Description of experiment
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Needed compounds:
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sodium sulfite : Na2SO3
hydrochloric acid : HCl
sodium hydroxide : NaOH
potassium iodide : KI
sulphuric acid : H2SO4
ammonium molybdate : (NH4)6Mo7O24 . 4H2O
Class:
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elem=Mo
redox
Summary:
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Molybdate gives rise to all kinds of complexes, when combined with reducing
agents. These complexes can be yellow, green or blue. The composition of
all these complexes is not very clear.
Description:
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Add a solution of ammonium molybdate to a strong solution of sulfuric acid
(appr. 20% by weight): The liquid becomes pale yellow/green and clear. This
liquid is called LIQ1.
Add some KI to LIQ1: The solid dissolves and the liquid slowly turns darker
from pale yellow/green to dark blue/grey. The total change of color takes
approximately 10 minutes. The final color is not a pure blue, but 'dirty'
blue. This liquid is called LIQ2.
Add an excess amount of sodium hydroxide to LIQ2: The solid dissolves.
The intensity of the color of the liquid becomes less. The color shifts
towards green. When even more NaOH is added, then the color shifts back
to blue again. It is less intense than before, but still it is 'dirty' blue.
Add an excess amount of HCl (10% by weight): The liquid becomes clear and
pale blue/green.
Add some sodium sulfite to LIQ2: The color of the liquid changes from 'dirty'
blue to bright blue. The intensity of the color does not change very much.
Add an excess amount of NaOH: The solid dissolves and the intensity of the
blue color becomes much less. The color remains bright, it does not turn into
a greyish 'dirty' blue.
Add an excess amount of sodium sulfite to LIQ1: The solid dissolves and the
liquid becomes colorless.
Add some solid KI: This dissolves and the liquid turns deep yellow.
Remark: The yellow color in the solutions with iodide and sulfite is NOT
due to the presence of molybdenum, but it is due to formation of a complex
of iodide and sulphur dioxide. This complex is not well-known!