Studies on the kinetics and mechanism of the complex formation between hexa-aquachromium(III) and phthalic acid

Kinetic studies of the interaction of [Cr(OH2)6]3+ with phthalic acid have been carried out at 30, 35, and 40 °C, the hydrogen-ion concentration ranging from 0.25 × 10–3 to 3.16 × 10–3 mol dm–3. The variation of the pseudo-first-order rate constant (kobs.) with [H+] and ligand concentration indicates a mechanism based on ion-pair formation. The reaction is believed to be of the associative-interchange (Ia) type. Activation parameters calculated by the Eyring equation are ΔH‡= 93 kJ mol–1 and ΔS‡=–5 ± 2 J K–1 mol–1 for the formation of [Cr(OH2)5(O2CC6H4CO2)]+, and ΔH‡= 94 kJ mol–1 and ΔS‡=–2 ± 1 J K–1 mol–1 for the formation of [Cr(OH2)(O2CC6H4CO2H)]2+.