A
pH indicator is a
halochromic chemical compound added in small amounts to a
solution so the
pH (
acidity or
basicity) of the solution can be determined visually. Hence, a pH indicator is a
chemical detector for
hydronium ions (H
3O
+) or hydrogen ions (H
+) in the
Arrhenius model. Normally, the indicator causes the
color of the solution to change depending on the pH. Indicators can also show change in other physical properties; for example, olfactory indicators show change in their
odor. The pH value of a neutral solution is 7.0. Solutions with a pH value below 7.0 are considered acidic and solutions with pH value above 7.0 are basic (alkaline). As most naturally occurring organic compounds are weak protolytes,
carboxylic acids and
amines, pH indicators find many applications in biology and analytical chemistry. Moreover, pH indicators form one of the three main types of indicator compounds used in chemical analysis. For the quantitative analysis of metal cations, the use of
complexometric indicators is preferred, whereas the third compound class, the
redox indicators, are used in titrations involving a redox reaction as the basis of the analysis.