INTRODUCTION:
Determination of anatomical, physiological and pharmacological activities of any molecule is essential for knowing its significances and application in medicinal, pharmaceutical agricultural and industrial sciences. One of the important property in these studies is to establish drug activity drug effect, transmission of drug and absorption of drug all these four factors are directly depend stability of the molecule. The stability of molecules during this study can be easily determined from the values of stability constants. The formation of complex can also be determined from these values, hence pH-metric investigation have various significances in life, medicinal and pharmaceutical, agricultural and industrial sciences. Picolinic acid has been reported to possess various effects [1-5]. Potency of drugs can be increased activity in metal complexes and study of metal and nitrogen and sulphur atoms containing ligands have been reported [6-7]. Stability constant of mixed ions was also studied [8].
Proton-ligand and metal-ligand stability constants of Cu(II)-salicylic acid [9], Mn(II), Cu(II), Fe(III), Ni(II) with maleic [11], transition metal ions with 3-amino-5-methyl isoxazole[12] and lanthanide metal ions-pyrazoles [13] were determined. Binary, ternary and quaternary complexes were studied by pH–metric method[14]. Hence, stability constant of complexes formed by interactions of picolinic acid were studied with Cu(II), Co (II) and Fe(II) metal ions at 0.2 M ionic strength pH-metrically in water.
MATERIALS AND METHOD:
Picolinic acid (PA) was obtained from chemistry department of our college. Nitrates of Cu(II), Co (II) and Fe(II) were taken. Stock solution of picolinic acid was prepared in water and treated as ligand solution. Titrations of (i) free acid (0.01 M), (ii) free acid (0.01 M) and ligand (20 x 10-4M) and (iii) free acid (0.01 M), ligand (20 x 10-4) and metal ion (4 x 10-4M) against standard 0.1N NaOH solution were carried out. Ionic strength of all the solutions was maintained constant by adding appropriate amount of 2 M solution KNO3. All the titrations were carried out in water and readings were recorded for each 0.2 ml addition. Graph of volume of alkali added against pH were plotted. Ligands used in this work is monobasic acid having only one dissociable H+ ion from phenolic-OH group and it can therefore, be represented as HL. Dissociating equilibrium can be shown as,
By the law of mass action, we have,
Where, quantities in bracket denote activities of species at equilibrium.
RESULTS AND DISCUSSION:
Calculation of Proton-Ligand Stability
Constant i.e. (
)
Plots between volume of NaOH and pH of the
solution were used to determine the proton ligand stability constant.
Horizontal difference (V2-V1) was measured accurately
between titration curves of free acid and acid+ligand. It was used to calculate
formation number at various pH values and fixed ionic
strength μ = 0.2 M using equation 2.
Where, V0 is the initial volume of the
solution. E0 and TL0 are initial
concentrations of mineral acid and ligand respectively. V1 and V2
are volumes of alkali of normality N during acid and ligand titration at given
pH. ‘γ’ is replaceable proton from ligand. Values were determined at 32 (±1)°C in E0=1x10-2 M, TL0
= 20 x 10-4 M, V0 = 50 ml, N = 0.2 N respective ratios of
solutions.
Data of obtained at various pH along with
horizontal difference for some representative systems are represented in Table
1. Metal–ligand formation number (
) was determined using equation 3.
Where, notations have same meaning as given
in earlier equation. Horizontal difference (V3-V2)
between metal complex (A+M+L) and reagent (A+L) curve were used for
determination of values. and values obtained during study are given in Table-1
