Question

The best indicator for the detection of end point in titration of a weak acid and a strong base is: (a) Methyl orange ( \(3.1\) to \(4.4\) ) (b) Methyl red \((4.2\) to \(6.3)\) (c) Bromothymol blue (6 to 7.6) (d) Phenolphthalein ( \(8.2\) to 10 )

Short answer

The best indicator for the titration of a weak acid and a strong base is Bromothymol blue (c).

Step-by-step solution

Identify the pH Range of the Reaction

First, it is necessary to know that the neutralization of a weak acid by a strong base will end with a pH greater than 7, as the weak acid's conjugate base is left in solution, making it slightly basic.

Match the pH Range with Indicator Ranges

Now, compare this pH range with the ranges of the indicators given in the options. The indicator that should change color at a pH just above 7 would be the most suitable.

Select the Appropriate Indicator

Bromothymol blue, which has a range from 6 to 7.6, will change color in the basic range, just above 7, which is appropriate for a weak acid and strong base titration endpoint.

Key concepts

Weak Acid and Strong Base Titration

In an acid-base titration, the process involves a weak acid reacting with a strong base. A weak acid does not dissociate completely in water, meaning not every molecule of the weak acid breaks apart to release H+ ions. Conversely, a strong base dissociates completely, providing plenty of OH− ions.

Titrations aim to find the point where the amount of acid equals the amount of base, known as the equivalence point. At this stage, all the acid has reacted with the base to form a salt and water in what we call a neutralization reaction. However, because the salt formed is derived from a weak acid, it can make the solution slightly basic. As the titration progresses, the pH of the mixture increases and must be monitored carefully to determine the endpoint — the point at which an indicator changes color to signal that the equivalence point has been reached.

pH Range and Indicator Selection

Selecting the proper indicator for a titration is key to accurately determining the endpoint. Indicators are themselves weak acids or bases that change color over a specific pH range. This range is where the indicator switches from its acid form to its base form (or vice versa), each form having a different color.

For a titration involving a weak acid and a strong base, the pH at the equivalence point will be above 7, which means the indicator chosen must change color in a basic pH range. It is essential to match the endpoint pH range of the titration with the color change range of the indicator to ensure accurate detection of the endpoint.

Neutralization Reaction

A neutralization reaction is a type of chemical reaction where an acid and a base react to form water and a salt. In the context of a weak acid and strong base titration, the HA (weak acid) reacts with OH− (from the strong base) to yield water (H2O) and the conjugate base of the acid (A−).

The general equation for this reaction is:\[ HA + OH^- \rightarrow H_2O + A^- \]
Although the name 'neutralization' implies a pH of 7, this isn't always the case. For titrations with weak acids and strong bases, the resulting solution tends to be slightly basic due to the presence of the weak conjugate base, A−.

Titration Endpoint Detection

Detecting the endpoint of a titration accurately is crucial. It is the practical aspect that hints at when the equivalence point has been reached, although there might be a slight difference between the two points. The endpoint is marked by a persistent color change in the indicator used.

Choosing the correct indicator is vital, as different indicators change color at different pH ranges. For instance, phenolphthalein is a common indicator when the expected endpoint pH is around 8 to 10, which would be too high for our scenario involving a weak acid and strong base titration. The accuracy of the titration is compromised if an indicator with an inappropriate pH range is selected; therefore, it is important that the indicator's pH range is close to the expected pH at equivalence to ensure precision in determining the completion of the titration.