Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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hydrogen peroxide : H2O2
sodium dithionite : Na2S2O4 . 2H2O
sulphuric acid : H2SO4
ammonium molybdate : (NH4)6Mo7O24 . 4H2O
potassium metabisulfite : K2S2O5
Class:
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elem=Mo
redox
Summary:
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Molybdate forms blue or green compounds, when reduced with mild reducing
agents or when little quantitities of reducing agents are used.
Description:
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Add some solid K2S2O5 to a solution of ammonium molybdate in H2SO4 (1 mol/l):
The liquid turns blue, but this reaction takes several hours at appr. 15 C.
When the solid is added, at first glance nothing appears to happen besides
the dissolving of the solid and the formation of SO2 (pungent smell). After
a few days, white crystals have settled at the bottom and the liquid is blue.
Sequence 1 (based on white crystals):
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Decant the blue liquid from the white crystals. Keep the blue liquid aside and
add some water to the white crystals: The white crystals do not dissolve
(at least not visibly). After appr. 30 hours, however, the crystals have
dissolved and the liquid has becomes green with a bluish hue.
Add some sodium dithionite to the green liquid: The liquid becomes intensely
colored dark blue, almost black.
Add some H2O2 to the dark blue liquid: When some H2O2 is added to this liquid,
then it becomes lighter and its color shifts from blue to blue with a green
hue. The color, however, remains fairly intense.
Add an excess amount of H2O2 (10%) to the green liquid: The blue hue disappears,
the liquid attains a more pure green colour.
Sequence 2 (based on blue liquid of the initial experiment with K2S2O5):
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Add an excess amount of H2O2 (appr. 10%): The liquid becomes green. The change
of color from blue to green is immediate. After appr. 1 day the liquid has
become pale yellow.