IODINE.
Examine iodine, potassium iodide.
161. Preparation of I.
Experiment 98.--Put into a t.t. 2 or 3 g. of powdered KI mixed
with an equal bulk of MnO2, add H2SO4 enough to cover well, shake
together, complete the apparatus as for making Br, and heat.
Notice the color of the vapor, and any sublimate. The direct
product of the solidification of a vapor is called a sublimate.
The process is sublimation. Observe any crystals formed. Write
the reaction, and compare the process with that for making Br and
Cl. Compare the vapor density of I with that of Br and of Cl.
With that of air. What vapor is heavier than I? What acid and
what base are represented by KI?
162. Tests.
Experiment 99.--(1) Put a crystal of I in the palm of the hand
and watch it for a minute. (2) Put 2 or 3 crystals into a t.t.,
and warm it, meanwhile holding a stirring-rod half-way down the
tube. Notice the vapor, also a sublimate on the sides of the t.t.
and rod. (3) Add to 2 or 3 crystals in a t.t. 5 cc. of alcohol,
C2H5OH; warm it, and see whether a solution is formed. If so, add
5 cc. H2O and look for a ppt. of I. Does this show that I is not
at all soluble in H2O, or not so soluble as in alcohol?
163. Starch Solution and Iodine Test.
Experiment 100.--Pulverize a gram or two of starch, put it into
an evaporating-dish, add 4 or 5 drops of water, and mix; then
heat to the boiling-point 10 cc. H2O in a t.t., and pour it over
the starch, stirring it meanwhile.
(1) Dip into this starch paste a piece of paper, hold it in the
vapor of I, and look for a change of color. (2) Pour a drop of
the starch paste into a clean t.t., and add a drop or two of the
solution of I in alcohol. Add 5 cc. H2O, note the color, then
boil, and finally cool. (3) The presence of starch in a potato or
apple can be shown by putting a drop of I solution in alcohol on
a slice of either, and observing the color. (4) Try to dissolve a
few crystals of I in 5 cc. H2O by boiling. If it does not
disappear, see whether any has dissolved, by touching a drop of
the water to starch paste. This should show that I is slightly
soluble in water.
164. Iodo-Starch Paper.
Experiment 101.--Add to some starch paste that contains no I 5
cc. of a solution of KI, and stir the mixture. Why is it not
colored blue? Dip into this several strips of paper, dry them,
and save for use. This paper is called iodo-starch paper, and is
used as a test for ozone, chlorine, etc. Bring a piece of it in
contact with the vapor of chlorine, bromine, or ozone, and notice
the blue color.
Experiment 102.--Add a few drops of chlorine water to 2cc. of the
starch and KI solution in 10 cc. H2O. This should show the same
effect as the previous experiment.
165. Explanation.--Only free I, not compounds of it, will color
starch blue. It must first be set free from KI. Ozone, chlorine,
etc., have a strong affinity for K, and when brought in contact
with KI they unite with K and set free I, which then acts on the
starch present. Com- plete the equation: KI + Cl = ?
166. Occurrence.--The ultimate source of I is sea water, of which
it constitutes far too small a percentage to be separated
artificially. Sea-weeds, or algae, especially those growing in
the deep sea, absorb its salts--NaI, KI, etc.--from the water. It
thus forms a part of the plant, and from this much of the I of
commerce is obtained. Algae are collected in the spring, on the
coasts of Ireland, Scotland, and Normandy, where rough weather
throws them up. They are dried, and finally burned or distilled;
the ashes are leached to dissolve I salts; the water is nearly
evaporated, and the residue is treated with H2SO4, and MnO2, as
in the case of Br and Cl. I also occurs in Chili, as NaI and
NaIO3, mixed with NaNO3. This is an important source of the I
supply.
167. Uses.--I is much used in medicine, and was formerly employed
in taking daguerreotypes and photographs. Its solution in alcohol
or in ether is known as tincture of iodine.
168. Fluorine.--F, Cl, Br, I, are called halogens or haloids, and
exist in compounds--salts--in sea water. F is the most active of
all elements, combining with every element except O. Until
recently it has never been isolated, for as soon as set free from
one compound it attacks the nearest substance, and seems to be as
much averse to combining with itself, or to existing in the
elementary state, as to uniting with O. It is supposed to be a
gas, and, as is claimed, has lately been isolated by electrolysis
from HF in a Pt U-tube. Fluorite (CaF2) and cryolite (Al2F6 + 6
NaF) are its two principal mineral sources. The enamel of the
teeth contains F in composition.