Acid-Base Indicators and pH Color Changes

Q1. How does the color of methyl red indicator change with pH, and what does this indicate about the relative concentrations of its acid (HX) and base (X−) forms?

Background

Topic: Acid-Base Indicators

This question is about how acid-base indicators, such as methyl red, change color depending on the pH of the solution. The color change is due to the equilibrium between the protonated (acid, HX) and deprotonated (base, X−) forms of the indicator.

Key Terms and Concepts:

• Indicator: A weak acid or base that changes color depending on the pH of the solution.

• HX: The protonated (acidic) form of the indicator.

• X−: The deprotonated (basic) form of the indicator.

• pH: A measure of the hydrogen ion concentration in a solution.

• pKa: The pH at which the concentrations of HX and X− are equal.

Key Formula:

$\mathrm{HX} \rightleftharpoons \mathrm{H}^+ + \mathrm{X}^-$

$\mathrm{pH} = \mathrm{p}K_a + \log\left(\frac{[\mathrm{X}^-]}{[\mathrm{HX}]}\right)$

Step-by-Step Guidance

1. Recall that methyl red is an indicator that changes color over a specific pH range (approximately pH 4 to 6).

2. At low pH (acidic conditions), the concentration of HX (the acid form) is much greater than X−, so the solution appears red.

3. At the midpoint of the color change (around the pKa), the concentrations of HX and X− are approximately equal, and the solution appears orange.

4. At high pH (basic conditions), the concentration of X− (the base form) is much greater than HX, so the solution appears yellow.

5. Use the Henderson-Hasselbalch equation to relate the pH to the ratio of [X−] to [HX].

Try solving on your own before revealing the answer!

Final Answer:

The color of methyl red changes from red at low pH (where [HX] >> [X−]), to orange at pH ≈ pKa (where [HX] ≈ [X−]), to yellow at high pH (where [HX] << [X−]).

This color change visually indicates the relative concentrations of the acid and base forms of the indicator as the pH changes.