Abstract: The binding of alkaline earth metal ions with 18-crown-6 was studied by microcalorimetry. Equilibrium constant and changes of enthalpy, entropy, Gibbs free energy can be obtained from titration analysis. In the process of binding, the free metal ions bind to oxygen atom of 18-crown-6 by static force. The binding causes both energy and entropy in system to decrease, so reaction process was accompanied by release of heat and reduction of freedom. Having the similar size to ion radii (138 pm) of Ba2+, the cycle of 18-crown-6 can surround Ba2+ tightly. Metal ions with a smaller size have a weaker force to bind oxygen atom because of size effect. From Be2+, Mg2+, Ca2+, Sr2+ to Ba2+, more heat releases and equilibriumconstant increases in the process of the binding reaction.

Key words: Microcalorimetry, Titration, Crown ester, Binding constant, Enthalpy-driven