Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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ascorbic acid : C6H8O6
hydrochloric acid : HCl
potassium iodide : KI
ammonium molybdate : (NH4)6Mo7O24 . 4H2O
Class:
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elem=Mo
coordination
redox
Summary:
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Molybdate gives a yellow complex with iodide. Order of dissolving and
acidifying has influence on the reactions observed.
Description:
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Sequence 1:
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Add some solid ammonium molybdate to HCl (10% by weight): The solid slowly
dissolves. The liquid becomes colorless.
Add some KI: The liquid turns pale yellow.
Sequence 2:
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Dissolve some ammonium molybdate in water (results in colorless liquid) and
add this solution to some HCl (10% by weight): The liquid becomes pale
yellow.
Add some solid KI: The solid dissolves and the liquid becomes dark green.
The reaction takes a few minutes.
Add some ascorbic acid: The liquid turns bright blue.
Remark:
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Ammonium molybdate, when dissolved in acid, gives a colorless liquid. When
first dissolved in water and acidified lateron, it becomes pale yellow or
pale yellow/green.
Addition of iodide results in reduction and formation of a blue compound in
the case of the pale yellow liquid. The blue compound is polluted by iodine,
causing the blue color to be 'dirty' or even causing the color of the liquid
to be dark green instead of blue.
When iodide is added to the colorless acidic liquid, then it is not oxidized
quickly, under the formation of a blue compound.