The Chemical Educator
ISSN: 1430-4171 (electronic version)
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Abstract Volume 24
(2019) pp 126-132
Primary Kinetic Salt Effect on Fading of Phenolphthalein in
Strong Alkaline Media: Experimental Design for a Single Lab Session
Domingo González-Arjona*,†, Manuel M. Dominguez†,
Germán López-Pérez† and Willem H. Mulder‡
†Department of Physical Chemistry, Universidad de Sevilla, Sevilla, E-41012, Spain, dgonza@us.es; ‡Department of Chemistry, The University of the West Indies, Mona Campus, Kingston 7, Jamaica
Received July 18, 2019. Accepted August 29, 2019.
Published: 3 October
2019
Abstract. 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.
Key Words: Laboratories and Demonstrations; physical chemistry; phenolphthalein; kinetics; primary kinetic salt effect
(*) Corresponding author.
(E-mail: dgonza@us.es)
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