Techniques for obtaining thermodynamic data

Thermodynamic aspects of the interaction of metal ions with macrocyclic ligands have been well studied. In many instances such studies have involved a comparison of the behaviour of cyclic ligand systems with that of their open-chain analogues. In this manner, information concerning the thermodynamic consequences arising from the cyclic nature of the macrocyclic ligand has been obtained. Frequently these studies have been restricted to stability constant (log K) measurements and, for such studies, a variety of techniques has been employed (Izatt etal., 1985). 

When the value for K is small, or the ligand system is otherwise not suitable for pH titration, then ion-selective electrodes have frequently 

A wide variety of other methods have been used to obtain the concentration of either the metal or ligand in a particular equilibrium solution. These include polarography, spectrophotometry, magnetic resonance, cyclic voltametry and conductivity. Of course, if reliable results are to be obtained, the technique chosen must not significantly perturb the position of equilibrium in the solution. 

As has been mentioned previously, the approach to equilibrium can often be slow for macrocyclic complex formation indeed, equilibrium may take days, weeks or even months to be established. This may give rise to experimental difficulties in conventional titration procedures. Under such circumstances, it is usually necessary to carry out batch determinations in which a number of solutions, corresponding to successive titrations points, are prepared and equilibrated in sealed flasks. The approach to equilibrium of each solution can then be monitored at will. 

If the stability constant measurements are coupled with enthalpy determinations, then entropy values (as well as the corresponding free energy values) may be calculated  

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KEMS is a tremendously powerful technique for obtaining thermodynamic data on metals and alloys. Such information as heats of vaporization, thermodynamic activities of components, partial and integral molar heats of mixing, and phase transformation temperatures can be obtained with a high degree of accuracy. General experimental aspects of the technique have been discussed with particular emphasis on the Knudsen cell... 

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