Thermodynamic studies on metal complexes of Co2+, Ni2+, Cu2+ and Cd2+ with 8-hydroxy 5-quinolinesulfonic acid in water, methanol and water-methanol binary solvent systems at 303.15 K, 313.15 K and 323.15 K by conductometric method

Stability constants are measured for complexes of Cd, Cu, Co and Ni with 8-Hydroxy 5Quinolinesulfonic acid [HQS] in water, methanol and water-methanol binary solvent systems at 303.15 K, 313.15 K and 323.15 K by conductometric method. The results shows that the stoichiometry of the complexes HQS with Co, Cu and Cd metals in all binary mixed solvents is 1:2. The log Kf values of HQS-M complexes in H2O-MeOH binary mixtures increases with increasing % of MeOH and temperature. The ∆H and ∆S of the complexation reaction in the different H2O-MeOH binary mixtures were evaluated by the temperature dependence of the formation constants using a linear least square analysis according to van’t Hoff equation.

used. The specific conductance of this solution at various temperatures has been reported in the literature [1].

MEASUREMENT OF CONDUCTANCE AND EVALUATION OF STABILITY CONSTANT (K f )
The change in conductance is measured when a metal salt solution to which ligand solution is added, this provide valuable information about metal ligand interactions. Conductance measurement gives information about the affinity of the ligand and the stoichiometry of the complexation reaction. The measurement of conductance is generally carried out by titrating the metal salt solution with ligand solution in a suitable solvent system. The titration may give rise to two different conductometric effects. In the case when ligand titrated to the metal salt solution in which the metal salt is not fully dissociated, the opposite conductometric effect is observed. In the case when the metal salt is completely dissociated in the solvent, the complexation by a ligand leads to a large cationic complex of the metal ion and the ligand. In this case the complex has less mobility in the solution than the uncomplexed metal cation, resulting in the diminished capacity of charge transfer and hence the conductivity of the solution decreases. Since the ligand coordinates to the metal ion, leaving the anion undisturbed, dissociation of the metal salt increases and hence the conductivity of the solution increased [1]. In some cases, in spite of increase in the ligand concentration the change in conductance is negligible; consequently, determination of the formation constant by conductometric method becomes difficult. There are two factors responsible for a very small change in conductance in spite of an increase in the concentration of ligand: (1) when there is no complexation occurs between metal ion and ligand and (2) mobilities of a resulting metal-ligand complex and the corresponding metal ion are both equal at the same concentration. The first factor is responsible in the most of the cases. When the dielectric constant of the solvent is very high and conductance measurements are made at low concentration, the correction for viscosity changes and association between the metal ion i.e. an uncomplexed metal ion, an anion and the ligand metal complex are neglected. The stability constant can be described for a 1:1 metal-ligand binding is as follows; M + + L = ML + (1) Where, M + , L, and ML + represent the free solvated cation, the free ligand, and the complex respectively. The thermodynamic equilibrium constant K f for the association is given by (2) where, [ If α is the fraction of total metal cation that is uncomplexed with the ligand, then the following equations result: The dilute conditions used make it possible to neglect ion aggregation and corrections for viscosity changes. The molar conductivities Λ can be calculated by using the following equation, (7) where κ is the conductivity of the test solution in Siemens·cm −1 . On the other hand, Λ can be related to α by the following equation, where, Λ M+ and Λ ML represent the molar conductivities of the ligand free metal salt solution and the complexed cation solution at the same concentration, respectively. Rearranging Equation [8] gives an expression for α as; (9) Substituting Equations [8] and [9] into Equation [7] gives the following equation: The above equation can be simplified as where, a 1 = Λ M + − Λ, a 2 = Λ M + −Λ ML , a 3 = Λ−Λ ML . Here, Equation [18] can be rearranged to form a quadratic equation in terms of Λ as: .

VAN'T HOFF Plot for Complexation
For better understanding of the thermodynamics of complexation reactions of Cd 2+ , Cu 2+ , Co 2+ and Ni 2+ ions with HQS, it is useful to investigate the enthalpic and entropic contributions in these reactions. The ΔH o and ΔS o of the complexation reactions in different H 2 O-MeOH mixtures were evaluated by the temperature dependence of the formation constants using a linear least squares method according to the van't Hoff equation [7] . The result which are summarized in Table-2 show that in all the cases, the complexation reactions between HQS and the studied metal cations in H 2 O-MeOH binary solutions are entropy stabilized and enthalpy destabilized. According to the data in the table indicates that the thermodynamic quantities are very sensitive to the composition and the nature of the mixed solvent and non-monotonic behavior is observed for the variation of enthalpy and entropy changes versus the composition of the water-methanol binary solvent systems [8].  From Figure 6, the changes of the stability constant (log K f ) of HQS-Cd 2+ , HQS-Cu 2+ , HQS-Cu 2+ and HQS-Ni 2+ complexes versus the composition of H 2 O-MeOH binary system at various temperatures were observed. It is interesting to note that the formation constants of HQS-M 2+ (M = Cd, Cu, Co, Ni,) complexes increase with increase in the methanol percent in H 2 O-MeOH binary solvent systems.

Effect of Ionic Size on Complexation
The variation of log K f for formation of HQS-Cd 2+ , HQS-Cu 2+ , HQS-Co 2+ and HQS-Ni 2+ complexes versus the ionic radius in H 2 O-MeOH binary solvent systems are shown in Figure  7. It is evident from these Figures that, the order of selectivity of these complexes at 30 °C, 40 °C and 50 °C is HQS-Cd 2+ > HQS-Cu 2+ > HQS-Co 2+ > HQS-Ni 2+ Similar selectivity order was observed in all compositions of the binary solvent systems.

CONCLUSION
According to the results obtained in the present work, complexes formed between the ligand and the metal cation, it indicates the probability of changes in the stabilities and stoichiometry of the complexes due to the composition of the solvent systems. In most of the cases HQS formed 1:2 complexes but in the case of Ni +2 , HQS formed 1:1 complex. The results obtained in our studies suggest that stability of the complexes of HQS with the metal cations in solutions undergoes the chemical and physical properties of each solvent molecule such as donor number and polarity and with increasing methanol percent when mixed one another and therefore changing their solvating ability towards the ligand, metal cation and the resulting complex .The negative values of ∆G shows the ability of the studied ligand to form stable complexes and process trends to be spontaneously. However, the obtained positive value of ∆H indicates that the enthalpy is not driving force for the formation of the complexes. Furthermore, the positive value of ∆S indicates that entropy is a driving force for the complexation reaction.