Description of experiment
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Needed compounds:
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ammonium thiocyanate : NH4SCN
hydrogen peroxide : H2O2
hydrochloric acid : HCl
titanium : Ti
Class:
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elem=N,Cl,Ti
coordination
redox
Summary:
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Titanium metal slowly dissolves in concentrated hydrochloric acid and then
forms a very dark solution of a titanium/chloride complex. This solution is
much darker than an aqueous solution of titanium(III) in which the ions are
present as aqua complex.
With thiocyanate an even more intensely colored complex is formed. This complex
has the same color, but it is very dark.
Description:
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Add a small amount of high-purity titanium (99.9%) to 30% HCl and loosely
stopper the test tube: The metal very slowly dissolves. After three days there
is a very dark solution. It looks almost black, but in thin layers this
solution has a beautiful purple/indigo color.
Take a few drops of this solution and add them to half a ml of water: A pale
purple solution is obtained. The color of this solution is much weaker than one
would expect based on dilution of the drops solution. Apparently, the
concentrated acid solution contains a chloro-complex of titanium(III), and on
dilution with water, the chloride ligands are replaced by water ligands.
Add a small spatula of ammonium thiocyanate to this pale purple solution and
swirl: The ammonium thiocyanate quickly dissolves and the solution becomes deep
purple/indigo and the color becomes really intense again. The color is the same
as the original solution in conc. HCl and equally intense, but the
concentration of titanium ions is much lower than in the original solution. So,
thiocyanate forms an even more strongly colored complex with titanium(III).
Add a few drops of 10% H2O2 to the solution of the thiocyanate/titanium
complex: The deep purple/indigo color at once disappears and a deep orange/red
liquid is obtained. This is the color of a titanium(IV)-peroxo complex.