Indicators

In chemistry, natural or synthetic substance that changes color in response to the nature of its chemical environment. Indicators are used to provide information about the degree of acidity of a substance (pH) or the state of some chemical reaction within a solution being tested or analyzed. One of the oldest indicators is litmus, a vegetable dye that turns red in acid solutions and blue in basic ones. Other indicators include alizarin, methyl red, and phenolphthalein, each one being useful for a particular range of acidity or a certain type of chemical reaction.

Litmus, vegetable dye obtained from lichens, usually of the genus Variolaria, and used in chemistry to determine the presence of acids and bases in a solution. Strips of paper impregnated with a blue or red litmus solution, or small quantities of the solution itself, are used to indicate the presence of an acid or a base; acids turn blue litmus red, and bases turn red litmus blue.

pH, term indicating the hydrogen ion (positively charged hydrogen atom) concentration of a solution, a measure of the solution’s acidity. Hydrogen ions are usually represented by the symbol H⁺. The term (from French pouvoir hydrogène, “hydrogen power”) is defined as the negative logarithm of the concentration of H⁺ ions: pH = -log₁₀[H⁺], where [H⁺] is the concentration of H⁺ ions in moles per litre. Because H⁺ ions associate with water molecules to form hydronium (H₃O⁺) ions, pH also is often expressed in terms of the concentration of hydronium ions.