Are all acid/base indicators water-based?

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