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Metal complex formation may be considered to be due to the displacement of a proton from the ligand by metal causing a drop in the pH values of the solution. Equilibrium constants are related to the complex formation. The stepwise formation of mononuclear, binary and tertiary complexes may be described by a set of equilibrium constants. The change in concentration caused by a complex formation are reflected in the potential of a well chosen electrode where electrode reaction must be reversible. The electrode potential is well defined function of the concentration of the ion to be determined. Rossotti1 have given a method for calculation of stability constants of complexes by pH-metry (Potentiometry). Calvin and Bjerrum2 also followed pH-metric technique for study of stability constant of metal complexes. Martell and Calvin3 have done potentiometric study of metal complexes of a large variety of polycarboxylic acids, oximes and phenols. Various experimental methods are also used for determination of composition, and stability constant of complexes, such as potentiometric (pH-metry) method, spectrophotometry, polarography, ion exchange method, kinetic measurements, colorimetric and extraction methods. The number of methods are related with only to a well defined group of equilibrium system and so their application is restricted. Potentiometric technique is more suitable but it requires that electrode reaction must be reversible. The law of mass action strictly determines the concentration of the reactants and products in every reversible chemical reaction. The numerical values of equilibrium constant depends on the concentration.