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Acid–Base Equilibrium in a Lipid/Water Gel

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Predrag-Peter Ilich

Department of Chemistry and Biochemistry, Loras College, Dubuque, IA 52004

Kristina K. Streb

Department of Chemistry, Ohio Northern University, Ada, OH 45810

J. Chem. Educ., 2003, 80 (12), p 1464

DOI: 10.1021/ed080p1464

Publication Date (Web): December 1, 2003

Abstract

A new and original experiment in which partition of bromophenol blue dye between water and lipid/water gel causes a shift in the acid–base equilibrium of the dye is described. The dye-absorbing material is a monoglyceride food additive of plant origin that mixes freely with water to form a stable cubic phase gel; the nascent gel absorbs the dye from aqueous solution and converts it to the acidic form. There are three concurrent processes taking place in the experiment: (a) formation of the lipid/water gel, (b) absorption of the dye by the gel, and (c) protonation of the dye in the lipid/water gel environment. As the aqueous solution of the dye is a deep purple-blue color at neutral pH and yellow at acidic pH the result of these processes is visually striking: the strongly green-yellow particles of lipid/water gel are suspended in purple-blue aqueous solution. The local acidity of the lipid/water gel is estimated by UV–vis spectrophotometry. This experiment is an example of host-guest (lipid/water gel–dye) interaction and is suitable for project-type biophysics, physical chemistry, or biochemistry labs. The experiment requires three, 3-hour lab sessions, two of which must not be separated by more than two days.