Acid–base indicator

An acid–base indicator is a compound whose protonation state changes with solution pH, producing a visible color change over a characteristic range.

Henderson–Hasselbalch equation: An equation relating a buffer’s pH to the ratio of its conjugate base and acid concentrations. It predicts the protonation ratio that determines an indicator’s observed color.
Acid–base titration: A quantitative method that determines an acid or base concentration by neutralizing it with a measured solution. An indicator can signal when a titration has reached its endpoint.
pH: A logarithmic measure of hydrogen-ion activity in a solution. An indicator responds to pH because pH sets the balance between its protonated and deprotonated forms.
pH meter: An electronic instrument that estimates pH from the voltage measured by an electrode system. Unlike a color indicator, it can measure pH without relying on visual color judgment.
Methyl orange: An acid–base indicator that changes from red in acidic solution to yellow above its transition range. Its acidic transition range makes it useful for titrations with low-pH endpoints.
Conjugate acid–base pair: Two chemical species that differ by one proton and interconvert through proton transfer. An indicator’s differently colored forms constitute a conjugate acid–base pair.
Equivalence point: The point in a titration where reactants have been added in stoichiometrically equivalent amounts. Indicator choice matters because its color-change range should encompass the titration’s equivalence-point pH.
Acid–base reaction: A chemical reaction involving transfer of a proton between an acid and a base. Indicator color changes arise from acid–base reactions between its molecular forms and solution.
Potentiometric titration: A titration method that tracks electrode potential as titrant is added. It locates endpoints instrumentally rather than through an indicator’s color transition.
Bromothymol blue: An indicator that appears yellow in acidic solution and blue in alkaline solution, with green near neutral pH. Its transition near neutral pH makes it useful for observing near-neutral changes.

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