Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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sulphuric acid : H2SO4
chrome alum : KCr(SO4)2 . 12H2O
EDTA di sodium : (NaOOC CH2)(HOOC CH2) N CH2 CH2 N (CH2 COOH)(CH2 COONa)
sodium sulfite : Na2SO3
nitric acid : HNO3
potassium dichromate : K2Cr2O7
Class:
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elem=Cr
coordination
redox
Summary:
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Chromium (III) builds a purple complex with EDTA, both when it is created
from dichromate and when it already exists and is brought in contact with
EDTA. When the pH is too low (or is this due to formation of a complex with
sulfate?), the formation of the complex does not occur.
Description:
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Sequence 1:
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Dissolve some K2Cr2O7 in dilute HNO3 (appr. 2 mol/l) and divide in two parts.
To part 1 add an excess amount of EDTA-Na2 and do not add anything to part 2.
At this point we have two identical looking orange liquids.
Add a solution of sodium sulfite to part 1: The liquid becomes blue/purple
(blue, when viewed under TL; purple, when viewed under tungsten light). Light
smell of SO2. After a few minutes, the liquid has become more purple.
Add a solution of sodium sulfite to part 2: The liquid becomes green (a very
common color for chromium (III)).
Heat both liquids and boil gently for a while: Liquid 1 remains purple, but
its color intensifies and it now also looks purple, when viewed under TL.
Liquid 2 remains green. The smell of SO2 disappears (all is driven off).
Add an excess amount of EDTA-Na2 to the still hot green liquid 2 and boil
for a while: Now this liquid also becomes deep purple.
Sequence 2:
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Dissolve some chrome alum in dilute H2SO4 (2 mol/l): Liquid becomes blue/
violet, depending on the light source, used for observation.
Add a little amount of sodium sulfite: No visible changes, smell of SO2.
Add some EDTA-Na2: The solid dissolves, no further visible changes, not even
when the liquid is boiled for a while.
Add a little amount of NaOH: The solid dissolves, no further visible changes.
Add some concentrated HNO3 (appr. 60% by weight): No visible changes.