The most suitable indicator to be used when reacting ethanedioic acid with potassium hydroxide is

1. Analyze the question

The question asks for the most suitable indicator for a titration between ethanedioic acid (also known as oxalic acid, $H_2C_2O_4$) and potassium hydroxide ($KOH$).

2. Explain the concept

To choose the right indicator, we must identify the type of acid-base reaction:

• Ethanedioic acid ($H_2C_2O_4$): This is a weak acid.
• Potassium hydroxide ($KOH$): This is a strong base.

When a weak acid reacts with a strong base, the resulting salt undergoes hydrolysis, making the solution at the equivalence point alkaline (basic), typically with a pH greater than 7 (usually around 8–9).

Indicators change color over a specific pH range. We need an indicator that changes color within the pH range of the equivalence point:

• Phenolphthalein: Changes color in the pH range of 8.2 to 10.0. This perfectly matches the alkaline equivalence point of a weak acid-strong base titration.
• Methyl orange/Methyl red: These change color in the acidic range (pH 3.1–4.4 and 4.4–6.2, respectively), making them unsuitable for this specific reaction because they would change color before the equivalence point is reached.

3. Show the solution

• Step 1: Identify the reactants. Ethanedioic acid is a weak organic acid, and potassium hydroxide is a strong inorganic base.
• Step 2: Determine the nature of the salt formed. The reaction produces potassium ethanedioate and water. Because the base is stronger than the acid, the salt solution is basic.
• Step 3: Match the indicator to the pH. Since the equivalence point is in the basic region (pH > 7), we require an indicator that functions in the basic range.
• Step 4: Evaluate the options. Phenolphthalein is the only indicator listed that operates effectively in the basic pH range (8.2–10.0).

4. Identify the answer

5. Exam Tip

A simple rule of thumb for choosing indicators:

• Strong Acid + Strong Base: Any indicator (Phenolphthalein or Methyl Orange).
• Strong Acid + Weak Base: Methyl Orange.
• Weak Acid + Strong Base: Phenolphthalein.