A test reaction from macrocyclic chemistry for calorimetric titrations

Titration calorimeters are controlled by standard reactions. One of these reactions is the protonation of trishydroxy methylaminomethane in aqueous solution. However, from this reaction only the reaction enthalpy is measured. Calorimetric titrations are used to study the interactions between macrocyclic (e.g. crown ethers) and macrobicyclic ligands (e.g. cryptands) with neutral or charged gust molecules. From the experimental measurements sometimes the stability constants and the reaction enthalpies are calculated. Thus, a test reaction should not only enable the calculation of the reaction enthalpy but also of the stability constant. A suitable reaction is the complexation of Ba2+ by the macrocyclic ligand 18-crown-6. This reaction has been studied in detail. The possible influence of the concentrations of ligand and salt, of the acidity of the solution, of the ionic strength and of the anion upon the stability constant and reaction enthalpy has been measured. The different anions do not influence the reaction enthalpy. However, they are able to change the values of the reaction entropy for the complexation of Ba2+ by 18-crown-6 within certain limits. (C) 1999 Elsevier Science B.V. All rights reserved.