76 HALOGEN COMPOUNDS.
oxygen gas until the air has all been replaced, then add a little
concentrated sodium nitrite solution and insert the stopper tightly
into the neck of the flask. To prevent any difficulty from the
liquid backing up into the gas-delivery tubing, it is well to provide
an empty gas-washing flask. On shaking the contents of the large
bottle, an energetic reaction takes place and there is a noticeable
consumption of oxygen. Shake, at first carefully, then more
strongly and continuously. When there is no more consumption
of oxygen, raise the stopper a little, fill the flask with fresh oxygen,
add a little more sodium nitrite solution and repeat the operation.
Continue until there is no further evidence of any consumption
of oxygen. Allow the iodine to settle out in the bottom of the
flask, pour off the supernatant liquid, which is usually colorless,
wash the residue with a little water and filter off the crude
iodine.
Purify the product by distillation with steam. It is best to
avoid the use of a water-cooled condenser. Lead the vapor of
steam and iodine into the middle of a large Erlenmeyer flask.
The stopper of this flask should contain two holes, one to receive
the tube through which the vapors enter and the other a tube
extending about half a meter upwards, to serve as an air condenser.
Clamp the flask in a dish containing cold, running water.
Filter off the purified iodine, drain it with suction and dry in a
vacuum desiccator over calcium chloride. It requires a long time
to accomplish a thorough drying of the iodine in this way. It is
quicker to heat the iodine on the water bath in an evaporating
dish, which is covered with a watch glass. Iodine and some water
will collect on the watch glass. After some minutes, replace the
watch glass with another one, and scrape off the iodine on it.
When the condensate on the watch glass appears to be iodine
without any drops of water, the iodine is practically dry.
To further purify the iodine, place it in a large beaker, cover the
beaker with a Kjeldahl flask filled with cold water, and heat the
bottom of the beaker so that the iodine sublimes and collects on
the flask.