Description of experiment
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Needed compounds:
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hydrochloric acid : HCl
sodium dithionite : Na2S2O4 . 2H2O
sodium hydroxide : NaOH
ferric chloride : FeCl3 . 6H2O
Class:
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elem=Fe
redox
Summary:
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Dithionite is not capable of reducing ferric oxide/hydroxide to an iron (II)
compound in alkaline environments.
Description:
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Add a solution of ferric chloride to an excess of a solution of sodium
hydroxide: A rusty brown precipitate is formed (hydrous ferric oxide).
Add some solid sodium dithionite: The solid dissolves, the rusty brown
precipitate does not change.
Add an excess amount of HCl (10%): The precipitate dissolves. The liquid
becomes slightly turbid and pale yellow. Sulphur is separated from the
liquid and a strong smell of SO2 can be observed. It is not clear whether
all iron (III) is reduced to iron (II) or not.