Chemical Meanings of Acid-Base Indicators
Acid-base indicators are handy chemicals that change color over a given pH range. This property makes them useful as a means to monitor the pH of a solution during a chemistry experiment. The color change and pH range are different for different indicators and stem from the underlying chemistry of each indicator.
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Types of Indicators
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Many different indicators are available. The simplest is one you can make easily at home: red cabbage juice. Cabbage juice is especially cool because it exhibits a variety of shades of color over different pH ranges. In the lab, a number of different chemicals are often used. The following are common indicators, together with the pH range at which they change color:
Thymol blue: 1.2 - 2.8. Red in more acidic solutions, yellow in more basic solutions.
Methyl orange: 3.2 - 4.4. Red in more acidic solutions, yellow in more basic solutions.
Bromophenol blue: 3.0 - 4.6. Yellow in more acidic solutions, blue in more basic solutions.
Bromocresol green: 3. 8 - 5.4. Yellow in more acidic solutions, blue in more basic solutions.
Methyl red: 4.8 - 6.0. Red in more acidic solutions, yellow in more basic solutions.
Litmus: 5.0 - 8.0. Red in more acidic solutions, blue in more basic solutions.
Bromothymol blue: 6.0 - 7.6. Yellow in more acidic solutions, blue in more basic solutions.
Phenol red: 6.6 - 8.0. Yellow in more acidic solutions, red in more basic solutions.
Thymol blue: 8.0 - 9.6. Yellow in more acidic solutions, blue in more basic solutions.
Phenolphthalein: 8.2 - 10.0. Colorless in more acidic solutions, pink in more basic solutions.
Alizarin yellow R: 10.1 - 12.0. Yellow in more acidic solutions, red in more basic solutions.
Alizarin: 11.0 - 12.4. Red in more acidic solutions, purple in basic solutions.
Meaning
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Basically, the acid-base indicator serves as an easy-to-spot signal during the experiment. Once the pH strays into the specified range, the solution starts to change color. If you're titrating a solution -- using an acid to neutralize a base, for example, or conducting an experiment that involves a change in pH -- the indicator helps you know once the acid/base originally present has been neutralized. For this reason, it's important to select an indicator whose color change range approximately matches the pH you expect as an endpoint in your experiment.
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Underlying Chemistry
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Aside from its meaning in your experiment, the change in color of the indicator has a more fundamental meaning as well. Quantum theory is the branch of physics that describes the behavior of matter down at the atomic scale. We know from quantum mechanics that atoms and molecules can only exist in certain energy states; in other words, they can only have certain discrete energies. A molecule will only absorb a photon of light if the energy of the photon is equal to the difference between two allowed states.
Conjugated Systems
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When atoms in molecules form double-bonds, they form both a sigma-bond, where the bond is centered along the axis between the two nuclei, and a π-bond, which is parallel to but not centered on the internuclear axis. A series of alternating single and double bonds in an organic compound creates what chemists call a π-conjugated system where electrons are delocalized or spread out over multiple atoms. Many more energy levels are available to electrons in this kind of system than in a molecule without this kind of bonding, so molecules with "delocalized" electrons can sometimes absorb light in the visible range, giving them color. When a molecule of this kind forms a bond with a hydrogen atom, one of the double bonds is broken. This changes the extent of delocalization in the molecule, changing in turn the wavelengths of light it can absorb -- and hence its color. The more acidic the solution, the more hydrogen ions are present, so a change in pH can cause the indicator molecules to pick up or lose hydrogen ions -- and thereby change color as a result.
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References
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