of Mo(VI) in the concentration range of 0.4-10 g/ml at 385 nm. The optimal concentration range for the effective spectrophotometric determination of molybdenum as evaluated by Ringbom plot was found to be 3-8 g ml-1. The Molar Absorptivity and Sandell’s sensitivity were calculated and found to be 1.153104 l mol-1 cm-1 and 0.0083 g cm2- at 385 nm respectively. The standard deviation and coefficient of variation in absorbance for an amount of 6 g ml-1 of Mo(VI) were 0.0023 and 0.33% respectively. The composition of the complex responsible for the observed yellow colour in the reaction between the metal ion and the TPBH is established from the slopes of pH vs. log D plot (R2=0.9837) and log [TPBH] vs. log D plot (R2=0.9952) of the extracted species, Job’s method and mole ration method was found to be 1:1 (M:L).

Effect of Diverse Ions: Under optimum conditions of the above procedure, the tolerance of various metal ions and common anions, which are usually associated with molybdenum is determined by measuring absorbance at of a solution at 6 g ml-1 of Mo(VI) in presence of various foreign ions above which the deviation in absorbance in not more than 2%. Na+, K+, Ba(II), Zn(II), Pb(II), Mn(II), Mg(II), Al(III), Bi(III) fluoride, chloride, bromide, iodide, sulphate, nitrate, bromate, thiosulphate, EDTA, tartrate, citrate, acetate did not interfere even present in large quantities. The tolerance limits (in g ml-1) of foreign ions in the determination of Mo(VI) taken 6 g/ml are Mg(II) – 350, Pb(II) – 100, Mn(II) – 100, Co(II) – 140, Ni(II) – 250, Cu(II) – 500**, Al(III) – 800, Bi(III) – 100, Iron (III) – 250*, Ti(IV) – 500**, V(V) – 250**, Cr(VI) – 200* . Fluoride – 2000, Bromide – 1700, Nitrate – 1000, Sulphate – 2500, Phosphate – 750, Oxalate – 500, acetate – 2000, Tartrate – 1000, citrate – 1000 and EDTA – 1200. [*masked with 50 mg of ascorbic acid] [**masked with 10 mg of disodium salt of EDTA].

Application: The proposed method is more sensitive ( = 1.153104 l mol-1 cm-1) and colour intensity is more stable than the methods proposed with Gallacetophenone oxime [15] ( = 0.3104 l mol-1 cm-1), Resacetophenone benzoic hydrazone [8] ( = 0.36104 l mol-1 cm-1), 2’,4’ dihydroxy acetophenone benzoyl hydrazone [9] ( = 0.94104 l mol-1 cm-1), 1,4 dihydroxy phthalimide dithiosemicarbazone [16] ( = 0.94104 l mol-1 cm-1) and a similar stoichiometry for the complex was observed with resacetophenone benzoic hydrazone [8], 2’,4’ dihydroxy acetophenone benzoyl hydrazone [9], but this method is simple, rapid and the Beer’s law limit is more.

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The validity of the proposed method was established by the determination of Mo(VI) in standard steel samples and the results are reported in Table 1. The results obtained are in good agreement with the certified values.

Table 1 – Determination of molybdenum in steel samples

Standard Steel % Mo(VI) Relative Error %

Standard Deviation Certified Value Founda

NIST 1761 NIST 1762

0.103 0.350

0.1022 0.3471

0.776 0.828

0.00105 0.00079

a

Mean value of five determinations; Fe masked by citrate

4. Conclusion

The proposed method permits the determination of trace amounts of molybdenum(VI) without any prior separation. The major advantage of the proposed method is that the colour development is instantaneous at room temperature and stable. The proposed method has higher sensitivity and selectivity than that of the methods reported earlier.

Acknowledgments

The authors are thankful to the authorities of NIT, Warangal and Andhra University, Visakhapatnam for the facilities provided.

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