These are called acid-base indicators. They are usually weak acids or bases, but their conjugate base or acid forms have different colors due to differences in their absorption spectra. Did you know that the color of hydrangea flowers depends on the pH of the soil in which they are grown?
Indicators are organic weak acids or bases with complicated structures. This is a very important formula, and its derivation is very simple. Start from the definition of the equilibrium constant K ; you can easily derive it. In other words, when the pH is the same as p K ai , there are equal amounts of acid and base forms. When the two forms have equal concentration, the color change is most noticeable.
Colors of substances make the world a wonderful place. Because of the colors and structures, flowers, plants, animals, and minerals show their unique characters. Many indicators are extracted from plants. For example, red cabbage juice and tea pigments show different colors when the pH is different. Practice Watch the video at the link below and answer the following questions: What was the acid used?
What was the base used? What color was methyl orange in acid? What color did indigo carmine turn in acid? Review What is an indicator? What would be the best indicator to use around pH 1? What indicator besides phenolphthalein might you use for a pH 7 experiment? Show References References Flickr: brittgow. Licenses and Attributions. Well, see this old answer. An indicator is a large, weak organic acid whose acid and base forms have distinctive colours.
And we can represent this species by HIn. As with any aqueous acid we may assess the acid base equilibrium:. The graph shows poorly the titration curve of a strong acid, when titrated by a strong base. Because the rise in pH is so precipitous, i. As the pH increases - the intensity of the colour of HIn decreases and the equilibrium is pushed to the right.
Therefore the intensity of the colour of In - increases.
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