Question

Would the indicator bromocresol green, with a transition range of pH 3.8–5.4, ever be useful in the titration of a weak acidwith a strong base?

Short answer

When titrating weak acid with a strong base, bromocresol green is not the proper indicator to use.

Step-by-step solution

Define Equivalence point acid-base titration

An acid-base titration is a titration between acid and base. It is also known as a neutralization reaction.

There are primarily four types of acid-base titrations -

• Strong base vs strong acid

• Strong base vs weak acid

• Weak base vs strong acid

• Weak base vs weak acid

The equivalence point ensures the titration reaction becomes completed and at this point, the number of moles of titrant and the number of moles of analyte remains equal. We cannot see with naked eyes the completion of the titration reaction - the titrant and the analyte becoming equal in stoichiometric proportion, hence we use a chemical compound called indicator called which indicates the end of the reaction by color change. The endpoint refers to the color change and indicates the equivalence point has been reached.

Observe the Table

An indicator called bromocresol green is employed throughout the titration. It becomes blue in color above the pH value 5.4 and becomes yellow in color at lower pH- below 3.8. A weak acid is not dissociated into individual ions completely the dissociated and non-dissociated forms tend to remain in solution and they exist in equilibrium. Thus, while titrating the weak acid with a strong base the conjugate base of the weak acid also remains in the titrating mixture. Weak acids furnish conjugate bases with higher basicity that the conjugate base formed in the titrating mixture obviously provides pH greater than 7. Since Bromocresol green functions as an

indicator in the pH range of 3.8-5.4, it is not suitable to use for weak acid and strong

base totration