Description of experiment
Below follows a plain text transcript of the selected experiment.
Needed compounds:
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sodium nitrite : NaNO2
potassium bromide : KBr
sodium sulfite : Na2SO3
hydrochloric acid : HCl
sulphuric acid : H2SO4
manganese(II) chloride tetrahydrate : MnCl2.4H2O
manganese (II) sulfate : MnSO4 . H2O
Class:
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elem=Cl,Mn
redox
Summary:
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When a solution of manganese(II)chloride in moderately concentrated
hydrochloric acid is electrolysed, then at the anode a remarkable very dark
compound is formed. The liquid remains clear, but it becomes very dark. This
must be a compound of manganese in higher than +2 oxidation state.
When a similar experiment is performed with manganese(II)sulfate in a 20%
solution of sulphuric acid, then a drk brown precipitate is formed at the
anode.
Description:
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Sequence 1: Electrolysis of manganese(II)sulfate in 20% sulphuric acid
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Dissolve 1 spatula full of solid manganese(II)sulfate in 15 ml of 20% sulphuric
acid: This gives a clear and colorless solution.
Pour the solution in a small U-tube and put one electrode in each arm of the
U-tube. The electrodes must be either platinum wire, or clean and smooth
graphite. Apply an electrolysis current of approximately 1 Ampere. In this
experiment this was done by taking a 10 Ohm power resistor and putting this in
series with a 13.8 V power supply: As soon as the current flows, a brown
precipitate is formed at the anode. At the cathode, a lot of colorless gas is
formed (this is hydrogen). Around the anode, there also
Let the electrolysis run for a few minutes and then let the liquid stand for a
while: The brown precipitate settles at the bottom, above the precipitate a
somewhat turbid liquid remains. It is expected that on much longer standing
this turbidity also will disappear.
Add some sodium sulfite to the liquid with the brown precipitate: The brown
precipitate dissolves at once and the liquid becomes colorless again.
Put the anode, covered with the dark brown layer in the colorless liquid, in
which there still is excess sodium sulfite: The dark layer slowly dissolves,
making the anode clean again. This cleaning, however, is not instantaneous, it
takes quite some time before the anode is clean again.
Sequence 2: Electrolysis of manganese(II)chloride in 20% hydrochloric acid
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Dissolve 1 spatula full of solid manganese(II)chloride in 15 ml of 20%
hydrochloric acid: This gives a clear and colorless solution.
Pour the solution in a small U-tube and put one electrode in each arm of the
U-tube. The electrodes must be either platinum wire, or clean and smooth
graphite. Apply an electrolysis current of approximately 1 Ampere. In this
experiment this was done by taking a 10 Ohm power resistor and putting this in
series with a 13.8 V power supply: As soon as the current flows, a brown/green
compound is formed at the anode, which goes in solution. This is not a
precipitate, nor a colloid. When light of a laser beam is
Let the electrolysis run for a few minutes: The liquid in the arm of the
U-tube, with the anode in it, is almost black. The liquid really is dark. The
liquid in the arm with the cathode in it still is colorless. Somewhere halfway,
near the bottom of the U-tube, there is a fairly sharp transition from dark
green/brown (almost black) to colorless.
Stop the electrolysis and pour the liquid from the anode arm in a separate
beaker: There is a strong smell of chlorine and the liquid is fuming somewhat.
Dilute 1 ml of the dark liquid with a few ml of water: A brown/yellow solution
is obtained. Again, this is a true solution, not a colloid (checked with a
laser beam).
Take another 1 ml of the dark liquid and add some solid KBr: The dark color
disappears at once, the liquid becomes orange/yellow and a faint yellow/orange
color of bromine vapor can be observed above the liquid. The dark compound
oxidized the bromide and itself is reduced to (almost) colorless manganese(II).
Take another 1 ml of the dark liquid and add some solid NaNO2: The dark color
disappears at once. The liquid becomes orange and a yellow/orange gas can be
observed above the liquid. The nitrate immediately reduces the dark compound.
Excess nitrite reacts with the fairly concentrated acid to form notrosyl
chloride, which makes the liquid orange and which appears as yellow/orange gas
above the liquid. Some solid material remains, which is NaCl from reaction of
HCl with NaNO2.
Take another 1 ml of the dark liquid and shake this with some dichloromethane:
The dark compound is not extracted into the DCM layer. The DCM layer remains
colorless.
Pour the remaining dark liquid into a beaker, filled with warm water: The
liquid quickly mixes with the water and the liquid becomes turbid and brown.
Now it seems that the dark material hydrolyses and a hydrous oxide of manganese
is formed.
Add a small amount of sodium sulfite to the beaker with the turbid brown liquid
and stir with a glass rod: Almost instantly, the brown precipitate dissolves
and the liquid becomes colorless.
Remark: In the presence of chlorine, manganese forms a very dark complex, when
it is in a higher than +2 oxidation state. On strong dilution, this complex
hydrolyses, and hydrous manganese oxide is formed. It is not clear whether this
is mannganese in the +3 or the +4 oxidation state. This complex is a strong
oxidizer, it oxidizes nitrite and bromide at once.
In the presence of sulfate, no such complex is formed. Even in 20% sulphuric
acid, a precipitate of hydrous manganese oxide is formed (most likely
Mn2O3/MnO2 mix).