Different indicators are used for neutralization reactions between acids and bases because each indicator has a specific pH range over which it undergoes a color change. The choice of indicator depends on the expected pH range of the reaction mixture during the neutralization process.
An indicator is a substance that changes color in response to a change in pH. In acid-base titrations, where an acid is reacted with a base, indicators are used to determine the endpoint of the reaction, which is when the stoichiometric equivalence point is reached. At the equivalence point, the moles of acid are equal to the moles of base.
Since different acids and bases have different strengths, the pH at the equivalence point can vary. For example, strong acids and strong bases will result in a neutral pH at the equivalence point (pH 7), while weak acids and weak bases will have different pH values at the equivalence point.
To detect the endpoint of a specific neutralization reaction, an indicator is chosen that has a color change within the expected pH range of the reaction mixture. Some common indicators and their corresponding pH ranges include:
Phenolphthalein: This indicator is colorless in acidic solutions (pH below 8.2) and pink in basic solutions (pH above 10). It is often used for strong acid-strong base titrations.
Methyl orange: This indicator is red in acidic solutions (pH below 3.1) and yellow in basic solutions (pH above 4.4). It is commonly used for titrations involving weak acids and strong bases.
Bromothymol blue: This indicator is yellow in acidic solutions (pH below 6) and blue in basic solutions (pH above 7.6). It can be used for a wide range of acid-base titrations.
By selecting the appropriate indicator, chemists can visually observe the color change and determine when the neutralization reaction has reached its endpoint. The choice of indicator depends on the expected pH range of the reaction, ensuring that the color change occurs within that range for accurate endpoint detection.