Henry E. (Henry Enfield) Roscoe.

A treatise on chemistry (Volume 2) online

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and becomes black, but regains the green colour on moistening.
Other oxychlorides of copper also are known, some being
minerals and others artificial products. Cupric chloride
also unites with other chlorides forming crystalline double salts.
Cupric Bromide, CuBr 2 . When cupric oxide is dissolved in
hydrobromic acid and the solution allowed to stand in a vacuum
over sulphuric acid, the anhydrous bromide separates out in
dark 'crystals very similar to those of iodine. These are very
deliquescent, and on heating in absence of air decompose into
cuprous bromide and bromine.

VOL. II. F F



434 COPPER



Cupric Sulphide, CuS, occurs in nature as covellite or indigo-
copper, occurring sometimes in hexagonal crystals but more
frequently in the massive state as at Mansfeld, on Vesuvian lavas,
and in Chili. It has a semi-metallic lustre and an indigo-blue
colour, and a specific gravity of 4'6.

This compound is prepared artificially by heating cuprous
sulphide with flowers of sulphur to a temperature not above
that of boiling sulphur (Hittorf). It may also be prepared by
triturating finely-powdered cuprous sulphide in a mortac with
cold strong nitric acid until the action ceases; the powder is
then washed, cupric sulphide remaining behind. 1 It may like-
wise be obtained as a blackish-brown precipitate by passing
a current of sulphuretted hydrogen gas into a solution of a
cupric salt. The finely-divided moist precipitate easily oxidises
on exposure to the air. When cupric sulphide is gently heated
in absence of air or in a current of hydrogen, it decomposes into
sulphur and cuprous sulphide, and this method is often used
for the estimation of the metal (Rose).

Several polysulphides of copper have been described having
the formulae Cu 2 S 5 , 2 Cu 2 S 6 , Cu 4 S 5 and Cu 2 S 3 . 3 These are all
unstable bodies, which, on keeping, slowly decompose into cupric
sulphide and sulphur.

Cupric Sulphate, CuS0 4 . This salt, commonly termed
copper sulphate, copper vitriol, or blue vitriol, has long been
known, being found in solution in the drainage water of copper
mines. It was for a long time confounded with green vitriol or
iron sulphate, this being partially due to the fact that both
frequently occur in the same drainage water, and are capable
of crystallising together. In the works attributed to Basil
Valentine the fact is recognised that both iron and copper
vitriol can occur together, for he says : " Venus and Mars can be
brought back into a virtuous vitriol." The alchemists frequently
experimented on such mixed vitriols, as they believed that they
contained the materia prima employed for the preparation of
the philosopher's stone. Thus, in the above-mentioned work we
read, " where copper and iron are found together gold will not
be far distant."

The artificial production of copper sulphate is first described
by Van Helmont in 1G44, who obtained it by heating together

1 Faraday, Quart. Jour. Sci., 21, 183.

2 Bodroux, Compf. rend., 1900, 130, 1397.

3 Rossing, Zdt. anorg. Chem., 1900, 25, 407.



CUPRIC SULPHATE 435

copper and sulphur, and moistening the residue with rain-water.
Glauber in 1648 proved that it might be readily obtained by
boiling copper with oil of vitriol.

Copper sulphate is obtained on the large scale chiefly from
copper pyrites, which is carefully roasted, the copper under
suitable conditions being oxidised to copper sulphate, whilst
the iron is chiefly converted into oxide. The residue is lixivi-
ated and the copper sulphate separated from the solution by
crystallisation. The mother-liquors contain both copper and iron
sulphates, from which the copper may be precipitated by scrap
iron. For agricultural use the presence of iron in the copper sul-
phate is not injurious, and for sulphate intended for this purpose
the mother-liquor may be directly evaporated. The precipitated
copper, as well as copper scale and metallic copper obtained by
other processes or as scrap, is roasted in reverberatory furnaces,
and the product is treated with dilute sulphuric acid. If the
copper contains gold or silver it is treated with sulphuric acid
mixed with its own volume of water, the silver and gold being
left undissolved.

Where argentiferous copper ores are used, the pasted ore is.
added in small quantities to sulphuric acid and digested until
the solution contains but little free acid. Lead and gold remain
behind as an insoluble powder, and the clear liquor is run into
wooden tanks lined with lead, and containing strips of copper
on which the whole of the silver and part of the arsenic and
antimony are deposited, whilst the greater portion of the bis-
muth separates as a basic sulphate. The copper sulphate solu-
tion, thus freed from other metals, is then crystallised in large
lead-lined coolers, the crystals being deposited on strips of lead
hung up in the liquid.

The commercial sulphate always contains small quantities of
iron ; this may be removed by boiling the solution with a
little nitric acid, the ferrous sulphate being thus converted
into ferric sulphate, which remains in the mother-liquor on
recrystallisation.

Cupric sulphate is also obtained as a secondary product in
the refining of silver ; the silver is precipitated from a solution
of the sulphate in the metallic form by strips of copper, and
pure cupric sulphate remains behind.

Copper sulphate crystallises from warm saturated solu-
tions on cooling in transparent blue triclinic crystals
having the formula CuS0 4 ,5H 2 O, and a specific gravity of

F F 2



436 COPPER



2' 28. 100 parts of water dissolve, according to Poggiale,
as follows :

At 10 20 30 50 70 90 100

CuSO 4 ,5H 2 O 36-95 42'31 48-81 65-83 94'60 156*44 203'32.

The salt is insoluble in absolute alcohol, and only slightly
soluble in dilute spirits. When it is heated for some time to
100, the hydrate CuS0 4 ,H 2 O remains as a bluish-Avhite
powder, which at a temperature of from 220 to 260 loses
nearly all its water. Even at 360, however, a small amount
of water (0'04 per cent.) is retained ; this is lost only at a
temperature at which the salt begins to lose sulphur trioxide
(Richards). The residue at 260 forms a white mass which is
extremely hygroscopic, becoming blue on absorption of water.
This reaction is sometimes used for the purpose of ascertaining
the presence of water in organic liquids, and also for de-
hydrating the same. The anhydrous compound may be obtained
in the form of grey prismatic needles by heating the hydrated
sulphate with ammonium sulphate out of contact with the
flame gases. 1

The anhydrous salt is stable up to 341, but above this
temperature, evolution of sulphur dioxide begins. 2 Between
341 and 621 small quantities of a brown oxy-salt, 8CuO,3S0 3 ,
are formed. Between 621 and 670 sulphur dioxide and sulphur
trioxide are evolved and an orange-coloured oxy-salt, 2CuO,S0 3 ,
is formed. This orange-coloured salt begins to decompose
at 704, the decomposition being incomplete at 850.

The anhydrous as well as the hydrated sulphate absorbs two
molecules of hydrogen chloride with evolution of heat and
formation of cupric chloride and sulphuric acid. The same
decomposition takes place when copper sulphate is treated with
an excess of aqueous hydrochloric acid ; the temperature falls
and the salts dissolve with formation of a green liquid which on
concentration yields crystals of cupric chloride. This reaction of
copper sulphate is employed for separating hydrochloric acid
from mixtures of gases, as, for instance, from chlorine, or from car-
bon dioxide which lias been prepared by means of hydrochloric
acid. Copper sulphate is largely used in calico-printing, and in
the preparation of the pigments of copper, as Scheele's green
and emerald green. It is also used in very large quantity in the

1 Klobb, Compt. rend., 1892, 114, 836.

a W. Uanjukoff, J. RUM. /'A//-. Cfcem, 8oc. t I'.M'J, 41, 688.



CUPRIC NITRATE 437



processes of electro-metallurgy. A crude copper sulphate con-
taining ferrous sulphate is used in agriculture for preventing
" smut " in seeds.

Hydrates containing 2H 2 and 3H 2 O are also known in
addition to those described above, and copper also forms
a series of basic sulphates. When the normal sulphate is
kept for several hours at a dark-red heat, an amorphous
orange-yellow powder remains behind having the composition

,0-Ctu

S0 9 \ /O, which is decomposed in contact with cold

\0-Cu/

water into copper sulphate and an insoluble green basic
sulphate, CuS0 4 ,3Cu(OH) 2 . By the action of boiling water a
salt having the composition CuS0 4 ,2Cu(OH) 2 is formed. These
as well as other basic salts are also obtained when a solution
of copper sulphate is treated under certain conditions with
potash or ammonia. Some occur as minerals ; thus brochantite
is a native basic sulphate having the formula CuS0 4 ,3Cu(OH) 2 ,
and occurring in bright green rhombic tablets, whilst a basic
sulphate of composition CuS0 4 ,2Cu(OH) 2 has also been found
occurring in nature as a mass of small green crystals. 1 The
former can be artificially prepared by allowing a piece of
porous limestone to lie in a solution of copper sulphate.

When a solution of copper sulphate containing sulphuric
acid is shaken with metallic copper, some cuprous sulphate is
formed 2 and equilibrium is set up according to the equation :

CuS0 4 +Cu^Cu 2 S0 4 .

The amount of cuprous sulphate produced increases with the
temperature and the concentration ; thus at 100 in a solution
containing originally one gram-molecule per litre of cupric
sulphate, the concentration of the cuprous sulphate is 1/82 of
that of the cupric salt, when equilibrium has been attained.

Cupric Nitrate, Cu(N0 3 ) 2 . In his De furnis novis philoso-
phicis, published in 1648, Glauber mentions the fact that
the solution of copper in nitric acid on evaporation leaves a
green residue, and Boyle in 1664 observed that crystals could
be obtained from the solution, and that these had the power
of colouring the flame of a spirit-lamp green. In order to pre-
pare copper nitrate, copper scale or copper oxide is dissolved in

1 Cesaro and Buttgenbach, Ann. Soc. GeoL Belgique, 1897, 24, Bull.,
p. xli.
. 2 Abel, Ze.it. anorg. Chem., 1901, 26, 361.



438 COPPER



dilute nitric acid and the solution evaporated to crystallisation.
In this way fine blue prismatic crystals, having the composition
Cu(NO 3 ) 2 ,3H 2 O, arc obtained which melt at 114-5, possess a
caustic metallic taste, and cauterise the skin. This hydrate is
stable in contact with its solution from its melting point to
24'5, when it forms the hexahydrate, which in its turn passes
at about - 20 into a hydrate with 9H 2 O. The hydrated salt
when heated loses nitric acid, a basic salt of the composition
Cu(N0 3 ) 2 ,3Cu(OH) 2 being formed. Copper nitrate is deli-
quescent and very soluble in water, and is deposited as a fine
crystalline powder when its concentrated solution is mixed with
nitric acid of specific gravity T522. This salt, owing to the
ease with which it is decomposed, possesses strong oxidising
properties. If some crystals of it are wrapped up in tinfoil and
rubbed, decomposition begins, often accompanied by a rise of
temperature sufficient to cause emission of sparks. If a solution
of copper nitrate is evaporated with one of ammonium nitrate,
decomposition takes place at a certain degree of concentration
attended with violent detonations. Copper nitrate is used as
an oxidising agent in the processes of dyeing and calico-
printing, especially in the production of catechu-browns and of
some steam colours containing logwood.

Anhydrous copper nitrate l is prepared by the action of nitric
anhydride, or a solution of this in nitric acid, on hydrated salts
of copper. It is a white powder, is very deliquescent, and
decomposes at 155.

Cupric Nitrite, Cu(N0 2 ) 2 , is prepared in solution by double
decomposition between (a) cupric sulphate and barium nitrite
or (6) between cupric chloride and silver nitrite. The solution
obtained in this manner is bright grass green in colour.

According to Hampe, 2 it evolves nitric oxide even in the
cold, whilst Berzelius states that it absorbs oxygen from the air
and is slowly converted into nitrate.

When the solution is evaporated under diminished pressure
over sulphuric acid, nitric oxide is evolved ; the residue is
insoluble in water, is bluish-green in colour, and consists of a
mixture of nitrite and nitrate. 3

Cupric Phosphide, Cu 3 P 2 , is formed when phosphine is passed
over cupric chloride or when phosphorus is boiled with a cupric

1 Guntz and Martin, Bull. Soc. chim., 1909 [iv.], 5, 1004.

2 Annalen, 1863, 125, 345.

3 Ray, Journ. Chem. Soc., 1907, 91, 1405.



NORMAL CUPRIC PHOSPHATE 439

salt. It is a black powder, or, when prepared at a high tempera-
ture, a greenish-black metallic mass, which, when ignited in
hydrogen, is converted into cuprous phosphide.

Normal Cupric Phosphate, Cu 3 (PO 4 ) 2 ,3H 2 0, is best obtained
by dissolving the carbonate in dilute phosphoric acid and heating
the solution to 70, when a fine blue crystalline powder is de-
posited. On heating this with water in closed tubes it decom-
poses into phosphoric acid and a basic salt, Cu:P0 4 .Cu.OH
(Debray), or Cu 3 (P0 4 ). 2 ,CuO,H 2 O, 1 which latter occurs in
nature as libethenite. The artificial compound crystallises in
rhombic prisms, and libethenite in dark olive-green prisms,
having a waxy lustre. The same compound also occurs in the
massive state. Another basic copper phosphate is the mineral
phosphochalcite, or pseudomalachite ; this occurs in triclinic
emerald -green crystals having the composition P0 4 (CuOH) 3 .
Other basic phosphates of copper are known.

Copper Arsenides. These elements combine in several pro-
portions, whilst certain copper arsenides are found as minerals ;
thus whitneyite, Cu 9 As, occurs as a bluish-red or greenish -
white amorphous or crystalline malleable substance found in
Michigan, and also in California and Arizona. Algodonite,
Cn 6 As, found in Chili and Lake Superior, possesses a silver-
white or steel-grey lustre ; and domeykite, Cu 3 As, found at
Portage Lake, has a tin-white to steel-grey colour. According
to Reinsch, the grey deposit obtained when metallic copper is
placed in a solution of arsenious oxide in hydrochloric acid has
the composition Cu 5 As 2 , and this on heating is converted into
Cu 3 As. When arsenic vapour mixed with carbon dioxide or some
other inert gas is passed over copper heated to the boiling point of
sulphur, the arsenide Cu 5 As 2 is formed, and this, when the
temperature is raised, is transformed into Cu 3 As. 2 The freez-
ing point curve of copper and arsenic also points to the forma-
tion of these two arsenides. 3 When arsine is passed over
dry cupric chloride the compound Cu 3 As 2 is formed.

Cupric Arsenite, CuHAs0 3 , was first obtained by Scheele by
precipitating a solution of potassium arsenite with copper sul-
phate, and is still prepared by the same process. It is a siskin-
green precipitate, which is known as a pigment under the name



1 Caven and Hill, /. Soc. Chem. Ind., 1897, 16, 29.

2 Granger, Compt. rend., 1903, 136, 1397.



3 Richards, Ber., 1898, 31, 3163. See also Friedrich, Metallurgie, 1908, 5,
529.



440 COPPER



of Scheeles green. The salt forms a blue-coloured solution in
caustic potash which is quickly decomposed on heating, with
separation of copper oxide.

Arsenates of Copper. These salts correspond closely to the
phosphates, and several basic salts occur in the mineral kingdom.
The ortharsenate Cu 3 (AsO 4 ) 2 ,2H 2 is obtained as a blue
amorphous powder by heating together copper nitrate and
calcium arsenate. Olivenite, Cu:As0 4 .Cu.OH, crystallises
in olive-green or brown rhombic prisms, and can be obtained
artificially by heating a solution of the ortharsenate to 100.

Clinoclasite, As0 4 (CuOH) 3 , forms dark green monoclinic
prisms possessing a pearly or vitreous to resinous lustre.

Carbonates of Copper. We are only acquainted with basic
copper carbonates. Of these, two occur in large quantity in the
mineral kingdom. Malachite, CuC0 3 ,Cu(OH) 2 , forms mono-
clinic, frequently twin, crystals which are rarely perfectly
developed, simple crystals being seldom found. It also occurs
massive with botryoidal or stalactitic surface, often fibrous and
frequently granular or earthy. Its colour is a bright green, and
its specific gravity varies from 3'7 to 4'01. Green malachite
accompanies other ores of copper, occurring especially in the
Urals, at Chessy in France, in Cornwall and Cumberland in
England, and- at the copper mines of Nischne-Tagilsk. The
fibrous varieties are frequently deposited in different coloured
layers which take a high polish, and from these masses vases
and other ornamental articles are manufactured. Crystals of
malachite can be obtained artificially by allowing a piece of
porous limestone to lie in a solution of copper nitrate having a
specific gravity of 1*1, until the stone becomes covered with basic
nitrate, and then bringing it into a solution of sodium carbonate
having a specific gravity of 1*04, when after a few days malachite
crystals are formed (Becquerel).

Verdigris, or copper rust, formed by the joint action of
air and water on copper, possesses the same composition as
malachite.

Azurite or Azure Copper Ore, 2CuCO 3 ,Cu(OH) 2 , occurs
together with malachite and other copper ores in shining
monoclinic tablets or short prisms, and also as an amorphous or
earthy mass having a dark azure-blue colour. It possesses a
specific gravity of 3 '5 to 3'83. If crystallised copper nitrate be
heated with pieces of chalk under a pressure of from 3 to 4
atmospheres a crystalline warty mass of azuritc is formed



SILICATES OF COPPER 441

(Debray). Azurite dissolves in a hot solution of sodium bi-
carbonate and the solution on boiling deposits a green powder
of malachite.

The following carbonates have also been isolated, 1 5CuO,C0 2 ,
a blue bulky precipitate obtained by the precipitation of copper
salts by sodium carbonate; 5CuO,3CO 2 , a light blue stable
precipitate obtained by precipitation with sodium hydrogen
carbonate, and 8CuO,3C0 2) 6H 2 0, a dark blue substance obtained
by the action of water on the double carbonate of copper and
sodium.

Silicates of Copper. Two silicates of copper occur as
minerals. Dioptase or emerald copper, H 2 CuSi0 4 , is found in
compact limestone in the Kirghese Steppes and also in Siberian
gold-washings. It forms emerald-green hexagonal crystals
(class 17, p. 201), having a specific gravity of 3*3.

Ckrysocolla, H 2 CuSi0 4 ,H 2 O, exists as a bluish botryoidal mass
occurring with other copper ores. The name of chrysocolla
occurs in old writers and serves to describe the most diverse
bodies. The word originally was used to signify the substance
employed for soldering gold (xpvvos, gold, and KoXXdco, to
cement) ; this, being prepared from urine, was probably
microcosrnic salt, which became coloured blue in the act of
soldering gold to copper or brass. The word then came to be
used for any green or blue substance, especially such as contained
copper ; the confusion thus created was great, all blue or green
minerals, such as emerald and malachite, as well as substances
which were employed for soldering, being termed chrysocolla.
Brochant, in the year 1808, first proposed to confine the use of
the name to this particular mineral.

AMMONIACAL COMPOUNDS OF COPPER.

211 By the action of ammonia on both cuprous, and cupric
salts, a large number of compounds have been obtained, the em-
pirical formulas of which show that combination has taken place
between the copper salt and a varying number of molecules of
ammonia. As to the manner in which the combination takes
place, and the exact constitution of the products, very little is
at present known, and they are usually formulated as additive
compounds or in accordance with Werner's co-ordination theory.

1 Pickering, Proc. Chem. Soc., 1909, 25, 188. See also Groger, Zeit. anorg.
Chem., 1900, 24, 127.



442 COPPER



Many other metals form similar series of ammoniacal derivatives,
these being often more complex than in the case of copper,
as for example those of cobalt and platinum. The compounds
appear to be formed chiefly by the metals occurring in the
middle of MendeleefFs double periods (p. 52).

The ammoniacal cuprous compounds are colourless when pure,
but very readily undergo oxidation, yielding the corresponding
ammoniacal cupric salts, which usually have a deep blue colour
and yield solutions of the same colour. The most important of
these are the derivatives of cupric chloride and sulphate.

Cuprous Ammonium Sulphate. Colourless crystals of the
composition Cu 2 S0 4 ,4NH 3 ,H 2 O have been obtained by pass-
ing a weak electric current through a strongly ammoniacal
solution of cupric sulphate. 1 In moist air this substance
changes rapidly to copper and a cupric compound.

The compound Cu 2 S0 4 ,4NH 3 is precipitated as a white
crystalline powder on the addition of alcohol to a solution of
cuprous oxide and ammonium sulphate in aqueous ammonia at
50 in an atmosphere of hydrogen. 2

Cupric Chloride and Ammonia. Anhydrous cupric chloride
absorbs ammonia, increasing in bulk and forming a blue powder
having the composition CuCl 2 ,6NH 3 ; this gradually dissociates
first into CuCl 2 ,4NH 3 , and then further to CuCl 2 ,2NH 3 . 3

When ammonia is passed into a hot saturated solution of
cupric chloride a dark blue solution is formed, which, on cooling,
deposits small dark blue octahedra or pointed tetragonal
prisms of cuprammonium chloride, CuCl 2 ,4NH 3 ,2H 2 0. 4 It is
converted at 150 into CuCl 2 ,2NH 3 , which is a green powder, and
on further heating decomposes as follows :

6(CuCl 2 2NH 3 ) = GCuCl + 6NH 4 C1 + 4NH 3 + N 2 .

Water converts it into cuprammonium chloride, ammonium
chloride, and a basic cupric chloride having the composition
CuCl 2 ,4CuO,6H 2 0.

Cupric Sulphate and Ammonia. When a solution of copper
sulphate is treated with ammonia a basic sulphate is first thrown
down. On further addition of ammonia this dissolves to a
deep pure blue liquid containing cuprammonium sulphate,

1 Foerster and Blankenberg, Ber., 1906, 39, 4428.

2 A. Bouzat, Compt. rend., 1908, 146, 7">.

3 Bouzat, Compt. rend., 1902, 135, 292; Ann. Chim. Phy*., 1903, (7), 29,
305.

4 Sabbatani, Ann. Chim. Farm., 1897, 26, 337.



DETECTION AND ESTIMATION OF COPPER 443

CuS0 4 ,4NH 3 ,H 2 O, which was first described by Stisser in
1693 as an arcanum epileplicum, and afterwards termed cuprum
ammoniacale. In order to obtain this compound in fine crystals
a layer of strong alcohol is poured on to the concentrated
aqueous solution and the whole allowed to stand : in this way
very long and thin transparent azure-blue rhombic prisms are
deposited. These on exposure to the air lose ammonia and are
gradually transformed into ammonium sulphate and basic copper
sulphate. Heated gently to 150 an apple-green powder of the
composition CuS0 4 ,2NH 3 is obtained. Anhydrous copper
sulphate absorbs dry ammonia gas with evolution of heat
forming a fine blue powder consisting of CuSO 4 ,5NH 3 , which
like the foregoing compound on heating to 200 forms the com-
pound CuSO 4 ,NH 3 (Graham).

It will be noticed that in several of these compounds the total
number of molecules of water and ammonia in combination with
the copper sulphate molecule is five, and they may therefore be
regarded as crystallised copper sulphate, CuS0 4 ,5H 2 0, in which
either the whole or a certain number of the water molecules
have been replaced by an equal number of ammonia molecules.
The same relation is found still more prominently in the
ammoniacal compounds of some of the other metals.

DETECTION AND ESTIMATION OF COPPER.

212 The halogen compounds and nitrate of copper colour the
non-luminous gas flame green, and the same coloration is
obtained from the other salts when mixed with a chloride or
moistened with hydrochloric acid.

The spectrum of this flame is a banded one containing a large
number of lines of which two in the violet, lying between the
rubidium and cassium lines, are the most characteristic. The
spark spectrum of cupric chloride is also rich in lines and con-
tains very bright lines in the green (Lecoq de Boisbaudran).
When borax is moistened with any copper salt and heated in
the oxidising flame a bead is obtained which, when hot, is green,
and when cold possesses a blue colour ; in the reducing flame,
especially with the addition of a small quantity of tin or a tin



Online LibraryHenry E. (Henry Enfield) RoscoeA treatise on chemistry (Volume 2) → online text (page 39 of 135)