Ion-pair formation in water and extraction into benzene of 18-crown-6–sodium ion complex with nitrated phenolate ions Original Research Article

The elementary equilibria in the extraction into benzene of a 18-crown-6 (18C6)–Na+ complex with nitrated phenolate anions have been studied at 25°C. The 1:1 ion-pair formation constants in water of the phenolate anions with Na+ (KMA) and 18C6–Na+ (KMLA) were measured by potentiometry with a Na+-selective electrode. The overall extraction constants (Kex) were determined, and the KMLA and Kex values were used to calculate the distribution constants of the neutral 18C6–sodium phenolate 1:1:1 complexes. The KMA values of all the phenolates are nearly equal, regardless of their difference in basicity. The KMLA value is much larger for picrate than for 2,4- or 2,6-dinitrophenolate. Such trends of the ion-pair formation in water are consistently explained on the basis of the thermochemical cycle including the ion-pairing process in the gas phase. The Kex value decreases in the order, picrate⪢2,6-dinitrophenolate>2,4-dinitrophenolate. The highest extractability of the picrate ion is attributed mostly to its highest ion-pairing ability in water for the 18C6–Na+ complex. The difference in the extractability between the dinitrophenolate isomers depends entirely on the lipophilicity of the 18C6–sodium phenolate complexes.