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  Home > JCE Print > Journal of Chemical Education > Issues > 2001  > March  >
In the Laboratory
Micelles in the Physical/Analytical Chemistry Laboratory. Acid Dissociation of Neutral Red Indicator
Kathryn R. Williams and Loretta H. Tennant
Department of Chemistry, University of Florida, Gainesville, FL 32611-7200
Cover
March 2001
Vol. 78 No. 3
p. 349
Abstract
This paper describes the third in a series of laboratory experiments demonstrating the effects of micelles on physicochemical properties. The acid dissociation constant for neutral red indicator is measured spectrophotometrically in water and in solutions of Tween-80, a neutral surfactant, and sodium dodecyl sulfate (SDS), an anionic surfactant. The DpKaS (pKa in the surfactant minus pKa in water) is given by DpKaS = DpKai - Y/59.16, where pKai represents the intrinsic pKa in the dielectric constant of the micellar interface, modeled as the DpKaS in the neutral Tween-80 micelles, and Y is an electrostatic potential, equal to -100 ± 5 mV for SDS at the experimental ionic strength of 0.100 M. Students observe that the measured DpKaS for SDS is much larger than the value calculated from the equation. This discrepancy is due to a specific molecular interaction between the cationic indicator and the anionic headgroups of SDS.
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*  Citation
 Williams, Kathryn R.; Tennant, Loretta H. J. Chem. Educ. 2001 78 349.
*  Keywords
 AcidÐBase Chemistry; Analytical Chemistry; Instrumental Methods; Laboratory Instruction; Micelles; Surface Science; Physical Chemistry
*  History
 Created:
Last Updated:		 February 6, 2001
August 31, 2005
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