Primary Kinetic Salt Effect on Fading of Phenolphthalein in Strong Alkaline Media

The fading of phenolphthalein color in an alkaline medium is well-suited as a model reaction for use in a relatively simple experiment to illustrate a number of basic concepts and methods of kinetics and thermodynamics. The experiment is relatively inexpensive, safe and can be completed in a single lab session. The reaction is second-order, reversible and involves the breaking-up of a quinoid group due to a reaction between carbinol and OH–. The OH– ion exerts a dual influence on the reaction rate, both directly as a reactant and indirectly, via its effect on ionic strength. The fading reaction only becomes irreversible at pH > 12.5 so the medium ionic strength is inevitably high. This fact renders the Debye-Hückel approximation inapplicable for use in the Brønsted-Bjerrum relationship. Satisfactory data fits and the correct value for the charge of the activated complex are only achieved by using the semi-empirical Davies equation for single-ion activity coefficients. Different data sets collected by students during several lab sessions, repeating the reaction at varying ionic strengths give results at a reasonable level of reproducibility. The proposed experiment can be completed in a single lab day using one set of stock solutions.