Description of experiment
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Needed compounds:
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ammonia : NH3
potassium bromate : KBrO3
sulphuric acid : H2SO4
sodium bisulfite : NaHSO3
potassium permanganate : KMnO4
citric acid : COOH CH2 COH(COOH) CH2 COOH
Class:
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elem=Br;Mn
redox
Summary:
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When bromate, manganese (II), citric acid and sulphuric acid are mixed at
certain propertions, then an oscillating reaction occurs, finally resulting
in an oily compound, which separates from the aqueous liquid.
Description:
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Prepare a solution, containing a fairly high concentration of Mn2+ ions
from potassium permanganate, dissolved in warm dilute H2SO4 (appr. 2 mol/l),
adding solid sodium metabisulfite, until all permanganate has reacted.
After the reaction, the liquid is boiled for some time, in order to expell
an excess amount of sulphur dioxide.
Using this liquid, the following sequence is performed:
Dissolve some citric acid in a 1:1 mixture of dilute H2SO4 (2 mol/l) and the
manganese (II) solution, as prepared above: The resulting liquid is almost
colorless, just a very faint pink color can be observed.
Dissolve some potassium bromate in a small volume of water, by heating the
liquid and shaking, until all of the potassium bromate has dissolved.
Add both liquids to each other: The resulting liquid is dark brown and turbid.
This probably is due to the formation of a manganese (IV) compound.
Heat the liquid, while constantly shaking: The liquid becomes lighter and
quickly becomes yellow and turbid. After a while, a fairly vigorous evolution
of a gas starts and the liquid becomes dark brown again. The evolution of
gas ceases again and the liquid becomes yellow again. A few seconds later,
the liquid becomes brown again and a gas is evolved. The second time, the
liquid, however, does not become as brown as the first time. This sequence
is repeated a third time, but the intensity of the brown color and the
evolution of the gas is even less pronounced. Finally, the liquid is turbid
and yellow. At the bottom of the test tube, there is a drop of a brown oily
liquid. There is a strong smell of an organic compound, probably a brominated
compound.
Decant the liquid above the oily drop and add a lot of water: The drop does
not dissolve and remains brown.
Decant the water again and add some dilute ammonia (appr. 5% by weight): The
oily drop becomes solid and dark brown.