= pH indicator
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= Acid-base indicator
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A <pH indicator> changes color as its <protonation> distribution changes. In a simple <weak acid>/<conjugate base> model, its two forms have different colors and their ratio depends on <pH> through the <acid dissociation constant>. The visible transition spans a range of <pH>; it is not a direct signal that the sample has reached an <equivalence point>. An indicator is useful when its transition lies in a steep part of the <titration curve>, so a modest range of <pH> corresponds to a small added <volume (thermodynamics)>.<Chemistry 2e by Paul Flowers and others/14.7 Acid-Base Titrations>[Chemistry 2e, §14.7, Acid-Base Indicators, Figures 14.19–14.21: indicator equilibrium, transition ranges, and selection]{ref}
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