Sensitization of Eriochromeazurol-B in Presence of Cetyldimethylethylammonium Bromide for the Microdetermination of Some Lanthanides

Cetyldimethylethylammonium bromide, a cationic surfactant has been used to decolorize eriochromeazurol B, an anionic triphenylmethane type of dye. Addition of specific lanthanide metal ion to this decolorized solution resulted into intense colored stable ternary complex with large bathochromic shift from 540 nm (binary complex) to 650 nm (ternary complex) with increase in absorbance values at shifted wavelength. CDMEAB thus decreases the color intensity of ECAB and increases the absorbance value of ternary complexes. This two fold advantage resulted into enhancement in molar absorptivities and sensitivities at shifted wavelength of ternary complexes with stoichiometric composition 1:(1:3)2, [Ln : (R:S)] for all lanthanides understudy namely yttrium, neodymium, europium, terbium and ytterbium. The ternary complexes at pH 6.0 exhibited absorption maxima at 650 nm with molar absorptivities 69000 L.mol.cm for Y(III), 66000 L.mol.cm for Nd(III), 69000 L.mol.cm for Eu(III), 64000 L.mol.cm for Tb(III), 70000 L.mol .cm for Yb(III). Beer’s law were obeyed in concentration range 0.11-0.94, 0.19-1.53, 0.2-1.41, 0.21-1.69 and 0.23-1.11 ppm for Y(III), Nd(III), Eu(III), Tb(III) and Yb(III) respectively. Conditional formation constants and various analytical parameters have been evaluated and compared the results of newly formed ternary complexes with binary complexes. Finally newly suggested modified method have been recommended for the microdetermination of lanthanides understudy.


Introduction
The lanthanide metal ions have found an ever increasing use in many areas of research such as synthesis, co-ordination chemistry, material science, photonic devices, catalysis etc.The metals like europium and yttrium are used in the production of red phosphor that is found in all color television screens.Yttrium is also used in heating elements alloys, super alloys and high temperature semiconductors.Ytterbium can be alloyed with stainless steel to improve some mechanical properties in fiber optic cables where it can be used as an amplifier and doping agent.Terbium is used in X-ray machines and green phosphor activator.Neodymium appears in many electronics such as fluorescent lights and color televisions.
Organic reagents play very important role in analytical and complex chemistry of lanthanides for its determination.With increasing demand of more sensitive reagents for their use in determination of metal ions by spectrophotometric technique, the attempts have been made in the past to intensify the color using third component like surfactant by converting the binary complex of lanthanides to ternary complex.It was observed that the addition of surfactant to dye solution decreases the color intensity of the reagent.The addition of specific metal ion to this decolorized solution resulted into the formation of intense colored stable ternary complexes with bathochromic shift in the wavelength with heightened molar absorptivity and sensitivity which primarily decides the usefulness and importance of the reagents developed for spectrophotometric determination of metal ions.
Very few ternary or mixed ligand complexes appear in the literature for their spectrophotometric microdeterminations.The ternary complexes of gallium (III) and lead (II) were reported with pyrocatechol violet in presence of some cationic surfactants 1 .Shokrollahi A. reported a spectrophotometric method for determination of sub-micro-molar amounts of aluminium (III) ion with eriochromecyanine R in presence of DTAB as surfactant 2 .Spectrophotometric determination of scandium (III) in monazite after separation in presence of reagent eriochromecyanine R and surfactant CTAB have been reported by Chan-il-park and coworkers 3 .Application of chromeazurol S and benzyldodecyldimethylammonium bromide for gallium (III) determination in mineral fertilizers have been studied by Buhl F. and coworkers 4 .Spectrophotometric determination of uranium (VI) with pyrocatechol violet in cetyltrimethylammonium bromide and thorium (IV) and uranium (VI) with methyl thymol blue in presence of cetyldiethylammonium bromide have been reported earlier [5][6] .
Attempts have been made by using cationic surfactants like cetyltrimethylammonium bromide and cetylpyridinium bromide to improve the photometric sensitivity for the microdetermination of some lanthanides using some organic dyes [7][8][9][10][11][12] .The ternary association of chromeazurol S and cetyltrimethylammonium ion in presence of triton-100 was useful for the sensitive spectrophotometric determination of lanthanide in mixtures and in hydroxide concentrates as reported by Preisler et al 13 .Spectrophotometric study of ternary complex forming systems of dysprosium (III) and holmium (III) with pyrogallol red and some lanthanide metal ions with eriochromecyanine R in presence of cetylpyridinium bromide for microdetermination have been reported by A. S. Dhepe and A. B. Zade 14,15 .Magda Ali Akl et al have discussed the extraction, preconcentration and determination of microamounts of lanthanum and yttrium using alizarin red sulfonate (ARS) of a metal complex via surfactant-mediated phase separation 16 .The ternary interaction of naphthochrome green (NCG) with cetyltrimethylammonium bromide (CTAB) and lanthanides (Res: Yb, Dy, Er and Eu) has been investigated with the microsurface adsorption spectral correction technique (MSASC) 17 .The surfactant (CPB) sensitized analytical reaction of cerium (IV) with some triphenylformazan derivatives was studied by Ahmed and his coworkers 18 .
The present investigation describes the systematic study on the use of eriochromeazurol B (ECAB) as spectrophotometric reagent for the microdetermination of lanthanides i.e. yttrium (III), neodymium (III), europium (III), terbium (III) and ytterbium (III) in absence as a binary complex and in presence of cationic surfactant cetyldimethylethylammonium bromide (CDMEAB) as a ternary complex and has not been reported so far.

Experimental
Instrumentation and reagent solutions All the chemicals used were of analytical grade purity.Eriochromeazurol-B used was supplied by Sigma Chemical Company, U.S.A. and cetyldimethylethylammonium bromide (CDMEAB) by Aldrich Chemical Company, U.S.A. and its purity was estimated by argentometric titration for the determination of bromide ion content 19 .All the lanthanide oxides used were supplied by Indian Rare Earth Ltd., India, of 99.9% purity.The stock solutions of lanthanides, CDMEAB and ECAB were prepared of strength 1.0 x 10 -3 M and subsequently diluted with double distilled water.
The CDMEAB solution was first added to ECAB solution and was kept for half an hour.The metal ion solution was then added to dye-surfactant solution and again kept for half an hour to reach complete equilibrium.This order of mixing of solutions was maintained.Standard HCl and NaOH solutions were used for adjustment of pH of desired concentration.Beckman model B spectrophotometer was used for the measurement of absorbance with glass cuvettes of light path 10 mm.All the experiments have been carried out at room temperature of 30 + 2 0 C.

Absorption spectra
It has been considered necessary to have prior information on the nature of interaction between anionic ECAB and cationic CDMEAB before evaluating the ECAB as a sensitive reagent for the estimation of lanthanides in presence of CDMEAB.Therefore absorption spectra of ECAB in absence and presence of CDMEAB, composition of dye-surfactant complex, absorption spectra of lanthanide complexes in absence and presence of CDMEAB, effect of pH, composition and stability constants of the complexes in absence and presence of CDMEAB have been studied.

Study of absorption spectra of ECAB in absence and presence of CDMEAB
The color of ECAB has been found to be different at different pH values.Addition of CDMEAB brings about a change in color of ECAB at the same pH value.The absorption spectra of ECAB has been therefore, studied at different pH values (1.0 to 12.0) in absence and presence of CDMEAB.The wavelength of maximum absorbance of ECAB in absence and presence of CDMEAB are summarized in Table 1.
Absorption spectra of alkaline ECAB solution at pH 11.5 show a characteristic maximum at 600nm.After the addition of CDMEAB, this maximum decreases and second absorption peak appears at 440nm.Visually this change represents a discoloration of the original pale violet solution to pale grayish or in more dilute solution to almost colorless.
Notable shifts are observed in the acidic medium.At pH 1.0,  max 470nm shifted to 520nm.In the pH range 2.0 to 4.0 peaks shifted from 490 nm to 520 nm.There is no shifting of absorption maximum in the pH range 4.5 to 5.5.It has been observed that the absorbance value increases from pH 1.0 to 5.0 and then decreases up to pH 12.0.These changes in absorbance maximum in presence of CDMEAB may be attributed due to interaction between anionic ECAB and cationic CDMEAB might be resulting into the possible formation of so called "Dye-Surfactant complex".

Composition of ECAB-CDMEAB Complex
The effect of varying CDMEAB concentration on the absorbance of ECAB has been studied in acidic medium at pH 6.0 and alkaline medium at pH 11.5 at  max 600 nm where the maximum discoloration has been observed.The absorbance of different concentrations of ECAB solution is plotted against variable concentration of CDMEAB.The concentration of ECAB taken were 2.0x10 -5 M, 1.6x10 -5 M and 1.33x10 -5 M and have been represented by curve A, B and C respectively in Figures 1 and 2. It is observed that the maximum decolorizing effect reached at the minimal ECAB: CDMEAB ratio of 1:3.When this ratio was reach the absorbance of the reagent remains unaltered even when excess of five times of CDMEAB has been added.The modified reagent species thus formed may therefore be written as [ECAB (CDMEAB) 3 ].The anionic ECAB dye and cationic surfactant reaction may be anionic in nature forming a new ECAB surfactant complex.This newly formed complex is useful for further microdetermination of metal ions.

Absorption spectra of lanthanide complexes in absence and presence of surfactant
Series of solutions were prepared by keeping the ratio of Metal: ECAB: CDMEAB as 1:1:10 and 4:1:10.Number of sets was prepared for each ratio and pH was adjusted to 3.0, 3.5, 4.0, 5.0, 5.5, 6.0 and 6.5.The absorption spectra were recorded in the entire visible region from 400 to 700nm.Absorbance maxima of ECAB and its complexes with lanthanide in absence and presence of CDMEAB have been summarized at different pH values in Table 2.In the pH range 3.0 to 3.5 in absence and presence of CDMEAB, the wavelength maxima of ECAB shows small change in  max and absorbance values; indicating poor complexation.However in presence of CDMEAB only one sharp peak has been observed at 650nm in the pH range 4.5 to 6.5 with high absorbance values.It has been observed that the maximum complexation takes place at pH 6.0 by comparing the absorption spectra and the absorbance values of the reagent and complex in absence and presence of CDMEAB as reported in the Table 2.But broad peak is observed in pH range 4.0 to 6.0 in the wavelength region around 540 nm indicating weak complexation in absence of CDMEAB for all lanthanides.
In Figure 3 to 7, the metal ions Eu (III), Nd (III), Tb (III), Y (III) and Yb (III) indicates binary complexation with ECAB and ternary with ECAB and CDMEAB at pH 6.0.Curve D represents the absorption spectra of ECAB alone at pH 6.0 showing a peak at 430 nm and second peak was also observed at 600 nm.ECAB in presence of CDMEAB has been indicated by curve C showing the λ max at 440 nm (single peak).Curve B indicates a broad peak around 540 nm indicating weak complexation.Thus shifting in  max from 430 to 540 nm in absence for binary and from 440 to 650 nm in presence of CDMEAB for ternary have been observed indicating a bathochromic shift of 110 nm in absence and 210 nm in presence of CDMEAB for all lanthanides.
Thus the complex formation of lanthanides in presence of CDMEAB shows large bathochromic shift and increased absorbance value causing marked sensitization of above color reaction resulting into high molar absorptivity of ternary complex which may be applied as useful tool in microdetermination of metal ions understudy.

Effect of pH
In Figure 8 variation of λ max of europium complex at different pH value was studied in absence and presence of ten fold excess of CDMEAB.Curve A represents variation of λ max with change in pH of solution of ECAB in absence of CDMEAB.It shows λ max at 510 nm in pH range 4.5 -5.5 and suddenly decreases to 430 nm from where it remains constant upto 6.5.Curve B represents variation of λ max with change in pH of the solution of ECAB in presence of CDMEAB show the constant λ max at 520 nm in the pH range 4.0 to 5.5 which then shifted at 440 nm and remains constant upto pH 6.5.Curve C represent the variation of λ max with change in pH of Eu complex in absence of CDMEAB.The horizontal portion of curve C in figure shows that the λ max of binary complex remains constant in the pH range 4.0 to 6.5 indicating pH range of stability of constant wavelength of the complexes.Curve D represents variation of λ max with change in pH of complexes of europium metal ion in presence of CDMEAB and indicate that the λ max of ternary complex remains constant in the pH range 4.5 to 6.5 i.e pH range of stability of constant wavelength.In figure 9 Curve B represents the plot of absorbance of Eu complex of ECAB at the wavelength of maximum absorbance i.e. 650 nm in presence of CDMEAB with the changes in pH in the range 4.0-6.5.The curves clearly shows that the absorbance increases almost linearly in the pH range 4.0-5.5, however it attains the maximum value at 5.5 and remain constant upto pH 6.5 as indicated by straight line portion of the curves.This pH range where the absorbance of the complex at the λ max is termed as optimum pH range of stability of complex formation.Curve A represents the plot of absorbance of lanthanide complexes at range 6.0 -6.5 as optimum pH range of constant absorbance.

Composition of Complexes
Quantitative information on complex formation is obtained by determining its composition and stability constants of complexes formed.From the spectral studies, it has been found that the reagent ECAB forms only one type of complex with lanthanide metal ions understudy.The composition of complexes has been studied by the Job's method of continuous variation 20 and it has been further confirmed by Mole ratio method.For Job's method, solutions of lanthanide and ECAB have been taken in three equimolar concentrations of 2.5x10 -4 M, 2.x10 -4 M, and 1.66x x10 -4 M; five times excess of CDMEAB has been then added for studying the composition in presence of surfactant.
The stoichiometric composition between the metal ion and ECAB in presence and absence of CDMEAB has been found to be 1:2.It has been observed that ECAB reagent at pH 6.0 exists as [ECAB (CDMEAB) 3 ] and therefore the composition of complexes in presence of CDMEAB may be written as Ln[ECAB(CDMEAB) 3 ] 2 for all lanthanide complexes.The composition has been further confirmed by Mole ratio method.This is just a tentative reaction and further studies in this regard is necessary for confirmation of the composition as the emphasis has been given in the present investigation on microdetermination of metal ions in presence of CDMEAB.The formation of complexes may therefore be expressed by equation (omitting charges).

Stability Constant
Values of log K of complexes of lanthanides understudy in absence and presence of CDMEAB have been calculated from Job's method 20 are reported in Table 4.The result shows that the value of log K for particular metal ion in presence of CDMEAB is greater than in absence of CDMEAB.This increase in log K values in presence of CDMEAB is due to formation of stable complexes.

Analytical Applications of Ternary Complexes of Lanthanides (in Presence of CDMEAB)
The large shift of 110 nm in absorption maxima of the complex from 540 nm in absence to 650 nm in presence of CDMEAB at pH 6.0 where the absorption of reagent is comparatively negligible and facilitate the analytical measurements making the reagent very sensitive for particular color reaction.

Order of addition of reactants
Sequence of addition of reactants must be followed strictly.In all the experiments, CDMEAB was first added to ECAB solution of desired concentration.This solution was kept for at least 30 minutes for equilibration.To this solution of modified ECAB, the metal ion solution was then added which again kept for 30 minutes for complete formation of ternary complex.

Rate of color formation and stability of color at room temperature
The color formation does not depend on reaction time and is almost instantaneous in presence of CDMEAB.However the mixtures were kept for 30 minutes for equilibration.A mixture containing 2.5 x 10 -4 M ECAB, 1.0 x 10 -4 M ECAB, and 2.5 x 10 -4 M lanthanide solutions at pH 6.0 retained its absorbance value even after 24 hours standing at room temperature.The temperature was found to have no effect on color intensity of ternary complexes from 20 0 C to 60 0 C.

Effect of reagent concentration
Different volumes of 1.0 x 10 -4 M of ECAB were taken in different flasks to which equal volume of 5.0 x 10 -4 M CDMEAB was added.One ml of 1.0 x 10 -4 M metal ions was then added in each flask.Total volume was maintained at 50 ml at pH 6.0.Absorbance readings were recorded at 650 nm.It was found that ECAB in all the cases of metal ions understudy must be present at least two times more than metal ion to have maximum color development.However in absence of CDMEAB reagent needed was six times that of metal ion for full color development.

Beer's Law and effective photometric ranges
The linearity between the absorbance of complexes and concentration of metal ion has been tested by taking the different volumes of metal ion solution (1.0x 10 -3 M in absence and 6.66 x 10 -4 M in presence of CDMEAB).The final concentration of ECAB taken was 2.0 x 10 -4 M, of CDMEAB was 1.0 x 10 -3 M. Total volume was kept constant at 50ml at pH 6.0.The absorbance values were measured in absence of CDMEAB at 540 nm.However in presence of CDMEAB all the spectral measurements were made at 650 nm.The range of Beer's law is given in Table 4 in absence and presence of CDMEAB.The effective range for photometric determination was also calculated from these data by Ringbom plot 21 (log of metal ion concentration versus percentage transmittance).Thus the range as derived by the slope of curve is selected to be range for the effective photometric determination as given in Table 5.

Conclusion
The spectrophotometeric determination of lanthanides with eriochromeazurol-B in absence and presence of cetyldimethylethylammonium bromide have been studied.Following are the merits of modified method.
1) The sensitization of ECAB by addition of CDMEAB is clear from the fact that formation of stable ternary complexes with lanthanides occurs at pH 6.0 with bathochromic shift in the  max of metal ion-ECAB complexes in presence of cationic surfactant.This change is attributed due to formation of ternary complex system in presence of CDMEAB in acidic medium compared to the binary system in absence of CDMEAB.
2) The formation of stable ternary complexes with sharp peak in wider pH range as compared to weak binary complexes with broad shoulder in small pH range.
3) The increase in value of log K in presence of CDMEAB for particular metal ion is greater than in absence of CDMEAB shows the formation of stable complexes.4) Due to the shifted  max towards higher wavelength (From 540 to 650 nm) and comparatively negligible absorbance of reagent gives the large difference in absorbance between the reagent blank (ECAB-CDMEAB) and its ternary complex which results in enhancement of sensitivities and molar absorptivities again indicates the sensitization of color reaction.In present investigation which was used for the determination of metal ions understudy when present in small concentration.5) Further, the modified method requires smaller molar concentration of ECAB over the metal ion concentration for full color development and is instantaneous in presence of CDMEAB again indicates the stability of color reaction.6) The modified reagent i.e. [ECAB (CDMEAB) 3 ] has also been found to be extremely useful in complexometric titration of lanthanide metal ions.This modified reagent act as sensitive metallochromic indicator giving a very sharp color change at the end point.
7) These observations suggest that a very useful application of these colored reactions in presence of ECAB or CDMEAB can be made in spectrophotometric determination of the metal ions with much enhanced sensitivity.It may be mentioned here that ECAB in presence of CDMEAB have been suggested for the first time as a sensitive reagents for spectrophotometric determination of metal ions understudy.

Table 1 .
Wavelengths of maximum absorbance of ECAB in absence and presence of CDMEAB.

Table 2 .
Absorbance maxima (nm) of ECAB and its complexes in absence and presence of CDMEAB at different pH.

Table 3 .
pH range of stability of ECAB complexes in absence and presence of CDMEAB.

Table 4 .
Composition and log K values of lanthanide complexes of ECAB in absence and presence of CDMEAB.

Table 6 .
Determination of Individual Metal Ion.