Copyright
Dionysius Lardner.

Hand-book of natural philosophy and astronomy (Volume 2) online

. (page 39 of 45)
Online LibraryDionysius LardnerHand-book of natural philosophy and astronomy (Volume 2) → online text (page 39 of 45)
Font size
QR-code for this ebook


portion of acid gradually from one to ten or fifteen per cent.,
the decomposition will require a less and less intense current.

It appears, therefore, that the acid without being itself affected
by the current, renders the water more susceptible of decom-
position. It seems to lessen the affinity which binds the
molecules of oxygen and hydrogen, of which each molecule
of water consists.

Various other acids and salts soluble in water produce the
same effect.

The electrolyte, properly speaking, is therefore in these cases
the water alone. The bath in which the electrodes are im-
mersed, and in which the phenomena of the electrolysis are
developed, may contain various substances in solution; but so
long as these are not directly affected by the current, they must
not be considered as forming any part of the electrolyte,
although they not only influence the phenomena as above
stated, but are also involved in important secondary phenomena,
as will presently appear.

Cases in which the matter of the electrodes combines with
the constituents of the electrolyte. The process of the elec-
trolysis of water has been presented here in its most simple
form, no other effect save the mere decomposition of the elec-
trolyte being educed. If, however, the platinum electrodes
which have no sensible affinity for the constituents of water be
replaced by electrodes composed of any metal having a stronger



ELECTRO-CHEMISTRY. 401

affinity for oxygen, other phenomena will be developed. The
oxygen dismissed by the water at the positive electrode, instead
of being liberated, will immediately enter into combination with
the metal of the electrode, forming an oxyde of that metal.
This oxyde may adhere to the electrode, forming a crust upon
it. In that case, if the oxyde be a conductor, it will itself
become the electrode. If it be not a conductor it will impede
and finally arrest the course of the current, and put an end to
the electrolysis. If it be soluble in water it will disappear
from the electrode as fast as it is formed, being dissolved by
the water ; and in that case the water will become a solution of
the oxyde, the strength of which will be gradually increased as
the process is continued.

If the water composing the bath hold an acid in solution, for
which the oxyde thus formed at the positive electrode has an
affinity, the oxyde will enter into combination with the acid,
and will form a salt which will either be dissolved or preci-
pitated, according as it is soluble or not in the bath.

While the oxygen disengaged from the water at the positive
electrode undergoes these various combinations, the hydrogen
is frequently liberated in the free state at the negative elec-
trode, and may be collected and measured. In such case it will
always be found that the quantity of the hydrogen developed
at the negative electrode is the exact equivalent of the oxygen
which has entered into combination with the metal at the
positive electrode, and also that the quantity of the metal oxy-
dated is exactly that which corresponds with the quantities of
the two gases which are disengaged, and with the quantity of
water which is decomposed.

2064. Secondary action of the hydrogen at the negative
electrode. In some cases the hydrogen is not developed in
the form of gas at the negative electrode, but in its place the
pure metal which is the base of the oxyde dissolved in the bath,
is deposited there. In such cases the phenomena become more
complicated, but nevertheless sufficiently evident. The hy-
drogen developed at the negative electrode, instead of being
disengaged in the free state, attracts the oxygen from the
oxyde, and combining with it forms water, liberating at the
same time the metallic base of the oxyde which is deposited on
the negative electrode.

Thus there is in such cases both a decomposition and a re-



402 VOLTAIC ELECTRICITY.

composition of water. It is decomposed at the one electrode to
produce the oxyde, and recomposed at the other electrode to
reduce or decompose the same oxyde.

2065. Its action on bodies dissolved in the bath. This
effect of the hydrogen developed at the negative electrode is
not limited to the oxyde or salt produced by the action of the
positive electrode. It will equally apply to any metallic oxyde
or salt which may be dissolved in the bath. Thus, while the
oxygen may be disengaged in a free state and collected in the
gaseous form over the positive electrode, the hydrogen deve-
loped at the negative electrode may reduce and decompose any
metallic salt or oxyde which may have been previously dis-
solved in the bath.

2066. Example of zinc and platinum electrodes in water.
To render this more clear, let it be supposed that while the
negative electrode is still platinum, the positive electrode is a
plate of zinc, a metal eminently susceptible of oxydation. In
this case no gas will appear at the zinc, but the protoxyde of
that metal will be formed. This substance being insoluble in
water will adhere to the electrode if the bath contain pure
water ; but if it be acidulated, with sulphuric acid for example,
the protoxyde so soon as it is formed will combine with the
sulphuric acid, producing the salt called the sulphate of zinc, or
more strictly the sulphate of the oxyde of zinc. This being
soluble, will be dissolved in the bath.

2067. Secondary effects of the current. In all these cases
the primary and, strictly speaking, the only effect of the current
is the decomposition of water, and the only substances affected
by the electric agency are the constituents, oxygen and hy-
drogen, of the water decomposed. All the other phenomena
are secondary and subsequent to the electrolysis, and depend
not on the current but on the affinities of the electrodes and of
the substances held in solution by the electrolyte for the con-
stituents of the electrolyte and for each other. The phenomena,
however, though successive as regards the physical agencies
which produce them, are practically simultaneous in their mani-
festation, and are often so complicated and interlaced in their
mutual relations and dependencies, that it is extremely difficult
to discover a clear and certain analysis of them.

2068. Compounds which are susceptible of electrolysis.
The electrolysis of water is, in all its circumstances and con-



ELECTRO-CHEMISTRY. 403

ditions, a type and example of the phenomena attending the
decomposition of other compounds by the same agency.

Compounds are susceptible of electrolysis or not, according
to the nature, properties, and proportion of their constituents.

It has been ascertained by direct experiment that most of the
simple bodies are capable of being disengaged from compounds
of which they may be constituents by electrolysis, and analogy
renders it probable that all of them have this property. Those
which have not yet been ascertained to be capable of elimi-
nation by this agency, include nitrogen, carbon, phosphorus,
boron, silikon, and aluminium. The difficulty of obtaining
these substances in compounds of a form adapted to electrolysis,
has alone rendered them exceptions to the otherwise uni-
versally aascertained law.

2069. Electrolytic classification of the simple bodies. At-
tempts have been made to classify bodies according to the ten-
dencies they manifest to pass to the one or the other electrode
in the process of electrolytic decomposition, those which evince
the strongest tendency to go to the positive electrode being
considered in the highest degree electro-negative, and those
which show the strongest tendency to go to the negative elec-
trode in the highest degree electro-positive. Although expe-
rimental research has not yet supplied very extensive or ac-
curate data for such a classification, the following proposed by
Berzelius will be found useful as indicating in a general
manner the electrical characters of a large number of simple
bodies, subject, however, to such corrections and modifications
as further experiment and observation may suggest.

2070. I. ELECTRO-NEGATIVE BODIES.



1. Oxygen.
2. Sulphur.
3. Nitrogen.
4. Chlorine.
5. Iodine.
6. Fluorine.
7. Phosphorus.


8. Selenium.
9. Arsenic.
10. Chromium.
11. Molydenum.
12. Tungsten.
13. Boron.
14. Carbon.


15. Antimony.
16. Tellurium.
17. Columbium.
18. Titanium.
19. Silicon.
20. Osmium.
21. Hydrogen.



2071. II. ELECTRO-POSITIVE BODIES.



1 . Potassium.


7. Magnesium.


13. Zinc.


2. Sodium.


8. Glucinium.


14. Cadmium.


3. Lithium.


9. Yttrium.


15. Iron.


4. Barium.


10. Aluminium.


16. Nickel.


5. Strontium.


11. Zirconium.


17. Cobalt.


6. Calcium.


12. Manganese.


18. Cerium.



404 VOLTAIC ELECTRICITY.

19. Lead. 23. Copper. 27. Platinum.

20. Tin. 24. Silver. 28. Rhodium.

21. Bismuth. 25. Mercury. 29. Iridium.

22. Uranium. 26. Palladium. 30. Gold.

All the bodies named in the first series are supposed to be
negative with relation to those in the second. Each of the
bodies in the fii-st series is negative, and each of the bodies in
the second positive, with relation to those which follow.

The meaning is, that if an electrolyte composed of any two
of the bodies in the first list be submitted to the action of the
current, that which stands first in the list will go to the po-
sitive electrode ; if an electrolyte composed of any body in the
first and another in the second list be electrolyzed, the former
will go to the positive electrode; and, in fine, if an electrolyte
composed of any two of the bodies named in the second list be
electrolyzed, the first named will go to the negative electrode.

It has been objected that sulphur and nitrogen occupy too
high a place in the negative series, these bodies being less
negative than chlorine and fluorine, and that hydrogen ought
rather to be placed in the positive series.

2072. The order of the series not certainly determined. It
must be observed that the order of the simple bodies in these
series has not been determined in all cases by the direct obser-
vation of the phenomena of the electrolysis. It has been in
many cases only inferred from the analogies suggested by their
chemical relations.

2073. Electrolytes which have compound constituents.
When the constituents of an electrolyte are compound bodies,
the decomposition proceeds in the same manner as with those
binary compounds whose constituents are simple. Most of the
salts which have been submitted to experiment prove to be elec-
trolytes, the acid constituent appearing at the positive, and the
base at the negative electrode. Acids are therefore in general
regarded as electro-negative bodies analogous to oxygen, and
alkalies and oxydes electro-positive bodies analogous to hy-
drogen.

2074. According to Faraday, electrolytes whose constituents
are simple can only be combined in a single proportion. It
appears to result from the researches of Faraday, that two
simple bodies cannot combine in more than one proportion so
as to form an electrolyte.



ELECTRO-CHEMISTRY. 405

When hydrochloric acid, whose constituents are chlorine and
hydrogen, is submitted to the current, electrolysis ensues, the
chlorine appearing at the positive and the hydrogen at the
negative electrode.

The protochlorides of the metals composed of the metallic
base and one equivalent of chlorine are also easily electrolyzed,
the chlorine always appearing at the positive electrode ; but the
perchlorides of the same metals which contain two or more
equivalents of chlorine are not susceptible of electrolyzation.

In general, compounds which consist of two simple elements
are only electrolyzable when their constituents are single equi-
valents. Hence sulphuric acid which has three, and nitric
acid which has five equivalents of oxygen, are neither of them
susceptible of electrolyzation.

2075. Apparent exceptions explained by secondary action.
In the investigation of the chemical phenomena which attend
the transmission of the current through liquid compounds,
results will be occasionally observed which will at first seem
incompatible with this law. But in these cases the phenomena
are invariably the consequences, not of electrolysis, but of se-
condary action. Thus, nitric acid submitted to the current is
decomposed, losing one equivalent of its oxygen, and reduced
to nitrous acid. In this case the real electrolyte is the water,
which always exists in more or less quantity in the acid. This
water being decomposed, the oxygen is delivered at the positive
electrode, and the hydrogen developed at the negative electrode
attracts from the nitric acid one equivalent of its oxygen, with
which it combines and forms water, reducing the nitric to
nitrous acid.

Ammonia, which consists of one equivalent of nitrogen and
three of hydrogen, is not properly an electrolyte, though in
solution it is decomposed by the secondary action of the current.
In this case, as in the former, the real electrolyte is the water
in which the ammonia is dissolved. Nitrogen and not oxygen
is disengaged at the positive electrode. The oxygen, which is
the primary result of the electrolysis of the water, attracts the
hydrogen of the ammonia, with which it reproduces water and
liberates the nitrogen.

2076. Secondary effects favoured by the nascent state of the
constituents : results of the researches of Becquerel and Crosse.
It is a general law in chemistry that substances in the



406 VOLTAIC ELECTRICITY.

nascent state, that is, when just disengaged from compounds
with which they have been united, are in a condition most
favourable for entering into other combinations. This explains
the great facility with which the constituents of electrolytes
combine with the electrodes where even a feeble affinity pre-
vails, and also the various secondary effects. When oxygen
is evolved against copper, iron, or zinc, chlorine against gold, or
sulphur against silver at the electrode, oxydes of copper, iron,
or zinc, chloride of gold, or sulphuret of silver are readily
formed. If the current producing these changes be of very
feeble intensity, so that the new compounds are very slowly
formed, so slowly as more to resemble growth than strong
chemical action, they will assume the crystalline structure. In
this manner Becquerel and Crosse have succeeded in ob-
taining artificially mineral crystals, and exhibiting on a small
scale effects similar to those which are in progress on a scale
so vast in the mineral veins which pervade the crust of the
globe, and which, doubtless, result from feeble electric currents
established for countless centuries in its strata by the vicis-
situdes of temperature and other physical causes.

2077. The successive action of the same current on different
vessels of water. If the same current be conducted succes-
sively through a series of vessels containing acidulated water,
by connecting the water in each vessel with the water in the
succeeding vessel by platinum wires i, i', i", i'", &c., as repre-
sented \nfig. 661., the current will enter each vessel at the ex-



Fig. 661.

tremity o, and will depart from it at the extremity h. The water
in each vessel will in this case constitute a separate electrolyte,
and will be decomposed by the current. The ends o will be all
positive, and the ends h all negative electrodes. Oxygen will
be disengaged at all the ends o, and hydrogen at all the ends h ;
and if the gases disengaged be collected, the same quantity of
oxygen will be found to be disengaged at the ends o, and the
same quantity of hydrogen at the ends h, the volume of the
latter being double that of the former. The weight of the



ELECTRO-CHEMISTRY. 407

oxygen produced will be eight times that of the hydrogen, and
the weight of the water decomposed will be nine times that of
the hydrogen.

2078. The same current has an uniform electrolytic power.
Since it is ascertained by reometric instruments that the
same current has everywhere the same intensity, it follows
that this constant intensity is attended with an electrolytic power
of corresponding uniformity. From this and other similar
results it is inferred that the quantity of electricity which passes
in a current is proportional to the quantity of a given elec-
trolyte which the current decomposes.

2079. Voltameter of Faraday. On this ground Faraday
gave the name of VOLTAMETER to an apparatus similar in prin-
ciple to that described in (2060.), taking water as the standard
electrolyte by which the quantity of electricity necessary to
effect the decomposition of any other electrolytes might be
measured. Thus, if it is found that a current which decomposes
in a given time an ounce of water, will in the same time de-
compose two ounces of one electrolyte (A), and three ounces of
another electrolyte (B), it is inferred that the quantity of elec-
tricity necessary to decompose a given weight of A is half that
which would decompose an equal weight of water, and that the
quantity necessary to decompose a given weight of B is a third
of that which would decompose the same weight of water, and,
in fine, that the quantities of electricity necessary to decompose
equal weights of A and B are in the ratio of 3 to 2.

2080. Effect of the same current on different electrolytes
Faraday's law. If the series of vessels represented in

fig. 661., connected by metallic conductors i, i', &c., instead of
containing water, contain a series of different electrolytes, each
electrolyte will be decomposed exactly as it would be if it were
the only electrolyte through which the current passed. Let us
suppose that the first vessel of the series which the current
enters from P contains water, and that means are provided by
which the quantities of oxygen and hydrogen liberated at o and
h shall be indicated, and that in like manner the quantities of
the constituents of each of the other electrolytes disengaged at
the respective electrodes can be determined. It will then be
found that for every grain weight of hydrogen liberated in the
first vessel, the number of grains weight of each of the consti-



408 VOLTAIC ELECTRICITY.

tuents of the several electrolytes disengaged will be expressed
by their respective chemical equivalents.

Thus, if e, e', e", e", &c. be the chemical equivalents of the
several constituents of the series of electrolytes, that of hydrogen
being the unit, and if A express the number of grains weight of
hydrogen evolved in the voltameter tube over the first vessel
in a given time, then the number of grains weight of each of
the constituents of the several electrolytes which shall be
evolved in the same time will be

ex h, e' x h, e" x h, e'" x h, &c., &c.

2081. It comprises secondary results. This remarkable law
extends not only to the direct results of electrolysis, but also to
all the secondary effects of the current. Thus, it applies to the
quantities of the several metallic electrodes which combine with
the constituents , which are the immediate results of the elec-
trolysis, and also to all combinations and decompositions which
result from the affinities which may exist between the results
primary or secondary of the electrolysis and any foreign sub-
stances which the electrolyte may hold in solution.

2082. Practical example of its application As a practical

example of the application of this electro-chemical law, let us
suppose the first vessel which the current enters at p to contain
water, the next iodide of potassium, the succeeding one proto-
chloride of tin, the next hydrochloric acid, and the last sulphate
of soda. The current will severally decompose these, the
oxygen, iodine, chlorine and acid appearing at the five positive
electrodes, and the hydrogen, potassium, tin and soda at the
five negative electrodes. If the electrode against which the
oxygen is evolved be zinc, the oxyde of zinc will result as a
secondary product ; and if the electrode against which the
chlorine is evolved be gold, the chloride of that metal will like-
wise be produced by secondary action. The chemical equi-
valents of the several substances involved in this process are as
follows:



Hydrogen

Oxygen

Water

Iodine

Potassium



1-00
8-00
9-00

126-30
39-26



Iodide of potassium - - 165-56

Chlorine - - - - - 35 -47



ELECTRO-CHEMISTRY. 409

Tin - ... 57.90

Protochloride of tin - . . _ - 93 -37

Hydrochloric acid - - . _ - 36 -47

Sulphuric acid - . 40-1O

Soda - - - - 31 -30

Sulphate of soda - - - 7 1 -4O

Zinc ....... 33.30

Gold- . 199-20

Oxyde of zinc - .... 40-30

Chloride of gold - - 234-67

It will follow, therefore, from the general electrolytic law
above stated, that for every grain of hydrogen evolved at the
negative electrode in the first vessel, the following will be the
quantities of the chemical results produced in the several
vessels :

I. Oxygen evolved at positive electrode - . 8'00

Water decomposed - - - - 9 00

Zinc oxy dated - ... 32-3O

Oxyde of zinc produced - ... 4Q-30

IT. Iodine evolved at the positive electrode - - 126-3O

Potassium evolved at the negative electrode - 39-26

Iodide decomposed . . 165-56

III. Chlorine evolved at the positive electrode - - 35-47
Tin evolved at the negative electrode - - 57-90
Gold combined at positive electrode - - 199-2O
Chloride of gold produced - 234~67
Protochloride of tin decomposed ... 93-37

IV. Chlorine evolved at positive electrode - - 35-47
Hydrogen evolved at negative electrode - - 1 -OO
Hydrochloric acid decomposed ... 36-47

V. Sulphuric acid evolved at positive electrode . 40-10

Soda evolved at negative electrode - 31-30

Sulphate decomposed - 71-40

2083. Sir H, Davy's experiments showing the transfer of the
constituents of electrolytes through intermediate solutions. If
the series of vessels containing different electrolytes be con-
nected by liquid conductors by means of capillary siphons,
instead of the metallic conductors by which they are supposed
to be connected in the cases just described, phenomena are pro-
duced, respecting which a remarkable discordance has arisen
between the highest scientific authorities.

From some of the early experiments of Sir H. Davy, con-
firmed by those of Gautherot, Hesinger, and Berzelius, it ap-
peared that the voltaic current was not only capable of decom-

II. T



410 VOLTAIC ELECTRICITY.

posing various classes of chemical compounds, but of trans-
ferring or decanting their constituents successively through
two or more vessels, to bring them to the respective electrodes
at which they are liberated. Davy pushed this inquiry to its
extreme limits, and by various experiments, characterised by all
that address for which he was so remarkable, arrived at certain
general results which we shall now briefly state.
Let a series of cups

P -> ABODE -> N

be connected by capillary siphons, which may be conveniently
formed of the fibres of asbestos or amianthus. Let any elec-
trolyte, a solution of a neutral salt for example, be placed in c,
and let the other cups be filled with distilled water. Let a
plate of platinum connected with the positive pole of a voltaic
battery be immersed in the cup A, and a similar plate connected
with the negative pole be immersed in E. The voltaic current
will then enter the series of cups at A, and passing successively
from cup to cup through the siphons, will issue from them at E,
as indicated by the arrows. Let the water in the cups A, B, D
and E be tinged by the juice of red cabbage, the property of
which is to be rendered red by the presence of an acid, and
green by that of an alkali.

The current thus established will, according to Sir H. Davy,
decompose the salt in the cup c. The acid will be transported
through the two siphons, and the water in B to the positive
electrode in A, where it will be liberated, and will enter into
solution with the tinged water. At the same time the alkali
will pass through the two siphons, and the cup D to the ne-
gative electrode, and will enter into solution with the water
in D.

The presence of the acid in A and of the alkali in E will be
rendered manifest by the red colour imparted to the contents of
the former, and the green to the latter.

2083*. While being transferred they are deprived of their
chemical property. Although to arrive at A and E respec-
tively, the acid must pass through B and the alkali through E,
their presence in these intermediate cups is not manifested by
any change of colour. It was therefore inferred by Sir
H. Davy, that so long as the constituents of the salt are under
the immediate influence of the current, they lose their usual



Online LibraryDionysius LardnerHand-book of natural philosophy and astronomy (Volume 2) → online text (page 39 of 45)