Skip to main content
Library homepage
 
Chemistry LibreTexts

The Iodine Clock or Landolt Reaction

  • Page ID
    222027
  • \( \newcommand{\vecs}[1]{\overset { \scriptstyle \rightharpoonup} {\mathbf{#1}} } \) \( \newcommand{\vecd}[1]{\overset{-\!-\!\rightharpoonup}{\vphantom{a}\smash {#1}}} \)\(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\) \(\newcommand{\id}{\mathrm{id}}\) \( \newcommand{\Span}{\mathrm{span}}\) \( \newcommand{\kernel}{\mathrm{null}\,}\) \( \newcommand{\range}{\mathrm{range}\,}\) \( \newcommand{\RealPart}{\mathrm{Re}}\) \( \newcommand{\ImaginaryPart}{\mathrm{Im}}\) \( \newcommand{\Argument}{\mathrm{Arg}}\) \( \newcommand{\norm}[1]{\| #1 \|}\) \( \newcommand{\inner}[2]{\langle #1, #2 \rangle}\) \( \newcommand{\Span}{\mathrm{span}}\)\(\newcommand{\AA}{\unicode[.8,0]{x212B}}\)

    Chemical Concepts Demonstrated

    • Kinetics
    • Concentration effects on reaction rate

    Demonstration

    The 5 beakers contain 4 solutions in varying amounts: The solutions are poured into the beakers simultaneously.
    • H2O
    • Starch solution
    • KIO3 solution
    • Na2SO3

    Beaker 5 contains the most amount of water and the least amount of KIO3 solution; Beaker 1 is the opposite. The remaining beakers are in a series relative to these two.

    lando.gif

    Observations

    The beakers turn blue-black in a specific order.

    Explanation (including important chemical equations)

    This demonstration is based on the following mechanism:

    1. IO3-(aq) + 3 HSO3-(aq) --> I -(aq) + 3 SO42-(aq) + 3 H+(aq)

    2. IO3-(aq) + 5 I -(aq) + 6 H+(aq) --> 3 I2 (aq) + 3 H2O (l)

    3. I2 (aq) + HSO3-(aq) + H2O (l) --> 2 I -(aq) + SO42-(aq) + 3 H+(aq)

    4. I2 (aq) + I -(aq) + starch (aq) --> starch-I3-(aq)

    Because steps 1 and 2 are much slower than step 3, no I2 accumulates as long as HSO3- remains in solution. When all of the HSO3- is consumed, I2 accumulates and the blue-black starch-triiodide complex is formed in step 4.

    Contributors


    The Iodine Clock or Landolt Reaction is shared under a not declared license and was authored, remixed, and/or curated by LibreTexts.

    • Was this article helpful?