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The Chemical Educator

ISSN: 1430-4171 (electronic version)

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Abstract Volume 16 (2011) pp 323-325

The Determination of the pKa of Red Cabbage Anthocyanin by the Spectrophotometric Method and Nonlinear Curve Fitting

Ampaporn Munmai and Ekasith Somsook*,

Institute for Innovative Learning, Mahidol University, 999 Buddhamonthon Sai 4 Rd. Salaya, Buddhamonthon, Nakornpathom 73170, Thailand; NANOCAST Laboratory, Center for Catalysis, Department of Chemistry and Center for Innovation in Chemistry, Faculty of Science, Mahidol University, 272 Thung Phaya Thai, Rama VI Rd. Ratchathewi, Bangkok 10400, Thailand,
Received May 26, 2011. Accepted September 21, 2011.

Published: 21 December 2011

Abstract. A linear plot based on the spectrophotometric method and Henderson-Hasselbalch equation has a drawback of choosing the appropriate pH for acid and basic forms of the indicators which may lead to the incorrect pKavalues especially in the systems with many involved reactions at the equilibrium. Herein, a nonlinear curve fitting based on the Henderson-Hasselbalch equation was proposed to solve this problem for the determination of the pKa values of red cabbage anthocyanin solution.

Key Words: Laboratories and Demonstrations; general chemistry; indicator; pKa; red cabbage; spectrophotometry

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