Description of experiment
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Needed compounds:
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sulphuric acid : H2SO4
vanadyl sulfate : VOSO4.5H2O
Class:
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elem=V
coordination
Summary:
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Vanadyl sulfate n-hydrate is dehydrated by hot concentrated sulphuric acid,
giving insoluble solid anhydrous VOSO4. This solid does not dissolve in the
concentrated acid, nor in water.
Description:
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Add some solid deep blue VOSO4.nH2O to concentrated sulphuric acid and strongly
heat the test tube, while shaking (CAREFUL, BEWARE OF BUMPING!): The solid
partially dissolves. First, the contents of the test tube is dark blue, but at
a certain point it becomes very turbid and pale blue. On even stronger heating
it becomes somewhat grey.
Let the liquid cool down and then add 20 times its volume of cold water: The
acid mixes with the water, and a light blue solution is obtained, which is very
turbid. Allowing the solution to stand for a few hours results in a light blue
solution, with a grey finely powdered precipitate (like grey concrete, no blue
hue at all).
Decant as much as possible of the light blue solution from the precipitate and
add a new amount of water and shake well: A grey turbid liquid is obtained.
Set this liquid aside for one day: After one day the liquid is clear and very
light blue. At the bottom, there is a layer of grey solid. Just above the
solid, there is a cm of somewhat deeper blue color.
Apparently, the grey solid does dissolve in water, but very slowly. It first
needs to be hydrated, which is a very slow process, and after this hydration,
it can dissolve, giving the familiar blue color of the vanadyl sulfate
solution.