Spectrophotometric Determination of the Thermodynamic  Ionisation Constant of 3,5-Dinitrobenzoic Acid in Water from  10° to 50° and Related Thermodynamic Quantities

The thermodynamic ionisation constant (K_a) of 3,5-dinitrobenzoic add In water from 10° in 50° has been investigated. spectrophotometrically. The resettle are shown as a function of temperature (T) in ºK by pk_a=-1147.01/T 6.9381-10^-4 T. The changes of free energy (Δ F), entropy (Δ H), entropy (AS) and heat capacity (Δ C_p) for the dissociation process have been caleulated from the temperature coefficient of the dissociation constant. The equations relating them thermodynamic functions with absolute temperature are also even. The'pK_a' of the acid increases linearly with increases In ionic strength of the buffer solution end ma be related by the equation pK_a= 2.0297 I+ 2.9255= up to an Ionic strength (I) of 0.1. The value of K_a have also been found out for venom methanol-water mixtures.