Thermochimica Acta, 89 (1985) 133-139 Elsevier Science Publishers B.V., Amsterdam - Printed in The Netherlands 133 ASSOCIATION, DISSOCIATION AND HYDROGEN BONDING OF SALICYLIC ACID IN WATER-NJV-DIMETHYLFORMAMIDE MIXTURES E.A. GOMAA, M.A. MOUSA and A.A. EL-KHOULY Chemistry Department, Faculty of Science, Mansoura University, Mansoura (Egypt) (Received 2 January 1985) ABSTRACT The free energy of transfer (AC,,) of salicylic acid from water to N,N-dimethylformamide (DMF) and their mixtures was calculated from the solubilities of salicylic acid in different H,O-DMF mixtures. The maximum value of AC,, was found to be -7.008 kcal for pure DMF. In addition to AC,, calculations, the values of the association constant (K,,,) of salicylic acid were calculated from the acid concentration (solubility, S) and the pH values of the saturated solution. The highest K,,, value was found to be 1016.16 in pure DMF. Moreover, the free energies of H-bonding of salicylic acid in these mixtures were calculated from the values of AC,, and its individual terms ( AGs,,). INTRODUCTION Many publications have reported on the behaviour of weak acids in anhydrous solvents. Interesting work has been done by Kolthoff et al. [1,2]. Aleksandrov et al. [3,4] studied the dissociation of salicylic acid in butan-2- one. Kreshkov et al. [5] studied the dissociation of amino acids (as weak acids) in mixtures of formic acid and ethyl methylketone and in mixtures of acetic acid-ethyl methylketone. Rao et al. [6] reported some results about the dissociation constants of benzoic, salicylic, o- and m-nitrobenzoic acid in 20-50 (w/w) acetonitrile-water mixtures using the direct potentiometric method. They found that salicylic acid has an abnormal character due to the intermolecular hydrogen bonding whose values have not been reported. EXPERIMENTAL The aim of the present work is to study the association, dissociation and hydrogen-bonding character of salicylic acid in aqueous-aprotic solvents (i.e., H,O-DMF mixtures). The salicylic acid and N, N-dimethylformamide 0040-6031/85/$03.30 0 1985 Elsevier Science Publishers B.V.