Question

The acid-base indicator bromcresol green changes color from \(\mathrm{pH} 3.8\) to \(\mathrm{pH}\) 5.4. It appears yellow in strongly acidic solutions and blue in basic solutions. Predict the color of the indicator in a solution having a \(\mathrm{pH}\) of 4.6 .

Short answer

The indicator will appear green at pH 4.6.

Step-by-step solution

Understanding the indicator's color range

Bromcresol green changes color between the pH of 3.8 and 5.4. At pH levels below 3.8, it appears yellow, while above 5.4, it appears blue.

Analyzing the given pH

The problem states that the solution has a pH of 4.6. This pH value is between 3.8 and 5.4.

Determining the indicator's color

Since 4.6 is between 3.8 and 5.4, bromcresol green will display a greenish color, as it transitions from yellow to blue within this pH range.

Key concepts

Bromcresol Green

Bromcresol green is a type of chemical compound known as an acid-base indicator. It is particularly valued in chemistry due to its ability to change color depending on the acidity or basicity of the environment it is placed in. This property makes it extremely useful for determining the pH of a solution visually.
Bromcresol green is an organic dye, and it contains characteristics that allow it to interact with hydrogen ions in a solution. When these interactions occur, they cause shifts in electronic structures, resulting in different perceived colors.
In practical laboratory settings, bromcresol green is used less for precise pH measurements and more for quick assessments where pH resolution isn't critical. This makes it an effective tool in educational environments to demonstrate the concept of pH visually.
When using bromcresol green, it is important to know the specific pH range over which the color change occurs. This helps in making accurate predictions and interpretations of the indicator's color at given pH levels.

pH Range

The pH range of an acid-base indicator is a crucial aspect as it determines the specific conditions under which the indicator will change color. For bromcresol green, this range is between pH 3.8 and 5.4.
Within this range, bromcresol green transitions from yellow to green to blue. Each color corresponds to a specific state of the indicator:

• At a pH below 3.8, the solution will appear yellow.
• Between pH 3.8 and 5.4, the indicator transitions to a green color.
• Above pH 5.4, it turns blue.

This means that bromcresol green is ideal for detecting environments where the acidity is moderately low to neutral. Understanding the pH range helps chemists choose the right indicator for their specific application, allowing them to monitor pH changes visually within this window.

Color Change

The color change in acid-base indicators, such as bromcresol green, is due to the protonation and deprotonation of the indicator molecule. At different pH levels, the indicator will surrender or receive hydrogen ions (H⁺), thus altering its molecular structure.
For bromcresol green:

• In acidic solutions (low pH), the excess hydrogen ions cause the indicator to appear yellow.
• In basic solutions (high pH), the lack of hydrogen ions means the indicator appears blue.

When the pH is between 3.8 and 5.4, bromcresol green exhibits a range of colors from yellow to green to blue. At pH 4.6, for example, it will appear greenish as it is halfway transformed between yellow and blue.
This dynamic color transformation not only provides a visual cue but also allows the tracking of pH changes over a specified range, making the indicator an invaluable tool in many chemical and biological experiments.