Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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hydrogen peroxide : H2O2
sodium chloride : NaCl
disodium phosphate : Na2HPO4
nitric acid : HNO3
phosphoric acid : H3PO4
potassium dichromate : K2Cr2O7
Class:
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elem=Cr
coordination
redox
Summary:
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Chromium (III) builds a nice brightly colored green complex with phosphates.
This compound has no bluish hue, like sulfate gives with chromium (III).
Chloride also builds a complex. Formation of these complexes is not on
simple addition of a chromium (III) salt to the corresponding anions.
Heating is required.
Description:
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Sequence 1:
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Dissolve some potassium dichromate in phosphoric acid (appr. 85% by weight)
by heating: The solid dissolves and the liquid becomes dark red/orange.
Without the heating, only a small part of the potassium dichromate dissolves
and the liquid remains light orange.
Dilute the liquid with water and then add some hydrogen peroxide (appr. 10%
by weight) in small quantities per step until all dichromate is reduced: For
each quantity added, the liquid starts foaming violently. At a certain point,
hydrogen peroxide can be added without further foaming, meaning that all
dichromate is reduced. At this point, the liquid has a pure dark green color.
The green color has no bluish hue, nor a brown hue, it is very pure green.
Heat the liquid for a while: The excess hydrogen peroxide decomposes, but no
change of color can be observed.
Sequence 2:
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Dissolve some potassium dichromate in dilute nitric acid (appr. 2 mol/l)
and heat this liquid in order to speed up dissolving of the dichromate:
When all of the solid has dissolved, the liquid is orange/red.
Add some hydrogen peroxide (appr. 10% by weight) in small quantities per
step until all dichromate is reduced: For each quantity added, the liquid
starts foaming violently. At a certain point, hydrogen peroxide can be added
without further foaming, meaning that all dichromate is reduced. At this
point, the liquid is greyish blue, hard to describe, the typical color of
aqueous chromium (III), coordinated to water only. This liquid is called
CRH2O.
Sequence 2a:
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Add some disodium phosphate to part of CRH2O: The solid dissolves, no further
visible changes.
Heat the liquid: At first glance nothing changes, but when the liquid starts
boiling, it slowly changes color, until it gets the green color of sequence 1.
Sequence 2b:
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Add some sodium chloride to part of CRH2O: The solid dissolves, no further
visible changes.
Heat the liquid: At first glance nothing changes, but when the liquid starts
boiling, it slowly changes color, until it becomes green. The green color,
observed here differs from the color of sequences 1 and 2a. The green has
a greyish hue, it is a much duller green. Even when a large excess amount
of sodium chloride is used, the color remains dull green.
The green has no bluish, nor a brown hue, it is just duller than the color,
observed in sequences 1 and 2a.