pH indicator

A substance whose color or another measurable property changes over a characteristic pH range, allowing acidity or alkalinity to be estimated.

Acid–base indicator equilibrium: An equilibrium between differently protonated forms of an indicator, often with distinct colors. The shift between these forms produces the characteristic color change.
Acid–base titration: A quantitative method that determines an acid or base concentration by reacting it with a measured titrant. An indicator can signal the titration endpoint through a visible color change.
pH: A logarithmic measure of hydrogen-ion activity in a solution. An indicator’s color is interpreted as evidence about this quantity.
Redox indicator: A substance whose color changes with the oxidation–reduction state of a solution. It reports redox conditions rather than the acidity or alkalinity targeted by pH indicators.
Henderson–Hasselbalch equation: An equation relating solution pH to the ratio of a weak acid’s conjugate base and acid forms. It estimates the pH range across which an indicator’s two forms become visible in comparable amounts.
Universal indicator: A mixture of indicators that displays different colors across a broad pH range. Its broad color scale estimates pH when a narrow-range indicator would be unsuitable.
Acid–base chemistry: The study of proton transfer, acids, bases, and their reactions. Indicator color changes arise from acid–base reactions of the indicator itself.
Fluorescent pH indicator: A pH-sensitive dye whose fluorescence changes with protonation state. It detects acidity through emitted light rather than ordinary visible color alone.
Absorption spectrum: A record of how strongly a substance absorbs light at different wavelengths. Different indicator forms absorb different wavelengths, creating a measurable color shift.
Litmus: A dye mixture that turns red in acidic conditions and blue in alkaline conditions. It offers a simple qualitative test without giving a precise pH value.

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