Expt7-07 - _University of Puget Sound Department of...

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Natural Pigments as Acid-Base Indicators 1 _____University of Puget Sound Department of Chemistry Chem 110 E XP . 7–N ATURAL P IGMENTS AS A CID -B ASE I NDICATORS INTRODUCTION An acid-base indicator is a substance which changes color according to how acidic or basic its environment is. There are numerous natural acid-base indicators that can be obtained from common flowers, fruits and vegetables. Most people are surprised at the color changes in plant juices. How many people would guess that blueberry juice is bright red in acid or that cherry juice is dark green in base? Especially interesting are the extracts from red cabbage, radish skin, rhubarb skin, turnip skin, and flowers that act as universal indicators. The plant pigments known as anthocyanins (see Figure 1) are responsible for many of the red, blue and violet colors seen in plants. The red leaves of autumn trees, red lettuce, and the flowers of black tulips and pansies all owe their distinctive coloration to the presence of anthocyanins. The deep purple color of the leaves of red cabbage is caused by the presence of anthocyanin pigments. While the yellow and orange carotenoids and the green chlorophylls are contained in plastids within the plant cell, the anthocyanins are found dissolved in the cell sap and, therefore, appear uniformly distributed throughout the cell. Since they are water soluble, they are easily extracted for use in the laboratory. + OH OH O HO OH + OH O HO OH OH OH O OH O HO OH OH OH C 15 H 10 O 7 ) Quercetin ( C 15 H 11 O 5 + Pelargonidin ( ) ) Cyanidin (C 15 H 11 O 6 + Figure 1. Anthocyanins ( Quercetin technically is a flavonol rather than an anthocyanin.) The color the above compounds exhibit depends upon the pH of the solution in which they occur. The same compound (e.g., cyanidin) will appear red or blue (and other colors) depending on the pH (for an explanation of pH see Appendix.) These properties are the basis for the following exercise. O
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This note was uploaded on 04/08/2008 for the course CHEM 110 taught by Professor Neeba during the Fall '08 term at Puget Sound.

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Expt7-07 - _University of Puget Sound Department of...

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