IODINE.
Iodine belongs to what may be called a chemical family, the other
members being chlorine and bromine. All three of these elements are
found in sea-water, but in very different quantities. Thus chlorine
is extremely abundant; bromine is in the water in minute quantities,
while iodine exists there in amounts that are exceedingly small. They
all exist as salts, of which of course chloride of sodium is by far the
most abundant. It has already been shown that bromine is obtained from
sea-water, after enormous amounts of the water have been concentrated
by evaporation. But iodine, the third element of this group, exists
in sea-water in quantities so very minute that it cannot be extracted
from it at any practicable cost. Even the concentration method, just
alluded to, is not applicable in the case of iodine. It happens however
that sea-weeds have the power of extracting from sea-water even the
exceedingly minute amount of iodine, or of iodides, that the water
contains; and moreover when sea-weeds are burned, iodides are found in
their ashes.
The Discovery of Iodine.
The discovery of iodine is associated with the history of certain of
the most important and interesting products of the chemical arts. It
also has a striking connection with some of the political and military
affairs in France, and indeed in Europe, in the early years of the
present century. Finally, its great usefulness to mankind is in marked
contrast with the misfortunes that overtook its discoverer.
[Illustration: FIG. 26.—Gathering the harvest of sea-weed for the
manufacture of soda-ash.]
The discovery of iodine is directly referable to the old soda industry.
The term soda is a general one, and it was formerly used to include
several different chemical compounds manufactured from the ashes of
sea-weed. Decidedly the most important of these is sodic carbonate.
This substance has a well marked alkaline reaction, and although not
an alkali in the strictest chemical sense, it is yet the principal
product of that greatest of all the chemical industries known as the
alkali trade. (See pp. 94 and 99.) During the last sixty years, and
after many early trials and failures, the production of the various
alkaline compounds of sodium has risen to enormous proportions, such
that in England alone the daily product of sodic carbonate, the
principal one, is probably more than two thousand tons. This vast
amount of alkali is consumed by civilized peoples in some of their most
extensive industries such as the manufacture of soap and of glass,
and in many processes of bleaching. The extension of these branches of
business has of course gone hand in hand with the increased production
of alkali. Indeed, on the one side there has been a steady diminution
in price, and on the other a steady increase in consumption; probably
each circumstance may be considered as both cause and effect of the
other. Prior to 1793 however, the demands for alkali—vastly smaller
than to-day—were all satisfied by the material obtained from the ashes
of marine plants. Thus along the coasts of Great Britain, France,
and especially of Spain, sea-weed of various kinds was gathered as a
very important harvest. Some of the weed was used as a fertilizer of
the soil; more was dried and burned for the sake of the ashes. On the
British coast the ash was known as _kelp_; that produced on the coasts
of Normandy was called _varech_; and that produced on the Spanish coast
went by the name of _barilla_.
[Illustration: FIG. 27.—Varieties of sea-weed used to produce varech.]
Now one of the important indirect effects of the French Revolution was
that felt by the consumers of the old-fashioned alkali. In 1793 an
embargo was put upon the supply of alkaline ashes, such as kelp and
barilla, into France. But the French demand for alkali, not only for
ordinary purposes, but also for the production of the great amounts of
saltpetre required for the manufacture of gunpowder, was imperious.
The immediate effect, therefore, was to create the sudden development
of a process called the Leblanc method, by which alkaline compounds
of sodium are made from common salt. Notwithstanding the stimulus of
the prohibitory embargo and the fostering help of the government of
Napoleon Bonaparte, the complexity of the Leblanc process was such that
it was slow in gaining a foothold as a practical industrial method.
But after its first successful establishment as a regular business
and up to almost the present day the application of this process has
continually widened, and the method has held undivided sway in its
important field. In the year 1811 Bernard Courtois, a French chemist,
was engaged, just as other manufacturers were, in the production of
nitrate of potash, or saltpetre, for use in gunpowder. In the course of
this work he employed soda obtained from varech. In order to separate
the alkali from the varech in a more refined condition the raw varech
was subjected to a very careful purification. At certain stages of
his experiments Courtois discovered that the addition of sulphuric
acid gave rise to the production of a magnificent violet vapor. He
did not make the matter public however until late in the year 1813,
when he brought the subject to the attention of Sir Humphry Davy, the
distinguished English chemist, who was then visiting Paris. The next
year, 1814, the substance was carefully investigated by Gay-Lussac,
who gave to the world a very full description of its properties, and
who called it iodine from a Greek word (ἰοειδής, _ioeides_, violet
colored), suggesting the striking and characteristic color of its
vapor. The political events of 1815 ruined the business of Courtois,
and he sunk into poverty from which he was not able to recover, until
finally he died in 1838, poor and almost forgotten, leaving a widow who
was forced to win her bread by the labor of her hands.
Present Sources of Iodine.
Although kelp, varech and barilla are no longer used for the direct
purpose of affording alkali, they are still produced with a view to
their yielding iodine. On the rough and stormy coasts of Scotland,
Ireland, France and Spain, large quantities of sea-weeds are cast
ashore. They are collected, they are dried in the sun, they are then
burned, and their ashes are employed—but principally in the manufacture
of iodine. Thus on the coasts of Brittany and Normandy the occupation
of collecting weeds occupies three or four thousand families for the
larger part of the year.
Experimental Method of Preparing Iodine.
Iodine may be prepared in a manner closely resembling the process
already described for bromine; that is, by placing in a suitable glass
vessel a small amount of manganese dioxide, some potassic iodide
(commonly known as iodide of potassium), then some water, and finally
a small quantity of chlorohydric acid. Iodine is almost instantly
liberated, and shows its presence by imparting to the liquid a brownish
color. If the vessel is covered lightly and then gentle heat is applied
to it the iodine will be expelled and appear in the vessel above the
liquid as a heavy vapor of a rich violet color. This vapor readily
condenses on the upper and colder portions of the vessel in the form of
minute crystals of a color almost black. This is almost precisely the
method employed on the large scale for the production of iodine from
kelp.
[Illustration: FIG. 28.—Changing iodine to a violet vapor by means of
heat.]
Chemical Properties of Iodine.
The chemical characteristics of iodine are throughout closely allied to
those of chlorine and of bromine, only in general iodine may be said to
have weaker chemical affinities than either of the other two.
[Illustration: FIG. 29.—Apparatus used in the manufacturing process for
obtaining iodine. The retorts C, C, are surrounded by sand (sand-bath);
the heat drives iodine, in form of vapor, into the receivers, A, A,
where it solidifies.]
Iodine produces compounds of the same general type as the others,
and of this an example is found in argentic iodide. The following
method of producing it can be followed by almost any one. Prepare a
solution of nitrate of silver in water, and then add a water solution
of potassic iodide; a chemical change takes place, with the production
of a yellowish-white precipitate. This precipitate is argentic iodide.
Upon exposure to sunlight it readily changes in color, becoming almost
black. This is an important characteristic and is made use of, as is
the same property possessed by argentic bromide and also by argentic
chloride, in the production of the photograph. And while it is a fact,
and one well known, that many of the salts of silver blacken more or
less upon exposure to sunlight, it is found that the chloride, the
bromide, and the iodide, have properties particularly fitting them for
the purposes of photography. In discussing bromine, reference was made
to the influence of the great expansion of the photographic business;
and this circumstance has stimulated the demand for iodine just as for
bromine. It was also pointed out, that potassic bromide is an important
remedial agent; potassic iodide is likewise of great medicinal value.
Starch as a Test for Iodine.
Iodine, when in the free or uncombined condition, has a remarkable and
very peculiar way of attaching itself to granules of starch.
This property may be demonstrated by a simple and attractive
experiment. Thus if starch is boiled with water and then the hot mass
is poured into cold water, minute particles of starch distribute
themselves through the liquid. If to this liquid a very small amount
of free iodine, in the form of a solution, is added, the starch
instantly takes on a deep blue color. If to another portion of the same
or similar starch suspended in water, iodine is added _in a combined
form_—that is as potassic iodide for example—absolutely no change of
color is detected. These two experiments show that the iodine only
attacks starch when the iodine is _free and uncombined_.
READING REFERENCES.
Chlorine, Bromine, Iodine, and Fluorine.
=Mylius=, E.—Chem. News. xxxiii, 244, 253; xxxiv, 5, 13, 25,
33, 45, 55, 66, 78, 86, 118, 139, 149, 166, 180,
188, 197, 215, 233.
Iodine, Manufacture of
=Schmidt=, T.—Chem. News. xxxvii, 56.
=Stanford=, E. C. C.—_Loc. cit._ xxxv, 172.