V. (Victor) Regnault.

Elements of chemistry : for the use of colleges, academies, and schools (Volume 2) online

. (page 4 of 87)
Online LibraryV. (Victor) RegnaultElements of chemistry : for the use of colleges, academies, and schools (Volume 2) → online text (page 4 of 87)
Font size
QR-code for this ebook


ment gives the quantity of acid which lias remained free, and shows,
when compared with the former experiment, the quantity of acid
required by the oxide of manganese.*

* The following is a shorter method of testing peroxides of manganese. The
chlorine disengaged from a weighed quantity of the oxide is conducted into the
solution of a given quantity of a protosalt of iron, an equivalent quantity of which
it oxidizes to peroxide ; so that, if the remaining quantity of protoxide of iron
which is determined with permanganate of potassa (as will be described in 1 g 804)
be subtracted from the quantity contained in the protosalt employed, the differ-
ence will be in proportion to the chlorine disengaged.

The protosalt of iron best adapted to the purpose is the protosulphate of iron
and ammonia, which is easily obtained by mixing equal volumes of saturated solu-
tions of sulphate of iron and sulphate of ammonia, when the liquid on evaporating
yields prismatic crystals of the salt, the formula of which is FeO,S0 3 -f-NH 4 0,
g0 3 -{-6HO. One hundred grammes of the salt are dissolved in 1837 cubic centi-
metres of water, so that the solution contains 5.44 per cent, of the salt; or, 544
parts of the salt corresponding to 184 parts of pure protoxide, exactly one per
cent, of protoxide of iron : and the standard solution thus obtained, which is best
prepared in larger quantities at a time, is used for all chlorometric determina-
tions, as well as for that of chrome.

Supposing the quantity of oxide subjected to the test to be exactly one gramme,
and the substance to be pure peroxide, which gives one equivalent of chlorine ;
then will the quantity of chlorine developed be 0.807 gm. ; and supposing the
quantity of the standard solution of iron employed to be 200 cubic centimetres,
which contain 2 gm. of protoxide, only 1.63 of which are oxidized by the chlorine;



TESTING THE OXIDES OF MANGANESE. 35

then will the 0.37 gm. of protoxide, determined directly by permanganate of po-
tassa, and subtracted from the 2 gm. employed, give the quantity of protoxide
which was oxidized, viz. 1.63 gm., which correspond to 0.807 gm. of chlorine, as one
equivalent of chlorine oxidizes two equivalents of protoxide of iron. W. L. F.

Another method of determining the commercial value of peroxide of manganese,
better than that described in the text, is to employ dry oxalate of soda, which is
easily prepared and preserved, and of which 152^ grains are just sufficient for
100 grs. of pure binoxide, in order that its oxalic acid may be wholly converted
into 100 grs. of carbonic acid. 76 grs. of the dry oxalate and 50 grs. of the per-
oxide are introduced with about oz. of water into a small flask containing two
tubulures, through one of which an S-tube passes, and through the other a small
tube connected with a tube of sulphuric-pumice or chloride of calcium. The
whole apparatus being weighed at once, together with about 200 grs. of oil of
vitriol, the latter is gradually poured through the S-tube into the little flask.
The oil of vitriol disengages the oxalic acid, which is oxidized into carbonic acid
by the excess of oxygen over that in the protoxide, and since it cannot pass through
either escape-tubes without being dried, the loss of weight of the whole apparatus
indicates the loss of carbonic acid alone. The number of grains of loss being
doubled, gives the percentage of peroxide equivalent to pure binoxide. The dif-
ferent methods of arranging the apparatus will be found in the analytical chemis-
tries of Rose and Fresenius, and others, and in the Encyclop. of Chem. The best
commercial varieties contain from 80 to 98 per cent, of binoxide. J. 0. B.



36



IRON.

EQUIVALENT = 28.0 (0=100 ; 350.0).

766. On account of its numerous technical applications, iron
is the most important of all the metals. It is used in three states :

1. Bar or malleable iron.

2. Steel.

3. Crude or cast-iron.

Steel and cast-iron are combinations of iron with small but vari-
able quantities of carbon and silicium.

The bar-iron of commerce is not chemically pure, as it contains
a small quantity of carbon, and often traces of silicium, sulphur, or
phosphorus, which latter remarkably affects its quality. The iron
used in fine locksmith's work approaches a state of purity ; but the
purest iron is found in piano-forte wires, or ordinary wire, because
only iron of great purity can be drawn out into very fine threads.

In order to obtain iron chemically pure, some wire is cut into
pieces of the same length, and tied in bundles ; when their surface
is oxidized, by heating them for a few moments exposed to the air,
or better still, in a porcelain tube through which steam is passed.
The bundles of oxidized iron are then placeM in a small porcelain
crucible with a small quantity of powdered glass ; and the crucible
being set in a second earthen crucible, luted externally with clay,
is heated in a blast-furnace at the highest temperature that can be
produced. The small quantities of foreign matter contained in the
iron, are burned by the oxygen of the oxide, while the excess of oxide
of iron, combining with the glass, forms a slag. If the temperature
be sufficiently elevated the purified iron fuses to a single lump.
Pure iron is whiter and more malleable than the iron of commerce,
but less tenacious.

Pure iron may likewise be obtained by the reduction of one of
its oxides by hydrogen, which takes place at a dull red-heat, and
may be effected in the small apparatus described (fig. 473) for
the preparation of the protoxide of manganese. The metallic iron
remains in the tube, in the form of a grayish-black powder, which
may be preserved by closing hermetically both ends of the tube
while it is filled with hydrogen gas ; for very finely divided iron
has so great an affinity to oxygen that it is inflamed by contact
with the air ; a property which has given to it the name of pyro-
phoric iron. If the reduction be made in a porcelain tube at a
high temperature, the metal becomes solid, assuming a metallic
lustre, and no longer oxidizing in dry air.



IRON. 37

Perfectly pure iron may also be procured, by heating protochlo-
ride of iron in a glass tube, through which a current of hydrogen
gas is passed ; when the iron forms on the sides of the glass a glit-
tering, brilliant coating, in which small cubic crystals may often
be seen.

767. The texture of commercial iron varies greatly, according
to its mode of manufacture. Pure iron which has been forged and
rolled equally in all directions, exhibits a texture of very small,
brilliant grains ; but, when drawn out into bars, its texture is often
decidedly fibrous, the fibres always running in the direction of the
bar, which may be readily proved by breaking the latter. The
fibrous texture is highly esteemed, because the iron possessing it
is much more tenacious than granular iron, and bears a greater
weight without breaking. The fibrous texture of iron is generally
regarded as an index of its go6d quality ; however, skilful work-
men can impart this quality also to bars of an inferior sort. Iron
of fibrous texture does not always remain in that state, but after
some time changes into the granular, or even the laminated tex-
ture ; which change ensues most rapidly when the bars are sub-
jected to vibration, as, for instance, when they support the floor
of a suspension-bridge. The tenacity of the metal diminishes at
the same time in a remarkable manner, and it frequently breaks
with a load which the bar would easily have borne when its tex-
ture was fibrous. A change of this kind is frequently observed in
the axles of locomotives and railway-cars.*

The specific gravity of wrought-iron varies from 7.7 to 7.9. Iron
is the most tenacious of all the metals, a cylindrical iron-wire of
2 millimetres in diameter being able to sustain a load of 250 kilogs.

768. The highest temperature that can be produced in a blast-
furnace is required for the fusion of iron, which, however, is more
easy when it can be combined with carbon. Iron passes from the
fluid to the solid, through the doughy state, and therefore belongs
to that class of substances which crystallize with difficulty by fusion.
However, if large masses of iron, heated to a very high temperature,
be allowed to cool very slowly, indications of crystallization of the
cubic form are found in the interior of these masses. f Heated to a
white-heat, iron becomes sufficiently soft to assume any form under
the hammer; and two bars, when heated to redness, can be readily
soldered to each other without the interposition of another metal,
when the surfaces to be joined are completely free from oxide.

* The fibrous texture of iron is also changed to the granular by heating the
metal to redness, and immersing it while hot into cold water. W. L. F.

f Some species of cast-iron, as, for example, that made from the mangauiferous
sparry iron-ore of Muesen in Westphalia, and that made at Easton, in Pennsyl-
vania, the latter of which is remarkable for its extreme ductility when converted
into bar-iron, show a laminated texture, which is owing to its being an aggregated
mass of laminated prismatic crystals, the angles of which are about 112. W. L. F.



38 IRON.

Now, as it is known that iron heated in the air soon oxidizes, the
blacksmith generally throws a small quantity of sand upon the
bars he wishes to solder, which, by combining with the oxide of
iron, produces a very fusible silicate, which, forming a kind of
varnish on the surface of the metal and preventing its further oxi-
dization, is afterward, from its extreme fluidity, entirely driven off
by the blows of the hammer.

769. Iron, cobalt, and nickel are the only metals which are
remarkably magnetic at the ordinary temperature. A piece of pure
iron immediately becomes a magnet, either by contact with or at
a short distance from a native magnet, its magnetic properties dis-
appearing again as soon as the magnet is removed ; but if the iron
is combined with a small quantity of carbon, if it is steely, the
magnetism is slower of development, but continues longer after the
removal of the magnet. A bar of steel, rubbed against a magnet,
acquires permanent magnetic properties, and becomes a true mag-
net. The magnetic properties of iron diminish rapidly with the
temperature, an iron ball heated to a whitish red-heat no longer
exerting any influence over the needle, but recovering its magnetic
virtue on cooling.

770. Iron remains unchanged for an indefinite time in dry air,
and even in dry oxygen, at the ordinary temperature ; but soon
alters in moist air, by becoming covered with rust. The rust of
iron, which consists of an oxidation of its surface, is most readily
formed in the presence of carbonic acid, of which the air always
contains a small quantity. Under the influence of the carbonic
acid and the oxygen, the surface of the iron is converted into proto-
carbonate, which, on absorbing a new portion of oxygen, is trans-
formed into hydrated peroxide of iron, while the carbonic acid
disengaged favours the oxidation of an additional quantity of metal-
lic iron. It has been observed, that when iron has begun to rust
at any particular point, it changes very rapidly around this point,
which is produced by a galvanic phenomenon accelerating the oxi-
dation. The iron and thin layer of oxide which forms on its surface
constitute the two elements of a pile in which the iron becomes posi-
tive, and thus acquires an affinity for oxygen sufficiently great to
decompose water at the ordinary temperature, with the evolution of
hydrogen gas. This phenomenon is rendered very evident by allow-
ing moist iron-filings to rust in the air, when, after some time, the
odour exhaled by hydrogen gas* made from the carburetted metals
is easily recognised. Rust almost always contains a small quantity
of ammonia, the presence of which may be recognised by heating it
with potassa, and is explained as follows : It has been shown (122)

* This peculiar odour is not exhaled by hydrogen gas, but is that of a certain
substance called ozone, and shown by Bunsen to be a combination of one atom of
hydrogen with three of oxygen, which forms under almost all circumstances where
a, galvanic current is active. W. L. F.



IRON. 39

that when hydrogen and nitrogen meet in the nascent state in a
liquid, they combine and form ammonia : now, the water which
moistens the rust, being in contact with the air, contains nitrogen
in solution, and on the other hand, hydrogen is disengaged by the
decomposition of the water. The circumstances under which am-
monia can form by the direct combination of hydrogen and nitro-
gen are therefore realized. The peroxide of iron, which acts with
very powerful bases the part of a feeble acid, retains the ammonia
and prevents it from being disengaged.

It is important to be aware of the presence of ammonia in rust,
as it has been long since admitted, that when spots of rust which
were found on sidearms or steel weapons, suspected to have been
used in the commission of a crime, evolved ammonia by contact with
potassa, it was a proof that the rust was formed by contact with
animal matter, and these spots of blood were the cause of its pre-
sence. This presumption was erroneous ; for as we have just seen,
steel-rust formed by the contact of air alone may contain an appre-
ciable quantity of ammonia.

Rust soon changes in fresh water, but very slightly in water
containing a few thousandths of carbonate of soda or potassa.
During the last few years, iron has been preserved from rust by
covering its surface with a very thin layer of metallic zinc,* and
iron thus coated is called galvanized iron. This phenomenon was
explained in 305.

Iron soon oxidizes by contact with the air when heated to red-
ness, becoming covered with a black pellicle of oxide, which falls
off under the hammer. To this easy combustion of iron in the air
may be attributed the property which it possesses of giving out
sparks when struck by a flint, in which case small particles are
detached, which, being strongly heated by friction against the flint,
become incandescent by combining with the oxygen of the air, and
may easily inflame combustible substances, such as tinder. If
the steel be struck for some time over a sheet of white paper, the
latter will be covered with small black particles, which are attracted
by the magnet, and are, in fact, small spherical globules of mag-
netic iron.

771. Iron is readily acted on by chlorohydric acid, protochlo-
ride of iron being formed, and hydrogen disengaged. Dilute cold
sulphuric acid dissolves it with the evolution of hydrogen, while
the concentrated acid also attacks it, but disengages sulphurous
acid. Concentrated nitric acid attacks it sharply with a copious

* A patent has lately been taken out in Europe (Vienna ?) for preserving iron
from rust by a coating of metallic cadmium, which at the same time imparts a
silvery lustre to the surface. Silicate of potassa, the German wasserglas, has also
been employed. W. L. F.



40 IRON.

disengagement with nitrous fumes,* while the dilute acid dissolves
it without any apparent evolution of gas, forming at the same time
protonitrate of iron and nitrate of ammonia (122).

COMPOUNDS OF IRON WITH OXYGEN.

772. Three compounds of iron with oxygen are known :

1. A protoxide FeO, which is a powerful base, isomorphous with
the bases of which the formula is RO.

2. A sesquioxide Fe 2 3 , being a very feeble base, analogous to
alumina, and isomorphous with the oxides of which the formu]a is
K S S .

3. Lastly, an acid Fe0 3 , analogous to manganic acid.

A fourth compound of iron with oxygen, of the formula Fe 3 4 , is
also known, and is called magnetic oxide', but as it behaves like a
compound of protoxide and sesquioxide FeO,Fe a 3 , it is regarded
as such.

Protoxide of Iron FeO.

773. Protoxide of iron has hitherto not been obtained in a
state of purity. When a large bar of iron heated to redness is
allowed to cool slowly in the air, its surface oxidizes, and a black
pellicle of a metallic lustre is formed, which falls off under the
hammer, and is called finery cinder. If a thin piece of cinder be
examined with a lens, it is seen to be composed of several layers ;
the outer stratum showing nearly the composition of magnetic
oxide Fe 3 4 , while the inside layer, or that immediately in contact
with the metal, resembles the protoxide very closely.

If a solution of caustic potassa be added to a protosalt of iron,
a white precipitate of hydrated protoxide is obtained, which soon
turns green on exposure to the air, by forming hydrated sesqui-
oxide by absorption of oxygen. If boiling solutions be used, and
the ebullition prolonged for some time, the white precipitate loses
its water of hydration and becomes black ; but the oxide has such
an affinity for oxygen that it is impossible to collect it unchanged.
It even decomposes water at the boiling point, and is ultimately
converted into magnetic oxide.

French bottle-glass owes its hue to the presence of this oxide
( 684), which imparts a deep green colour to fluxes.

* Very concentrated nitric acid "will not dissolve pure iron at all, owing to an
electrical phenomenon by which the iron is brought to the passive state, and changes
its electropositive power. The iron will continue in this state, and not be attacked
by the acid, even on diluting the latter to almost any degree ; but on touching the
piece of passive iron, lying in the diluted acid, with a piece of common iron, such
as a key, the galvanic current produced by the contact of the two pieces, whose
electromotive power is yet different, instantly changes the passive iron back to
its natural state, and renders it soluble. W. L. F.



OXIDES OF IRON. 41

Sesquioxide of Iron Fe 3 3 .

774. The sesquioxide Fe 3 3 , or peroxide, is a substance abun-
dantly met with in nature, occurring either in the anhydrous or the
hydrated state. The anhydrous peroxide forms flattened rhombo-
hedral crystals, very brilliant and nearly black, while their powder
is of a deep red colour. Mineralogists call it specular iron: it is
found in veins in the old rocks. In the fissures of volcanic lavas,
thin and brilliant laminae of peroxide of iron are often found,
having the form of regular hexagons, and also belonging to the
class of specular iron. Anhydrous peroxide of iron, which is also
found in compact masses, of an intense red colour, is called by mine-
ralogists red hematite, and is known in the arts by the name of
bloodstone, a substance extensively employed for polishing metals.

Peroxide of iron is prepared artificially by calcining protosul-
phate of iron, when sulphurous and sulphuric acids are disengaged,
and the peroxide remains in the form of a red powder :

2(S0 3 ,FeO)=Fe 3 3 +S0 3 +S0 3 .

Peroxide of iron thus prepared is known by the name of colco-
thar, and used for painting, for polishing silver, and for giving the
last polish to mirrors. The intensity of colour of peroxide of iron
is in proportion to its compactness.

Peroxide of iron may be obtained in the form of small crystal-
line lamellae, of great lustre and nearly black, by calcining in a
crucible 1 part of sulphate of iron with 3 parts of sea-salt. The
calcined matter is treated with boiling water, which leaves the per-
oxide.

775. Hydrated peroxide of iron is prepared by adding potassa
or ammonia to the solution of a sesquisalt of iron, when a copious
brown precipitate is formed. When the reaction has been effected
by caustic potassa, the precipitate always retains a small quantity
of alkali, which is removed with difficulty only by prolonged boil-
ing with pure water. The precipitation may be made by a solution
of carbonate of potassa or soda, in which case the precipitate is
also hydrated peroxide of iron, the carbonic acid being disen-
gaged, or combining with the excess of neutral carbonate, which it
transforms into bicarbonate/

Hydrated peroxide of iron parts readily with its water by the
application of heat, but when heated still further, a temperature is
soon attained at which the oxide suddenly becomes incandescent
from a spontaneous evolution of heat. This incandescence is only
momentary, and the temperature of the oxide again falls to that
of the vessel in which it is heated ; but its physical and chemical
properties have been remarkably modified, as it has become more
compact, and dissolves with great difficulty even in highly concen-



42 IRON.

trated acids. Sesquioxide of iron, heated to a high white-heat,
loses a portion of its oxygen, and is converted into magnetic oxide
Fe 3 4 .

Peroxide of iron colours fluxes of a reddish yellow, but a consi-
derable quantity is necessary to produce this effect in glass. The
small quantity of protoxide which imparts a deep green hue to a
vitreous flux, does not colour it appreciably when converted into
peroxide ( 674).

Magnetic oxide of iron Fe 3 4 .

T76. A native oxide of iron, intermediate between the prot-
oxide and peroxide, is often found in very regular, brilliant octa-
hedrons, of a fine metallic lustre. At other times it is found in
the old rocks in compact masses, often very large, and is worked
as an iron ore. Large quantities of it are found at Dannemora,
in Sweden, and from this ore the best quality of iron is obtained.
This compound has been called magnetic oxide, from its possessing
very highly developed magnetic properties. Native loadstone is
formed of this oxide of iron.

Magnetic oxide of iron is only produced when iron burns at a
high temperature in the air, or in oxygen; for example, by the
rapid combustion of iron-wire in pure oxygen ( 64). But the most
certain method of obtaining it in the laboratory consists in heating
iron-wire in a porcelain tube, in a current of steam, as in the ex-
periment described in 68, when the surface of the wire becomes
covered with an infinite number of small, very brilliant crystals,
which by the aid of a lens are seen to be regular octahedrons, resem-
bling those of the native magnetic oxide.

This oxide may also be obtained in the hydrated state, by dis-
solving the magnetic oxide in chlorohydric acid, and adding a
large excess of ammonia, when a deep green precipitate, becoming
black by desiccation, is formed. This hydrate is magnetic, like the
anhydrous oxide. Hydrated magnetic oxide may likewise be pre-
pared by pouring into ammonia a mixture of equal equivalents of
persulphate and protosulphate of iron. In order to make this
mixture, two equal volumes of the same solution of protosulphate
of iron are used, one of which is transformed into persulphate by
evaporating it to dryness with nitric and sulphuric acids, and then
redissolved in the other volume of protosulphate.

The magnetic oxide does not behave like an oxide per se, but
rather like a compound of protoxide and peroxide. Its formula is
properly FeO,Fe 3 3 , analogous to that of red oxide of manganese
MnO,Mn 3 3 . The solution of magnetic oxide in an acid possesses
the properties of a mixture of a protosalt with a sesquisalt ; and
if an alkali is dropped into the liquid, the peroxide is precipitated
before the protoxide. In order to precipitate the two oxides in



OXIDES OF IRON. 43

combination the proceeding must be inverted, and the solution of
the salt of iron be poured into the alkaline liquid. We shall, more-
over, soon see several compounds presenting a similar chemical
formula, and affecting identical crystalline forms, but in which
the peroxide of iron is 'often replaced by alumina or by oxide of
chrome, while magnesia, protoxide of manganese, or oxide of zinc
often take the place of the protoxide.

Ferric acid Fe0 3 .

777. The third compound of iron with oxygen possesses the
properties of an acid corresponding with manganic acid, and is
formed under the same circumstances. A mixture of iron filings
and nitrate of potassa is heated to redness in an iron crucible, when
a beautiful red solution of ferrate of potassa is obtained by treating
the mass with water, resembling permanganate of potassa in colour.
It is also procured by passing chlorine through a concentrated
solution of caustic potassa, containing hydrated peroxide of iron in
suspension. Pieces of caustic potassa are added from time to time,
in order constantly to maintain a large excess of alkali in the liquid.
Ferrate of potassa, being nearly insoluble in a concentrated solu-
tion of potassa, is deposited in the form of a black powder, which
may be almost entirely separated from the mother liquid by drying
it on unglazed porcelain. Ferrate of potassa is stiU less fixed than



Online LibraryV. (Victor) RegnaultElements of chemistry : for the use of colleges, academies, and schools (Volume 2) → online text (page 4 of 87)