ELECTROLYTIC METHODS
One of the most noteworthy developments of modern chemical industry has
been the increasing use of electricity as an agent for bringing about
changes in matter. This has followed naturally from the reduction in the
cost of electricity, due in great measure to the utilization of natural
sources of energy which for untold ages had been allowed to run to
waste.
This last achievement is likely to produce such a change in economic
conditions that it is worth while giving a little thought to what may be
called a newly-discovered asset of civilization. One example will make
this clear. In the bed of the Niagara river, which flows from Lake Erie
to Lake Ontario, there is a sudden drop of 167 ft. over which the water
rushes with tremendous force and expends its energy in producing heat
which cannot be utilized. This is a waste of energy, but it cannot be
circumvented because no method has yet been found to control the waters
of the Falls themselves. Nevertheless, by leading the head waters
through suitable channels from the high level to the low, it is possible
to use the energy to drive turbines, which, in their turn, drive dynamos
which produce the current. This is merely the conversion of the energy
of running water into electrical energy; and while the sun remains, this
supply of energy will be forthcoming in undiminished quantity, because
by the heat of the sun the water is lifted again as vapour, which
descends as rain to replenish the sources from which the Niagara flows.
Electricity is employed in chemical industry in two ways. In the first
place, it may be used to produce very high temperatures required for the
reduction of some metallic ores, for melting highly-refractory
substances, and for making steel. It is, however, rather with the second
method, called electrolysis, that we are here mainly concerned.
[Illustration: Fig. 15. THE ELECTROLYSIS OF SALT SOLUTION]
Solutions of acids, bases, and salts, and in some cases the fused
substances themselves, conduct the electric current; but at the same
time they suffer decomposition. This method of decomposing a substance
is known as _electrolysis_, or a breaking up by the agency of
electricity.
The apparatus required in a very simple case is shown in Fig. 15. It
merely consists of some suitable vessel to contain the liquid; two
plates—one to lead the current into the solution, the other to lead it
away again—and wires to connect the plates to the poles of a battery,
storage-cell, or dynamo. Each plate is called an _electrode_, and
distinguished as positive or negative according as it is joined to the
positive or negative pole of the current generator. By convention,
electricity is supposed to “flow” from the positive pole of the battery
to the positive electrode or _anode_, and then through the solution to
the negative electrode or _cathode_, and so back to the negative pole of
the generator, thus completing the circuit external to the battery.
When acids, alkalis, and salts are dissolved in water, there is strong
evidence to show that they break up to a greater or less extent into at
least two parts called _ions_. These are atoms, or groups of atoms,
which have either acquired or lost one or more _electrons_.[5] They move
about quite independently of one another and in any direction until the
electrodes are placed in the liquid. Then they are constrained to move
in two opposing streams—those which have acquired electrons all move
towards the negative electrode, and those which have lost electrons
towards the other. At the electrodes themselves, the former give up and
the latter take up electrons, and become atoms again. Let us now
consider a concrete example. Common salt is composed of atoms of sodium
and atoms of chlorine paired. When a small quantity of this substance is
dissolved in a large quantity of water, the pairing no longer obtains.
The chlorine atoms move away independently accompanied by an extra
satellite or electron, and the sodium atoms move away also but with
their electron strength one below par. When the current is introduced
into the liquid, the sodium ions travel towards the cathode and chlorine
ions towards the anode, and when they reach the goal, sodium ions gain
one electron and chlorine ions lose one, and both become atoms again.
Chlorine atoms combine in pairs forming molecules and escape from the
solution in the greenish yellow cloud that we call chlorine gas. The
sodium atoms react immediately with water, forming caustic soda with the
liberation of hydrogen.
To return now to practical considerations. The electrolysis of salt
solution appears to be an ideally simple method of obtaining caustic
soda and chlorine from sodium chloride. As a manufacturing process, it
would seem to be perfect, for the salt is broken up directly into its
elements and a secondary reaction gives caustic soda automatically.
There is no “waste” as in the Leblanc process, and it does not require
the use of any expensive intermediary substance afterwards to be
recovered, as in the Solvay process. But, as very often happens when
working on a large scale, difficulties arise, and these up to the
present have only been partially overcome.
Some of the chlorine remains dissolved in the liquid and reacts with the
caustic soda, forming other substances which, though valuable, are not
easy to separate from the caustic soda. It is possible to get over this
difficulty to some extent by placing a porous partition between the
anode and the cathode, and in that way dividing the cell into cathodic
and anodic compartments. As long as the partition is porous to liquids,
it will allow the current to pass, but at the same time it will greatly
retard the mixing of the contents of the two compartments. Porous
partitions or cells which are in common use for batteries are made of
“biscuit” or unglazed porcelain.
It must be remembered, however, that porous partitions only retard the
mixing of liquids; they do not prevent it. Moreover, a further
difficulty arises from the fact that chlorine is a most active
substance, and therefore it is difficult to find a material which will
resist its corrosive action for any length of time, and the same
difficulty arises in the case of the anode where the chlorine is given
off.
Castner Process for Caustic Soda. The following is the most successful
electrical process for the manufacture of caustic soda yet devised. It
was introduced in 1892, and is known as the Castner process. It should
be noted that the use of the porous partition has been avoided in a very
ingenious way.
[Illustration: Fig. 16. THE CASTNER PROCESS]
The cell (see Fig. 16) is a closed, rectangular-shaped tank divided into
three compartments by two non-porous partitions fixed at one end to the
top of the tank, while the other end is free and fits loosely into a
channel running across the tank. The floor of the tank is covered with a
layer of mercury of sufficient depth to seal the separate compartments.
The two end compartments contain the brine in which are the carbon
anodes; the middle compartment contains water or very dilute caustic
soda in which the cast-iron cathode is immersed.
The current enters the end compartments by the carbon anodes and passes
through the salt solution to the mercury layer which in these
compartments are the cathodes. The current then passes through the
mercury to the middle compartment, and then through the solution to the
cathode, thence back to the dynamo. It is important to note that in the
middle compartment the mercury becomes the anode.
Chlorine is liberated at the carbon electrodes, and when no more can
dissolve in the liquid it escapes and is conveyed away by the pipe P.
Sodium atoms are formed at the surface of the mercury cathodes in the
outside compartments and dissolve instantly in the mercury, forming
sodium amalgam.
While the current is passing, a slight rocking motion is given to the
tank by the cam E. This is sufficient to cause the mercury containing
the dissolved sodium to flow alternately into the middle compartment,
and there the sodium amalgam comes into contact with water; the sodium
is dissolved out of the mercury and caustic soda is formed. Water in a
regulated stream is constantly admitted to the middle compartment, and a
solution of caustic soda of about 20 per cent. strength overflows.
The production of caustic soda by an electrical method still remains to
be fully developed. A process which gives only a 20 per cent. solution
cannot be looked upon as final. In the meantime, other methods have been
tried, in some of which fused salt is used in place of brine in order to
give caustic soda in a more concentrated form. For a description of
these methods, the reader must consult some of the larger works
mentioned in the preface. Here we can only say that very great
difficulties have been encountered, particularly in the construction of
a satisfactory porous diaphragm or, alternately, in devising methods in
which this can be dispensed with.
Another interesting application of electrolysis is furnished by the use
of copper sulphate in industry. When this salt is dissolved in water, it
breaks up into copper ions (positive) and an equal number of negative
ions, composed of 1 atom of sulphur and 4 atoms of oxygen (SO″4). Under
the influence of the current copper ions travel to the cathode, and
there by the gain of two electrons become copper atoms. Now, since
copper is not soluble in copper sulphate solution, and is not volatile
except at very high temperatures, it is deposited on the cathode in a
perfectly even and continuous film when the strength of the current is
suitably adjusted. This film continues to grow in thickness as long as
the conditions for its deposition are maintained. If the current
employed is not suitable, the metallic film is not coherent, and the
copper may appear as a red powder at the bottom of the cell. Any other
metal or impurity which might be present in the unrefined copper falls
to the bottom of the tank.
Other metals are deposited electrolytically in exactly the same way. The
metal to be deposited is joined to the positive pole and the article to
be plated to the negative pole of the battery. Both are suspended in a
solution of salt, generally the sulphate, of the metal which is to be
deposited. Thus, for nickel plating, a piece of sheet nickel would be
used in conjunction with a solution of sulphate of nickel or, better, a
solution of nickel ammonium sulphate, made by crystallizing ammonium and
nickel sulphates together. The current required is small; indeed, if it
is too strong, the deposit adheres loosely to the article, and the
result is, therefore, not satisfactory.
Electrotype blocks are also made by a similar process. An impression of
the article to be reproduced is made in wax, or some suitable plastic
material, and polished with very fine graphite or black lead, in order
to give a conducting surface. It is then suspended in a solution of
copper sulphate and joined to the negative pole of the battery; a plate
of copper connected with the positive pole is suspended in the same
solution. When a weak current is passed, copper is deposited on the
black-leaded surface and grows gradually in thickness, until at length
it can be stripped off, giving a positive replica of the object.
INDEX
A
Acetic acid (glacial), 73
Acids, early notions of, 1
——, fatty, 78
——, mineral, 68
——, vegetable, 68
Agate, 61
Air-saltpetre, 42
Alkali Acts, 44
——, caustic, 96
——, metals, 95
——, mild, 80
—— waste, 87
Alkalis, properties, 3
Aluminium acetate, 73
Alums, the, 26
Amethyst, 61
Ammonal, 36
Ammonia, 97
——, synthetic, 99
Ammonite, 99
Ammonium carbonate, 99
—— chloride, 98
—— nitrate, 99
—— sulphate, 99
Anhydride, an, 21
Anode, 103
Argol, 76
Asbestos, 63
——, platinized, 19
Ash, black, 84
——, pearl, 93
——, soda, 10, 92
Atolls, 51
Atomized water, 18
B
Bacon, Roger, 32
Basic slag, 58
Basil Valentine, 12
Beryl, 63
Black liquor, 74
Blasting gelatine, 35
Bleaching powder, 46
Blue-john, 47
Boiler scale, 54
Bonbonnes, 31
Bone, 56
—— ash, 57
—— black, 56
—— meal, 56
Borax, 59
Bordeaux mixture, 7
Boric acid, 58
Boyle, Robert, 2
Burgundy mixture, 6
C
Calcium acetate, 5
—— bicarbonate, 54
—— carbonate, 50
—— fluoride, 47
—— nitrate, 29
—— phosphate, 56
—— sulphate, 27
Calc spar, 50
Caliche, 29
Calico printing, 26
Carbon, 49
Carbonic acid, 49
—— —— gas, 49
Castner process, 105
Catalytic action, 20
Cathode, 103
Cat’s-eye, 61
Cavendish, H., 40
Cellulose, 46
Chalcedony, 61
Chalk, 50
Chert, 66
Chili-saltpetre, 29, 39
China clay, 62
Citric acid, 77
Chlorides, 47
Chlorine, 46
Chrome yellow, 28
—— red, 28
Compound, 7
Compounds, binary, 8
Contact action, 20
—— process, 18
Copper refining, 107
—— sulphate, 5, 27
Coral reefs, 51
Cordite, 34
Cream of tartar, 76
Crops, rotation of, 37
Crystallization, water of, 9
Crystals, 9
D
Davy, Sir Humphry, 95
Derbyshire spar, 47
Devitrification, 65
Dynamite, 35
E
Efflorescence, 82
Electrode, 103
Electrolysis, 102
Electrons, 103
Electrotype blocks, 107
Element, definition of, 7
Elements, list of, 8
Explosives, 32
F
Felspars, 62
Ferrous acetate, 74
—— sulphate, 25
Flint, 61
Fluorspar, 48
Formic acid, 78
Fur in kettles, 54
G
Garnet, 63
Gas, laughing, 99
—— lime, 12
—— liquor, 98
Gay Lussac tower, 16
Glass, 64
——, annealing of, 65
——, Bohemian, 63
——, etching on, 47
——, flint, 63
——, lead, 63
——, soda, 63
——, water, 66
Glauber’s salt, 10
Glover tower, 17
Glue, 56
Graphite, 108
Greek fire, 32
Guncotton, 34
Gunpowder, 32
Gypsum, 27
H
Haber process, 100
Halogen, 43
Hardness, permanent, 53
——, temporary, 53
Hartshorn, salt of, 99
——, spirits of, 97
Hornblende, 63
Hydriodic acid, 48
Hydrobromic acid, 48
Hydrochloric acid, 43
Hydrofluoric acid, 47
I
Iceland spar, 50
Ions, 103
Iron pyrites, 11
J
Jade, 63
Jasper, 61
K
Key industries, 10
L
Lake, 26
Lead acetate, 75
—— chambers, 17
—— chamber process, 14
——, sugar of, 75
—— sulphate, 27
——, white, 75
Leblanc soda process, 82
Leguminosae, 37
Lemon, salts of, 77
Lime burning, 51
——, caustic, 97
—— kiln, 51
Limestone, 50
Litmus, 2
Lupin root, 37
M
Marble, 50
Marking ink, 28
Meerschaum, 63
Mica, 63
Mordants, 26
Mycoderma aceti, 68
N
Neutralization, example of, 4
——, explanation of, 3
Niagara, 101
Nitre, 29
—— pots, 14
Nitric acid, 30
—— ——, from air, 40
—— ——, importance of, 28
—— —— manufacture of, 30
—— ——, properties, 31
—— ——, red fuming, 31
—— oxide, 16
Nitrogen cycle, 37
——, fixation of, 100
—— peroxide, 16
Nitroglycerine, 34
O
Olein, 78
Onyx, 61
Opal, 61
Orthoclase, 62
Oxalic acid, 77
P
Palmitin, 78
Pearls, 51
Peregrine Phillips, 21
Philosopher’s stone, 2
Phosphoric acid, 57
Plaster of Paris, 27
Potash, caustic, 97
——, mild, 93
Potassium, 95
—— bicarbonate, 94
—— nitrate, 29
Propellants, 33
Prussian blue, 25
Pyrites burners, 14
Pyroligneous acid, 73
Q
Quartz, 61
—— fibres, 62
——, smoky, 61
Quicklime, 5, 51
R
Red liquor, 73
Rock crystal, 61
Rupert’s drops, 65
S
Sal ammoniac, 99
—— prunella, 29
Salt cake, 84
——, common, 47
——, formation of a, 4
Saltpetre, 29
Salts, from carbonates, 5
——, from oxides, 5
——, from metals, 4
——, insoluble, 6
Sandstone, artificial, 66
Saponification, 79
Schweinfurt green, 27
Shells, egg, 51
——, oyster, 51
Silica, 61
—— ware, 62
Silicic acid, 62
Silver bromide, 48
—— chloride, 48
—— iodide, 48
—— nitrate, 28
—— sand, 61
Soap, hard, 79
——, soft, 79
Soda, baking, 88
——, bicarbonate of, 6, 88
——, bread, 88
——, caustic, 96
——, mild, 80
——, natural, 82
——, washing, 3, 5, 81
—— water, 49
Sodium, 95
—— nitrate, 29
—— sulphate, 27
Soil bacteria, 38
Solvay process, 90
Sorrel, salts of, 77
Spent oxide, 11
Stalactite, 53
Stalagmite, 53
Stearin, 78
—— candles, 79
Stone ammonia, 99
Suffioni, 60
Sulphur, 11
—— dioxide, 11
—— trioxide, prep. of, 19
Sulphuric acid, properties, 20, 24
—— anhydride, 21
Sulphurous acid, 11
Superphosphate, 57
T
Tallow, 79
Tartaric acid, 76
Tinkal, 61
Trinitrotoluene, 35
V
Verdigris, 74
Vert de Montpellier, 74
Vinegar, 68
——, malt, 70
——, wine, 70
Vitriol, blue, 5
——, nitrated, 16
——, oil of, 12
W
Ward, Dr., 12
Water, hard, 53
——, soft, 53
——, softening of, 54
Wood ashes, source of potash, 3
—— ——, used as soap, 2
Z
Zinc chloride, 5
THE END
Footnotes
[1]An anhydride is a substance which unites with water to form an acid.
[2]See Frontispiece.
[3]Now £13 a ton.
[4]Basic lead carbonate.
[5]An electron is probably an “atom” of negative electricity detached
from matter.
_Printed by Sir Isaac Pitman & Sons, Ltd. Bath, England_
(v—1468c)
Transcriber’s Notes
—Silently corrected several palpable typographical errors.
—Retained publication information from the original source.
—In the text versions, included italicized text in _underscores_.