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Chemistry LibreTexts

The Blue Bottle Demonstration

  • Page ID
    3049
  • Chemical Concept Demonstrated

    • Reversible oxidation-reduction reactions

    Demonstration

    • KOH is dissolved in water.
    • Dextrose (glucose) is dissolved in this solution and methylene blue is added.
    reverse.gif

    Observations

    When the indicator is initially added to the solution, the solution turns blue.  The color gradually fades and becomes colorless.  The color reappears when the flask is vigorously shaken.  The color then begins to fade again. 

    Explanations (including important chemical equations)

    The color change results from the reversible oxidation-reduction reaction of the methylene blue indicator.  In alkaline solutions, glucose is oxidized to D-gluconic acid or alpha-D-gluconolactone

    HOCH2(CHOH)4CHO + 3 OH - ---> HOCH2(CHOH)4CO2 + 2 H2O + 2 e -

    In the course of this reaction, methylene blue is reduced from the blue (oxidized) form to the colorless (reduced) form.

    blue.gif

    Shaking the flask dissolves O2 in the solution, which oxidizes the indicator back to the blue (oxidized) form.

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