Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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glycerol : CH2OH CHOH CH2OH
sodium hydroxide : NaOH
potassium dichromate : K2Cr2O7
sulphuric acid : H2SO4
nitric acid : HNO3
tartaric acid : HOOC CHOH CHOH COOH
Class:
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elem=Cr
coordination
redox
Summary:
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Dichromate is capable of oxidizing tartaric acid and a colorless gas is
formed in this reaction (probably CO2). The liquid becomes purple/grey
(hard to describe color, depending on viewing illuminant).
The reaction product does not form a special complex in alkaline environment,
the familiar green color of chromium (III) in alkaline environments is
created. Addition of glycerol does not result in formation of a special
coordination complex.
When, however, dichromate is reduced by an excess amount of glycerol, then
a special coordinate complex appears to be formed, when the solution is
made alkaline. Even addition of acid does not destroy this complex.
Description:
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Sequence 1:
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Add an excess amount of tartaric acid to a solution of potassium dichromate
in dilute nitric acid (appr. 1 mol/l): The solution becomes slightly darker,
the orange liquid gets a brown hue.
Boil the solution gently for a while: Evolution of a gas, the liquid becomes
purple/grey, when viewed under TL-light, purple, when viewed under tungsten
light.
Add an excess amount of a solution of sodium hydroxide (excess, relative
to the acid): The liquid becomes moss-green.
Add some glycerol: The glycerol slowly sinks along the glass to the bottom
of the test tube and forms a colorless layer. When the liquid is shaken,
it dissolves. No visible change of color.
Sequence 2:
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Add an excess amount of glycerol to a solution of potassium dichromate in
dilute nitric acid (appr. 1 mol/l): The solution slowly changes color. When
the liquid is heated to appr. 60 C, then it becomes purple. A faint odour
is produced, this probably is the odour of an oxidation product of glycerol.
Add an excess amount of a solution of sodium hydroxide (excess, relative to
the acid): The liquid becomes dark green. The color is not moss-green, but
it is green with a fairly strong blue hue.
Add an excess amount of dilute nitric acid (appr. 2 mol/l): The liquid
remains dark green. Turning back to purple was expected here, hence one
can conclude that a new coordination complex is formed.
Add an excess amount of dilute sulphuric acid (appr. 2 mol/l) to the alkaline
dark green liquid: The same result as on addition of dilute nitric acid.