Which of the following is a common laboratory indicator for bases?

Analysis of the Question

The question asks for a "common laboratory indicator for bases." While many indicators change color in both acidic and basic solutions, we are looking for the one most characteristically associated with identifying or titrating bases in a standard Nigerian secondary school laboratory setting.

Concept Explanation

An acid-base indicator is a substance that changes color depending on the pH of the solution it is added to.

• Phenolphthalein is a weak organic acid. In acidic solutions, it is colorless. However, as the pH increases (becoming basic), it undergoes a structural change that turns it pink/magenta. Because its color change interval (pH 8.2 – 10.0) falls within the basic range, it is the standard choice for titrating strong bases against weak acids.

Step-by-Step Solution

1. Evaluate the options: • Methyl orange: Changes color in the acidic range (pH 3.1 – 4.4). It is typically used for titrating strong acids against weak bases. • Phenolphthalein: Changes color in the basic range (pH 8.2 – 10.0). It is the classic indicator for detecting the presence of a base or the endpoint of a base-related titration. • Bromothymol blue: Changes color near neutral pH (pH 6.0 – 7.6). It is often used for strong acid-strong base titrations. • Litmus: A general-purpose indicator that turns blue in bases and red in acids. While it works for bases, it is less precise for quantitative laboratory titrations compared to phenolphthalein.

2. Conclusion: In the context of laboratory chemistry (especially for WASSCE/JAMB practicals), phenolphthalein is the most specific and commonly used indicator for identifying the presence of a base or determining the endpoint in a titration involving a base.

Verification

This aligns with standard chemistry curricula where phenolphthalein is taught as the primary indicator for basic solutions in titration experiments.

Exam Tip

Remember the "P-P" rule: Phenolphthalein is Pink in a base. If you are doing a titration and the solution turns pink, you have reached the basic endpoint!