Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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sodium hydroxide : NaOH
sodium fluoride : NaF
hydrochloric acid : HCl
titanium : Ti
Class:
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elem=Ti,F
coordination
redox
Summary:
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Titanium dissolves in dilute hydrochloric acid with great difficulty. When
hydrofluoric acid is present as well, then the metal dissolves easily. A
tinanium (III) fluoro complex is formed.
Description:
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Sequence 1:
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Add some powdered titanium metal to dilute hydrochloric acid (appr. 12% HCl
by weight): The metal does not dissolve, no gas is produced, no visible
changes.
Heat the liquid, until it boils and then take out of flame: After boiling
stops, very slow formation of a colorless gas at the metal particles can be
observed.
Add a spatula full of sodium fluoride to the still hot liquid: The solid
dissolves. A colorless gas is formed quickly at the metal particles. After
a few minutes the metal has dissolved completely. The liquid turns light green
and a little turbid. After a while, a flocculent white precipitate has settled
at the bottom of the test tube and a clear green liquid is above it.
The glass of the test tube has been attacked considerably in this experiment.
The hydrofluoric acid has etched the glass quite strongly.
Prepare a concentrated solution of sodium hydroxide and add some of the clear
green liquid to this: A dark blue/black precipitate is formed. A colorless
gas is produced and while this gas is produced, the precipitate turns lighter.
Finally, the precipitate is white (from dark blue through lighter shades of
blue to white) and the evolution of the gas ceases. The dark precipitate
probably is titanium (III) hydroxide, which reacts with water, forming hydrogen
gas and hydrous titanium (IV) oxide, which is white.
Add the remaining part of the green liquid to the liquid with the white
precipitate: Again, the liquid becomes dark blue. Now, the evolution of gas
is very slow and the color does not change appreciably within a few minutes.
Apparently the reduction of water by the titanium (III) hydroxide is faster
at higher pH.
Sequence 2:
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Dissolve some titanium metal in concentrated HCl, by adding the metal to the
acid and leaving the liquid alone for one day: After one day, the liquid has
become deep purple/blue. This is due to presence of a Ti3+ species in the
liquid. Not all of the metal has dissolved after one day.
In a separate test tube, take some dilute HCl (10%) and add a spatula of NaF
and dissolve the solid. Add some of the deep blue/purple solution with Ti3+
contained in it: The deep blue/purple color disappears at once and the liquid
becomes green. So, titanium(III) forms a green complex with fluoride in acidic
media.