Define indicator. Why are different indicators used in different \(\mathrm{pH}\) ranges?

An indicator is a substance that changes its color depending on the pH of the solution it is in. These substances, usually weak acids or bases, are often used to determine the endpoint of titration or to visually identify the acidity or alkalinity of a solution.

The color change of an indicator is due to the presence of the ionizable hydrogen atom in its molecular structure. When this hydrogen atom is lost (or gained) due to changes in the pH of the solution, the electronic structure of the indicator is also altered, leading to a change in color. Since the color change occurs at a specific pH range, an indicator can only be used in that specific pH range to accurately determine the pH of the solution.

Different indicators are used in different pH ranges because the pH at which an indicator changes color is unique to that specific indicator. Therefore, one indicator might be suitable for a particular pH range, while another indicator might be suitable for a different pH range. For instance, phenolphthalein is a common indicator used in acid-base titrations and turns colorless in acidic solutions and pink in basic solutions with a transition pH range of about 8.2 to 10.0. However, it would not be useful in a titration involving solutions with a very low pH because its color change would not be detectable. In such cases, another indicator like methyl orange with a transition pH range of 3.1 to 4.4 would be more appropriate.

In summary, an indicator is a substance that changes color depending on the pH of a solution. Different indicators are used in different pH ranges because the color change of each indicator occurs at a specific pH range unique to that indicator. Selecting the appropriate indicator for a particular pH range is crucial for accurate pH determination and ensuring the success of experiments such as titrations.