The phenol red turns yellow as a result of the carbon dioxide (CO2) generating carbonic acid in the water (H2O).
The experiment can be carried out in a number of different ways, but commonly four tubes are prepared, each of which is filled with phenol red and initially turned yellow by adding carbon dioxide through a straw. The Elodea would be in two test tubes, and not in the other two. Both one of the non-elodea tubes and one of the elodea tubes would have foil coverings to exclude light.
The vials' CO2 content is estimated using the pH indicator phenol red. Aqueous solutions with higher CO2 concentrations also have higher amounts of H+ ions, which lowers the pH. This happens as a result of the synthesis of the intermediate molecule carbonic acid. Substances known as pH indicators change color when the pH level changes.
For example, phenolphthalein, which is pink in a base and clear in an acid, bromothymol blue, which is yellow in an acid, greenish near neutral, and blue in a base, and phenol red, which is yellow below pH 6.8, red near neutral, and pink above pH 7.4 are some of the often used indicators. When carbon dioxide is added to test tubes for an experiment utilizing Elodea sprigs as plants and phenol red as a pH indicator, the phenol red will change color first.
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