Hydrogen ion–1,10-phenanthroline system. Calorimetric determination of the equilibrium constants and heats of formation of the species H(phen)n+, where n= 1 or 2

Abstract

A method for the simultaneous determination of equilibrium constants and of enthalpies of reaction from calorimetric data only has been applied to the investigation of the reactions of 1,10-phenanthroline (phen) with hydrogen ion in water and in 20% dioxan–water containing 0·5M-KCl at 25 °C. The data were treated by assuming the existence of one, two, or three species of general formula H(phen)_n⁺(n= 1,2, and 3) by use of a method of calculation based on ‘pit-mapping’ due to Sillén. Although the constants obtained agree with the potentiometric ones in the literature, the existence of the species H(phen)₃⁺ appears in doubt. The enthalpies and entropies of reaction indicate that the hydrogen ion in the species H(phen)⁺ is bound directly to the nitrogen atom through N–H covalent bonding, and not through hydrogen bonding requiring water molecules, as claimed previously.