Description of experiment
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Needed compounds:
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sodium sulfite : Na2SO3
sodium iodide : NaI
formic acid : HCOOH
Class:
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elem=S,I
coordination
Summary:
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Iodide ion and sulphur dioxide form a deep orange/yellow complex at high
concentration. At somewhat lower concentration this complex looks deep yellow.
The complex formed in this experiment is {I.nSO2]-.
Description:
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Take equal amounts of sodium iodide and sodium sulfite and add a small amount
of formic acid (85% by weight) to this mix: Part of the solids dissolves, the
liquid becomes yellow and the solid mass becomes orange.
Heat the liquid for a while: A gas is evolved (probably SO2) and all of the
solids dissolves. The liquid becomes deep orange. A thin layer of the liquid,
sticking to the glass still looks deep yellow. The liquid is clear.
Let liquid cool down: The liquid becomes somewhat turbid and a fine crystalline
mass is separated. It is not possible to see whether the crystals are white or
orange, due to the intense color of the liquid.
Add ten times the amount of water: The liquid becomes deep yellow and remains a
little turbid, also after some shaking. This turbidity most likely is due to
formation of some sodium salt, which separates from the liquid.