Problem 40 - Chemistry: The Central Science

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 \(\mathrm{pH}\) 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 its pH is below 6.0 based on the color changes with phenolphthalein and bromthymol blue. (b) The maximum pH of the solution is 6.0, as this is the color change threshold for bromthymol blue. (c) Methyl orange or universal indicator can be used to determine the pH of the solution more precisely, as they have color change ranges around the solution's pH value.

Step 1: Understand the Indicators and their Color Changes

Phenolphthalein is a pH indicator that is colorless in acidic solutions (pH < 8.2) and turns pink in basic solutions (pH > 10). Bromthymol blue is another pH indicator that changes color between different pH ranges; it is yellow below pH 6.0, green at neutral pH (6.0 to 7.6), and blue above pH 7.6.

Step 2: Determine Solution's Acidity or Basicity

The given exercise states that the colorless solution did not undergo any color change when phenolphthalein was added, indicating that the pH of the solution is less than 8.2 (since the solution remained colorless). Additionally, when bromthymol blue was added, the solution turned yellow, meaning the pH of the solution is below 6.0 (as bromthymol blue is yellow below pH 6.0). These observations can be combined to conclude that the solution is acidic, as its pH is less than 6.0.

Step 3: Determine pH Range of the Solution

The color changes of the two indicators provide us insight into the possible pH range of the solution. Since the solution had the following properties: - No color change with phenolphthalein (pH < 8.2) - Yellow color with bromthymol blue (pH < 6.0) We can establish that the pH of the solution has a maximum pH of 6.0.

Step 4: Suggest Suitable Indicator(s) for Precise pH Determination

Considering the solution has a pH below 6.0, we can recommend an indicator that has a color change around that pH range for more precise determination. One such indicator is methyl orange, which changes color from red to yellow between pH 3.1 and pH 4.4. This will help us accurately determine the pH of the acidic solution. Another possibility is the universal indicator, which has a wide range of color changes corresponding to different pH values, thus allowing more precise determination of the solution's pH. In summary, (a) The solution is acidic. (b) The maximum pH of the solution can be established as 6.0. (c) Methyl orange or universal indicator can be used to determine the pH of the solution more precisely.

Key Concepts

Phenolphthalein

Phenolphthalein is a widely used pH indicator known for its distinctive color change in basic solutions. It remains colorless in acidic and neutral solutions, specifically when the pH is below 8.2. However, once the pH increases beyond 8.2, phenolphthalein exhibits a vibrant pink color. This property makes it exceptionally useful for identifying the basicity of a solution.

In practical applications, such as titrations, phenolphthalein signals the endpoint by changing color when the solution shifts from acidic to basic. It is important to note that once the pH exceeds 10, the color deepens to a magenta hue. This specific range of color change limits its use in strongly basic solutions.

• Colorless: pH < 8.2
• Pink: pH between 8.2 and 10
• Magenta: pH > 10

By not causing any color change in the unknown solution, phenolphthalein indicates that the solution's pH is less than 8.2.

Bromthymol Blue

Bromthymol blue is another versatile pH indicator which transitions colors between different pH levels, ranging from acidic to basic. In acidic solutions, bromthymol blue presents a yellow color making it an excellent choice for detecting lower pH environments. As the pH approaches neutral, specifically between 6.0 and 7.6, it shifts to green, and then turns blue in more basic conditions above pH 7.6.

Its property of changing from yellow to blue makes bromthymol blue useful in a variety of experimental setups including educational demonstrations for distinguishing acidic and neutral solutions from basic ones.

• Yellow: pH < 6.0
• Green: pH 6.0 - 7.6
• Blue: pH > 7.6

In the exercise presented, the appearance of a yellow color indicates that the pH of the solution is below 6.0, confirming its acidic nature.

Methyl Orange

Methyl orange serves as a useful pH indicator, particularly effective in identifying strongly acidic environments. It showcases a vivid color change from red in acidic solutions to yellow as the solution becomes less acidic. The transitional pH range for this indicator is between 3.1 and 4.4.

This distinct and narrow range makes methyl orange suitable for precision in a lower acidic pH range. Often used in titrations, it provides clear visual cues for approximating the end point in the context of strong acids and bases.

• Red: pH < 3.1
• Orange: pH between 3.1 and 4.4
• Yellow: pH > 4.4

Choosing methyl orange can help achieve a more accurate pH reading for acidic solutions with a pH below 6.0, complementing the insight gained from indicators like phenolphthalein and bromthymol blue.

Acidic Solutions

Acidic solutions are characterized by a pH level below 7, meaning they have a higher concentration of hydrogen ions \(H^+\) than what is found in neutral water. These solutions can vary widely in strength from very weak acids to strongly acidic substances, which can significantly affect the behavior and outcome of chemical reactions.

Recognizing whether a solution is acidic is invaluable for conducting experiments and understanding chemical properties. Various pH indicators such as phenolphthalein, bromthymol blue, and methyl orange provide visual cues to determine the acidity of a solution.

• Phenolphthalein: colorless in acidic solutions
• Bromthymol blue: yellow in acidic conditions
• Methyl orange: changes from red to yellow below pH 4.4

Utilizing these indicators effectively helps define the acidic character of substances, aiding in scientific investigations.

Universal Indicator

The universal indicator is a comprehensive tool for assessing pH levels, known for its ability to present a spectrum of colors corresponding to a wide range of pH values from very acidic to very basic. This versatility makes it a preferred choice when a broad overview of a solution’s acidity or basicity is required.

Unlike single-range indicators, the universal indicator undergoes multiple color changes as the pH varies, providing an easy-to-read visual representation across the pH scale.

• Red: strongly acidic (< 3)
• Orange/Yellow: weakly acidic (3-6)
• Green: neutral (around 7)
• Blue: moderately basic (8-11)
• Purple: strongly basic (> 11)

When more precise pH measurement is needed within an unknown substance, the universal indicator can be an excellent tool to help pinpoint the solution's characteristics across a broader spectrum. Coupling with other indicators like phenolphthalein and methyl orange enhances its precision in characterizing solutions.