Thermochromic Behavior of Buffered Indicator Solutions I. Determination of the Heat of Proton Ionization of Phenolphthalein

Department of Chemistry, University of Jordan, Amman, Jordan  

Manuscript received 25 March 1980, accepted 27 July 1980

The apparent heat of proton ionization, ∆ H_^', for phenolphthalein in aqueous solutions containing 6 per cent ethanol by weight was determined by two methods, (a) the conventional spectrophotometric method in which apparent dissociation constants are obtained as a function of temperature and (b) the novel thermochromic method. The reasonably good agreement between the results of the two methods establishes the latter as a powerful and unique tool for obtaining ∆ H' values of weak acids.