Take
approximately ⅓ of the indigo liquid and dilute with the same volume of water
and then add some hydrogen peroxide to this, drop by drop, until a steady deep
red/brown color is observed. When the test tube is shaken, then the glass will
be tinged with a yellow/orange color. The pictures below show this deep red
compound and a dilution of this.
Dilute
the remaining part of the indigo liquid with the its volume of water and then
add some sodium persulfate or ammonium persulfate. The liquid slowly becomes
colorless. This shows the oxidation of titanium in the +3 oxidation state to
titanium in the +4 oxidation state and it shows that without a complexing agent
like hydrogen peroxide, titanium in the +4 oxidation state is colorless.
The deep red liquid is added to 50 ml of water in an
erlenmeyer. This results in a red/orange liquid. Dissolve a spatula full of
sodium sulfite in a separate test tube with a few ml of dilute hydrochloric acid
(approximately 10% HCl by weight), and add the contents of this test tube to the
erlenmeyer as well.
Now the erlenmeyer contains as active compounds the
titanium-peroxide complex, acid and sulphur dioxide (from the sulfite). The
sulphur dioxide slowly reduces the peroxide in the complex and leaves titanium
(IV) in solution, which is colorless. Titanium (IV) cannot be reduced to
titanium (III) by sulphur dioxide, otherwise the violet/blue color of titanium
(III), as shown in the pictures above, should appear.
Reduction of the complex is slow. The sequence below shows a
set of pictures, taken at different times. The time from left, with the initial
red/orange liquid to right is approximately 10 minutes. The temperature was
around 15 °C during this experiment.
Discussion of results
Titanium metal is very corrosion resistant in an oxidizing
environment, in spite of the fact that it is relatively electropositive. Nitric
acid for instance is not capable of dissolving the metal. The reason for this
probably is that the metal forms a passivating oxide layer in an oxidizing
environment. In a non-oxidizing acidic environment, the metal, however is slowly
attacked. Concentrated hydrochloric acid slowly attacks the metal, according to
the following equation:
2Ti(s) + 6H+(aq) →
2Ti3+(aq) + 3H2(g)
The Ti3+(aq) ions have a
deep violet/blue color.
On addition of hydrogen peroxide, the titanium in the +3
oxidation state is oxidized to titanium in the +4 oxidation state. The latter in
turn forms the deep red complex with hydrogen peroxide.
Aqueous titanium in the +4 oxidation state exists as a
dipositive cationic species, which only remains in solution at very low pH.
Whether the cation is TiO2+(aq) or Ti(OH)22+(aq)
is not yet clear. At higher pH, a precipitate of hydrous TiO2 is
formed.
When hydrogen peroxide is added to an acidic solution with
titanium (IV) in it, then the red cationic species [Ti(O2)(OH)]+(aq)
is formed. Here the peroxo ligand O22- is connected to the
metal through both of the oxygen atoms, hence a better formula for the complex
is [Ti(η2–O2)(OH)(H2O)x]+.
In the second part of the experiment, the peroxo-complex
of titanium is broken down slowly by sulphur dioxide and plain colorless
titanium (IV) is in solution again. |