|dc.description.abstract||The experimental work conducted in this thesis was aimed at determining the thermodynamic
quantities associated with the formation of complexes formed between various metal ions and
KELEX 100, a ligand which is used commercially for liquid - liquid extraction.
In order to accomplish this, the beats of protonation of the ligand KELEX 100 were
determined calorimetrically at 25°C in a partially aqueous medium of75% (v/v) l,4-dioxane
and at an ionic strength of 0.1 mol dm·3• Extraneous heat effects that usually accompany
complex formation were accounted for by measuring each heat contribution separately.
The complexation of the lead (II), cadmium (II) and nickel (II) ions with KELEX 100 was
studied in 75%(v/v) l,4-dioxane medium. The enthalpies of complexation were measured
calorimetrically and calculated using the program LETAGROP KALLE. These results were
combined with the values of the Gibbs free energies available in the literature for these
systems to yield the entropies of complexation. In all calorimetric determinations a constant
ionic strength of 0.1 mol dm'3 and a temperature of 25°C was maintained.
The calculated enthalpies and entropies are discussed in terms of a number of factors that
affect the thermodynamics of the systems. These factors include the structure of the ligand
molecule, the nature of the donor atoms, the degree of substitution on the ligand and the
properties of the metal atom such as charge and size, and the nature of the solvent.
The cumulative enthalpies and entropies of formation of the metal-ligand complexes are
favourable, Le. complexation is accompanied by a decrease in enthalpy and an increase in
entropy. However, the enthalpy changes contribute more to the stability of the complexes
and hence are the driving forces for complex formation. In the case of the Cd(II) ion, the
enthalpy and entropy changes are similar.
The cumulative enthalpies of formation increase (Le., become more exothermic) in the order:
Ni > Pb > Cd
while the reverse order is found for the entropies of formation. The smaller enthalpy change
for the Cd(II) complex is possibly due to the weak interaction between the 'soft' Cd(II) ion
and the 'hard' KELEX 100 ligand while the decrease in entropy for the Ni(II) may be due
to the loss of fewer solvent molecules from the Ni(II) hydration sphere on complexation with
The stepwise entropy and enthalpy changes for the formation of the ML+ complexes
are dependent on the ionic radius of the metal ion. However, no linear correlations
exist between the cumulative entropy or enthalpy changes of formation of the ML complexes
and the size of the metal ion.
The stepwise and cumulative enthalpies of formation appear to be largely independent of
steric effects of the bulky alkyl substituent on the KELEX 100 ligand. The increased steric
hindrance of the substituent decreases the cumulative entropy change for the formation of the
Ni(II) complexes. However, in the case of the Pb(II) complexes, steric effects do not appear
to affect the cumulative entropies of complex formation.||en