Ion Pair Formation between Tertiary Aliphatic Amines and Perchlorate in the Biphasic Water/Dichloromethane System

The ability of aliphatic amines (AAs), namely, tripropylamine (TPrA), trisobutylamine (TisoBuA), and tributylamine (TBuA), to form ion pairs with perchlorate anion (ClO₄^-) in biphasic aqueous/dichloromethane (CH₂Cl₂) mixtures containing ClO₄^- 0.1 M has been demonstrated by GC with flame ionization (FID) and mass detectors (MS) and by NMR measurements. The extraction efficiency of the AAs to the organic phase was modeled by equations that were used to fit the experimental GC data, allowing us to determine values for K_P (partition constant of the free AA), K^IP (formation constant of the ion pair), and K_P^IP (partition constant of the ion pair) for TPrA, TisoBuA, and TBuA at 25 °C. Ion pairs were shown to form in CH₂Cl₂ also when ClO₄^- is replaced by other inorganic anions, like NO₃^-, ClO₃^-, Cl^-, H₂PO₄^-, and IO₃^-. No ion pairs formed when CH₂Cl₂ was replaced by n-hexane, suggesting that aliphatic amine ion pairs can form in polar organic solvents but not in nonpolar ones.