POTASSIUM IODIDE. 75
prepared without the intermediate formation of iodate, by the interaction of
ferrous iodide (which can be obtained synthetically from the elements) and
potassium carbonate.Shake 7 or 8 g. iron filings and 50 c.c. water in an Erlenmeyer
flask with 25 g. iodine added in small portions. Warm the mix-
ture somewhat until all of the iodine has combined, and the color
of the solution has become deep yellow (ferrous iodide); then pour
off the liquid from the excess of iron. Add five grams more of
iodine to the solution and heat until it is dissolved.
1
Pour this
solution into a boiling solution of 17 g. potassium carbonate in
50 c.c. water. The mixture, which at first is very thick, becomes
more fluid upon further heating, since the precipitate assumes a
more compact form. A little of the solution when filtered must
be perfectly colorless, and free from iron; if this is not the case,
add a little more potassium carbonate to the boiling solution.Evaporate the filtrate to a small volume in a porcelain dish, filter
again, and evaporate further in a beaker until crystals begin to
separate. Then allow the solution to evaporate slowly by placing
the beaker in a warm place (as on top of the hot closet). Drain
the crystals in a funnel, wash them with a little cold water, and
save the mother liquor for another crop of crystals. Yield, 25
to 35 g.
The potassium iodide when dissolved in a little water and acidi-
fied should not show any yellow color (free iodine), which would
indicate the presence of iodate in the salt. Dependent prepara-
tion: Potassium lead iodide, No. 106.
Iodide residues, such as result from chemical work in organic
laboratories, can be easily worked up into iodine by oxidation with
oxygen gas in acid solution, with oxides of nitrogen as catalyzer.
Half fill a 5-liter flask with the solution of iodide residues and fit
the flask with a stopper carrying a glass tube which reaches down
to about the middle of the flask. Make the solution strongly
acid with commercial sulphuric acid, which should heat the mix-
ture somewhat. With the flask loosely stoppered, introduce(^1) The extra addition of the iodine serves to partially oxidize the ferrous
salt, and the subsequent precipitate thus contains hydrated ferric ox-
ide and is' readily filtered; ferrous carbonate would be very difficult to
filter.