Spectrophotometric determination of Neodymium(III), Samarium(III), Gadolinium(III), Terbium (III), Dysprosium (III), Holmium (III) in micellar media

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1 Indian Journal of Chemical Technology Vol. 19, September 2012, pp Spectrophotometric determination of Neodymium(III), Samarium(III), Gadolinium(III), Terbium (III), Dysprosium (III), Holmium (III) in micellar media Asha Mathew, AV Krishna Kumar, P Shyamala*, A Satyanarayana & I M Rao Department of PNCO, School of Chemistry, Andhra University, Visakhapatnam , India Received 2 August 2011; accepted 21 February 2012 Spectrophotometric procedure for the determination of metal ion representatives from the three subgroups of lanthanide series, namely Neodymium (III) & Samarium (III) of Cerium group; Gadolinium (III) & Terbium (III) of Terbium family and Dysprosium (III) & Holmium (III) of Yttrium family has been studied using 1-(2-pyridylazo)-2-naphthol (PAN) as a chromogenic reagent in micellar media. The reported procedure avoids the use of carcinogenic molecular solvents. The method is based on the formation of an intense red colored ML 3 type of complex (L=PAN and M= Ln 3+ ) in the ph range , soluble in all the three micellar media under study, viz. Sodium dodecyl sulphate (SDS), Cetyl trimethyl ammonium bromide (CTAB) and Triton X-100. All the systems show ( ) 10 4, ( ) 10 4 and ( ) 10 4 mol -1 cm -1 dm 3 molar absorptivity and ( ) 10-4, ( ) 10-4, ( ) 10-4 μg cm -2 Sandell s sensitivity for Triton X, CTAB and SDS respectively. The tolerance limits towards several foreign ions are also reported. Keywords: Dysprosium, Gadolinium, Holmium, Micelles, Neodymium, Samarium, Spectrophotometry, Terbium The determination of lanthanides is important in nuclear chemistry and metallurgy and there is a growing need to develop various methods for their rapid determination and separations. Several reagents such as 1-phenyl-3-methyl-4-benzoylpyrazol-5-one (ref.1), thenoyltrifluoroacetone (ref. 2), 1-(2-pyridylazo)-2-naphthol (refs 1-5) and disodium1, 2-dihydroxybenzene-3,5-disulphonate (refs 6) have been used for the formation of metal complexes followed by their spectrophotometric determination. All these methods suffer from the limitations of either the use of organic solvents or the low sensitivity of determination. The use of organic solvents to solubilize the reagents or to extract the metal complexes by the traditional liquid-liquid extractions are time consuming and labour intensive approaches, requiring large amounts of high purity and toxic solvents, which are expensive, carcinogenic and cause environmental pollution. It is therefore important to develop methods that do not use solvents for solubilization or extraction. Surfactant molecules and micelles, designed as organized assemblies exhibit several peculiar properties. They solubilize, *Corresponding author. shyamalapulipaka@rediffmail.com concentrate and compartmentalize chemical species. They also alter dissociation constants, oxidation reduction properties, provides unique reaction media and influence transport properties 7. Surfactants improve the analytical characteristics of chromophoric chelating organic reagents and thus increase the sensitivity of the determination, decrease the equilibration time of the reaction and increase the stability of the complex. Hence, the micellar solution can be used as a medium for spectrophotometric determination of metal ions Little work has been reported on the systematic studies of Ln 3+ -PAN complexes in the presence of micelles. Therefore, we have taken up the analytical study of these lanthanides with PAN in different micellar media which would determine these metal ions at trace levels. The rare earth elements Nd 3+, Sm 3+, Gd 3+, Tb 3+, Dy 3+ and Ho 3+ in the subgroups of lanthanide series viz. Cerium, Terbium and Yttrium families have been selected. Hereafter, we will quote Ln 3+ for Nd 3+ /Sm 3+ /Gd 3+ /Tb 3+ /Dy 3+ /Ho 3+. Experimental Procedure Reagents and apparatus All absorbance measurements and the spectra of Ln 3+ PAN [1-(2-pyridylazo)-2-naphthol] complex

2 332 INDIAN J. CHEM. TECHNOL, SEPTEMBER 2012 solutions were taken at 30 C with a double beam UV-VIS spectrophotometer (Shimadzu UV-1800). Measurements of ph were made using control dynamics digital ph meter equipped with a combination electrode. Double distilled water was used to prepare all the solutions. All reagents used were of analytical grade. Cetyl trimethyl ammonium bromide (CTAB), sodium dodecyl sulphate (SDS), TritonX-100 (Merck) were used without further purification. A 0.01 mol. dm -3 solution of PAN (Reidal) was prepared by dissolving an appropriate amount of reagent in respective surfactant solution and was standardized. A 0.01 mol. dm -3 solution of Ln 3+ was prepared by dissolving an appropriate amount of oxide in 1:3 nitric acid by heating on a sand bath and diluting with water to the mark in volumetric flask. The stock solution was then standardized by titrating against EDTA with xylenol orange as indicator 13. Buffer solutions of ph 6-8 (borax) and 8-10 (ammonical) were prepared according to National Bureau of Standards (NBS) for use in the study. medium varies in the range , show that the optimum ph for complex formation lies between 8.5 and 9.5 for all the systems under investigation (Figs 1a-c). This indicates that the complexation is favored in weakly basic conditions. Based on these observations, other investigations were carried out at ph ~9.2 using ammonium chloride-ammonium hydroxide buffer. Effect of PAN concentration The effect of concentration of the chelating reagent PAN has been studied for the metal-to-ligand ratios between 1:1 and 1:20 by fixing the metal ion Analytical procedure and optimization conditions A typical experiment was carried out as follows for optimizing the experimental conditions: Requisite volumes ( mol dm -3 ) of solution of PAN in surfactant and mol dm -3 of Ln 3+ solution were mixed using [PAN]/[ Ln 3+ ] of 20 and 10 ml of buffer (ph 9.2). The mixture was then made up to the mark in a 25 ml volumetric flask with double distilled water. The solution was allowed to stand for 20 min before recording the absorbance against a reagent blank at 545, 535, and 540 nm in CTAB, Triton X and SDS respectively. Results and Discussion Optimization of conditions Time of stability Absorbance of the solution has been measured at various time intervals and the complex was found to be stable for 24 hour as against the stability of only 30 min in aqueous media 14. Effect of ph The effects of various parameters on the sensitivity of Ln 3+ determination have been investigated one at a time. ph is one of the most important parameters for this determination as the complex formation depends largely on the ph. Experiments in which the ph of the Fig. 1 Effect of ph on absorbance of Nd 3+, Sm 3+, Gd 3+, Tb 3+, Dy 3+, Ho 3+ -PAN complexes in different surfactants [[Ln 3+ ]= mol dm -3, [PAN]= mol dm -3, and [surfactant]= mol dm -3 ]

3 MATHEW et al.: SPECTROPHOTOMETRIC DETERMINATION OF METAL IONS 333 concentration at mol dm -3 and varying the PAN concentration from mol dm -3 to mol dm -3. A plot (Figs 2a-c) of absorbance vs (L/ M) shows an initial increase up to 10.0 (L: M) and remains constant thereafter upto 20:1. Therefore, in subsequent studies the ligand concentration was kept more than ten times of the metal ion concentration so that the consumption of PAN by other metal ions could be avoided. PAN complexes have λ max between 538 nm and 546 nm in all the three surfactants while PAN has no appreciable absorption in that wavelength range. Composition of complex PAN (LH) is known to exist in three forms in solution 15, depending on the ph (Fig. 4). The LH 2 + Absorption spectra of complex Absorption spectra of Ln 3+ -PAN complex were taken at the optimum conditions (Figs 3a-c). From the absorption curves it is evident that all the lanthanide Fig. 2 Effect of [PAN] on absorbance of Nd 3+, Sm 3+, Gd 3+, Tb 3+, Dy 3+, Ho 3+ -PAN complexes in different surfactants [[Ln 3+ ]= mol dm -3, ph=9.2, and [surfactant]= mol dm -3 ] Fig. 3 Visible spectra of Nd 3+, Sm 3+, Gd 3+, Tb 3+, Dy 3+, Ho 3+ - PAN complexes in different surfactants [[Ln 3+ ]= mol dm -3, [PAN] = mol dm -3, [surfactant]= mol dm -3, and ph= 9.20]

4 334 INDIAN J. CHEM. TECHNOL, SEPTEMBER 2012 form (Fig. 4), in which the pyridine nitrogen is protonated, exists below a ph of 2. The neutral LH form exists between 3 and 11 ph and the anionic L - form in which hydroxyl group is deprotonated appears in solution above a ph of 11. PAN acts as a tridentate ligand bonded to the metal ion through hydroxyl oxygen, pyridine nitrogen and azo nitrogen. The composition of Ln 3+ -PAN complexes in surfactant media has been determined using Job s continuous variation method 19 and is ascertained by mole ratio method. The results indicate the formation of LnL 3 type of complexes. As the ligand exists in neutral LH form in the ph region of study (~9.2 ph), the complexation reaction may be represented as shown below: Ln 3+ + LH Ln L 2+ + H + Ln L 2+ + LH + Ln L 2 + H + Ln L LH Ln L 3 + H + However, the bonding centers of the ligand cannot be ascertained by Job s method as it gives only the information regarding the composition of the complex. Effect of micelles The effect of all the three kinds of surfactants, namely SDS (anionic), CTAB (cationic) and Triton X-100 (non-ionic), have been studied to determine their effect on absorption properties of the complexes. The effect of surfactant concentration is studied within the range from mol dm -3 to mol dm -3 (Figs 5a-c). The absorbance at λ max of the complex shows a sharp increase with increasing surfactant concentration. The increase in absorbance in the presence of micelle is due to the binding of reactants in a small volume of stern layer of the micelle, thus leading to considerable concentration effect. The maximum in the plot of absorbance vs. concentration of surfactant 20 is considered to be produced by two opposing effects. With increase in surfactant concentration, binding of reactants in the stern layer begins and they are transferred into small volume of the micellar pseudophase. There is thus a concentration effect which is responsible for increase in absorbance. This concentration effect is opposed by continuous dilution of the reactants within the micellar pseudophase with increasing surfactant concentration. The former effect is predominant at lower surfactant concentration, whereas the latter becomes important at higher concentrations of Fig. 4 Protonated and deprotonated forms of PAN Fig. 5 Effect of [different surfactants] on absorbance of Nd 3+, Sm 3+, Gd 3+, Tb 3+, Dy 3+, Ho 3+ -PAN complexes [[Ln 3+ ]= mol dm -3, ph=9.2 and [PAN] = mol dm -3 ] surfactant, resulting in a maximum in the absorbance vs. [surfactant] profile. The binding constants of the chelating agent PAN has been calculated in all the three micelles by taking absorbance at different surfactant concentration above CMC and under the conditions [Surfactant]>> [PAN] using the following equation 21 : 1/ (A M A 0 W) = 1/ (A 0 M A 0 W) (1+1/K s C) where A 0 W and A M are the absorbances in water and micellar media respectively. A 0 M is the limiting absorbance in micellar media. The binding constants

5 MATHEW et al.: SPECTROPHOTOMETRIC DETERMINATION OF METAL IONS 335 Sandells sensitivity 10-4, μg cm -2 RSD, % Table 1 Molar absorptivity, Sandells sensitivity and RSD of the determinations in three surfactant solutions Metal ion Molar absorptivity 10 4 mol -1 cm -1 dm 3 Triton X CTAB SDS Triton - X CTAB SDS Triton - X CTAB SDS Nd Sm Gd Tb Dy Ho are found to be 104 dm 3 mol -1 in Triton-X, 84 dm 3 mol -1 in SDS and 100 dm 3 mol -1 in CTAB. Under the present experimental conditions, (ph ~9.00), PAN exists in neutral form and is more bound to the neutral surfactant rather than with anionic surfactant (SDS). Water-insoluble compounds containing polar groups are orientated with the polar group at the core surface interface of the micelle, and the hydrophobic group buried inside the hydrocarbon core of the micelle. In addition to these sites, solubilisation in non-ionic polyoxyethylated surfactants can also occur in the poly oxyethylene shell (palisade layer) which surrounds the core. Hence, the binding constant of neutral surfactants is more than cationic or anionic. In the case of metal-pan complexes, the extent of formation is high in neutral surfactant when compared to SDS. This may be explained based on the fact that the attractive forces between the negative heads of the anionic micelles and the free metal cations decrease the metal ion availability for complexation. In the case of non-ionic micelles, the entire metal ion is available for complexation by the ligand. Therefore, the lower molar absorptivity and sensitivity is observed in SDS compared to that in Triton-X and CTAB (Table 1). Stoichiometry, sensitivity, molar absorptivities and detection limits Beer-Lambert law has been studied over the metal ion concentration range from mol.dm -3 to mol.dm -3 and the molar absorptivity and Sandells sensitivity 22 are calculated (Table 1). The results show an increase in sensitivity in micellar media when compared to the aqueous medium 15. Precision of the method was determined from replicate concentration measurements. Relative standard deviation is also calculated (Table 1). Effect of foreign ions The effect of presence of foreign ions on the trace level determination of Ln 3+ in aqueous phase has also Table 2 Effect of foreign ions on Ln 3+ PAN complex in three surfactants Foreign ions Molar tolerance limit NO - 3, ClO - 4, Br -, CI -, I -, and SCN - >2000 Ca(II), Mg(II), Mo(VI), Cr(VI) >2000 Oxalates, V(V) 100 F - 50 Acetate, tartarate, thorium 10 Uranyl(VI), Al 3+, citrate, phosphate, Equal Ni(II), Cu(II), Cd(II), Fe(III) been determined. The criterion for the studies is kept as ±4.0% change in absorbance. The amount of foreign ion tolerated (i.e. which changes absorbance by ±4.0%) is given in Table 2. Metal ions Fe (III), Cd(II), Cu(II), Ni(II), Zn(II), and U(VI) are found to interfere. The complexation reaction between PAN and Ln 3+ is completely masked by EDTA, citrate, phosphate at low concentrations, whereas oxalates, tartarates, Br -, CI -, I - and SCN - have negligible effect at these concentrations levels. Conclusion One of the most important aspects of the present work is the simple and rapid detection and trace level determination of Ln 3+ ions in an aqueous solution of a surfactant. The sensitivity and selectivity considerably improves in presence of micelles. The method does not require the use of expensive, toxic and carcinogenic organic solvents. The method is not only sensitive but also economical and less complicated when compared to other methods. References 1 Cheng K L & Goydish B L, Anal Chim Acta, 34 (1966) Eskandari H & Saghseloo A G, Anal Sci, 19 (2003) Navratil O, Co Czech Chem Commun, 31(1966) Popova T V, Tolmachev V L, Ansari & Shcheglova N V, J Anal Chem, 56 (2001) Shibata S, Anal Chim Acta, 28 (1963) Rose E D, Drabek V R & Larsen R P, Talanta,16 (1969) Pramauro E & Pelezetti E, Surfactants in Analytical Chemistry, Applications of Organized Amphiphilic Media (Elsevier), 1996.

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