Universal indicator

A mixture of acid–base indicators that changes through a sequence of colors across a broad pH range, allowing approximate pH estimation.

Acid–base indicator: A substance that changes color over a characteristic pH range as it gains or loses hydrogen ions. Universal indicator combines several indicators with different transition ranges.
pH paper: Paper impregnated with an indicator or indicator mixture for estimating a solution’s pH by color. It is a common portable format for using universal indicator.
Litmus: A natural dye mixture that turns red in acidic conditions and blue in alkaline conditions. Unlike universal indicator, it distinguishes broad acid and base conditions rather than estimating a range.
Arrhenius acid–base theory: A theory defining acids as substances that increase hydrogen ions in water and bases as substances that increase hydroxide ions. It provides an introductory framework for interpreting acidic and alkaline solutions.
pH: A logarithmic measure of hydrogen-ion activity in a solution. The mixture’s color is used to estimate this quantity.
Acid–base titration: A quantitative method that determines an acid or base concentration by reaction with a measured counterpart. Universal indicator can show broad pH change, though it is usually too gradual for precise endpoints.
Phenolphthalein: An acid–base indicator that is colorless in acidic solution and pink in alkaline solution. Its sharp transition suits some titrations better than universal indicator’s gradual color scale.
Brønsted–Lowry acid–base theory: A theory defining acids as proton donors and bases as proton acceptors. Proton transfer between indicator forms underlies their pH-dependent colors.
Acid–base equilibrium: A chemical equilibrium involving transfer of hydrogen ions between acids and bases. Indicator molecules change between differently colored forms as this equilibrium shifts.
Soil pH: A measure of the acidity or alkalinity of soil water and its chemical environment. Universal indicator offers a quick, approximate check of soil extracts.

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