Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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ammonia : NH3
hydrogen peroxide : H2O2
sodium hydroxide : NaOH
hydrochloric acid : HCl
ascorbic acid : C6H8O6
copper sulfate penta hydrate : CuSO4 . 5H2O
Class:
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elem=Cu
coordination
redox
Summary:
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Copper builds a remarkable complex with ascorbic acid and is easily
reduced by ascorbic acid in alkaline environments. Copper (II) builds
a brown compound with hydrogen peroxide in alkaline environments.
Description:
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Add a little amount of a solution of copper sulfate to a solution of ascorbic
acid and sodium hydroxide (effectively being an alkaline solution of sodium
ascorbate): Formation of a blue/green precipitate, which quickly becomes
orange/yellow. Small blue/green particles stick on the glass. After vigorous
shaking all particles have become orange/yellow. The liquid, with the
orange/yellow precipitate dispersed in it is called (Liq1).
Add an excess amount of HCl (10% by weight) to some of Liq1: The liquid
becomes very pale yellow/green and clear. No ordinary copper (II) color,
this yellow/green compound may be a coordination complex with ascorbic
acid or its oxidation product or a compound, consisting of copper (I),
copper (II) and chloride.
Add an excess amount of NH3 (5% by weight) to some of Liq1: The orange
precipitate does not dissolve. This liquid, containing the orange/yellow
precipitate and ammonia is called Liq2.
Slowly add some H2O2 (3% by weight) to some of Liq2: Liquid immediately turns
dark green and turbid. After a few seconds, a vigorous evolution of a gas
starts and a lot of foam is produced (almost going out of the test tube).
When the foam has disappeared again, a clear and moss green liquid remains.
Add much more H2O2 (3% by weight) to the moss green liquid: Liquid becomes
clear and brown.
Very slowly add H2O2 (3% by weight) to some of Liq2 and every time when a
little amount is added, shake well: The liquid first becomes turbid and
greenish (probably yellow particles floating around in a blue liquid), but
at a certain point it has a beautiful deep blue color and it is clear. At
this point all ascorbic acid is oxidized and all copper (I) is oxidized
and there is deep blue copper tetrammine complex [Cu(NH3)4]2+. When more
H2O2 is added slowly, the color gradually shifts from deep blue through
moss green to brown (color of dark tea, with a little greenish hue).