First and second dissociation constants of deuterio-o-phthalic acid in D₂O from 5 to 50°C

Abstract

The first and second dissociation constants of deuterio-o-phthalic acid in deuterium oxide have been determined by the emf method over the temperature range of 5 to 50°C. The pD values for potassium deuterium phthalate have been calculated from these two constants and experimentally verified. The thermodynamic properties for the dissociation of deuterio-o-phthalic acid have been evaluated. At 25°C, these values in the molality scale are: pK _1A =3.505, pK _2A =5.890, and pD=4.518. From K _1A and K _2A , respectively: ΔG^o=20.003, 33.582 kJ-mol⁻¹; ΔH^o=2.851, 2.208 kJ-mol⁻¹; ΔS^o=−76.7, −105.2 J-mol⁻¹-K⁻¹; and ΔC ^o _p =−52.7, −315.6 J-mol⁻¹-K⁻¹. The isotope effect is discussed.