Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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sodium sulfite : Na2SO3
hydrochloric acid : HCl
sodium thiosulfate : Na2S2O3 . 5H2O
sodium dichromate dihydrate : Na2Cr2O7.2H2O
Class:
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elem=S,Cr
redox
Summary:
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Dichromate ion oxidizes thiosulfate easily in acidic media, but in neutral
media it only is oxidizing incompletely, and a solid compound is formed.
Description:
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Mix a solution of sodium thiosulfate with a solution of sodium dichromate.
Excess sodium thiosulfate is used: No visible reaction occurs. The liquid
remains clear and orange, at least for a several minutes.
Heat the liquid and boil for a while: Slowly the color changes from orange to
dark brown. A precipitate is formed as well. The liquid becomes turbid and dark
brown.
Let liquid settle for one day: A clear yellow liquid with a faint brown tinge
is obtained, above a brown precipitate.
Decant the clear yellow liquid into another test tube and add some dilute
hydrochloric acid to this: The liquid becomes green at once, green like moss.
The liquid does not become turbid at once, it remains clear at least for a few
minutes.
Add some dilute hydrochloric acid to the remaining brown precipitate, which was
left over after decanting the yellow liquid: Thew precipitate does not dissolve
and remains brown. A faint smell, reminiscent of hydrogen sulfide appears, but
it is very faint.
Heat the liquid with the brown precipitate: Hardly any change occurs. It looks
as if the liquid becomes slightly more green, but the effect is not clearly
visible.
Add a pinch of sodium sulfite and keep heating the liquid: Slowly, the liquid
shifts from brown to green, but it remains turbid. It is not clear whether the
turbidity is due to a precipitate of some chromium compound, or due to finely
divided sulphur.