Infrared spectra of phthalic acid, the hydrogen phthalate ion, and the phthalate ion in aqueous solution

The infrared spectra of a series of aqueous solutions containing phthalic a cid (1,2-benzenedicarboxylic acid) and varying pH were examined using atten uated total reflection Fourier transform infrared spectroscopy and potentio metry. The basis spectra of phthalic acid, the hydrogen phthalate ion, and the phthalate ion were isolated using a factor analysis in which the absorb ance of these species varies with pH and total phthalate concentration acco rding to equilibrium and mass balance relations. Assignments of these basis spectra were made by comparison with spectra calculated ab initio. The con ditional formation constants of phthalic acid and the hydrogen phthalate io n were determined at 25.0 +/- 0.1 degreesC in 0.6 M NaCl ionic media using infrared spectroscopy and in 1.5 M NaCl ionic media using both infrared spe ctroscopy and potentiometry. (C) 2001 Elsevier Science B.V. All rights rese rved.