V. (Victor) Regnault.

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

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and the residue treated with ammonia, which dissolves the molybdic
acid during the evaporation of the liquid. The molybdate of am-
monia, which separates in crystals, is converted into molybdic acid
when heated in the air. Molybdic acid may also be separated by
pouring chlorohydric acid into a solution of a molybdate.

Molybdic acid is a white powder, which sublimes at a strong red-
heat in white crystalline spangles ; which operation can be well

* Discovered by Scheele, in 1778.


performed only in a current of gas. Although molybdic acid is
very feebly soluble in water when freshly precipitated by an acid,
it readily dissolves after calcination. It is easily soluble in the

Protoxide of molybdenum MoO is obtained by pouring chloro-
bydric acid into the solution of an alkaline molybdate, until the
molybdic acid, which is at first precipitated, is redissolved, when a
Hade of zinc is plunged into the liquid, which is turned black, after
passing through the shades of blue and brownish-red successively.
Ammonia is then carefully added to the liquid containing proto-
chloride of molybdenum and chloride of zinc ; and, as the protoxide
of molybdenum is precipitated first, the addition of ammonia is ar-
rested as soon as the liquid becomes clouded. The precipitate
should be washed rapidly, and protected as much as possible from
the air, because it is a great absorbent of oxygen.

Binoxide of molybdenum Mo0 3 is prepared by decomposing molyb-
date of ammonia by heat, protected from the air, or by calcining a
mixture of molybdate of soda and sal-ammoniac. This oxide, a red-
dish-brown crystalline powder, forms a reddish-brown hydrate,
which resembles the hydrate of sesquioxide of iron.

By adding ammonia to the blue liquid obtained by partially re-
ducing by zinc a chlorohydric solution of molybdic acid, a blue pre-
cipitate is formed, which is a saline oxide resulting from the com-
bination of molybdic acid with binoxide of molybdenum.


1034. Both the protoxide and binoxide of molybdenum form
salts by combining with acids.

These two classes of salts present the following reactions : The
alkalies and ammonia yield brown precipitates, while the alkaline
carbonates afford the same coloured precipitate, which dissolves
in a large excess of the carbonate of ammonia. Sulf hydric acid
precipitates them completely after some time as a black deposit,
the same precipitate being formed with the alkaline sulf hydrates ;
in an excess of which it is soluble. The salts of the protoxide im-
part to their solutions a brown colour approaching a black, while
those of the sesquioxide produce a deep red colour.


1035. Molybdic acid forms two series of salts : neutral molyb-
dates RO,Mo0 3 and bimolybdates RO,2Mo0 3 ; the former of which
are obtained by dissolving molybdic acid in an excess of alkali, and
the latter by boiling a solution of an alkali or an alkaline carbonate
with an excess of molybdic acid. The bimolybdates generally crys-
tallize during the cooling of the liquid.



1036. Metallic molybdenum combines directly . with chlorine,
yielding at a high temperature a red vapour, which condenses in
the form of crystals closely resembling those of iodine. The for-
mula of the chloride, which dissolves freely in water, is MoCl a .

A protochloride of molybdenum is obtained by dissolving the hy-
drated protoxide in chlorohydric acid.

By passing chlorine over heated binoxide of molybdenum, small
and very soluble spangles are sublimed, the formula of which is
Mo0 3 Cl, corresponding to chlorochromic and chlorotungstic acids.


EQUIVALENT = 68.6 (857.5; = 100).

1037. Vanadium* is an exceedingly rare metal, found in very
small quantities in certain Swedish iron-ores, and also occur-
ring in the state of vanadate of lead. Vanadium is obtained by
heating vanadic acid with potassium in a platinum crucible ; when
active reaction takes place, after which the substance is treated
with water to dissolve the potassa, and the metal remains in the
form of a black powder. It may also be prepared by decomposing
chloride of vanadium by ammoniacal gas at a red-heat, in which
case it presents the appearance of a flaky, silvery-white mass.

1038. Vanadium forms three compounds with oxygen : the
protoxide VO, the binoxide V0 2 , and vanadic acid V0 3 .

Vanadic acid is readily obtained from the native vanadate of
lead, by heating the mineral with nitric acid, when vanadic acid is
set free, while nitrate of lead is formed. It is treated with water,
which leaves the vanadic acid. The acid is dissolved in ammonia,
and the vanadate of ammonia crystallized by the evaporation of
the liquid, after which it is converted into vanadic acid by calcina-
tion in the air. Vanadic acid is an orange-coloured or brown
powder, nearly insoluble in water. It is reduced to a lower degree
of oxidation by many reducing substances, such as alcohol, sugar,
oxalic and sulphurous acids. It dissolves in cold chlorohydric acid,
while, if heat be applied, chlorine is disengaged, and the solution
contains chloride of vanadium VC1 3 . By pouring carbonate of po-
tassa into this solution, hydrated binoxide of vanadium is precipi-
tated as a gray flaky substance, which dissolves readily in acids,
and produces crystallizable salts, of which the solutions are blue.

* Vanadium was discovered in 1830, by M. Sefstrom, a Swedish chemist.


By heating vanadic acid in a current of hydrogen gas, a black
powder of protoxide of vanadium VO is obtained, no saline com-
pounds of which are known.

If a mixture of vanadic acid and charcoal be heated in a current
of chlorine, a volatile chloride VC1 3 is formed, which condenses as
a yellow liquid. It boils at a few degrees above 212, and exhales
copious fumes in the air.


EQUIVALENT = 31.7 (396.25; =

1039. Copper has been known from the earliest times. Although
it sometimes occurs in the native state, it exists more frequently in
combination with oxygen, sulphur, or arsenic. Some salts of the
oxide of copper, chiefly carbonates, are also found.

Some kinds of commercial copper are nearly pure ; the Russian
containing only a trace of iron. Native copper is often crystal-
lized in the form of small, regular octahedrons, which form it also
assumes when precipitated slowly from its solutions by galvanic
processes, or on being allowed to cool slowly after fusion in a smali
quantity in a crucible, the liquid portion having been poured off
Chemically pure copper is obtained by reducing pure oxide of cop-
per heated in a tube by means of hydrogen, the reduction taking
place at a temperature below a red-heat, and leaving the metal in
the form of a red powder, which assumes a brilliant metallic lustre
under the burnisher.

Copper has a characteristic red colour, and becomes transparent
when reduced to a very thin pellicle ; in which case it displays, by
transmitted light, a beautiful green colour. Coppery pellicles suit-
able for the experiment are obtained by, reducing by hydrogen, in a
heated glass tube, a small quantity of oxide or chloride of co ."->> r :
when a very thin layer of metallic copper, which displays a red
colour by reflected, and a beautiful green by transmitted light, is
deposited in certain parts of the tube.

Copper possesses a sufficient degree of malleability to allow its
being hammered into thin sheets or drawn out into very fine wire ;
and at the same time is considerably tenacious, as it requires a
weight of 140 kilog. to break a wire of 2 mm. in diameter. The
density of copper varies from 8.78 to 8.96, according to the greater
or less degree of aggregation it has received during its manufacture.
By rubbing, copper acquires a disagreeable smell and a peculiar
taste. It fuses at a strong red-heat, and at a white-heat gives off
vapours which burn with a green flame in the air.


At the ordinary temperature copper does not oxidize in dry air,
but soon changes in a moist atmosphere, especially if acid vapours
be present, becoming covered with a green substance commonly
called verdigris. A blade of copper, moistened by an acid, and
exposed to the air, combines with the oxygen of the air, and first
produces a neutral salt, which after some time is converted into a
basic salt. A blade of copper also oxidizes in the air when moist-
ened with an ammoniacal solution ; and dilute solutions of sea-salt
attack copper very powerfully, while concentrated solutions exert
less influence on it. Copper decomposes aqueous vapour at a strong
white-heat, while hydrogen gas is disengaged. A concentrated solu-
tion of chlorohydric acid attacks finely divided copper with disen-
gagement of hydrogen, while it scarcely affects the metal in a solid
form. Copper does not decompose water in the presence of pow-
erful acids : concentrated sulphuric acid dissolves it with disengage-
ment of sulphurous acid ; and it dissolves readily in cold nitric acid
of any degree of concentration, with disengagement of deutoxide of


1040. Copper forms four compounds with oxygen :

1. The suboxide Cu 3 0,* or red oxide.

2. The protoxide CuO, or black oxide.

3. The binoxide Cu0 2 .

4. Cupric acid, the composition of which is not yet known.

The first two compounds are basic, and form well-defined and
crystallizable salts, while the third is an indifferent oxide; and
lastly, the fourth is an acid.

Suboxide of Copper Cu 3 0.

1041. Suboxide of copper 'is found in nature in masses of a
beautiful red colour, possessing occasionally a vitreous lustre, and
sometimes consisting of beautiful red crystals. It may be obtained
artificially by several processes : 1st, by heating in an earthen
crucible equivalent parts of black oxide of copper CuO and finely
powdered metallic copper ; which mixture aggregates when fused at
a high temperature ; 2d, by heating in a crucible a mixture of chlo-
ride of copper Cu 2 Cl with carbonate of soda, and then treating the
substance with water, which dissolves the chloride of sodium and ex-
cess of carbonate of soda, leaving the suboxide of copper in the
form of a deep red crystalline powder ; 3d, by adding to a solution
of a salt of copper, for example, the sulphate CuO,S0 3 , sugar and
potassa, until the oxide of copper, which is at first precipitated, is

* The name of protoxide of copper is often given to the suboxide Cu a O, and that
of binoxide of copper to the oxide CuO. We shall not adopt this nomenclature
because it does not agree with that which we have thus far adopted.


redissolved, and by then boiling the liquid ; when suboxide of copper
is deposited in the form of small bright-red crystals.

Hydrated suboxide of copper is obtained by adding potassa to a
solution of protochloride of copper, in the form of a yellow powder,
which soon absorbs oxygen from the air, and which, when dried in
vacuo, presents the formula 4Cu a O+HO. Hydrated suboxide of
copper dissolves in ammonia without colouring the liquid, but by its
rapid absorption of oxygen from the air soon changes the colour of
the solution to a beautiful blue.

Suboxide of copper imparts a beautiful red colour to fluxes ( 702).
When heated with concentrated acids it is generally decomposed
into protoxide of copper CuO which dissolves, and metallic copper
which is separated.

Protoxide of Copper CuO.

1042. On heating metallic copper in the air, its surface first
becomes covered with suboxide Cu 2 0, which subsequently changes
into the black oxide CuO. Although protoxide of copper is often
prepared by roasting copper turnings, or better still, the very finely
divided copper which remains after the calcination of the acetate
with access of air, it is obtained more readily by decomposing the
nitrate by heat, when the oxide remains in the form of a black pow-
der, which rapidly condenses the moisture of the atmosphere.

When caustic potassa is poured into the solution of a protosalt
of copper, a grayish-blue precipitate of hydrated protoxide is formed,
the water of which is readily driven off by heat : it suffices to boil
the solution in which it has been precipitated to convert it into a
black powder of anhydrous oxide. Hydrated protoxide of copper
dissolves in ammonia, producing a solution of a slightly purple-blue
colour, called celestial water.

Deutoxide of Copper.

1043. This oxide is prepared by treating the hydrated prot-
oxide of copper with oxygenated water, when the blue matter is
changed into a brownish-yellow substance, from which a slight ele-
vation of temperature easily abstracts one-half of its oxygen.

Cupric Acid.

1044. An intimate mixture of very finely divided copper, po-
tassa, and nitre, heated to redness and then treated with water,
yields a blue solution which appears to contain a combination of an
oxide of copper containing more oxygen than the preceding with
potassa. This compound, however, is so evanescent that, if the
liquid be heated, oxygen is disengaged, and the copper is precipi-
tated in the state of black oxide CuO.

SALTS. 239


1045. The salts of the suboxide of copper are obtained by dis-
solving hydrated suboxide in dilute acids, which, when they are con-
centrated, decompose the suboxide into metallic copper which sepa-
rates, and protoxide which combines with the acids.

A subsulphite of copper Cu 3 0,S0 3 , is prepared by decomposing a
solution of protosulphate of copper CuO,S0 3 by a solution of sul-
phite of soda, when an orange precipitate is formed which is con-
verted, by boiling, into a red crystalline powder.

When acetate of copper is distilled, a small quantity of a white
sublimate, consisting of sub-acetate of copper, is found in the upper
part of the retort.

The soluble subsalts of copper produce colourless solutions, from
which alkalies throw down an orange-yellow precipitate. Ammo-
nia gives the same reaction, but an excess of the reagent redissolves
the precipitate, producing a colourless liquid which soon turns blue
in the air. Sulf hydric acid throws down a black precipitate of these
salts, for the study of whose reactions the subchloride CuCl is ex-
actly suitable.


1046. These salts, which are obtained by dissolving protoxide
of copper, or better still, its hydrate or its carbonate, in acids, are
blue or green, when they contain water of crystallization, while in
the anhydrous state they are of a dirty white, when the acid is
colourless, and their solutions are blue or green. They exhibit the
following characteristic reactions :

Caustic potassa and soda yield a grayish-blue precipitate of hy-
drated protoxide, which is converted into a brown precipitate by
boiling. The blue precipitate, which is insoluble in weak alkaline
liquids, dissolves with a blue colour in the latter when they are con-

Ammonia throws down the same precipitate, while an excess of
the reagent dissolves the precipitate and produces a beautiful blue
solution, which then contains a double salt of copper and ammonia,
from which caustic potassa precipitates oxide of copper.

Sulf hydric acid and the sulf hydrates throw down black precipi-
tates, which are insoluble in an excess of sulf hydrate.

Prussiate of potash forms, with protosalts of copper, a chestnut-
brown precipitate, which assumes a purplish shade when the precipi-
tate is very weak. The test is a very delicate one, and will detect
the presence of the smallest quantities of copper in a solution.

Iron and zinc precipitate metallic copper in the form of a brown
powder, which, when burnished, assumes the metallic lustre and
ordinary appearance of copper.

Protoxide of copper turns borax, and in general all vitreous


fluxes, green. If the glass be heated in the reducing portion of
the flame, it acquires a beautiful red colour, produced by the reduc-
tion of the protoxide of copper CuO into the suboxide Cu a O.

Sulphate of Copper.

1047. Sulphate of copper is found in commerce, where it is
known by the name of blue vitriol, in which state it generally con-
tains variable quantities of sulphate of iron. It may be obtained in
a state of purity by treating copper of the first quality with sul-
phuric acid diluted with one-half its weight of water ; when sulphur-
ous acid is disengaged, and sulphate of copper is formed which
contains only a trace of sulphate of iron. It is evaporated to dry-
ness, and, toward the close of the evaporation, a few drops of nitric
acid are added, which convert the iron into sesquioxide. By dis-
solving it in water the greater portion of the iron remains in the
state of an anhydrous basic sesquisulphate ; when, after boiling the
liquid with a small quantity of the hydrate or carbonate of the
protoxide of copper, which precipitates the least traces of iron, the
liquor is crystallized.

Sulphate of copper is soluble in 4 parts of cold and 2 parts of boil-
ing water, and crystallizes at the ordinary temperature in beautiful
blue crystals, which belong to the sixth system, and of which the
formula is CuO,S0 3 -h5HO. They are isomorphous with those pro-
duced by protosulphate of iron when crystallized at a temperature
of about 40, and which likewise contain 5 equiv. of water. When
these two sulphates are mixed together, and the compound solution
is crystallized, crystals are deposited containing the two sulphates
in different proportions, according to the respective quantities of the
salts in the solution. A crystal of sulphate of copper may even be
made to grow at pleasure, in a solution of sulphate of iron. The
crystal then increases by the superaddition of layers of sulphate
of iron, which are easily distinguished by their colour. The same
crystal, suspended in a solution of sulphate of copper, becomes
covered with layers of this latter sulphate, without any remarkable
change in its external appearance.

Sulphate of copper readily parts by heat with 4 equiv. of water,
but retains the fifth with more tenacity. It is entirely decomposed
at a high temperature, into oxide of copper which remains, and a
mixture of sulphurous acid and oxygen which is disengaged.

Sulphate of copper is manufactured in various ways ; and a cer-
tain quantity of this salt is obtained in copper furnaces. - When
sulphuretted copper ores or cupreous matts, are roasted, and the
roasted matter is sprinkled with water, a certain quantity of the
sulphates of iron and copper is dissolved, and separates by crystal-
lization. The sulphate of copper thus obtained, always contains a
large proportion of sulphate of iron.

Large quantities of sulphate of copper are manufactured from the

SALTS. 241

copper sheathing of ships which has been rendered useless by the
corrosive action of salt water. The copper is heated to a dull red-
heat in a reverberatory furnace, and sulphur thrown in, the doors
of the furnace being previously closed, when the sulphur attacks
the surface of the copper, covering it with sulphide of copper Cu a S,
after which it is roasted, and air allowed to enter the furnace freely.
A portion of the sulphur is then disengaged in the state of sulphur-
ous acid, while another portion changes into sulphuric acid, and
forms a basic protosulphate of copper. The sulphatized sheets are
then placed in large boilers filled with water, to which a certain
quantity of sulphuric acid has been added, when neutral protosul-
phate of copper dissolves, and is crystallized by evaporation as soon
as the liquid contains a sufficient quantity of it. This process is
repeated until the sheets of copper have disappeared.

Large quantities of sulphate of copper have been obtained in the
refining of old silver coin, as we shall mention hereafter.

If sulphate of copper be dissolved in a hot solution of ammonia,
a beautiful blue solution is obtained, which deposits on cooling deep
blue crystals, the composition of which is represented by the formula
CuO,S0 3 +2NH 3 +HO.

By digesting hydrated oxide of copper with a solution of proto-
sulphate of copper^ a green powder, consisting of a hydrated basic
sulphate of copper CuO,S0 3 -f-2CuO+3HO is obtained. Analo-
gous basic sulphates are precipitated when solutions of sulphate of
copper are incompletely precipitated by the alkalies.

Sulphate of copper forms with the alkaline sulphates double salts
which are readily crystallizable, and also produces double sulphates,
of various proportions, with the sulphate of magnesia, and with
those of the protoxides of iron, zinc, nickel, etc., which are all iso-
morphous. These double sulphates, crystallized at the ordinary
temperature, contain 5 equiv. of water when the sulphate of copper
predominates, and 7 equiv. of water, on the contrary, when the
other metallic sulphate is prevailing. In both cases, the sulphates
are isomorphous whenever they contain the same quantity of water.

Nitrate of Copper.

1048. This salt is prepared by dissolving copper in dilute nitric
acid, when the liquid yields on evaporation beautiful blue crystals,
which contain 3 or 6 equiv. of water, according to the temperature
at which the crystallization has been effected. It is used in dyeing.

The influence of heat changes nitrate of copper into the green
basic nitrate 4CuO,N0 5 , and subsequently decomposes it at a more
elevated temperature, leaving protoxide of copper. The same basic
nitrate is obtained by precipitating the neutral nitrate of ammonia.

Carbonates of Copper.
1049. By adding a solution of an alkaline carbonate to a solu-

VOL. IL 16


tion of sulphate of copper, a bright blue gelatinous precipitate is
obtained, which, after some time, changes into a green powder, the
composition of which is represented by the formula 2CuO,C0 3 -fHO ;
the blue gelatinous precipitate appearing to differ from it only in
containing more water. By boiling the liquid with the precipitate,
the latter is converted into a brown powder of anhydrous protoxide
of copper. The green carbonate of copper is used in oil-painting,
under the name of mineral green.

A hydrocarbonate of copper, of the formula CuO,C0 2 -f-CuO,HO,
called malachite, is found in nature in the form of green concrete
masses, which are often very compact and of considerable size, and are
fashioned into ornamental objects, such as vases, shafts of columns,
and table and chimney tops, which are of great value. When
polished they display veins of different shades of colours, which are
produced by the mammillary structure of the material, and impart
a very beautiful appearance to the polished surfaces. Malachite is
sufficiently abundant in Siberia to be worked as an ore of copper.

Another hydrocarbonate of copper, of which the formula is
2CuO,C0 3 -f CuO,HO, and which yields fine blue crystals, also
occurs in nature, which substance existed in great abundance in
the mines of Chessy, near Lyons, where it was long smelted as an
ore of copper. When finely powdered it is of a beautiful blue
colour, in which state it is used in the manufacture of coloured
wall-paper, and is called mountain blue, or native blue ashes, (bleu
de montagne, or cendres bleues naturelles.) Artificial blue ashes,
of a more brilliant shade than the native product, are made in En-
gland, by a process which is kept secret.

Arsenite of Copper.

1050. Arsenite of copper, which is used in oil-painting, under
the name of Scheeles green, is prepared by dissolving 3 kilog. of
carbonate of potassa, and 1 kilog. of arsenious acid in 14 litres of
water, and pouring the solution, by small quantities at a time, into
a boiling solution of 3 kilog. of sulphate of copper in 40 litres of
water, the solutions being stirred constantly during the precipita-
tion. The shade of colour is modified by varying the proportions
of arsenious acid.

Silicates of Copper.

1051. By means of fusion the oxide of copper combines in all
proportions with silicic acid, forming green vitreous substances. A
crystallized silicate of copper, called dioptase by mineralogists, is
found in nature, and presents the formula 3CuO,2Si0 3 -f-3HO.

Acetates of Copper.

1052. By dissolving protoxide of copper in acetic acid, a green
liquid is obtained, which, when evaporated at a proper temperature,


deposits beautiful green crystals of the formula CuO,C 4 H 3 3 +HO,
and which are soluble in 5 parts of boiling water. It is known in
commerce by the name of verdigris, and is manufactured by dis-

Online LibraryV. (Victor) RegnaultElements of chemistry : for the use of colleges, academies, and schools (Volume 2) → online text (page 24 of 87)