A pH indicator is a chemical compound that is added in small amounts to a solution so that the pH (acidity or alkalinity) of the solution can be determined easily. Hence a pH indicator is a chemical detector for protons (H+). Normally, the indicator causes the color of the solution to change depending on the pH.
pH indicators themselves are frequently weak acids or bases. When introduced into a solution, they may bind H+ or OH- ions. The different electronic configuration of the bound indicator causes the indicator's color to change.
Because of the subjective determination of color, pH indicators frequently do not give very precise readings. For applications requiring precise measurement of pH, a pH meter is frequently used.
pH indicators are frequently employed in titrations in analytic chemistry and biology experiments to determine the extent of a chemical reaction.
Common pH indicators litmus paper universal indicator phenolphthalein