Problem 39 - Chemistry The Central Science In Si Units

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?

Step-by-Step Solution

Answer

(a) The solution is acidic. (b) The pH range is 4.5 to 5.9. (c) Consider using phenol red.

Step 1: Understand the Color Indicators

Both methyl orange and bromthymol blue are pH indicators. Methyl orange is yellow at pH values above 4.4, turning red below this pH. Bromthymol blue is yellow below pH 6.0 and turns blue above pH 7.6. Our solution is yellow with both indicators.

Step 2: Analyze pH Based on Given Data

The solution being yellow with methyl orange implies it is above pH 4.4. Being yellow with bromthymol blue indicates it is below pH 6.0. Thus, the pH of the solution must be greater than 4.4 and less than 6.0.

Step 3: Determine acidity, neutrality, or basicity

pH below 7 is acidic, 7 is neutral, and above 7 is basic. Since the pH of this solution is between 4.4 and 6.0, it is acidic.

Step 4: Consider Another Indicator

To narrow the pH range, we could consider an indicator with a range overlapping our current range, like phenol red, which transitions from yellow to red between pH 6.8 and 8.4. However, it is better to use one at the lower end of the transition range, like thymol blue, which covers 1.2 - 2.8 and 8.0 - 9.6, or Alizarin yellow, operating from 10.0 - 12.0.

Key Concepts

Methyl Orange

Methyl orange is a well-known pH indicator that is particularly sensitive within a specific pH range. It is one of the most common indicators used in titrations.
Methyl orange operates effectively within the pH range of 3.1 to 4.4.

• When the pH of a solution is lower than 3.1, methyl orange exhibits a red hue.
• As the pH surpasses 4.4, the color transitions to yellow.

This property makes methyl orange an excellent choice for detecting slightly acidic environments.
When methyl orange is added to a solution and shows a yellow coloration, it confirms that the pH is above 4.4, indicating that the solution is not very acidic. Understanding this color transition is crucial in determining the acidic, neutral, or basic nature of a solution using common laboratory indicators.

Bromthymol Blue

Bromthymol blue is another reliable pH indicator extensively used in chemistry labs. It provides a clear visual signal when analyzing the pH of a solution.
This indicator changes color across the pH spectrum of 6.0 to 7.6.

• Below a pH of 6.0, bromthymol blue appears yellow, indicating acidity.
• Between 6.0 and 7.6, it turns green, representing neutrality.
• Above 7.6, it shifts to a blue color, indicating a basic solution.

When the solution remains yellow with bromthymol blue, we can deduce that the pH is below 6.0. This means the solution is still within the acidic range. By combining observations from both methyl orange and bromthymol blue, we can narrow down the pH more effectively.

Phenol Red

Phenol red is a versatile indicator that helps in identifying a broader range of pH values. Used widely in biological and chemical testing, it provides a visible change in color between pH 6.8 and 8.4.
Here's how phenol red reacts across its pH range:

• Below pH 6.8, phenol red takes on a yellow color, just like when methyl orange and bromthymol blue indicate acidity.
• Above pH 8.4, it shifts to a more intense red shade.

Phenol red’s transition range overlaps slightly with the ranges of bromthymol blue. If one needs to confirm whether the pH value lies slightly closer to the neutral mark or to further confirm acidity, phenol red can be a viable choice.
It is, however, important to remember that phenol red might not be ideal for narrowing down the pH within the acidic range, which often requires indicators sensitive to lower pH levels.

Acidic Solutions

In chemistry, an acidic solution is defined by its pH value being less than 7.
The pH scale, which ranges from 0 to 14, measures the acidity or basicity of a solution.

• A pH less than 7 indicates acidity.
• A pH of 7 is considered neutral, like pure water.
• A pH greater than 7 represents a basic or alkaline environment.

Our analysis of the solutions with methyl orange and bromthymol blue shows both indicators turning yellow. This suggests the solution’s pH lies between 4.4 and 6.0.
This indicates that the solution is indeed acidic.
Knowing the specific pH value of an acidic solution helps scientists and chemists determine the behavior and reactivity of compounds in various environments.