Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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sodium sulfite : Na2SO3
sodium hydroxide : NaOH
hydroquinone : HO C6H4 OH
Class:
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elem=C,S
photography
redox
Summary:
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Hydroquinone is a very strong reductor in alkaline environments. Oxygen
from the air is readily absorbed by an alkaline solution of hydroquinone.
Sulfite, however, makes an alkaline solution of hydroquinone much more
stable, because of formation of special hydroquinone/sulfite compounds.
Description:
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Sequence 1
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Add some solid hydroquinone to a dilute solution of sodium hydroxide (just
three 2mm size granules of NaOH in 5 ml water): The solid hydroquinone
dissolves and the liquid turns dark brown.
Add some sodium sulfite to the dark brown liquid: The sodium sulfite
dissolves, the liquid remains dark brown. The sulfite is not capable
of undoing the effect of oxidation of the hydroquinone.
Sequence 2
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Dissolve some solid hydroquinone in a solution of NaOH, with a little crystal
of sodium sulfite also dissolved in the NaOH solution: The hydroquinone
dissolves. The liquid now only has a pale yellow color after dissolving
of the hydroquinone. The sulfite protects the hydroquinone from oxidation
by oxygen from the air.