T. E. (Thomas Edward) Thorpe.

A dictionary of applied chemistry online

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AuEnsr, and Teiphenylmethanb (lOLOUEiirG

HATTBBS).

CHROMITE or Chrome - lion - oie. A

member of the spinel group of minerals, con-
sisting essentially of ferrous oxide and chromic
oxide PeO'OrjOj, or PeftjOj, analogous to the
aluminates. The ferrous oxide is, however,
often partly replaced by magnesia, and the
chromic oxide by alumina, so that there is a
passage to chrome-spinel or piootite ; on the
other hand, with a replacement of chromic oxide
by ferric oxide, there may be a transition to
magnetite (FejOj or FeFejDj). AU these
minerals are cubic in crystallisation -and belong
to the same isomorphous group — the spind
group. The actual amount of chromic oxide
varies considerably, as shown by the following
analyses ; the ordinary ore, as mined, contains
on an average 45 p.c. CrsOj : —



V 5L203


AI2O3


FejOa


I'eO


MgO


I. 6ji9






32-1





XL 4-15


22-41


5-78


11-76


15-67


in>69-20


7-15


n.d.


25-02


4-42


IV. 41-23


24-58


2-28


16-99


14-77


V. 56-54


12-13


— .


18-01


14-08



ni.



IV.



I. Calculated for FeCraO 4.
H. Tranklin, Macon co.. North Carolina.

Price's Creek, Yancey co.. North Carolina (J. H.
Pratt, Amer. J. Sol., 1899, vii. 281). Also
SiOa 2-80, MnO 0-69.
Tampadel, Lower Silesia (H. Icaube, Zeits.
Deutsch. Geol. Ges. 1894, xlvi. 60).
V. Dun Mountain, New Zealand.
(For analyses of ehromite isolated from meteorites,
Bee W. Tassin, Proc. U.S. Nat. Museum, 1908,
xxxiv. 685.)

In general appearance, chrome-iron-ore is
very Hke magnetite, but it is readily distinguished
from this by the dark-brown colour of its streak
or powder, and by the fact that it is only slightly,
if at all, magnetic. Sp.gr. about 4-5. Crystals
are of rare occurrence, the mineral being usually
found as grains disseminated in basic rocks of
igneous origin, especially those rich in olivine
(viz. the peridotites). Sometimes these grains
are segregated into nodular masses of con-
siderable size and with a granular to compact
texture. Rocks of this kind, when subjected to
weathering processes, become hydrated and
altered into serpentine, and for this reason most
of the workable deposits of chrome iron-ore are
in serpentine rocks. With the denudation and
Vol. II.— r.



breaking down of these rocks, the heavy grains
of the chemically resisting ehromite collect in
the beds of streams and rivers ; and some of the
deposits worked in the Ural Mountains are of
this nature.

ehromite is the only commercial source of
chromium and its compounds. It is used for tho
preparation of the oxides and chromates em-
ployed as pigments, and in dyeing, calico-
printing and tanning ; in the manufacture of
chrome-steel, and for chrome-bricks and furnace
linings. It is mined in European and Asiatic
Turkey, Greece, the Ural Mountains, New
Caledonia, India, Rhodesia, and Quebec ; and
formerly, to a considerable extent, at Bare HUls,
near Baltimore in Maryland, and in Lancaster
CO. in Pennsylvania. Quarries were, at one
time, worked on Unst, one of the Shetland Isles.
The world's output amounts to about 80,000
tons per annum. . L. J. S.

CHROMIUM. (Ft. Ghrome; Ger. Ohrom.)
Symbol Cr. At. wt. 52-0. Chromium occurs
principally as ohrome-iron-ore, or ehromite
Iq.v.). As sesquioxide Cr^Oa, it is found in
small quantities in chrome ochre, generally
mixe^ with clay and associated with ehromite,
in EVance and Siberia. As lead ohromate it
occurs in crocoisite and malanchroite PbCrOa,
and ,as a basic lead chromate in the rare
mineral phcenicite 3PbO-2Cr03. It is usually
present in the form of ehromite in meteorites, but
rarely to the extent of more than 1 p.c. The
greenish colour of the emerald, verd-antique
marble, serpentine, sapphire, and many other
minerals, is due to the presence of chromium
sesquioxide.

Preparation. — Metallic chromium may be
prepared by several methods. DeviHe pro-
duced it in combination with a small quantity
of carbon, in ingots weighing as much as 100
grams, by the ignition of a mixture of chromium
sesquioxide and sugar in » lime crucible, at
the highest temperature of a blast furnace.

Wohler (Annalen, 111, 230) prepared chro-
mium by fusing a mixture of the violet chromium
sesquichloride with twice its weight of zinc under
a layer of equal parts of potassium and sodium
chlorides. The mixture of zinc and chromium
so produced was treated with dilute nitric acid
to remove the' zinc, and the chromium remained
as a light-greenish powder consisting of minute
octahedra belonging to the quadratic system
(BoUey), of sp.gr. 6-81 according to Wohler, 7-3
according to Bunsen.

Chromium may be prepared by the electro-
lysis of its salts, but the purity and condition of
the deposit vary greatly with current strength
and solution concentration. A bright deposit,
capable of receiving a polish, can be obtained
by the use of a hot 25 p.c. acid solution of
chromic chloride, with a current density of
about 40-50 amperes per square foot. Aa in
the case of nickel, the deposits have a. strong
tendency to peel (Cowper Coles, Ohem. News,
81, 16).

It is most easily obtained in small quantities
by heating equivalent parts of chromic oxide
(or mixture of the oxide and anhydride) with
aluminium powder. When started, the heat of
the reaction causes it to continue and a fused
mass of chromium of a high degree of purity is
obtained (Goldschmidt, Annalen, 301, 19).

B



50



CHROMIUM.



In large quantities it is best prepared by
heating the oxide -with carbon in the electric
furnace. The crude metal thus obtained always
contains cmiideti, of which two definite com-
pound? appear to exist : CrjCa and OjC. It
may be refined by further heating in the electric
furnace on a bed of lime mixed with chromic
oxide. The refined metal crystallises in cubes
or octahedra and stiU contains about 1-5 p.c.
carbon. By further refining, however, with a
double oxide of chromium and calcium, the
pure metal is obtained (Moissan, The Electric
iFurnace).

Pure chromium is a steel-grey metal, slightly
harder than glass ; the carbides, however, are
harder than quartz, so that a metal with between
1 -5 and 3 p.c. carbon can only be cut and polished
by diamond-dust. Its density is" 6-92, and it
oxidises superficially in moist air ; the melting-
point is probably above 2000°. The pure
finely divided metal bums rapidly when heated
in the blowpipe flame, and a pyrophorio powder
may be prepared by distillation of chromium
amalgam in vacud \v. infra). It also unites
with nitrogen when heated in nitrogen or am-
monia, forming a very stable nitride. Crude
chromium does not burn until heated to above
2000°. It combines with sulphur vapour at
700°, and when heated to redness in hydrogen
chloride, forms chromous chloride. Electrolytic
chromium occludes 250 times its volume of
hydrogen. Aqueous hydrochloric acid acts slowly
on the metal, but the action is accelerated if
the chromium is made the anode in electrolytic
hydrochloric acid. Heating with sulphuric acid
causes evolution of sulphur dioxide ; concen-
trated nitric acid and aqiM regia have no action.
As in the case of iron, active and passive varieties
of chromium exist ; in general, the metal is
rendered passive by oxidising solutions, whilst
hydrogen ions make it active. In the electro-
motive series, the active chromium immediately
follows zinc, the inactive metal is near platinum.
The cause of the passive state is usually ascribed
to the existence of a superficial film of oxide, but
this view has been disputed.

Alloys. Various alloys of chromium have been
prepared, chiefly by means of the electric furnace,
by the ' thermit ' method, or by the mixture of
the molten metals.

Molten zinC' dissolves but little chromium ;
a hard and brittle alloy has been obtained in the
form of hexagonal lamellee. Aluminium and
chromium mixtures, containing between 5 and
55 p.c. chromium, separate into two liquid
layers, and probably contain a compound,
CrgAl. The alloys with a low percentage of
chromium are brittle, one containing 13 p.c.
chromium can be powdered in a mortar.

Alloys with antimony are brittle, and com-
pounds represented by SbOr and SbjCr are
known. Chromium alloys with copper ivith
difficulty, but an alloy can be obtained by
stirring in copper oxide into a molten alloy of
aluminium and chromium (Moissan). The
liquid metals are misoible only to a small
extent.

Cobalt and chromium are miscible in all pro-
portions, both in the liquid and the solid states,
and yield a mixture of minimum melting-point
(1320°) with 47 p.c. Cr. Homogeneous alloys,
containing above 30 p.c. Cr, when cooled, separate



into two sets of crystals, distinguishable by
electroljftic etching.

Nickel chromium alloys with less than 90 p.c.
nickel are non-magnetic.

Silver and o&omium, although partially
miscible in the liquid state, form no solid solu-
tion. Molten cadmium does not dissolve any
chromium. Molten mixtures of lead and
chromium, containing more than 27 p.c. lead,
separate into this mixture and pure lead.

Chromium also forms silicides and borides in
the electric furnace ; of the former, compounds
represented by SiCr,, SiCr„, SijCr,, and SiaCr,
have been described. They are very hard —
SiCrj is harder than corundum — and are not
attacked by ordinary acids, though attacked
readily by hydrofluoric acid and by aqiia regia.
The borides CrjB^ and CrB are metalUc-loolung
.crystalline sohds, not attacked even by a mixture
of aqua regia and hydrofluoric acid. The com-
pound CrB is stated to be weakly magnetic.
The phosphide CrP, obtained from the metal
and copper phosphide, forms duU-grey crystals,
sp.gr. 5-71, which burn in oxygen, forming
chromium phosphate, but are unattacked by
all acids except a mixture of hydrofluoric acid
and aqua regia. .

By electrolysis of a strongly acid solution of
chromic chloride, using a platinum anode and
a mercury cathode, an amalgam HgjCr is
obtained, soft and brilliant, and altering but
slightly in air. By pressure, a harder amalgam
HgCr is obtained, which alters in air more
rapidly. By distillation in vacud, both lose
mercury arid yield pyrophoric chromium {v.
supra). An amalgam may also be obtained
by the action of sodium amalgam on a chro-
mium salt.

With iron the alloys of chromium are of
great interest. The presence of chromium in
iron or steel produces a much finer texture,
greater hardness, tenacity, and elasticity, and
greater smoothness of fracture. Chromium can-
not be used to replace carbon in steel, as has
been asserted (Boussingault).

When crystalline, the tendency of these
alloys is to produce needles instead of plates,
as in the case of manganese. Two to 4 p.c.
chromium with 1-2 to 1'4 p.c. carbon, renders
it so hard that it is bored with difficulty, and
when tempered has a fracture resembling porce-
lain. In these alloys of steel and chromium,
the hardness is probably due to the forma-
tion of a definite ferro-chrom-carbide of great
hardness, which is completely misoible with its
components. All alloys containing between 10
and 90 p.c. chromium appear to consist of two
structural elements, that separating primarily
being the softer. Chrome - steel is now very
largely used in railway work for the manufacture
of tyres and springs. It also possesses great
penetrating power and penetrating resistance, and
is therefore largely used for the manufacture of
armour plates and armour-piercing projectiles ;
the addition of nickel further increases its
toughness and use in armour plating. Chrome
steel is also used in the manufacture of special
Idnds of files. The alloy is usually prepared of
the requisite composition by the addition of a
definite amount of ' ferroohrome,' containing
from 40 to 85 p.c. chromium to the molten steel.
The ' ferroohrome ' is prepared from rich chrome-



CHROMIUM.



51



Irou ores in blast furnaces by the use of coke and
hot high-pressure blast.

The addition of chromium decreases the
magnetic properties, but all alloys, to 80 p.o.
chromium, are magnetic. A carbide of chromium
and tungsten of great hardness and of sp.gr. 8-41,
has been produced in the electric furnace, and to
it the special properties of chrome tungsten steels
are probably due. These latter (as well as
chrome molyhdenum steels), containing up tp 3 ji.c.
chromium and 8 p.c. tungsten, are used for the
manufacture of machine tools.
. Detection ' of Chromium. When heated
strongly, all compounds of chromium impart a
green colour to a borax bead in both the reduc-
ing and oxidising flames. On ignition with
potassium nitrate, all chromium compounds pro-
duce a yellow colour, due to the presence of
potassium chromate. If this is dissolved in
water, the addition of a solution of lead acetate
produces a precipitate of chrome yellow. A
similar oxidation occurs when chromium com-
pounds are fused with sodium peroxide, or when
solutions of chromium salts are boUed with
sodium peroxide.

Solutions of chromic salts or salts of ses-
quioxide of chromium have an aeid reaction.
With caustic alkalis they produce a green pre-
cipitate of hydrated oxide partially soluble in
excess of the reagent, but reprecipitated on
boiling the solution. With carbonates, a green
precipitate is produced, hkewise soluble in
excess.

Chromates (in which the chromium exists as
an acid) are usually strongly coloured. Soluble
chromates are reduced, when warmed with sul-
phuric acid and a reducing a.gent such as alcohol,
with the production of a green colour. The
chromium is then present as a base, and may
be jaecipitated, as already .mentioned. When
heated with hydrochloric acid, they are partially
reduced, with evolution of chlorine and formation
of chromium chloride and chloride of the metal
present as chromate, together with the forma-
tion of chromyl chlofide CrOjCla.

Solutions of chromates containing no free
acid except acetic acid give a yellow precipitate
with a salt of lead, a red precipitate with silver
nitrate, and a yellow precipitate with a salt of
barium.

Estimaiion. — ^When present as a base, chro-
mium is usually estimated as sesquioxide CraOj.
For this purpose, the solution is heated nearly to
boiling, ammonia solution added in slight excess,
and the temperature maintained until the liquid is
perfectly colourless, indicating that the hydroxide,
which is slightly, soluble in excess of ammonia,
is completely precipitated. The precipitate is
well washed by decantation and transferred to
a filter, washed thoroughly with hot water, dried,
and ignited. The oxide produced contains 68'63
p.c. of chromium. The precipitation is not com-
plete in presence of organic matter, and when a
glass vessel is used, the precipitate always con-
tains a small percentage of silica. Chromium,
when present as a chromate, cannot be estimated
directly by that method. It requires to be first
reduced to a base. Tor this purpose, hydro-
chloric acid and a small quantity of alcohol are
added, and the solution heated until the odour
of alcohol is dispelled. The chromium may then
be precipitated as above. For the direct estima-



tion of chromium in chromates, the precipitation
with lead acetate is most satisfactory. The
solution is mixed mth sodium acetate and acidi-
fied strongly with acetic acid. A solution of
normal lead acetate is then added, and the pre-
cipitate of PbCrOj is ooUeoted on a weighed
filter, washed, dried at 100°, and weighed.
It contains 16-19 p.c. of chromium, or 31-06
p.c. of chromic anhydride CrOj. The following
scheme may be adopted for the estimation of
chromium in chrome iron ore : A few grams of
the carefully sampled mineral are ground in an
agate mortar and passed through a fine muslin
sieve. The dust so produced is further ground
in small portions until all grittiness, on passing
a little between the fingers, has disappeared and
the ore cakes as an impalpable powder round the
pestle. About 0-5 gram of the powder is weighed
into a platinum crucible of about 60 c.o. capacity,
covered with twelve times its weight of recently
fused hydrogen potassium sulphate (potassium
bisulphate), and gently heated to just fuse the
sulphate. After keeping at this temperature for
15 or 20 minutes, the heat is gradually increased
until the crucible bottom becomes red hot. The
fused mass should not be allowed to rise above
haU-way up the crucible. The mixture soon
fuses quietly, and evolves dense fumes of
sulphuric aeid ; the heat is gradually increased
to bright redness. In about haU an hour, 6 parts
of powdered anhydrous sodium carbonate are
added and the mixtui'e again fused for an hour
at a red heat, 6 parts of nitre being added in
small portions. The temperature is then raised
to a full red heat for about 20 minutes ; the
crucible cooled and transferred to a porcelain
basin where the mass is boiled out with water.
The solution is filtered, and the residue washed
with hot water until the filtrate comes through
colourless. The filter and its contents are dried,
the precipitate detached and placed with that
still remaining in the basin, the filter paper
burned, and the ash also added. To ascertain
if the fusion has been satisfactory, this residue is
digested with moderately strong hydrochloric
acid, when the whole should dissolve. Any in-
soluble black residue indicates either imperfect
grinding or insufficient fusion. It must be col-
lected on a filter, dried, and the whole ignited in
a crucible and treated with potassium bisulphate,
&c., as in the first instance. The aqueous ex-
tract, after fusion, is mixed with the main solu-
tion. To the liquid, a few grams of ammonium
nitrate are added, and the whole evaporated to
dryness, taken up with water, and filtered from
the alumina, silica, &c., into a porcelain basin.
An excess of sulphurous acid solution is then
added, and the solution heated until that gas is
nearly expelled. The chromate, having thus
become reduced to chromium sulphate, a slight
excess of ammonia is added, and the solution
boiled until colourless. The precipitated chro-
mium hydroxide is washed by decantation with
hot water, transferred to the filter, and washed
with hot water six times. The use of the suction
pump is of great assistance ia this operation. The
precipitate and filter are then dried, transferred
to a weighed platinum crucible, and heated
gently until the paper is charred. The crucible
Ud is then placed at the edge of the crucible so
that the flame may reverberate into the crucible,
and the whole ignited strongly for 10 or 15



62



CHROMIUM.



minutes. The weighed precipitate should show
no yellowish ooloui on treatment with a few
drops of water ; if such a colour is produced it
indicates imperfect washing of the precipitate
from alkaU salts. The oxide contains 68'63 p.c.
of chromium.

The oxidation may also be effected by means
of sodium peroxide, and the ohrgmate pro-
duced, instead of being reduced to chromic oxide,
may be estimated as chromic acid volumetricaUy.
For this, various methods are available, e.g. : (1)
boiling with standard arsenious oxide and esti-
mating the excess by standard iodine ; (2)
adding potassium iodide and titrating iodine
liberated by standard thiosulphate ; (3) adding
standard ferrous ammonium sulphate solution
and titrating excess with standard potassium
dichromate.

A speedy and accurate method for the esti-
mation of chromium in ferrochrome or ohromite,
is the following : 1 part of the ore is finely
powdered, as described above, and fused with a
mixture of 6 parts of sodium peroxide and 8 parts
of caustic soda in a silver crucible. The aqueous
extract is filtered and again similarly treated ;
three or four such fusions are necessary to dis-
solve all the ore, when the only residue is a little
silver extracted from the crucible. The filtrate
is acidified with sulphuric acid, boiled for 40
minutes and diluted to a known volume. An
aliquot part is then titrated with potassium
^ iodide and standard sodium thiosulphate.

Chromates may also be readily estimated in
acid solution by hydrazine sulphate, whereby
nitrogen is liberated, 100 c.c. of nitrogen at
N.T.P. being equivalent to 0-59576 gram Cr.

COKPOnNDS ov Chrdmitjm.

Chromium forms a number of oxides ; of
these, chromous oxide CrO, and chromic oxide or
chromium sesquioxide CtjOj, are definite basic
oxides, yielding with acids salts in which chro-
mium is present as a divalent or trivalent cation,
forming the chromous and the chromic salts.
Chromic oxide can also combine with bases to
form salts, the chromitee, of the type MOrO a, in
which the chromium is present in the monovalent
anion CrO'j. Chromic anhydride CrOj is an acid
anhydride forming, with bases, salts of the type
MaOrOj, the chromates, or MaOcjO,, the di-
chromates, in which the chromium exists as the
divalent anion CrO", or Cr20",. Other oxides
have been prepared and are usually regarded as
con^KJunds of the basic and acid oxides, e.g'.
CiiO^-GiO^— chromic chromate. The chromous
salts are of no industrial importance at present.

Chiomic oxide. Chromium sesquioxide
Cr^Oa. This compound is produced by the oxi-
dation of metallic chromium and by ignition of
chromic hydroxide, chromic anhydSde, and
certain chromates.

Wohler prepared this oxide in fine, small
rhombohedral crystals by passing the vapour
of chromyl diohloride through a tube heated to
redness. The crystals are isomorphous with
corundum and of equal hardness ; their sp.gr. is
5'21. Crystalline chromic oxide is also obtained
when a mixture of potassium dichromate and
sodium chloride is heated to redness (Ditte,
Compt, rend. 134, 336). The finest coloured
amorphous chromium oxide is produced by heat-
ing mercurous ohromate HgjCrOj in a covered



crucible; mercury and oxygen escape, and the
oxide remains as a green powder. For the
preparation of this substance on the large scale,
a great number of methods are recommended.

BoU a solution of potassium dichromate with
half its weight of flowers of sulphur so long as
the green hydroxide is precipitated. The addi-
tion of a little potash solution, by forming
potassium sulphide, accelerates the decomposi-
tion. The precipitate is filtered from the
solution containing potassium sulphate, and
washed. The sulphur retained in the precipitate
may be removed by heating. Instead of
performing the reaction in the wet way, the
mixture may be ignited in a crucible, and the
resultant mass digested with water (Lassaigne,
Ann. Chim. Phys. [3] 14, 299). Wohler (Pogg.
Ann. 10, 46) heats a mixture of potassium
dichromate with its own weight of ammonium
chloride and a small quantity of sodium car-
bonate, and purifies the residue by washing.

According to Barian (Rev. Scient. 20, 425), a
very pxire colour, suitable for colouring fine por-
celain, is produced by igniting in a crucible a
mixture of 4 parts of potassium chromate and
1 part of starch. The mass is washed free from
potassium carbonate and reignited.

Chromic sesquioxide is a green pigment of
great permanence. It is not acted upon by
chlorine or sulphur gases or by an intense heat.
At a white heat it melts, and crystallises on
cooling.

Hydrogen and carbon monoxide are without
action, but it is reduced to the metallic state on
heating with carbon at a temperature of 1185°
or higher.

This oxide is largely used under the names
chrome green (mixtures of chroma yellow and
Prussian blue are also frequently called chrome
greens and must not be confounded with the
true chrome green), and ultramarine green for
imparting a green oolom: to glass, porcelain, &c.,
as a pigment, in oil and water colours and in
printing, and as a mordant in calico-printing and
dyeing. Certain hydiated oxides are also used
under various names ; their colours are, generally
speaking, brighter than that of the anhydrous
oxide, but they usually contain small quan-
tities of other substances besides the oxide and
water.

Guignet's Green ; Pennettier's Qreen ; Eme-
rald Green ; Veridian Crj03,2H20. This pig-
ment appears to be identical with that formerly
manufactured in secret by Pennettier. Accord-
ing to Guignet's method, 3 parts of boric acid
and 1 part of potassium dichromate are heated
to dull redness in a reverberatory furnace. The
mass swells up, evolves oxygen, and becomes of
a fine green colour ; it contains the borates of
potassium and chromium, or a double borate of
those two metals. It is boiled with water, whereby
the borate of chromium is decomposed into
boric acid and hydrated chromium sesquioxide,
potassium borate also remaining in solution.



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