Question

Addition of the indicator methyl orange to an unknown solution leads to a yellow color. The addition of bromthymol blue to the same solution also leads to a yellow color. (a) Is the solution acidic, neutral, or basic? (b) What is the range (in whole numbers) of possible pH values for the solution? (c) Is there another indicator you could use to narrow the range of possible pH values for the solution?

Short answer

(a) The unknown solution is acidic. (b) The range of possible pH values for the solution is 4 to 6. (c) An alternative indicator is phenol red, which would help narrow down the pH range if the solution turns yellow when it is added.

Step-by-step solution

Recognizing the color changes for each indicator

Methyl orange changes its color from red (in acidic solution) to yellow (in basic solution) at the pH range of approximately 3.1 to 4.4. Bromthymol blue changes its color from yellow (in acidic solution) to blue (in basic solution) at the pH range of approximately 6.0 to 7.6. The given unknown solution shows a yellow color for both indicators.

Determining the pH range of the unknown solution and its acidity/basicity

Based on the information about the color changes of the indicators, when the unknown solution turns yellow for both methyl orange and bromthymol blue, it suggests that the pH of the solution is within the overlapping pH range for both indicators. The overlapping range for the two indicators is \(4.4 \leq pH \leq 6.0\). (a) Since the pH range of the solution is between 4.4 and 6.0, the unknown solution is acidic. (b) The range of possible pH values for the unknown solution is 4 to 6 in whole numbers.

Finding another indicator to narrow down the range of possible pH values

(c) An alternative indicator that can be used to narrow down the range of possible pH values is one that has a color change within the range of 4 to 6. An example is the indicator phenol red, which changes its color from yellow (in acidic solution) to red (in basic solution) at the pH range of approximately 6.4 to 8.0. By adding phenol red to the unknown solution, we would expect it to turn yellow if the pH of the solution is below 6.4, which would further narrow down the range of possible pH values for the unknown solution.

Key concepts

pH range

The pH range is a numerical scale used to identify how acidic or basic a solution is. The scale ranges from 0 to 14, where 7 is considered neutral. Values below 7 indicate an acidic solution, while values above 7 suggest a basic or alkaline solution.
The range indicates the concentration of hydrogen ions (H⁺) in the solution. An acidic solution has more hydrogen ions, whereas a basic solution has fewer. Understanding the pH range is crucial in chemistry and biology because it affects chemical reactions and biological processes.
In the context of using indicators like methyl orange and bromthymol blue, the observed color change helps determine the pH range of a solution. This can be particularly useful in experiments and practical applications where precise measurement tools are not available.

methyl orange

Methyl orange is a popular acid-base indicator used in titrations due to its clear and distinct color change. It transitions from red to yellow as the solution changes from acidic to slightly basic conditions.
Its effective pH transition range lies between 3.1 and 4.4. Within this range, methyl orange exhibits a noticeable color change:

• Red in highly acidic solutions.
• Yellow in less acidic or near neutral solutions.

In experiments, a yellow color indicates that the pH is beyond 4.4, suggesting the solution is less acidic. Methyl orange is most effective for titrations involving strong acids and weak bases, allowing for precise end-point detection.

bromthymol blue

Bromthymol blue is another common pH indicator, valued for its ability to show a wide range of colors across different pH levels. It changes color from yellow to blue as a solution moves from acidic to basic.
Its pH transition range is approximately from 6.0 to 7.6, making it useful for identifying neutral to slightly basic solutions. The specific color changes are:

• Yellow in acidic conditions.
• Blue in basic conditions.

In practice, if a solution remains yellow when bromthymol blue is added, it suggests that the pH is likely below 6.0. This makes it a preferred choice for experiments needing to distinguish between neutral and weakly acidic solutions.

phenol red

Phenol red is an acid-base indicator used for solutions where a broader pH range needs to be covered. It helps refine pH observations, especially in conjunction with other indicators.
The transition range for phenol red is from 6.4 to 8.0. In this range, phenol red changes from yellow to red:

• Yellow indicates slightly acidic conditions.
• Red indicates basic conditions.

This makes phenol red particularly handy when trying to narrow down the pH of a solution that already reacts to other indicators like methyl orange and bromthymol blue. If added to a solution and it turns yellow, it can confirm a pH below 6.4, providing a more precise answer for experiments requiring accurate pH boundaries.