120 OXY-HALOGEN COMPOUNDS.
" Time Reaction."
If a solution of iodic acid is added drop by drop to an aqueous solution of
sodium sulphite acidified with sulphuric acid, the solution at first remains
colorless:
3 H 2 SO 3 + HIO 3 = 3 H 2 SO 4 + HI.
When, with further addition of iodic acid, all the sulphurous acid becomes oxi-
dized, the reduction then continues at the expense of the hydriodic acid
which has been formed:
HIO 3 + 5 HI = 3 H 2 O + 3 I 2.
Thus if an acidified sulphite solution is treated with more than one-third of a
mol. of iodic acid, a separation of iodine occurs; if the solution is concen-
trated this takes place immediately; if dilute, only after some time, and then
suddenly and completely.
This very remarkable retardation reminds one of the phenomena of super-
cooling, superheating, and supersaturation, but it is different inasmuch as the
reaction takes place in a homogeneous medium, and is not followed by a
separation of material in another state of aggregation.
Prepare one solution by dissolving 1.8 g. of iodic acid, or an
equivalent amount of potassium iodate, in water and diluting to
one liter; prepare a second solution with 0.9 g. of Na2SO 3 • 7 H 2 O,
5 g. of 10% sulphuric acid, and 9.5 g. of starch (the latter should
first be suspended in a little cold water and then stirred into a
beaker of boiling water to form a paste),' and dilute this likewise
to one liter. Measure 100 c.c. of each solution into separate
beakers, mix these two portions at a definite instant, and count
the seconds until a deep blue color suddenly appears. Check
this result by repeating the experiment. Then dilute each of the
solutions to i, 1,1, and £ of their former concentrations, and deter-
mine for each dilution the time which elapses before the blue color
appears. Plot graphically the dependence of the time on the dilu-
tion. The results are reproducible and comparable only when the
temperature of the solutions does not vary appreciably.
- Potassium Iodate from Potassium Chlorate.
2 KCIO3 + I 2 = 2 KIO 3 + Cl 2.
Place 30 g. of potassium chlorate in a 200 c.c. flask and dissolveit in 60 c.c. of warm water. Add 35 g. of iodine to the solution,
and while maintaining the mixture at a moderate temperature
introduce 1 to 2 c.c. of concentrated nitric acid. (Hood.) After
one or two minutes a vigorous reaction begins and a stream of
chlorine escapes, carrying with it a little iodine. When the re-
action moderates, boil to drive off the chlorine, and, when this is