The distribution of the anion and zwitterion forms of methyl orange between the disperse microemulsion pseudophase and continuous water phase

Abstract

A procedure is suggested for determining the distribution constants of methyl orange dye between water and direct microemulsion droplets (N-cetylpyridinium chloride + 1-pentanol + benzene in water) as the disperse phase, which allows one to circumvent difficulties associated with the formation of low-solubility salts and premicellar associates. Within the framework of the pseudophase model of equilibria, the distribution constants of the yellow monoanion and red zwitterion methyl orange forms between microscopic droplets and the continuous phase at an ionic strength of 0.2 mol/l (NaCl + HCl) were determined from the dependence of the apparent ionization constant of the indicator on the volume fraction of the disperse pseudophase. The constants on the molar concentration scale were \( P_{B^ - }^ \otimes \) = (253 ± 0.08) × 105 and \( P_{^ + HB^ - }^ \otimes \) = (1.7 ± 0.4) × 102.