DETERMINATION OF ASSOCIATION AND DISSOCIATION CONSTANTS OF Co(II), Ni(II), Cu(II), Zn(II), Cd(II) AND Hg(II) METAL ION COMPLEXES WITH 3-(((3-(2-HYDROXYPHENYL)-1-PHENYL- 1H-PYRAZOL-4-YL)METHYLENE)HYDRAZONO)INDOLIN-2-ONE

Siddappa K. The proton-ligand stability constants and metal-ligand stability constants of Co(II), Ni(II), Cu(II), Zn(II), Cd(II) and Hg(II) with 3-(((3-(2hydroxyphenyl)-1-phenyl-1H-pyrazol-4-yl)methylene)hydrazono)indolin-2one has been investigated potentiometrically using pH metric technique in aqueous, ethanol-water (50% v/v) and dioxane-water (50% v/v) mixtures at different ionic strengths (0.1 M and 0.2 M) of NaClO4 and at 25 ± 1 C temperature. Proton-ligand stability constants values fall in the range of 9.642 to 11.751 attributable to ionization of phenolic –OH. The method of Calvin and Bjerrum as adopted by Irving and Rossotti has been employed to determine log K and log β values.


Introduction:-
pH-metry has been effectively adopted for the determination of stability constant J. Bjerrum and Ido-Laden's work enlightened the interest in the investigation of equilibrium of metal-chelates and ionic complexes in solution. Schwarzenbach and Ackermann ( Schwarzenbach and Ackemann, 1948) found that stability of the chelates decreases as the increase of size ring. The present work provides a systematic solution study of the complex formation of Co(II), Ni(II), Cu(II), Zn(II), Cd(II) and Hg(II) with different acids. As the coordination chemistry of metal-complex play a crucial role in biological system of organism (Ayesha, 2014), hence the formation, stability and reactivity of these complexes have been an active field of research (Brij, 2009). The stability constant gives the formation of metal-ligand. Metal ions plays essential role in the development of stable complexes is of interest to the analytical chemists and bio-inorganic researchers (Ayesha, 2011).
Hydrazones are very significant group of analytical regents for the fortitude of various metal ions by using assorted analytical techniques. Besides this use of hydrazones are also having biological activities also (Lakshmi et al., 2012). Hydrazone compounds obtained by the reaction of aromatic and heterocyclic hydrazides with mono and dialdehydes or ketones have revealed very versatile behavior in metal coordination. Many researchers have synthesized a number of new hydrazones because of their relieve of synthesis. Hydrazones have been studied as a group of the most useful spectrophotometric reagents. Combining suitable starting materials (carbonyl compounds and hydrazine), the sensitivity as an analytical reagent and/or solubility of the hydrazones could be improved and the donating environment could be altered. The shortcoming of hydrazones was their lack of selectivity for metal ions. Much attempt has been devoted to emergent masking agents for use with hydrazones (Ajaykumar et al., 2009).

Apparatus:-
The titrations were carried out with a digital Elico pH-meter model L1-122 using combined electrode with the accuracy range ± 0.01 pH digits at 25 ± 1 o C. The magnetic stirrer for the continuous stirring of the solution during the titration, and a thermostat were used to maintain the temperature constant.
pH titrations:-Titrations were performed in aqueous, ethanol-water (50% v/v) and dioxane-water (50% v/v) mixtures and a different ionic strengths was maintained by using NaClO 4 at 25 ± 1 o C in an inert atmosphere. The experimental procedures involve the acid titration, ligand titration and metal titration. The details of titrations are shown in the Table 1 to Table 3. The total volume in all the cases was 50 mL. Table 1 to Table 3. The experimental procedures involve the acid titration, ligand titration and metal titration.  where Y = is the number of replaceable hydrogen ion, N = concentration of alkali, T 0 L = total concentration of ligand, V 0 = total volume 50 mL, V 1 = volume of alkali required by the acid V 2 = volume of akali used by acid and ligand V 3 and V 2 = volumes of alkali required for acid+ligand+metal ion, T 0 M = total concentration of the metal ion pH titration data was utilized for determination of , dissociation constants, association constants. The plot of pH vs volume of NaOH given in the Figures 2 and 3. Dissociation constant and stability constant were evaluated by Irving-Rossotti methods and by computer pregame.

Dissociation constants:-
The proton-ligand equilibrium constant for the ligand (HDPPPMHI) under experimental conditions were determined by Calvin-Bjerrum pH -titration, as modified by Irving and Rossotti for calculation of n and pH from proton-ligand formation. The proton-ligand formation curve was obtained by plotting n values against pH. This indicates that the ligand has one dissociable proton. The pK values were estimated from formation curve by noting the pH at which n = 0.5. In Irving and Rossotti method, the pH titration of the three sets of mixtures against a carbonate free standard alkali, were performed. Dissociation constants decrease with the increase in ionic strength and polarity of the media (Table 4).

Association constants:-
The formation constants have been determined by plotting a graph between n and pL value and in the case of a metal ligand curve by plotting n against pL, log K and log β were calculated from the formation curve by the known values of pL at which n = 0.5 and n = 1.5 corresponds to the values of log K and log β, respectively. The log K values decreases with increase ionic strength of the medium which is concordance with Debye-Hukel limiting law. It means that Bronsted relationship is valid for the dissociation of ligand and complex equilibria of chelates. The difference in the log K and log β is small which is due to the simultaneous formation of complexes. The thermodynamic metal-ligand stability constants are reported in Table 5.