Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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ammonia : NH3
hydrochloric acid : HCl
titanium : Ti
Class:
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elem=Ti
redox
Summary:
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Titanium(III) ions are fairly stable in acidic media, but in alkaline solution,
these become so strongly reducing that hydrogen gas is formed and water is
reduced!
Description:
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Take a small amount of very pure powdered titanium metal (99.9%) and add this
to 1.5 ml of concentrated hydrochloric acid (30% HCl) and heat this liquid
until it nearly boils: The metal slowly dissolves and the solution becomes
blue.
Let the liquid stand for a few days in a loosely stoppered test tube: All of
the metal has dissolved and a very dark liquid is obtained. The color of the
liquid is beautiful, it is deep purple/indigo.
Dilute the liquid by adding 5 ml of distilled water: The color becomes much
more pale, much more than one would expect on the basis of the dilution. The
color also shifts from purple/indigo to purple/rose.
Add a large excess of 5% ammonia, such that the test tube is completely filled
up to the rim (appr. 15 ml): A very dark blue (almost black) precipitate is
formed. There also is a slow production of gas. The precipitate tends to
collect in the upper part of the test tube.
Take a piece of glass wool and use this to close the test tube with the liquid,
which contains the black precipitate and the ammonia. Next, place the test tube
upside down in a beaker, which has a few cm of 1% ammonia on the bottom. The
test tube should rest on the plug of glass wool and should be completely filled
with the liquid, no bubble of air should be in the top. Let the beaker with the
dilute ammonia and the test tube stand for one day: After one day, the
precipitate has become completely white and
Turn the test tube (while it still is closed with the glass wool) such that the
rim is upside again. Remove the wet plug of glass wool near a flame of a candle
or an alcohol burner: A small whoop is produced, the gas, which has collected
in the test tube is flammable.
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In this experiment titanium is dissolved in concentrated hydrochloric acid, and
in this reaction titanium(III) is produced, which forms a deep indigo/purple
solution with the chloride. On dilution with water, ligand exchange occurs and
the chloride ligands of the titanium(III) ions are replaced by water ligands.
On addition of ammonia, a precipitate of dark blue hydrated titanium(III)
hydroxide is formed, which slowly reacts with water, giving hydrogen gas and
hydrous titanium(IV) oxide. The latter is white.