Acid-base indicators (Forensic Science)
The acidity or alkalinity of a substance is indicated by its pH, which is a measure of the concentration of hydrogen ions (H+) in a solution. The pH scale is logarithmic and ranges from 0 to 14. The lower the pH, the more acidic the solution, and the higher the pH, the more alkaline, or basic, the solution; pH 7.0 is neutral and is the pH of pure water.
Acid-base indicators are organic dyes that change color depending on the concentration of hydrogen ions present in a solution. The change does not become visible at a precise point; rather, it happens within a fairly narrow pH range. Many different acid-base indicators are available, and they change colors within different pH ranges. For example, phenolphthalein is colorless at a pH of 8.2 but turns red at a pH of 10. Methyl orange is red at a pH of 3.2 but turns yellow at a pH of 4.4.
The most common acid-base indicator is litmus paper. It comes in two forms, red and blue. When dipped into a solution, blue litmus paper turns red if the pH of the solution is 4.5 or below, indicating the solution is acidic. If the pH of the solution is 8.2 or above, blue litmus paper remains its original blue color. Conversely, red litmus paper remains red when dipped into an acidic solution but turns blue when dipped into a basic solution.
Most often, acid-base indicators are used with a technique called titration. Titration allows analytical chemists to make quantitative determinations of how...
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Further Reading (Forensic Science)
Blei, Ira. and George Odian. General, Organic, and Biochemistry: Connecting Chemistry to Your Life. 2d ed. New York: W. H. Freeman, 2006.
James, Stuart H., and Jon J. Nordby, eds. Forensic Science: An Introduction to Scientific and Investigative Techniques. 2d ed. Boca Raton, Fla.: CRC Press, 2005.
Oxlade, Chris. Acids and Bases. Chicago: Heinemann Library, 2007.
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