Acid-Base Indicators - SS2 Chemistry Lesson Note

Acid-base indicators are substances that undergo a colour change in response to changes in the acidity or alkalinity (basicity) of a solution. Indicators are typically weak acids or bases that have different colours in their protonated (acidic) and deprotonated (basic) forms. The colour change of an indicator occurs within a specific pH range called its "transition range" or "indicator range." Common examples of acid-base indicators include phenolphthalein, bromothymol blue, methyl orange, litmus, and universal indicator.

Phenolphthalein:

Phenolphthalein is a colourless indicator in acidic solutions and turns pink or magenta in alkaline solutions. It has a transition range around pH 8.2 to 10.0, making it useful for determining if a solution is acidic or alkaline.

Bromothymol Blue:

Bromothymol blue is yellow in acidic solutions, green in neutral solutions, and blue in alkaline solutions. It has a transition range around pH 6.0 to 7.6, allowing for the differentiation between acidic and alkaline solutions.

Methyl Orange:

Methyl orange is red in acidic solutions and yellow in alkaline solutions. It has a transition range around pH 3.1 to 4.4, making it suitable for distinguishing between acidic and slightly basic solutions.

Litmus:

Litmus is a mixture of dyes extracted from lichens. It is blue in alkaline solutions and red in acidic solutions. Litmus paper is often used as a simple and quick way to determine if a solution is acidic or alkaline.

Universal Indicator:

A universal indicator is a mixture of several indicators, displaying a range of colours across the entire pH scale. It is often used in laboratories to obtain more precise pH measurements.

Acid-base indicators are typically used in titration experiments to determine the endpoint of a reaction by monitoring the colour change. The choice of an indicator depends on the expected pH range of the solution being tested. It is important to select an indicator with a transition range that corresponds to the expected pH change in the reaction. Some indicators are more suitable for precise pH measurements, while others provide a quick visual indication of whether a solution is acidic or alkaline. It's important to note that some indicators may be sensitive to temperature, concentration, and other factors, which can affect their accuracy and reliability.

Acid-base indicators provide a valuable visual tool for determining the acidity or alkalinity of a solution. They are widely used in chemistry, biology, environmental science, and various other fields for qualitative and semi-quantitative analysis.