Equilibrium Constant Studies for Complexation between Ammonium Ions and 18-Crown-6 in Aqueous Solutions at 298.15 K

Abstract

Osmotic vapor pressure and density measurements have been carried out for bi- nary aqueous and ternary aqueous solutions containing a fixed concentration of 18-crown-6 (0.2 mol·kg −1 ) and ammonium chloride or ammonium bromide at 298.15 K. The concentra- tion of the ammonium salts was varied between 0.02 to 0.5 mol·kg −1 . The measured water activities were used to obtain the activity coefficient of water and the mean molal activity coefficient of the ions in binary as well as ternary solutions. Using the method developed by Patil and Dagade reported earlier in this journal and the McMillan-Meyer pair and triplet Gibbs energy interaction parameters, the thermodynamic equilibrium constant (K )f or the 18-crown-6:NH + complexes were determined. It is observed that the nature and polariz- ability of anions play important roles in imparting stability to the complexed species. The log10 K values for the 18-crown-6:NH + complexed species are lower than for the complexes involving alkali metal ions such as K + . The volume of complexation for the studied systems obtained from the apparent molar volumes of ammonium halides in ternary solutions are positive and of smaller magnitude than those reported for complexation with alkali ions. The results are further discussed in terms of water structural effects, complex formation, the role of counter anions and hydrophobic interactions.