Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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zinc : Zn
sodium persulfate : Na2S2O8
hydrochloric acid : HCl
sodium thiocyanate : NaSCN
stannous chloride : SnCl2
ammonium perrhenate : NH4ReO4
Class:
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elem=Re
coordination
redox
Summary:
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Rhenium forms complexes with thiocyanate in more than one oxidation state.
Here a rhenium (IV) thiocyanate complex is made and then oxidized to rhenium
(V). Historically these complexes have been used to detect minute traces of
rhenium due to the intense colours formed.
Description:
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Sequence 1
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Add 3ml of 6% HCl to a clean tube.
Add 50mg of tin (II) chloride and 150mg of thiocyanate. (I used the potassium
salt but ammonium or sodium thiocyanate should also work).
Swirl to dissolve the solids. The resultant liquid is white and quite turbid.
In a second tube add 50mg of ammonium perrhenate to 1.5ml water and dissolve.
Add one drop of the perrhenate solution to the first tube. Formation of a
bright green colour which very quickly turns yellow-brown and then red. Add
the rest of the perrhenate to the first tube. An intense blood-red complex is
formed. This is [Re(NCS)6]2-
Sequence 2
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Add 100mg of sodium persulfate to 2ml water and dissolve.
Add the persulfate solution to the red complex formed in sequence 1.
Heat the liquid until it just begins to boil then remove from the heat. As the
liquid warms it will change colour to green-yellow and then to a clear yellow
with a hint of orange. The persulfate oxidizes the complex to [ReO(NCS)5]2-
which contains rhenium (V)
Sequence 3
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Add a pich of zinc powder to the yellow complex. The zinc reacts with the acid
and reduces the complex back to the red Re(IV) species.
Riddle
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What is the transient green colour observed in sequence 1?
Could it be [ReCl6]2- which is said to be green?