Indicator, Acid-base

The forensic examination of tissues or other material can involve chemical testing. These tests help reveal the presence of contaminating chemicals and can help determine the cause of the incident or the death. Some tests rely on a color change to indicate the presence of the target compound. Acid-base indicators are often utilized to show this reaction.

An acid-base indicator is often a complex organic dye that undergoes the color change when the pH (a measure of the amount of acidic or basic components) changes over specific values. Many plant pigments and other natural products are good indicators. Synthetic compounds like phenolphthalein and methyl red are also good acid-base indicators.

Paper that has been impregnated with indicator chemicals and allowed to dry is a common site in laboratories that do acid-base testing. This paper, which is typically cut into thin strips, is called pH paper. The use of different chemicals allows the strips to detect different ranges of pH.

The pH at which the color of an indicator changes is called the transition interval. Forensic chemists use appropriate indicators to signal the end of an acid-base neutralization reaction. Such a reaction is usually accomplished by titration—slowly adding a measured quantity of the base to a measured quantity of the acid (or vice versa). When the reaction is complete and there is no excess of acid or base, but only the reaction products, this is called the endpoint of the titration. The indicator must change color at the pH which corresponds to that endpoint.

The indicator changes color because of its own neutralization in the solution. Different indicators have different transition intervals, so the choice of indicator depends on matching the transition interval to the expected pH of the solution just as the reaction reaches the point of complete neutralization.

The two most common pH indicators are phenolphthalein and methyl red. Phenolphthalein changes from colorless to pink across a range of pH 8.2 to pH 10. Methyl red changes from red to yellow across a range of pH 4.4 to pH 6.2. Other indicators are available through most of the pH range, and can be used in the titration of a wide range of weak acids and bases.

see also Chemical and biological detection technologies; Inorganic compounds; Toxicological analysis.