Question

When methyl red is added to an aqueous solution, a pink color results. When methyl orange is added to the same solution, a yellow color is produced. What is the approximate pH range of the solution? Use Figure \(18.24 .\)

Short answer

The approximate pH range of the solution is between 6 and 8, as determined by the observed color changes of both methyl red (pink) and methyl orange (yellow).

Step-by-step solution

Analyze the color change of methyl red

When methyl red is added to the aqueous solution, a pink color results. According to Figure 18.24, methyl red exhibits a pink color in solutions with pH values above 6 (approximately). So, we know that the pH of the solution is greater than 6.

Analyze the color change of methyl orange

When methyl orange is added to the same solution, a yellow color is produced. Based on Figure 18.24, methyl orange exhibits a yellow color in solutions with pH values above 4.4 (approximately). So ,we know that the pH of the solution is greater than 4.4.

Determine the pH range

Since the solution turns pink with methyl red and yellow with methyl orange, we now know that the pH of the solution is above 6 (due to the methyl red color change) and above 4.4 (from the methyl orange color change). Combining these constraints, the approximate pH range of the solution is between 6 and 8. So, the approximate pH range of the solution is 6 - 8.

Key concepts

Acid-Base Indicators

Understanding acid-base indicators is crucial for deciphering the mysteries of pH in solutions. These are unique substances that exhibit different colors depending on the pH level of the environment. Think of them as pH detectives; when dropped into a solution, they provide visual clues about the acidity or basicity of that solution by changing their color. For example, litmus is a commonly known indicator that turns red in acidic solutions and blue in basic ones. However, there are many more indicators, such as phenolphthalein, which is colorless in acidic solutions but turns pink in basic solutions.

Indicators are made from organic dyes and each has a specific pH range at which their color change occurs. The change is not random—it's a result of chemical reactions happening within the molecule of the indicator in response to the hydrogen ion concentration in the surrounding solution. The design of an experiment often calls for a particular indicator that will provide the clearest indication of pH change within the desired range, as seen in the exercise involving methyl red and methyl orange.

pH Scale

The pH scale is a numerical scale used to specify how acidic or basic an aqueous solution is. It ranges from 0 to 14, with 7 being neutral. pH is actually a measure of the relative amount of free hydrogen and hydroxyl ions in the water. A pH less than 7 signifies an acidic solution, while a pH greater than 7 indicates a basic or alkaline solution.

To get a bit technical, the pH is actually the negative logarithm (base 10) of the hydrogen ion concentration: \( pH = -\text{log}[H^+] \). That's why a small change in pH can represent a significant change in the concentrations of ions in the solution. Consider it a way to make very large or very small numbers more manageable and interpretable when we talk about the acidity or basicity of a solution. In the context of our textbook exercise, understanding the pH scale helped us to approximate the pH range of the solution using the color change presented by two different pH indicators.

Color Change in pH Indicators

pH indicators are not just one-trick ponies; they can exhibit a range of colors over a span of pH values. This range is known as the color transition range. Within this range, a gradual change occurs from the acid color to the alkaline color of the indicator. For instance, methyl red transitions from red (in acidic solution) to yellow (in basic solution). The color change typically occurs over a narrow pH range, making certain indicators more suitable for detecting a specific pH.

During the color transition range, what you are observing is the mixture of the colors of the acid and base forms of the indicator, which explains why you might see an intermediate color such as orange, between red and yellow, for methyl red in a pH-titration process. In our example, methyl red showed pink, an indicator of its transition nearing the basic side, and methyl orange showed yellow, hinting at a pH above its transition point but not yet fully basic. This illustrates how valuable the color change in pH indicators can be for pinpointing the pH of a solution.