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Why is Agar Pink?

Published in Chemistry Color Indicators 2 mins read

The pink color in certain agar preparations is due to the presence of a specific pH indicator.

Understanding the Pink Color of Agar

The pink color you see in some agar experiments isn't from the agar itself, but rather from a chemical reaction involving a pH indicator. Here's a breakdown:

  • The Key Ingredient: Phenolphthalein - This chemical acts as a pH indicator, meaning it changes color depending on the acidity or alkalinity (basicity) of a solution.
  • How it Works -
    • In acidic conditions, phenolphthalein is colorless.
    • In basic or alkaline conditions, phenolphthalein turns a vibrant pink.
  • The Role of Sodium Hydroxide (NaOH) - The pink solution often contains sodium hydroxide (NaOH), a strong base. This base raises the pH of the solution.

The Process

  1. Preparation: A sodium hydroxide solution is prepared, and phenolphthalein is added to it. This solution turns pink.
  2. Agar Diffusion: When this pink solution diffuses into the agar, it carries the indicator. If the agar is neutral or basic, the phenolphthalein remains pink.
  3. Observation: The pink color allows students to visually track how far the alkaline solution has diffused into the agar cube. This process is often used to understand the rate and extent of diffusion within a solid medium like agar.

Visualizing Diffusion

Component pH Condition Color Role in Experiment
Phenolphthalein Acidic Colorless pH indicator
Phenolphthalein Basic Pink Indicates diffusion of the alkaline solution
Sodium Hydroxide (NaOH) Basic N/A Creates alkaline environment, causing phenolphthalein to turn pink

Practical Insights

  • This method is used to visually demonstrate diffusion because the pink color is easily visible.
  • By slicing the agar, students can see the internal progression of the diffusing solution.