Description of experiment
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Needed compounds:
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ammonia : NH3
sodium hydroxide : NaOH
urea : NH2 CO NH2
copper sulfate penta hydrate : CuSO4 . 5H2O
Class:
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elem=Cu
precipitation
coordination
Summary:
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Copper builds a complex with urea, which has a pale purple color (or is
this due to contamination of the urea with traces of biuret?). The copper,
bound to urea, does not build the intense blue cuprammine complex with
ammonia.
Description:
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Dissolve some copper sulfate in water and add an excess amount of a solution
of sodium hydroxide: Formation of a bright blue precipitate.
Add a large excess amount of urea: The liquid turns pale purple. The largest
part of the precipitate remains present, it does not dissolve. Apparently a
small part of the precipitated copper (II) forms a complex with urea (or
with biuret, due to impurities in urea??).
Add an excess amount of ammonia (5% by weight): No visible change of color.
Add a large amount of water: Still no visible change, other than dilution
of the pale color. The liquid becomes almost colorless.
Add an excess amount of hydrochloric acid (10% by weight) to the pale purple
liquid, which is formed after adding urea to the copper hydroxide precipitate:
The liquid becomes almost colorless.