Explain the concept of acid-base indicators and... - SS2 Chemistry Acids, Bases, and pH Question

Acid-base indicators are substances that exhibit different colours in acidic and basic environments, allowing us to determine the pH of a solution. They are usually weak acids or bases that undergo a reversible colour change as the pH of the solution changes.

Acid-base indicators function through the concept of conjugate acid-base pairs. In their acidic form, indicators are protonated and have one colour, while in their basic form, they are deprotonated and have a different colour. The equilibrium between the acidic and basic forms shifts as the pH of the solution changes, resulting in a visible colour change.

The choice of an appropriate indicator depends on the desired pH range. Different indicators have different pH ranges over which they undergo a colour change. It is crucial to select an indicator that changes colour near the pH of the solution being tested. For example:

●     Methyl orange has a range of pH 3.1-4.4 and is suitable for determining the pH of acidic solutions.

●     Phenolphthalein has a range of pH 8.2-10.0 and is commonly used for neutralisation reactions or to determine the endpoint of acid-base titrations.

●     Bromothymol blue has a range of pH 6.0-7.6 and is useful for determining the pH of slightly acidic to slightly basic solutions.

The choice of indicator is based on the principle that the indicator's pKa (the pH at which it changes colour) should be close to the pH of the solution being tested. This ensures that the indicator is sensitive to the pH change and provides an accurate representation of the solution's acidity or basicity.