Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
-----------------
sodium hydroxide : NaOH
oxalic acid : HOOC COOH . 2H2O
sodium sulfite : Na2SO3
potassium dichromate : K2Cr2O7
Class:
------
elem=Cr
coordination
redox
Summary:
--------
Chromium (III) has many different colors, depending on coordinated ligands.
Description:
------------
In the experiments described below no other acid is used than the oxalic
acid.
Sequence 1:
-----------
Dissolve some K2Cr2O7 and a large excess amount of oxalic acid in water:
The resulting solution becomes fairly dark orange/brown. A tiny amount of
gas is evolved (probably CO2). The solution appears to be a little opaque.
Add a slight excess amount of sodium sulfite (relative to the amount of
K2Cr2O7 used): The sodium sulfite dissolves. The solution turns deep
purple and clear. When watched at SL-light, the solution seems to be dark
grey and clear. It becomes very dark, looking through 1.5 cm of solution,
one cannot see anything, although just a little amount of potassium
dichromate is used.
(If the same experiment is done with a solution of potassium dichromate
with some hydrochloric acid added, then the color of the resulting
solution after adding sodium sulfite is much less intense).
Add a little amount of a solution of sodium hydroxide: The liquid remains
deep purple.
Add much more solution of sodium hydroxide: The liquid turns green and it
becomes a little opalescent.
Sequence 2:
-----------
Dissolve a little quantity of K2Cr2O7 and a large amount of oxalic acid in
water and heat the liquid to approximately 65 C: The solution turns deep
purple. This takes several minutes. The color is much more intense than
one would expect on the basis of the amount of potassium dichromate which
is used in the experiment.
Remark:
-------
For the experiments, described here, tap water was used. This tap water
contains some calcium bicarbonate, which may cause the formation of
calcium oxalate (CaC2O4) which is highly insoluble. The opaqueness of
the liquid in the experiments can be explained by this. When the experiments
are repeated with distilled water, then the opaqueness is not observed.