Are all acid/base indicators water-based?

The indicator must be able to exchange hydrogen ions with the solution, and do so quickly. So indicators in aqueous acid-base chemistry must be water-soluble to some degree to be useful. However, indicators are usually large organic molecules, and some are only sparingly soluble in water. These indicators are prepared by dissolving the dry compound in a water-miscible solvent. For example, solid phenolpthalein is dissolved in ethanol, and the ethanol solution is what you find in dropper bottles labeled 'phenolpthalein'.

Many indicators can be used to monitor acid-base chemistry in solvents other than water. This is especially useful when you're trying to study a water-insoluble organic compound, or a compound with a pK_a or pK_b of less than 7. For example, phenols are weakly acidic compounds that can often be determined by titration with a quaternary ammonium hydroxide (like tetrabutylammonium hydroxide) in dimethyformamide, a nonaqueous solvent. Azo violet can be used as the indicator in this titration. o-nitroaniline is another indicator used in nonaqueous weak acid titrations.

Very weak bases are also titrated in nonaqueous solvents. For example, nicotine can be determined by titrating it with perchloric acid, using glacial acetic acid rather than water as the solvent. The indicator in this case could be methyl violet (which changes from violet to green and then to yellow, with the last disappearance of violet indicating the endpoint). Two other indicators often used in nonaqueous weak base titrations are crystal violet and methyl red.

Author: Fred Senese senese@antoine.frostburg.edu

General Chemistry Online! Are all acid/base indicators water-based?