Description of experiment
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Needed compounds:
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hydrochloric acid : HCl
hydrogen bromide : HBr
sodium selenite : Na2SeO3
tellurium dioxide : TeO2
cesium bromide : CsBr
cesium chloride : CsCl
Class:
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elem=Se,Te
precipitation
coordination
Summary:
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Cesium ion allows many very special complexes to be isolated as solid salts. In
this experiment it is demonstrated how this is done for selenium and tellurium
complexes.
Description:
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Prepare a concentrated solution of cesium bromide in 40% hydrogen bromide. The
CsBr dissolves fairly easily in the 40% hydrogen bromide.
In a separate test tube, dissolve some tellurium dioxide in 40% hydrogen
bromide solution: The resulting solution is clear and fairly dark red/brown,
somewhat like concentrated bromine water, albeit slightly more yellowish.
Add a few drops of the solution of cesium bromide in 40% HBr: A very fine
brown/yellow compact precipitate is formed. This precipitate is not flocculent,
it consists of very fine crystalline particles.
Heat the liquid with the precipitate till near boiling: The precipitate does
not dissolve, it becomes a little more compact. On cooling down, the
precipitate settles at the bottom. It has a yellow/brown/mustard color.
Add some of the liquid with the precipitate to water: The color of the
precipitate quickly fades, the liquid becomes somewhat opalescent/white. In the
neutral water, the material quickly hydrolyses to tellurium dioxide, HBr and
free cesium ions.
Part of the precipitate is filtered, pressed dry and allowed to dry. The
material has a yellow/mustard color and can fairly easily be isolated.
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Prepare a solution of a small amount of sodium selenite in 40% hydrogen
bromide: The solution becomes deep red and clear.
Add a few drops of a solution of cesium bromide in 40% HBr: A golden/yellow
precipitate is formed, which has a structure, very similar to the
mustard/yellow precipitate formed with TeO2 in 40% HBr. The precipitate has a
very nice bright color.
Add the liquid with the precipitate to a lot of water: The golden/yellow solid
quickly settles at the bottom and the liquid above it remains colorless. The
solid does not quickly hydrolyse, the yellow color persists in the water.
However, at further strong dilution the color does fade and all of the solid
dissolves, it takes a few minutes though.
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Prepare a solution of cesium chloride in concentrated hydrochloric acid.
Dissolve some tellurium dioxide in concentrated hydrochloric acid: A bright
yellow and clear solution is obtained.
Add the solution of cesium chloride to the bright yellow solution: A bright
lemon/yellow precipitate is formed, which has a structure very similar to the
precipitates, formed in the above experiments.
Add a lot of water to the liquid with the yellow precipitate: The yellow color
quickly fades and the liquid become opalescent/white. The yellow solid quickly
hydrolyses in neutral water.
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Remark: TeO2 and SeO2 form complexes in conc. HBr, which can be described as
TeBr6(2-) and SeBr6(2-). In conc. HCl, TeO2 forms a similar complex TeCl6(2-).
These complexes remain in solution. With cesium ions, however, very sparingly
soluble salts are formed with formula Cs2[TeBr6], Cs2[TeCl6] and Cs2[SeBr6].
These salts are stable and can be isolation, however, in contact with water
they hydrolyse to the oxide and free acid again and cesium ions go in solution
again. With the selenium complex this hydroly