Final answer:
An acid-base indicator is used to visually signal the acidity or basicity of a solution, through a color change caused by a shift in equilibrium between its protonated and deprotonated forms. Common examples are litmus, which is red in acidic and blue in basic solutions, and phenolphthalein, which is colorless in acids but pink in bases.
Step-by-step explanation:
Purpose of an Acid-Base Indicator
The purpose of an acid-base indicator is to provide a visual indication of the acidity or basicity of a solution. An acid-base indicator functions based on the color change that occurs when an indicator reacts with different pH levels. Typically, indicators are weak acids or bases, and the color change is due to the shift in equilibrium between the protonated and deprotonated forms of the indicator molecule.
Examples of Common Indicators
Two common indicators are litmus and phenolphthalein. Litmus turns red in acidic solutions and blue in basic solutions. Phenolphthalein is colorless in acidic conditions but turns pink when in a basic solution. These changes occur due to the shift in equilibrium described by the general equation:
HIn(aq) → H+(aq) + In−(aq).
Properties of a Good Indicator
To be effective, a good indicator must have a color change that is easily detected and rapid. Additionally, it should not react with the substance being titrated and have a pKin within one pH unit of the expected pH at the equivalence point of the titration.