Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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sodium thiocyanate : NaSCN
hydrogen peroxide : H2O2
sodium hydroxide : NaOH
ascorbic acid : C6H8O6
hydrochloric acid : HCl
ferric chloride : FeCl3 . 6H2O
Class:
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elem=Fe
coordination
precipitation
redox
Summary:
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The experiment described below suggests the formation of a coordination
complex between iron and ascorbate. The presence of the ascorbate induces
a completely different behavior of ferric/ferrous ions in alkaline
environments.
Description:
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Sequence 1
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Add an excess amount of a solution of ascorbic acid to a solution of
ferric chloride, which was slightly acidified with hydrochloric acid in
order to keep it clear: The liquid becomes colorless. Is the ferric
ion reduced to ferrous ion?
Add an excess amount of a solution of NaOH: Formation of a grey/green
precipitate, which quickly dissolves again (in a second try of this
experiment, this precipitate was not observed). The liquid turns dark
green/brown and slowly turns darker, until it is black. When spread,
the liquid looks dark grey/black.
Add an excess amount of H2O2 (3% by weight): Liquid turns intense red/
brown (looks like diluted blood) and is almost clear. When the liquid
is shaken, it develops gas bubbles.
Sequence 2
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Add an excess amount of a solution of ascorbic acid to a solution of
ferric chloride, which was slightly acidified with hydrochloric acid in
order to keep it clear: The liquid becomes colorless.
Add H2O2 (3% by weight): The liquid becomes yellow/brown (color of a
normal solution of acidified ferric chloride). Apparently the colorless
liquid contains ferrous ions, which are almost colorless at fairly low
concentrations.
Sequence 3
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Add an excess amount of a solution of slightly acidified ferric chloride
to a solution of NaSCN: The liquid turns intense red and looks like blood.
Add an excess amount of ascorbic acid: The solid dissolves and while it
dissolves, the liquid becomes colorless. The ascorbic acid removes free
ferric ions and doing so, it forces the equilibrium between Fe3+ and
the blood red [FeSCN]2+ to the side of Fe3+ completely. This experiment
shows that ascorbic acid removes all Fe3+ from a solution, probably
reducing it to Fe2+ (or coordinating to it, forming a colorless complex).
Add a solution of NaOH: The liquid turns dark green and slowly darkens,
until it is almost black. The NaOH is not capable of liberating Fe3+, such
that the blood red complex [FeSCN]2+ is formed again.
Add some H2O2 (3%) to the colorless liquid, containing thiocyanate: Where
the drops of H2O2 touch the liquid, a red spot appears, which disappears on
shaking. When much more H2O2 is added, the red color persists and the liquid
turns blood red again. Apparently H2O2 oxidizes the ascorbic acid and/or
ferrous ions, such that free ferric ions appear again, which coordinate to
thiocyanate, forming the blood red [FeSCN]2+.