Determination of Proton-Ligand Stability Constant and Metal-Ligand Stability Constant of Substituted Pyrazolines in 70% Ethanol-Water Mixture

The proton–ligand (pK) and metal–ligand stability constants (log K) of substituted pyrazolines were determined in a 70% ethanol–water medium using the potentiometric (pH-metric) method at constant ionic strength (μ = 0.1 M NaNO₃) and temperature (25 ± 0.1 °C). The study aimed to evaluate the complexation behaviour of pyrazoline derivatives toward transition metal ions such as Cu(II), Ni(II), Co(II), The proton–ligand stability constants were obtained by titrating the ligand solution with standard alkali and analyzing the titration curves using the Irving–Rossotti method. Metal–ligand stability constants were evaluated by comparing the titration curves of ligand alone and ligand–metal systems under identical conditions The results revealed that the pyrazoline ligands act as bidentate chelating agents, coordinating through the nitrogen atoms of the azomethine (–CH=N–) and pyrazoline ring. The values of log K were found to follow the order Cu(II) > Ni(II) > Co(II) which is in agreement with the Irving–William’s stability order. The moderately high values of the formation constants indicate the formation of stable 1:1 and 1:2 metal–ligand complexes in the 70% ethanol–water mixture. The study demonstrates that solvent polarity significantly influences both protonation and complexation equilibria, suggesting partial covalent character and strong coordination between the metal ions and the substituted pyrazolines.