George Fownes.

A manual of elementary chemistry: theoretical and practical online

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From both these mixtures the potassium pentasulphide may be extracted
by alcohol, in which it dissolves.

When the carbonate is fused with half its weight of sulphur only, the
trisulphide is produced, as above indicated, instead of the pentasulphide.

The effects described happen in the same manner when potassium
hydrate is substituted for the carbonate ; also, when a solution of the hy-
drate is boiled with sulphur, a mixture i>{ sulphide and hyposulphite al-
ways results.

Potassium-salts are colorless, when not associated with a colored metallic
oxide or acid. They are all more or less soluble in water, and may be
distinguished by the following characters :

(1.) Solution of tartaric acid, added in excess to a moderately strong solu-
tion of potassium-salt, gives, after some time, a white crystalline precipi-
tate of cream of tartar ; the effect is greatly promoted by strong agitation.

(2.) Solution of platinic chloride with a little hydrochloric acid, if neces-
sary, gives, under similar circumstances, a crystalline yellow precipitate,
which is a double salt of platinum tetrachloride and potassium chloride.
Both this compound and cream of tartar are, however, soluble in about 60
parts of cold water. An addition of alcohol increases the delicacy of both
tests.

(3.) Perchloric acid, and silicojluoric acid, give rise to slightly soluble white
prcipitates when added to a potassium-salt.

(4.) Potassium-salts usually color the outer blowpipe-flame purple or
violet : this reaction is clearly perceptible only when the potassium-salts
are pure.

(5.) The spectral phenomena exhibited by potassium compounds are men-
tioned at p. 88.



SODIUM.

Atomic weight, 23. Symbol, Na. (Natrium).

SODIUM is a very abundant element, and very widely diffused. It occurs
in large quantities as chloride, in rock-salt, sea-water, salt-springs, and
many other mineral waters ; more rarely as carbonate, borate, and sul-
phate, in solution or in the solid state, and as silicate in many minerals.

Metallic sodium was obtained by Davy soon after the discovery of po-
tassium, and by similar means. Gay-Lussac and Thenanl afterwards pre-
pared it by decomposing sodium hydrate with metallic iron at a white heat;
and Brunner showed that it may be prepared with much greater facility
by distilling a mixture of sodium carbonate and charcoal.

The preparation of sodium by this last-mentioned process is much easier
than that of potassium, not being complicated, or only to a slight extent,



300



MONAD METALS.



by the formation of secondary products. Within the last few years it has
been considerably improved by Deville and others, and carried out on the
manufacturing scale, sodium being now employed in considerable quantity
as a reducing agent, especially in the manufacture of aluminium and mag-
nesium, and in the silver amalgamation process.

The sodium carbonate used for the preparation is prepared by calcining
the crystallized neutral carbonate. It must be thoroughly dried, then
pounded and mixed with a slight excess of pounded charcoal or coal. An
inactive substance, viz. pounded chalk, is also added to keep the mixture
pasty condition during the operation, and prevent the fused sodium



carbonate from separating from the charcoal,
portions recommended by Deville :



The following are the pro-



For Laboratory Operations.
Dry sodium carbonate, 717 parts

Charcoal 175 "

Chalk 108 "



For Manufacturing Operations.
Dry sodium carbonate, 30 kilogr.

Coal 13

Chalk . . 3 "



These materials must be very intimately mixed by pounding and sifting,
and it is advantageous to calcine the mixture before introducing it into the
distilling apparatus, provided the calcination can be effected by the waste
heat of a furnace ; the mixture is thereby rendered more compact, so that
a much larger quantity can be introduced into a vessel of given size.

The distillation is performed, on the laboratory scale, in a mercury bottle
heated exactly in the manner described for the preparation of potassium.
For manufacturing operations, the mixture is introduced into iron cylin-
ders, which are heated in a reverberatory furnace, and so arranged that,
at the end of the distillation, the exhausted charge may be withdrawn and
a fresh charge introduced, without displacing the cylinders or putting out
the fire. The receivers used in either case are the same in form and di-
mensions as those employed in the preparation of potassium (p. 291).

When the process goes on well, the sodium collected in the receivers is
nearly pure; it may be completely purified by melting it under a thin layer
of naphtha. This liquid is decanted as soon as the sodium becomes per-
fectly fluid, and the metal is run into moulds like those used for casting
lead or zinc.

SODIUM CHLORIDE ; COMMON SALT, NaCl. This very important substance
is found in many parts of the world in solid beds or irregular strata of im-
mense thickness, as in Cheshire, Spain, Galicia, and many other localities.
An inexhaustible supply exists also in the waters of the ocean, and large
quantities are obtained from saline springs.

Hock-salt is almost always too impure for use. If no natural brine-spring
exists, an artificial one is formed by sinking a shaft into the rock-salt, and,
if necessary, introducing water. This when saturated is pumped up, and
evaporated more or less rapidly in large iron pans. As the salt separates,
it is removed from the bottom of the vessel by means of a scoop, pressed
while still moist into moulds, arid then transferred to the drying-stove.
When large crystals are required, as for the coarse-grained bay-saH used in
curing provisions, the evaporation is slowly conducted. Common salt is
apt to be contaminated with magnesium chloride.

Sodium chloride, when pure, is not deliquescent in moderately dry air.
It crystallizes in anhydrous cubes, which are often -grouped together into
pyramids, or steps. It requires about 1\ parts of water at 1-5-5 C. (60
F.) for solution, and its solubility is not sensibly increased by heat; it dis-
solves to some extent in spirit of wine, but is nearly insoluble in absolute
alcohol. It melts at a red heat, and is volatile at a still higher temperature.
The economical uses of common salt are well known.



SODIUM.



301



The iodide and bromide of sodium much resemble the corresponding potas-
um-compoumls: they crystallize in cubes which are anhydrous, and very



sium

soluble in water.



SODIUM OXIDES. Sodium forms a monoxide and a dioxide ; also
drate corresponding to the former.



hy-



Sodium Monoxide, or Anhydrous Soda, ONa 2 , is produced, together with
the dioxide, when sodium burns in the air, and may be obtained pure by
exposing the dioxide to a very high temperature; or by heating sodium
hydrate with an equivalent quantity of sodium : 20NaH -(- Na 2 = 20Na
-f- H 2 . It is a gray mass, which melts at a red heat, and volatilizes with
difficulty.

Sodium Hydrate, or Caustic Soda, ONaH or ONa 2 , OH 2 . This substance
is prepared by decomposing a somewhat dilute solution of sodium carbonate
with calcium hydrate: the description of the process employed in the case
of potassium hydrate, and the precautions necessary, apply word for word
to that of sodium hydrate.

The solid hydrate is a white, fusible substance, very similar in properties
to potassium hydrate. It is deliquescent, but dries up again after a time
in consequence of the absorption of carbonic acid. The solution is highly
alkaline, and a powerful solvent for animal matter: it is used in large
quantity for making soap.

The strength of a solution of caustic soda may be roughly determined
from a knowledge of its density, by the aid of the following table drawn
up by Dalton:

TABLE OF PERCENTAGE OP ANHYDROUS SODA, ONa 2 , IN SOLUTIONS OF
DIFFERENT DENSITY.



Density.
2-00
1-85
1-72
1-63
1-55
1-50
1-47
1-44



Percentage of
anhydrous soda.


Density.


. 77-8




40


63-6




36


. 53-8




32


46-6




29


. 41-2




23


36-8




18


. 34-0




12


31-0




06



Percentage of

anhydrous soda.

. 29-0

26-0

. 23-0

19-0

. 16-0

13-0

. 9-0

4-7



Sodium Dioxide, 2 Na 2 . Sodium, when heated to about 200 in a current
of dry air, absorbs oxygen, and is converted into dioxide : this substance is
white, but becomes yellow when heated, which tint it again loses on cool-
ing. It is soluble in water without decomposition: the solution maybe
evaporated under the receiver of the air-pump, and, when sufficiently con-
centrated, deposits crystalline plates having the composition 2 Na 2 .80H a .
These crystals left to effloresce over oil of vitriol for nine days lose three
fourths of their water, and yield another hydrate containing 2 Na 2 .20H 3
(Ilarcourt). The aqueous solution of sodium dioxide when heated on the
water-bath, is decomposed into oxygen and the monoxide.

SODIUM CARBONATES. The Neutral or Disodic Carbonate, C0 3 Na 2 .100H r
was once exclusively obtained from the ashes of sea-weeds, and of plants,
such as the Salsola soda, which grow by the sea-side, or, being cultivated
in suitable localities for the purpose, are afterwards subjected to incinera-
tion. The barilla, still employed to a small extent in soap-making, is thus
produced in several places on the coast of Spain, as Alicante, Carthagena,

. That made in Brittany is called varec.
26



302 MONAD METALS.

Sodium carbonate is now manufactured on a stupendous scale from com-
mon salt by a series of processes which may be divided into two stages :

(1.) Manufacture of sodium sulphate, or salt-cake, from sodium chloride
(common salt); this is called the salt-cake process.

(2.) Manufacture of sodium carbonate, or soda-ash; called the soda-ash
process.

(1.) Salt-cake process. This process consists in the decomposition of
common salt by sulphuric acid, and is effected in a furnace called the Salt-
cake furnace, of which fig. 164 represents a section. It consists of a large

Fig. 164.




covered iron pan, placed in the centre, and heated by a fire underneath ;
and two roasters, or reverberatory furnaces, placed one at each end, and on
the hearths of which the salt is completely decomposed. The charge of
half a ton of salt is first placed in the iron pan, and then the requisite
quantity of sulphuric acid is allowed to pass in upon it. Hydrochloric acid
is evolved, and escapes through a flue, with the products of combustion,
into towers or scrubbers, filled with coke and bricks moistened with a stream
of water; the whole of the acid vapors are thus condensed, and the smoke
and heated air pass up the chimney. After the mixture of salt and acid
has been heated in the iron pan, it becomes converted into a solid mass of
acid sodium sulphate and undecomposed sodium chloride :

2NaCl -f S0 4 H 2 = NaCl -f S0 4 NaH + HC1.

It is then raked on to the hearths of the furnaces at each side of the decom-
posing pan, where the flame and heated air of the fire complete the decom-
position into neutral sodium sulphate and muriatic acid :

NaCl + S0 4 NaH = S0 4 Na 2 -f HC1.

(2.) Soda-ash process. The sulphate is next reduced to powder, and
mixed with an equal weight of chalk or limestone, and half as much small
coal, both ground or crushed. The mixture is thrown into a reverberatory
furnace, and heated to fusion, with constant stirring, 2 cwts. are about the
quantity operated on at once. When the decomposition is judged complete,
the melted matter is raked from the furnace into an iron trough, where it
is allowed to cool. This crude product, called black ash or ball-soda, is
broken up into little pieces, when cold, and lixiviated with cold or tepid
water. The solution is evaporated to dryness, and the salt calcined with a
little sawdust in a suitable furnace. The product is the soda-ash, or British
alkali of commerce, which, when of good quality, contains from 48 to 52
per cent, of anhydrous soda, ONn 2 , partly in the state of carbonate, and
partly as hydrate, the remainder being chiefly sodium sulphate and common
salt, with occasional traces of sulphite or hyposulphite, and also cyanide
of sodium. By dissolving soda-ash in hot water, filtering the solution, and
then allowing it to cool slowly, the carbonate is deposited in large trans-
parent crystals.

The reaction which takes place in the calcination of the sulphate with
chalk and coal-dust seems to consist, first, in the conversion of the sodium
sulphate into sulphide by the aid of the combustible matter, and, secondly,



SODIUM.

in the interchange of elements between that substance and the calcium car-
bonate :

SNa 2 -f C0 3 Ca = SCa + C0 3 Na 2
Sodium Calcium Calcium Sodium

sulphide. carbonate. sulphide. carbonate.

Other processes have been proposed, and even carried into execution;
but the above, which was originally proposed by Leblanc, is found most
advantageous.

The ordinary crystals of sodium carbonate contain ten molecules of
water ; but by particular management the same salt may be obtained with
fifteen, nine, seven, molecules, or sometimes with only one. The common
form of the crystals is derived from an oblique rhombic prism; they
effloresce in dry air. and crumble to a white powder. Heated, they fuse in
their water of crystallization; when the latter has been expelled, and the
dry salt exposed to a full red heat, it melts without undergoing change. The
common crystals dissolve in two parts of cold, and in less than their own
weight of boiling water : the solution has a strong, disagreeable, alkaline
taste, and a powerfully alkaline reaction.

Hydrogen and Sodium Carbonate, Hydrosodic Carbonate, Monosodic Car-
bonate, Acid Sodium Carbonate, C0 3 NaH, or C0 5 Na 2 .C0 3 H 2 , commonly called
Bicarbonate of soda. This salt is prepared by passing carbonic acid gas
into a cold solution of the neutral carbonate, or by placing the crystals in
an atmosphere of the gas, which is rapidly absorbed, while the crystals
lose the greater part of their water, and pass into the new compound.

Monosodic carbonate, prepared by either process, is a crystalline wl;ite
powder, which cannot be re-dissolved in warm water without partial de-
composition. It requires 10 parts of water at 15-5 for solution : the liquid
is feebly alkaline to test-paper, and has a much milder taste than that of
the simple carbonate. It does not precipitate a solution of magnesia. By
exposure to heat, the salt is converted into neutral carbonate.

Dihydro-tetrasodic Carbonate, (C0 3 ) 3 Na 4 H 2 . 20H 2 . This salt, commonly
called sesquicarbonate of soda, may be regarded as a compound of the neutral
and acid salts just described (C0 3 Na 2 .2C0 3 NaH). It occurs native on the
banks of the soda lakes of Sokenna, near Fezzan, in Africa, where it is called
trona; also as urao, at the bottom of a lake in Maracaibo, South America.
It is produced artificially, though with some difficulty, by mixing the mo-
nosodic and disodic carbonates in the proportions above indicated, melting
them together, drying and exposing the dried mass in a cellar for some
weeks; it then absorbs water, becomes crystalline, and contains spaces
filled with the tetrasodic carbonate.

Sodium and Potassium Carbonate, C0 3 NaK . 60H 2 , separates in monoclinic
crystals from a solution containing the two carbonates in equivalent pro-
portions.

A mixture of these two carbonates in equivalent proportions melts at a
much lower heat than either of the salts separately; such a mixture is
very useful in the fusion of silicates, &c-

Alkalimetry. Analysis of Alkaline Hydrates and Carbonates.

The amount of alkali or alkaline carbonate in commercial potash, flpda,
or ammonia, is estimated by determining the quantity of an acid of gm-n
strength required to neutralize a given weight of the sample. The estim.-i-
tion depends upon the facts that the alkaline salts of strong acids (sul-
phuric, oxalic, &c.) are neutral to litmus; and that the violet solution of
litmus is colored blue by caustic alkalies or alkaline carbonates, wine-red
by carbonic acid, and light red by strong acids.



304 MONAD METALS.

The first step is the preparation of the standard acid. It is best to make
this liquid of such strength that 1000 cubic centimetres (1 litre) shall
contain exactly one J gram-molecule (i. e., 1 molecule expressed in J grams)
of the acid.

About 70 grams of concentrated sulphuric acid are diluted with about
600 grams of water ; when the mixture is cool, the volume of it necessary
to saturate 5-3 grams (one J-decigram-moleculc) of pure anhydrous sodium
carbonate, C0 3 Na 2 , is determined.* For this purpose 5-3 grams of freshly
ignited sodium carbonate are dissolved in hot water, the solution colored
blue with a few drops of litmus, and the acid added from a burette or al-
kalimeter (p. 305), at last drop by drop, till the color just passes from
wine-red to light red, and till strips of litmus-paper, moistened with the
solution begin to retain the color when dry. The volume of acid employed
is then noted, and the whole diluted so as to approximate to the required
strength. Suppose, for instance, 37 cubic centimetres of acid have been
used ; water is then added till every 100 volumes is diluted to 250 volumes,
and another determination is made. If 90 cubic centimetres are now re-
quired to saturate the J-decigram alkaline solution, every 90 volumes of the
acid must be diluted to 100, and the result controlled by a fresh determina-
tion; 100 cubic centimetres of this acid should exactly saturate 5-3 grams
of sodium carbonate, and will contain 1 half-dccigram-rnolecule of acid;
2 cubic centimetres will therefore contain 1 milligram-molecule (0-098
gram)f and will saturate 2 milligram-molecules of an alkali (OKH or
ONaH), or 1 milligram-molecule of an alkaline carbonate (C0 3 K 2 or C0 3 Na 2 ).

To estimate the proportion of alkali in a commercial sample, a weighed
portion of the substance is dissolved in water (if a solid), a few drops of
litmus added, and the standard acid added from a burette, until the first
permanent appearance of a light red color ; and the volume of acid em-
ployed is read off. Each cubic centimetre of acid corresponds to 1 milli-
gram-molecule of alkali, or 1 half milligram-molecule of alkaline carbonate ;
i. e., to - 053 gram sodium carbonate, C0 3 Na 2 , 0-069 gram potassium carbo-
nate, C0 3 K 2 , 0.040 gram caustic soda ONaH, 0-056 gram caustic potash OKH,
and 0-017 gram ammonia NH 3 ; and a simple proportion gives the amount
of alkali or alkaline carbonate present (e. g. 100 : 6-9 : : number of cubic
centimetres employed: potassium carbonate present). By operating on
100 times the ^-milligram-molecule (e. g. 6-9 grams in the case of potassium
carbonate, 5-3 grams in the case of sodium carbonate), all calculation is
saved : for as this amount, if present, would require 100 cubic centimetres
of acid for its saturation, the number of cubic centimetres actually required
at once indicates the percentage of alkaline carbonate. The burettes
commonly used contain 50 cubic centimetres, and are graduated into half
cubic centimeters; so that by operating on 50 times the ^-milligram-mole-
cule, the number of divisions employed indicates the percentage.

Sometimes, instead of exactly neutralizing the alkali with the standard
acid, it is better to add the acid till the litmus assumes a distinct light-red
color, then heat the solution to boiling, and add a small excess (5 to 10
cubic centimetres) of acid. The hot solution is freed from carbonic acid by
agitation and by drawing air through it with a glass tube ; and then neu-
tralized with a standard solution of caustic soda (100 cubic centimetres of
which exactly saturate 100 cubic centimetres of the standard acid) till the
color just changes from red to blue. Since the acid and alkaline solutions
neutralize each other volume for volume, it is only necessary to deduct the
number of cubic centimetres employed of the latter from that of the former,
and calculate the amount of alkali from the residue. This method, called
the indirect or residual method, is preferable to the direct method previously

* The molecule of sodium carbonate CO ? Na 2 weighs 12 -f 48 -f 46 :r 106.
f The molecular weight of sulphuric acid S0 4 IL. is 98 = 32 + 64 -f- 2.




SODIUM.



305



described, for the analysis of carbonates, since the change from blue to
red is more distinctly marked than that from one shade of red to another.

The standard solution of caustic soda must be kept in a flask, into the
cork of which is inserted a calcium chloride tube tilled with a mixture of
sodium sulphate and quicklime, which eifectually prevents the absorption
of carbonic acid. If the burette be closed with a similar tube, the soda so-
lution may remain in it for days.

The " alkalimeter " or "burette" is a glass tube (fig. 165) Fig- 165.
closed at one end, and moulded into a spout or lip at the other,
and marked with any convenient scale of equal parts, generally,
as above mentioned, into 100 half cubic centimetres.* A strip of
paper is pasted on the tube and suifered to dry, after which the
instrument is graduated by counterpoising it in a nearly upright
position in the pan of a balance of moderate delicacy and weigh-
ing into it, in succession, 5, 10, 15, 20, &c., grams of distilled
water at 4 C. (39-2 F.) until the whole quantity, amounting to 50
grams (50 cubic centimetres), has been introduced, the level of the
water in the tube being, after each addition, carefully marked
with a pen upon the strip of paper, while the tube is held quite
upright, and the mark made between the top and bottom of the
curve formed by the surface of the water. The smaller divisions
of the scale, of a half cubic centimetre each, may then be made
by dividing with compasses each of the spaces into 10 equal
parts. When the graduation is complete, and the operator is
satisfied with its accuracy, the marks may be transferred to the
tube itself by a sharp file, and the paper removed by a little
warm water. The numbers are scratched on the glass with the
hard end of the same file, or with a diamond. Or the glass is
covered with etching wax, the scale traced upon it with a fine
needle point, and the marks etched by exposing the tube to the vapor of
hydrofluoric acid.

fig. 166. Fig. 167. Fig- 168.





n


>





1


3


*


1







10
20


T


E




4




-|


20


30




|







30






430




-5


40


50




Ir




i




BO




In






5O


70




->sc




^


CO


80




f

1




^



TO

80

\


90

no

gf*


c


3
_=-




1


90 QP


"


Ni>




J


100




* It mav also be divided into 1000 grain-mcasim-H. th grain-measure being the capacity of a
ain of distilled water at 60 *V, 70,000 siu-h measure* go to an imperial gallon, and 8,7^0 to



a pint.

26*



306 MONAD METALS.

The alkalimeter, represented in fig. 165, is the simplest form of this in-
strument. The pouring out of minute quantities is, however, greatly facil-
itated by providing the measure with a narrow dropping tube, fig, 166,
the lower extremity of which is soldered into the measure, while the upper
one is bent outward and sharply cut off. This kind of burette, which is
known as Gay-Lussac's, is chiefly used in France. The liquid may be very
conveniently poured from it ; but it is rather easily broken, so that its
manipulation requires a good deal of care. This defect is greatly obviated
in the burette, fig. 167, in which the graduated tube is provided with
a spout at the top, there being at the same time an orifice for pouring in
the liquid.

A very elegant instrument has been contrived by Dr. Mohr of Coblentz.
It is a graduated tube, drawn out at one end to a paint, to which is at-
tached, by means of a narrow vulcanized caoutchouc tube, a short glass
tube, likewise drawn out to a point (fig. 108). There is a small space
(about inch) between the two tubes, upon which is fixed a metallic clamp,
a, represented in its actual dimensions in fig. 169. This clamp shuts off
the connection between the graduated cylinder and the small glass tube.
But by pressing with the fingers upon the ends, b 6, of this clamp, it opens,
and allows the liquid to flow out of the lower tube. It is evident that by
this arrangement the amount of liquid may be regulated with the greatest
nicety.

It is often desirable, in the analysis of carbonates, to determine directly
the proportion of carbonic acid: the following methods leave nothing to
be desired in point of precision:

A small light glass flask of three or four ounces capacity, with lipped



Online LibraryGeorge FownesA manual of elementary chemistry: theoretical and practical → online text (page 38 of 114)