Description of experiment
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Needed compounds:
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hydrochloric acid : HCl
ferric chloride : FeCl3 . 6H2O
hydroquinone : HO C6H4 OH
Class:
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elem=C,Fe
precipitation
coordination
redox
Summary:
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Hydroquinone can be oxidized by ferric chloride. If not too much ferric
chloride is added, then a coordination complex of the oxidation product
with hydroquinone is formed and crystals of this compound separate
from the liquid.
Description:
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Add a small amount of a solution of ferric chloride, acidified with a few
drops of hydrochloric acid in order to make the solution clear, to a
solution of hydroquinone: As soon as the iron (III) solution touches the
solution of hydroquinone, a green compound appears, which quickly dissolves
again. After the reaction, the liquid becomes yellow/brown and a little
bit opaque (milky).
Add a little more of a slightly acidified solution of ferric chloride: The
same effect is observed, a temporary appearance of a green substance,
which quickly disappears again. After shaking, the resulting liquid is
brown. After a few minutes of standing, however, little needle-like crystals
with a glittering metallic lustre appear. These needles grow, until a
network of needles/wires has grown throughout the liquid. Just a few minutes
later all wires/needles move to the bottom of the glass container, resulting
in a layer of many glittering needles. When the liquid is shaken, all
needles go up into the liquid and they quickly go down to the bottom of
the glass container (this takes just a few seconds). The color of the
needles is hard to observe, because of their metallic appearance. Their
color appears to be something between dark grey and dark green, depending
on the light source (SL-lamp: dark green, ordinary bulb: dark grey/black).
The liquid above the needles is clear and its color is pale yellow/green.
Add a lot of water (approximately 7 times the volume of the pale yellow/green
liquid): The needles move throughout the liquid and they slowly go back to
the bottom. While they are moving through the water, they slowly dissolve.
Most of the needles do not reach the bottom of the glass container, before
they do so, they have dissolved in the water. Finally the liquid is clear and
its color is pale yellow/green/brown.
Remark: The green needles probably are a coordination complex between
hydroquinone and quinone (an oxidation product of hydroquinone), which
does not dissolve in water as well as hydroquinone and quinone.