Problem 40 - Chemistry: The Central Science in SI Units, Global Edition

Question

Addition of phenolphthalein to an unknown colorless solution does not cause a color change. The addition of bromthymol blue to the same solution leads to a yellow color. (a) Is the solution acidic, neutral, or basic? (b) Which of the following can you establish about the solution: (i) A minimum \(\mathrm{pH}\), (ii) A maximum \(\mathrm{pH}\), or (iii) A specific range of pH values? (c) What other indicator or indicators would you want to use to determine the \(\mathrm{pH}\) of the solution more precisely?

Step-by-Step Solution

Answer

(a) The solution is acidic as the pH is below 6.0. (b) (i) Insufficient information is available to infer the minimum pH value. (ii) The maximum pH of the solution is 6.0. (iii) We cannot determine the specific range of pH values without additional information. (c) To determine the pH more precisely, litmus paper (range 4.5-8.3, red to blue) or methyl orange (range 3.2-4.4, red to yellow) can be used as indicators.

Step 1: (a) Determine the solution's nature (acidic, neutral, or basic)

Given that phenolphthalein did not cause any color change, it confirms that the pH of the solution is below 8.2. On the other hand, bromthymol blue turned the solution yellow, meaning the pH of the solution must be below 6.0. Considering both indicators, the unknown solution must have a pH below 6.0. As a result, the solution is acidic.

Step 2: (b) (i) Determine the minimum \(\mathrm{pH}\)

Since the given indicators did not reach a lower bound, there is insufficient information to infer the minimum pH value for the unknown solution.

Step 3: (b) (ii) Determine the maximum \(\mathrm{pH}\)

Bromthymol blue displayed a yellow color which indicates that the pH is less than 6.0. Hence, the maximum pH of the solution is 6.0.

Step 4: (b) (iii) Determine the specific range of pH values

In this case, the indicators provide us with partial information on the range of pH values. The solution's pH is less than 6.0, but we cannot determine the specific range of pH values without additional information.

Step 5: (c) Choosing other indicator or indicators

To determine the pH of the solution more precisely within the acidic range, indicators with a color change range that covers lower pH values are required. Some examples include litmus paper, which has a color change range between 4.5-8.3 (red to blue), or methyl orange, with a pH change range between 3.2-4.4 (red to yellow).

Key Concepts

Phenolphthalein

Phenolphthalein is a popular pH indicator used to detect changes in pH during titrations. It is particularly helpful in identifying basic solutions. When added to a solution, phenolphthalein is colorless in acidic to neutral conditions, generally below pH 8.2. However, as the pH of the solution rises and becomes slightly alkaline (around pH 8.2 to 10), it changes to a pink or fuchsia color.

Important points about phenolphthalein include:

• Colorless in solutions with a pH below 8.2.
• Turns pink or fuchsia as the pH reaches 8.2 and above.
• Not suitable for measuring the pH in strongly acidic solutions.

In acidic or neutral solutions, such as the case described in the exercise where no color change occurred, phenolphthalein remains colorless. This confirms that the pH of the unknown solution is below the threshold of phenolphthalein's active range, highlighting the need for other indicators to determine precise pH levels.

Bromthymol Blue

Bromthymol blue is another widely used pH indicator. It is particularly effective in the neutral pH range. This indicator shifts its color from yellow to blue as the pH changes from acidic to basic. Specifically, bromthymol blue is yellow in acidic solutions with a pH below 6.0, and turns blue when the pH rises above 7.6.

Key features of bromthymol blue include:

• Yellow when pH is less than 6.0.
• Greenish near a neutral pH of 7.0.
• Turns blue as the pH exceeds 7.6.

This indicator is useful for general testing and provides a visual representation of whether a solution is leaning towards being acidic or basic. In the exercise, bromthymol blue turns the solution yellow, confirming that the solution is indeed acidic, with a pH below 6.0. Bromthymol blue helps narrow down pH ranges effectively, but, similar to phenolphthalein, it is best complemented with other indicators for precise measurements.

Acidic Solutions

Acidic solutions, by definition, have a pH less than 7.0. They are characterized by the presence of a higher concentration of hydrogen ions (H⁺) compared to hydroxide ions (OH⁻). These solutions can range from being mildly acidic, such as lemon juice with a pH around 2, to extremely acidic, like battery acid with a pH close to 0.

To better understand acidic solutions, it's important to consider:

• The exact pH determines the acidic strength.
• Common indicators for acidic solutions include phenolphthalein, bromthymol blue, and methyl orange.
• Choosing the right indicator depends on the pH range within the acidic scale that needs to be measured.

For the exercise in question, since both phenolphthalein and bromthymol blue confirm the presence of an acidic solution without specifying the precise pH level, additional indicators like methyl orange could be employed. Methyl orange has a sharp color transition between pH 3.2 and 4.4, turning from red to yellow as the pH increases, which would help find the specific pH within this acidic territory.