Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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hydroxyl amine sulfate : (NH3OH)2 SO4
ascorbic acid : C6H8O6
ammonia : NH3
sodium hydroxide : NaOH
sodium dichromate dihydrate : Na2Cr2O7.2H2O
Class:
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elem=Cr
coordination
redox
Summary:
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Hydroxyl amine gives a brown coordination complex when it reduces hexavalent
chromium to trivalent chromium in alkaline environments. This coordination
complex is really due to the hydroxyl amine. When other reducing agents are
used in the presence of ammonia in alkaline environments, then no similar
reaction product can be obtained.
Description:
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Prepare two liquids:
I: Dissolve some sodium dichromate in excess solution of sodium hydroxide. A
bright and clear yellow liquid is obtained.
II: Do the same as in (I), but add some ammonia (25% by weight).
Now the following experiments are performed:
Experiment 1:
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Add an excess amount of ascorbic acid to liquid (I): The liquid becomes green/
yellow. When the liquid is heated, it becomes almost black. Transmitted light
from a tungsten lamp through a thin layer of liquid is red.
Experiment 2:
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Add an excess amount of ascorbic acid to liquid (II): The liquid becomes green/
yellow. When the liquid is heated, it becomes almost black. When viewed at TL-
light, the liquid looks very dark green (a little bit more so, than the black
liquid of experiment 1). Transmitted tungsten light is red as in experiment 1.
The result of experiment 2 does not significantly differ from the result of
experiment 1. The ammonia does not have a clear effect. The small differences
can be explained, because of small differences in concentrations of the
reactants.
Experiment 3:
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Add an excess amount of hydroxyl amine sulfate to liquid (I): The liquid
becomes brown. This reaction does not proceed quickly at room temperature,
but when it is heated, then the reaction completes quickly. The brown
compound is a trivalent chromium compound which is stable, even in strongly
acidic experiments (other experiments show this fact).
Experiment 4:
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Add an excess amount of hydroxyl amine sulfate to liquid (II): The same
result as experiment 3. The presence of ammonia has no visible effect in
this experiment.