Effect of the pH in the Color of an Alcoholic Extract of Red Hibiscus and pKh Determination

TCE

The Chemical Educator

ISSN: 1430-4171 (electronic version)

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Abstract Volume 8 Issue 1 (2002) pp 66-69

Effect of the pH in the Color of an Alcoholic Extract of Red Hibiscus and pK_h Determination

Armando Carrasquero-Durán* and Beatriz Navas^†

Universidad Pedagógica Experimental Libertador, Institute of Chemistry, POB 1017–Las Acacias, Maracay–Aragua, Venezuela, acarrasquerod@latinmail.com* and†Universidad Central de Venezuela, Facultad de Agronomía and Department of Chemistry, Maracay–Aragua, Venezuela^†
Received August 17, 2002. Accepted September 17, 2002

Published online: 13 November 2002

Abstract. The spectrophotometric study in the visible region of a methanolic extract of red hibiscus petals (Hibiscus rose quinensis) establishes the acid–base equilibrium of an anthocyanin. A three-step didactic strategy was followed in this exercise. Starting with a clearly defined problem, the students learn the chemical principles involved, the design and the execution of experiments to obtain reliable data, and data interpretation. The absorption spectra of cayena methanolic extract shows a maximum at 530 nm and remains unperturbed when an aluminum chloride solution is added to the extract, which indicates that the molecule responsible for the color lacks hydroxyl groups at ortho positions; thus, it cannot produce chelates between carbonyl C-4 and free OH at the C-3 or C-5 positions. The extract of the red flower changes its color from red to green when the pH increases from 2 to 4. This was associated with the flavilium-ion dissociation according to the equation HAn « H⁺ + An^–, which is usual in many monobasic acid–base indicators. The pK_h value obtained was 3.27.

Key Words: Laboratories and Demonstrations; general chemistry; anthocyanin; indicators; color; spectra; dissociation

(*) Corresponding author. (E-mail: acarrasquerod@latinmail.com)

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