Studies of the dissociation of 2,4,6-trinitrophenol in aqueous mixtures of methanol and ethanol at 25 °C

Abstract

have been determined at 25'C in aqueous mixtures containing 30,50,70, and 90% by weight of methanol and 30, 50,70,90, and 95% by weight of ethanol by spectrophotometric method. Solvent effect on the dissociation of the acid: 6(A Go) = 2 . 3 0 3 R T [~( X )~ -P(,K)~] shows an unusual behaviour, the value decreases and passes through a minimum and ultimately becomes positive as the proportion of organic component in the solvent is progressively increased. The results have been compared with those of other uncharged acids like acetic acid, benzoic acid available from literature. The difference of behaviour (minimum characteristic in F ( A G 9 curve) has been attributed to very strong dispersion interaction of trinitrophenolate anion with the organic component of the solvent. The overall behaviour of this acid indicates that solvent effect is an involved process being guided by electrostatic and dispersion interactions besides relative solvent basicities.A. L. DE et A. K. ATTA. Can. J. Chem. 63, 3 129 (1985). Faisent appel a des mkthodes spectroscopiques et optrant a 25"C, on a CtudiC les constantes thermodynamiques de dissociation (,K),, du trinitro-2,4,6 phenol dans des solutions aqueuses contenant 30, 50, 70 et 90% en poids de methanol et 30, 50, 70, 90 et 95% en poids d'tthanol. L'effet du solvant sur la dissociation de I'acide 6 ( A G 7 = 2 , 3 0 3 R T [~( . X )~ -P(,K)~] montre un comportement inhabituel, la valeur diminue, passe par un minimum et finalement devient positive lorsqu'on augmente progressivement la proportion de compose organique du solvant. On a compare les resultats avec ceux d'autres acides non charges tels l'acide acetique, I'acide benzoique, repertories dans la litterature. On a attribuk la difference de comportement (minimum caracteristique dans la courbe de 6(A Go) a une interaction de dispersion trks forte de I'anion trinitrophCnolate avec la partie organique du solvant. Le comportement global de cet acide indique que la participation de I'effet du solvant est gouvernie par des interactions de dispersion et Clectrostatiques en plus de la basicit6 relative du solvant.[Traduit par le journal] Introduction It is well known that the dissociation of acids is a complex phenomenon dictated by various types of interactions of the species involved in the ionization equilibrium. To understand the solvent effect on the dissociation equilibria, dissociation constants of some monopositively charged acids, nitroanilinium ions were determined earlier (1, 2) in some mixed solvents. This paper reports the dissociation constant of uncharged acid, 2,4,6-trinitrophenol, in aqueous mixtures of methanol and ethanol.

Studies of proton-transfer equilibria of different charge type acids in the glycerol-water solvent system at 25.degree.C

“…Figure 1 shows the widely different nature of variation of ( ß°) with mole percent GL for different acids. ( <3°) is determined by the interactions of H+, acid, and its conjugate base with the mixed solvent relative to that of water and can be expressed (11,12) by eq 6, where AG°,(\) is the Gibbs energy change ( °) = ß°,( +) + AG°,(conjugate base) -AG°,(acid) (6) accompanying the transfer of 1 mol of species i from the standard state in water to that In the mixed solvent.…”