Conductometric Study of Complex Formation Between Cu ( II ) Ion and 4-Amino-3-ethyl-1 , 2 , 4-triazol-5-thione in Binary Ethanol / Water Mixtures

The stability constant, Kf, for the complexation of copper(II) ion with 4-amino-3-ethyl-1,2,4-triazol-5-thione (AETT ) in 0, 20, 40, 60 and 80 (v/v) % ethanol-water mixtures were determined conductometrically at different tempratures. Stability constant of resulting 1:1 complexes were being larger by increasing of temprature and ethanol percent. Stability constants of complexes vary inversly with dielectric constant of solvents. The enthalpy and entropy of complexation were determined from the temprature dependence of the formation constant. In all cases, the complexation were found to be enthalpy unstablized but entropy stablized. ∆G of the studied complexes were evaluated at 25C using thermodynamic relations, the negative values of ∆G means that the complexation process is spantaneously.


Introduction
Copper(II) ion is a biologically active, essential ion, cleating ability and positive redox potential allow participation in biological transport reactions.Cu(II) complexes possess a wide range of biological activity and are among the most potent antiviral, antitumor and antiinflammatory agents 1 .On the other hand, condensed triazoles exhibit a range of pharamacological activities such as mitotic 2 , hypotensive 3 , CNS stimulant 4 , antiflammatory 4,5 and analgesic activites 6,7 .
Triazoles fused another heterocyclic ring has attracted wide spread attention due to their application as antibacterial, antidepressant, antiviral, antitumorial and antiiflammatory agents, pesticides, herbicides dyes, lubricant and analytical agents 17,18 .The present study deals with the conductometric determination of the stability constants, stiochiometric ratio and related thermodinamic parameters of AETT complexes with Cu(II).

Reagents
The CuCl 2 .2H 2 O and ethanol were purchazed from Merck.The ligand of AETT was prepared by reported method 19 .

Apparatus
Conductivity measurments were carried out with a Metrohm 644 Conductometer.A diptype conductivity cell, made of platinum black, with a cell constant of 0.69 cm -1 was used.In all measurments, the cell was thermostated at the desired temprature ±0.05ºC using a ATBIN immersion thermostat.

Procedure
In a typical run, 30 cm 3 of a copper(II) chloride solution(5×10 -4 M) was placed in a container having water circulating arrangement.The cell was dipped in the solution.The solution was stirred by using a magnetic stirrer.The desired temperature was maintained by thermostat connected to the water circulating arrangement of the container.
Conductance of the initial solution was measured after thermal equilibrium had been reached.Then a known amount of the ligand solution(5×10 -3 M) was added in stepwise manner using a calibrated micropipette.The conductance of the solution was measured after each addition, and then corrected to avoid the effect of dilution durning the titration by multiplying the measured value [(V + v) / V], where V is the original volume of the salt (metal ion solution) and v is the volume of titrant (The ligand solution).

Results and Discussion
In order to evaluate the influence of adding L in the molar conductance of the metal ion used in different ethanol-water (v/v)% mixtures, the conductivity at a constant salt solution (5×10 -4 M) was monitored while increasing the L concentration (5×10 -3 M) at different tempratures.The molar conductance were plotted against [L] / [M] mole ratio for reaction of Cu(II) with ligand of AETT at different tempratures, as an example, the molar conductance vs ([L] / [M]) curves for Cu(II)-AETT complexes in ethanol-water40 (v/v) % at different tempratures is shown in Figure 1.The plots (Figure 1) exhibit one abvious slopes, suggesting that the probable stiochiometric ratio of complexes are M : L. Incidentally, as is expected, the corresponding molar conductance increased with increasing the temprature (Figure 1), owing to decrease viscosity of the solvent and consequently, the enhance mobility of the charged species present in the solution.1 : 1 Complexes of transition and heary metal ions(M + ) with (L), can be expressed by the following equilibrium 20 : The corrospinding equilibrium constant, K f is given by: where [ML + ], [M + ], [L] and f represent the equilibrium molar concentration of complexes, free cation, free ligand and the activity coefficient of the species indicated, respectively.Under the dilute condition we used, the activity coefficient of the uncharged ligand, f (L) can be reasonably assumed as unity 20,21 .The use of Deby-Huckel Limiting Law 22 lead to the conclusion that f (M+) ≈ f (ML + ) , so the activity coefficients in equation 2 were canceled.Thus the complex formation constant in term of the molar conductance can be expressed as Takeda 23 , Zollinger et al 24 .
] )[ ( Here, Λ M is the molar conductance of the metal ion before addition of the ligand, Λ ML is the molar conductance of the complexed ion, Λ obs is the molar conductance of the solution during titration, C L is the analytical concentration of the L added, and C M is the analytical concentration of the metal ion, the complex formation constant, K f , and the molar conductanc of complex, Λ ML , were obtained by computer fitting of equnation 3 and 4 to molar conductance-mole ratio data using a non linear least-squares program Genplot.All calculated stability constants are summarized in Table 1.  1 shows that stability constant values increase with increasing temprature, i.e. the complexation is an endothermic process.Comparison of the formation constants(at idendical temprature) given in Table 1 reveales that relative stabilities of the complexes increased with increasing ethanol percent.Since, donor numbers of ethanol(19.0) and water(18.0)are approximately equal 25,26 , if we ignore negligible difference donor number of solvents, it seems to dielectric constant, ε, of solvents 25,26 play an important role in the formation of complexes.Therefore, stability constants of Cu(II)-AETT complexes vary inversely with dielectric constant of the solvents.In order to have a better understanding of the thermodyamics of complexation reaction discussed, it is useful to consider the enthalpic and the entropic contributions to these reactions.The ∆H 0 and ∆S 0 values for the complexation reaction were evaluated from the corresponding lnK f -temperiture data by applying a linear least-squares analysis according to the Van't Hoff equation.Plots of lnK f vs 1/T for Cu(II)-AETT complexes are shown in Figure 2. The enthalpies and entropies of complexation were determined in the usual manner from the slopes and intercepts of the plots, respectively.∆G o of the studied complexes were evaluated at 25 0 C using the relations: The computed results were collected in Table 2.The negative values of o G ∆ (Table 2) show the ability of the studied ligand to form stable complexes and the process trend to proceed spantaneously.However, the obtained positive values of ∆H º (table 2) means that entalphy is not the driving force for the formation of the complexes.The positive values of ∆S 0 (Table 2) indicate that entropy is responsible for the complexing process.For investigating the influence of solvents on the stability constant we draw (logK f ) against (V/V) % of ethanol-water for reaction of Cu(II) with AETT at different temperatures (Figure 3).These plots (Figure 3) were linear, and based of this, can get a result that there is not considerable interaction between components of solvents in binary systems of ethanolwater and this components cause to diluted each other in binary systems.

Conclusions
The stability constant for the complexation of copper(II) ion with 4-amino-3-ethyl-1,2,4triazol-5-thione in 0, 20, 40, 60 and 80 ethanol-water (v/v) % mixtures were determined conductometrically at different tempratures.Comparison of the formation constants(at identical temprature) revealed that relative stabilities of the complexes increased with increasing ethanol percent.Thermoynaic parameters of complexation were determined from the temprature dependence of the formation constant..The negative values of ∆G 0 show the = 45.0 °C t = 35.0°C t = 17.5 °C t = 25.0 °C t log K f ability of the studied ligand to form stable complexes and the process trend to proceed spantaneously.However, the obtained positive values of ∆H º means that entalphy is not the driving force for the formation of the complexes.Furthermore, the positive values of ∆S 0 indicate that entropy is responsible for the complexing process.The results of this work show that there is not considerable interaction between components of solvents in binary systems of ethanol-water and this components cause to dilute each other in binary systems.

Figure 1 .
Figure 1.Molar conductance vs ([L]/[M]) curves for Cu(II)-AETT complexes in 40 (v/v) % ethanol-water binary Mixture at different tempraturesTable1shows that stability constant values increase with increasing temprature, i.e. the complexation is an endothermic process.Comparison of the formation constants(at idendical temprature) given in Table1reveales that relative stabilities of the complexes increased with increasing ethanol percent.Since, donor numbers of ethanol(19.0) and water(18.0)are approximately equal25,26 , if we ignore negligible difference donor number of solvents, it seems to dielectric constant, ε, of solvents25,26 play an important role in the formation of complexes.