Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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manganese (IV) oxide : MnO2
sodium sulfite hepta hydrate : Na2SO3 . 7H2O
sodium persulfate : Na2S2O8
hydrochloric acid : HCl
hydrogen peroxide : H2O2
sodium hydroxide : NaOH
metol : (CH3 NH C6H4 OH)2 . H2SO4
pyrogallol : C6H3 (OH)3
hydroquinone : HO C6H4 OH
Class:
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elem=C
redox
Summary:
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Photography developers, based on phenol-like structures, are easily oxidized
by air in alkaline environments and the oxidation products are such, that
a reverse process does not occur anymore (probably the oxidation products
are large polymerized species).
Description:
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Sequence 1
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Add some solid NaOH to a solution of hydroquinone: The solid dissolves, the
liquid slowly turns darker. Finally, the liquid is almost black (very dark
brown).
Add an excess amount of H2O2 (3% by weight): Liquid hardly changes, it becomes
intense dark red/brown (dilution of dark brown liquid). On heating to appr.
60 C it becomes slightly lighter.
Add some MnO2: Liquid starts bubbling (this means that not all H2O2 was used
up).
Add an excess amount of HCl (10% by weight): The liquid starts bubbling
more vigorously, the liquid turns much lighter now.
Add some solid Na2SO3.7H2O: The liquid becomes less opaque (part of the MnO2
dissolves and disappears, reduced by sulfite). The color of the liquid does
not get much lighter anymore.
Sequence 2
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Add some solid NaOH to a solution of pyrogallol: The solid dissolves, the
liquid turns dark quickly. The liquid becomes almost black.
The darkening of the liquid is much faster than when hydroquinone
is used.
Add an excess amount of HCl (10% by weight): Liquid turns much lighter. Its
color becomes orange/brown.
Add some solid Na2S2O8: The solid dissolves, no further visible changes.
Sequence 3
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Add some solid NaOH to a solution of metol: The solid dissolves, the
liquid slowly turns dark (almost black). The darkening of the liquid
proceeds very slowly, compared to the speed, which could be observed
with hydroquinone. The darkening goes through an intermediate stage,
where the liquid has a red/purple color. When the liquid is dark, the
liquid has a strong unpleasant smell (not suffocating or irritating).
Add an excess amount of HCl (10% by weight): Liquid does not become
much lighter. The smell disappears.
Add some solid Na2S2O8: The solid dissolves, no further visible changes.
Remarks:
All three developers are based on a phenol-molecule, with an additional
hydroxy or amino group attached to it. Compounds having this structure
apparently polymerize when oxidized in strongly alkaline environments.
This polymerization causes the liquid to darken to almost blackness.
For the pure-hydroxy compounds these polymers apparently are broken
partially by acid, for the amino-compounds this is not the case.