Dissolving iodine in fuming sulphuric acid
This is a very simple experiment, just to show some of the
remarkable properties of a solution of iodine in oleum (sulphuric acid with free
SO3 dissolved in it).
Fuming sulphuric acid, also known as oleum, is extremely corrosive. Oleum
also reacts with explosive violence, when it comes in contact with water. This
compound must be handled with really great care! Use thick-walled gloves and
good protection of the eyes. Contact of the skin with oleum results in instant
charring.
Required
chemicals:
Required
equipment:
Safety:
- See above for the warnings about the extreme
corrosiveness of oleum. Fumes from oleum also are very corrosive and
inhalation of these should be avoided.
- Iodine is fairly toxic and vapor of iodine should not
be inhaled.
Disposal:
- The iodine in the brown liquid obtained at the end of
the experiment can be neutralized with some sodium sulfite. Slowly add
sodium sulfite while stirring, until the solution has become pale yellow.
The liquid then can go down the drain with a lot of water.
Doing the experiment
This experiment is very simple. It just involved dissolving a
small amount of iodine in a ml of oleum. Put a few, somewhat crushed crystals of
iodine in a perfectly dry test tube and then pour in some oleum. When the
oleum is handled use thick impervious gloves and be 100% sure that this liquid
cannot reach the skin!
When the oleum is added, then the colorless liquid soon turns
olive green and some of the iodine dissolves.
The iodine dissolves with a brown color, but the brown color
quickly turns green in the oleum. The brown color is nicely demonstrated in the
picture below, where some iodine is sticking on the glass wall of the test tube.
The solution also becomes very dark, almost black. The color of the solution
however can be observed well where it sticks to the glass walls of the test
tube.
Slowly, the color of the liquid changes from olive green to
blue/green. The two small pictures show how the color develops over a few
minutes.
Iodine, sticking to the glass in higher parts of the test
tube also liquefies and dissolves and it produces beautiful traces of blue/green
viscous liquid on the glass walls of the test tube.
When this green/blue liquid is poured in an erlenmeyer,
filled with cold water, then the green/blue compound immediately is destroyed
and free iodine is formed, which appears as brown particles and partially
dissolves as a brown solution.
This pouring of the
liquid in water must be done with really great care. The reaction between the
oleum and water is explosively violent and a scary crackling noise and a lot of
splashes are produced when the liquid comes in contact with water.
After the oleum with the dissolved iodine is poured in water,
the result is as follows:
Some solid iodine sticks to the glass. A faint purple color
of iodine vapor can be observed and there is the white fog, due to fuming of the
oleum.
|