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The Chemical Educator

ISSN: 1430-4171 (electronic version)

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Abstract Volume 16 (2011) pp 99-108

Litmus as a Natural Indicator: A Comprehensive Review

Manuel A. P. Segurado†,‡,*, Rita Castro,†,§ Margarida F. B. Silva†,§

Faculdade de Farmácia, Universidade de Lisboa, Av. Prof. Gama Pinto, 1649-003 Lisboa, Portugal; Centro de Ciências Moleculares e Materiais, Faculdade de Ciências da Universidade de Lisboa, Edifício C8, 1749-016 Lisboa, Portugal, asegurado@ff.ul.pt; §Research Institute for Medicines and Pharmaceutical Sciences (iMed.UL), Faculty of Pharmacy, University of Lisbon, Av. Prof. Gama Pinto, 1649-003 Lisbon, Portugal.
Received September 22, 2010. Accepted March 1, 2011.

Published: 18 March 2011

Abstract. Dyes and color indicators have been recurrent themes in chemical education because of their strong pedagogical interest and multidisciplinary nature. However, litmus, one of such indicators, has remained almost ignored in this respect, despite its important role in the History of Chemistry. In this paper, after a brief introduction to the origin and significance of natural indicators, some basic concepts about the biology and chemistry of lichens, the natural source of litmus and orcein, a related compound, are considered. Then, a comprehensive analysis is given on the primitive processes used for obtaining orcein and litmus, their archaeological significance, the methods of separation and identification of dyes, and interpretation of chromatic changes depending on the pH.

Key Words: Chemistry and History; acids / bases; plant chemistry; dyes / pigments; natural products

(*) Corresponding author. (E-mail: asegurado@ff.ul.pt)

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