Description of experiment
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Needed compounds:
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ammonia : NH3
iodine : I2
potassium iodide : KI
Class:
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elem=I,N
redox
Summary:
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Ammonia reacts with iodine, giving nitrogen iodide, coordinated to ammonia.
When acetone is present, however, then a so called haloform reaction
occurs, resulting in formation of CHI3.
Description:
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Experiment 1:
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Dissolve some iodine in a concentrated solution of potassium iodide and add
some ammonia (25% by weight) to the dark red/brown solution: A dark grey
precipitate is formed. This precipitate has constitution NI3.nNH3, where n
is a non-stoichiometric number, depending on concentration of ammonia, iodine
and other reaction conditions. The value of n is at least equal to 1.
Experiment 2:
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Dissolve some iodine in a concentrated solution of potassium iodide and add
some acetone: No reaction. When the liquid is shaken, the acetone mixes with
the aqueous liquid.
Add some ammonia (25%): Formation of a dark grey precipitate, just as in
experiment 1. After a few seconds, however, the color of this precipitate
changes. Approximately 15 seconds later the color has shifted from dark grey
to pale yellow (like sulphur). The yellow precipitate is iodoform (CHI3).