Question

As shown in Figure 16.8, the indicator thymol blue has two color changes. Which color change will generally be more suitable for the titration of a weak acid with a strong base?

Short answer

The second color change of thymol blue (from yellow to blue) with the pH interval 8.0 to 9.6 is more suitable for the titration of a weak acid with a strong base, as it lies within the basic range of the pH scale seen during such titrations.

Step-by-step solution

Understand Thymol Blue Color Changes

Thymol blue is a pH indicator with two color change intervals. The first interval is from pH 1.2 (red) to 2.8 (yellow), and the second interval is from pH 8.0 (yellow) to 9.6 (blue).

Identifying Weak Acid and Strong Base Conditions

A weak acid is a compound that only partially ionizes in water, while a strong base is a compound that fully dissociates in water. During a titration, a weak acid reacts with a strong base to form a weak base and a salt. The endpoint of the titration occurs when these two species are present in equal concentrations.

Compare pH Range of Titration and Thymol Blue Color Changes

In a titration of a weak acid with a strong base, the pH starts acidic and ends up in the basic range after the equivalence point. The equivalence point is the point at which the weak acid has been neutralized completely by the strong base.

Determine Suitable Thymol Blue Color Change for Titration

Since the titration of a weak acid with a strong base leads to a pH increase from acidic to basic range, it is essential to choose the color change interval that lies within this pH range. The second color change interval of thymol blue (pH 8.0 to 9.6) is more suitable because it lies within the basic range of the pH scale, as seen during weak acid-strong base titration. In conclusion, the second color change of thymol blue (from yellow to blue) is more suitable for the titration of a weak acid with a strong base.

Key concepts

pH indicators

pH indicators are special chemicals that help us determine the acidity or basicity of a solution. They do this by changing color at specific pH levels. These color changes are due to the molecular structure of the indicator, which alters at different pH levels. This structural change affects the wavelength of light that is absorbed and thus changes the color we see.
One popular pH indicator is thymol blue, which has the unique ability to show two separate color transitions across different pH ranges:

• The first occurs between a pH of 1.2 and 2.8, where it changes from red to yellow.
• The second transition is between a pH of 8.0 and 9.6, shifting from yellow to blue.

Choosing the right indicator is crucial during a titration, as it must be able to exhibit a visible color change close to the equivalence point to ensure accurate measurements.

equivalence point

The equivalence point in a titration is a milestone where the amount of acid equals the amount of base present in the solution. At this point, the reaction between the acid and base is complete, leading to the formation of a salt and water.
During a weak acid-strong base titration, the equivalence point is not neutral (pH 7) as it would be in a strong acid-strong base titration. Instead, it is usually in the basic pH range. This shift happens because the weak acid only partially ionizes in water, leaving more hydroxide ions ( ext{OH}^-) at the equivalence point.
Recognizing when the equivalence point has been reached is important, as it allows for the determination of the concentrations of the solutions involved. Therefore, a pH indicator that changes color around the equivalence point is necessary for proper titration results.

weak acid strong base titration

In the realm of chemistry, a weak acid-strong base titration involves adding a strong base to a weak acid solution. Here, the strong base fully dissociates, meaning it breaks apart into ions completely in the aqueous solution. The weak acid, however, does not dissociate fully.
Throughout the titration process, a base is added dropwise until the acid is neutralized. A notable feature of this type of titration is the dramatic increase in pH towards the end, resulting in the solution becoming increasingly basic.
To track this critical change, the use of an appropriate pH indicator, like thymol blue, is crucial. Thymol blue indicates an endpoint around the basic pH range (8.0 to 9.6) suitable for observing the equivalence point in a weak acid-strong base titration.
The capability to visually observe the endpoint significantly aids in accurately determining the concentration of the unknown solution, making pH indicators an essential tool in analytical chemistry.