Analytical grade reagents were used throughout the study. Fe(NH₄)₂(SO₄)₂・6(H₂O) was obtained from Merck and Methyldopa was obtained from Wild Wind, CO₂ free distilled deionized water was used for the preparation of buffer and complex solutions.

Absorbance maxima of the complex, was investigated by treating 0.5 mM of Fe(II) solution with adequate excess of Methyldopa solution prepared in Acetate buffer of desired pH. The pH of the complex was recorded by JENWAY370 pH meter and SCHIMADZU model number UV-160A was used for scanning complexes in visible region. Spectrum of the complex indicated presence of a broad peak centered at 615 nm and another peak at 430 nm, shifting of the peak with rise in pH was also observed that is why wavelength of 615 nm was selected for further study as illustrated in Figure 2.

Solutions of different dilutions were prepared in Acetate buffer of desired pH. Absorbance was recorded for all diluted solutions at selected wavelengths i.e. 615 nm. Table 1 demonstrates a plot of absorbance for different dilutions against metal concentration provided the slope for determining molarextinction coefficient [28] .

Accurate amounts of Iron (II) salt was used to prepare stock solution of metal in deionized distilled water. Stock solution of Methyldopa was prepared in acetate buffer of required pH. Different aliquots of ligand solution were

added in 0.5 mM metal solution in order to get various ligand metal ratios ranging from 0.5:1 to 9:1. The final volume was maintained with respective buffers in all cases. The absorbance was recorded at 615 nm and temperature maintained at 25˚C ± 1˚C is illustrated in Figure 3 [29] .

The slope ratio method was used to find the Stoichiometry of the complex. Two series of solutions were prepared. In first half constant volume of 0.5 mM Fe(II) was treated with variable volumes of 5 mM Methyldopa, where as in the other half of the analysis, MD was kept constant versus variable volumes of Fe(II) solution. Resulting complexes were scanned at selected wavelength of 615 nm and the recorded absorbance was plotted versus concentration of varying specie. Stoichiometry of the complex was interpreted by calculating ratio of slope of two straight lines [30] . Figure 4 is a plot of slope ratio at pH 5.5. The same method was used at pH 4.0, 4.5 and 5.0. All experiments were performed in triplicate in order to get consistent results.

The results show that Methyldopa-Fe have two distinct peaks at low pH, which appear at 430 and 730 nm. The absorbance increases at these wavelengths with the rise of pH. However, the peak at 730 shifts to lower wavelength, as pH is increased. The molar extinction coefficient values were evaluated by serial dilution of complex (standard curve method). The values found are indicated in Table 1 and are found to be very high, increasing with the pH. The high value indicates charge transfer band either LMCT or MLCT.

At all pH, work was carried on three wavelengths, 430, 615 and 730, selected purposely. At 430 nm the ε increases with pH. Same trend is found at all wavelengths. Considering ε on a single pH, it is interesting to note that at pH 4.0 and 4.5, the highest value is found at 430 nm, while at pH 5.0 and 5.5, ε is higher at 615 nm. This observation may correspond to the result, that, at low pH, 2 MD molecules chelate iron (II), while with the rise of pH, all six coordination sites of iron are occupied by MD, forming Fe(MD)₃. as suggested in Figure 5.

Stoichiometry is evaluated by mole ratio and further confirmed by slope ratio method. It has been found that at low pH Fe(H₂O)₂(MD)₂ forms while it converts to Fe(MD)₃ due to de-protonation of ligand at higher pH as showed in Table 2. Since the pH have a significant effect on complex formation, indicate that, the chelation of metal take place through catecholic side.

Results obtained by Slope ratio method are in good agreement.

β value of ML₁, ML₂ and ML₃ species formed gradually in the solution of varying stoichiometric ratio were calculated by using moleratio data applying single point statistical method where β is the ratio of complex concentration to the product of remaining concentration of metal and ligand at each data point. Overall formation constant of the complex was calculated by using same method; direct moleratiois also used to calculate K_f in this method formation constant is ratio of equilibrium concentrations of products and reactants [30] .

The overall formation constant for Fe(MD)₃ is found very high about 10¹⁰. The values of K_f remains unaffected by pH when determined in Table 3 and Table 4 at 615 nm. Step wise formation constant at each pH is also similar i.e. no significant change is observed with pH. K_f obtained by the two methods graphical and statistical handling of mole ratio data is consistent.