Henry S. (Henry Smith) Carhart.

Physics for university students (Volume 2) online

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current then simply transfers copper from the anode to
the cathode.


4 \ CuSOCu. Cathode.

There is no change in the density of the whole solution ;
the copper ions migrate toward the cathode and the S0 4
ions toward the anode.

If platinum electrodes are used, copper will be deposited
on the cathode, and the S0 4 at the anode will decompose
water and release oxygen with the formation of ff. 2 S0 4 .

206. Electrolysis of Sodium Sulphate. A salt of
one of the alkaline metals presents some secondary reac-
tions of interest. With a solution of sodium sulphate free
sodium cannot exist in water

JSl /*

at the cathode, but unites with
water, forming sodium hydrox-
ide and hydrogen ; at the anode
the S0 4 decomposes water, as
in the other cases, and liberates

Let the solution be placed in
a flat glass tank with a non-
conducting partition a extend-



: _^


I -..;













ing nearly to the bottom (Fig. > U #

Add to the solution a little extract of purple cal>-
When the current is passed the liquid will turn


red at the anode and green at the cathode, the former
color being due to the acid formed, and the latter to the


alkali. Stop the flow of the current and mix the liquids
on the two sides of the partition; both the red and the
green colors will disappear with the restoration of the faint
purple, showing that the acid and the alkali were produced
in chemically equivalent quantities which neutralize each
other. The final result is the decomposition of water.

2O7. Electrolysis of Lead Acetate. Place the solu-
tion, which may !>e made clear by the addition of a small
quantity of acetic acid, in a flat glass tank and electrolyze
between two lead wires. The lead separated from the
clear solution will be deposited on the cathode in the form
of shining crystals, which will grow rapidly, giving rise to
what is known as the "lead tree." If the process is not
conducted too rapidly, these crystals will assume very
beautiful forms. The lead goes into solution at one elec-
trode and comes out of solution at the other.

After a few minutes reverse the current ; the first crys-
talline deposit will gradually disappear, and another one
will form on the other wire. In this way the disappear-
ance of the lead at the one electrode and its appearance
at the other may both be observed at the same time. The
reaction is precisely like that of copper sulphate between
copper electrodes.

Cathode. Pb^Pb ( C,H, 6>,) 2 | Pt> ( C,H, 0,) 2 | PI. Anode.

208. Quantitative Laws of Electrolysis. Faraday
showed that the masses of the ions separated are connected
by a very simple relation with the quantity of electricity
which passes through the electrolyte. This relation is
expressed by the following laws :


/. The mews of an electrolyte decomposed by the passage
of an electric current is directly proportional to the quantity
of electricity that passes through it. M ' Q

If the current be kept constant, the mass of the ion
liberated in a given time will be directly proportional to
the strength of the current.

//. If the same quantity of electricity passes through
different electrolytes, the masses of the different ions liberated
at the electrodes are proportional to their chemical equiva-
lents. /nk A f t /j/f : &cpz^ * ,

Thus, if the same current passes through a series of elec-
trolytic cells, in which it liberates as ions hydrogen,
chlorine, copper, and silver, then for every gramme of
hydrogen set free, 35.46 gms. of chlorine, 31.7 of copper,
and 107.9 of silver will be separated.

The electro-chemical equivalent of an ion is the number
of grammes of it deposited by the passage of unit quantity
of electricity. When the current has unit strength, mi it
quantity flows through any cross-section of the conductor
in onelsecond of time. Faraday's laws may then be com-
bined TiPthe statement that the number of grammes of an
ion deposited by the passage of a current through an
electrolyte is equal to the continued product of the strength
of the current, the time in seconds during which it flows,
and the electro-chemical equivalent of the ion. ty ... ^

209. Electro-chemical Equivalents. - - The electro-
chemical equivalents of the several ions are proportional to
the relative masses of them which take part in equivalent
chemical reactions. The electro-chemical equivalents of
those ions which have a valency of one are proportional
to their atomic weights, and to half the atomic weights if
the ions have a valency of two. Elements which form two



series of salts, such as copper in cupric and cuprous salts,
and mercury in mercuric and mercurous salts, have different
electro-chemical equivalents according as they are deposited
from solution of cupric or cuprous, mercuric or mercurous

The following table of electro-chemical equivalents is
based on the practical unit of quantity of electricity called
the coulomb, which is one-tenth the C.G.S. electromagnetic
unit of quantity (see Chapter XXI.) :


Atomic weight.


Electro- chemical
equivalent in
grammes per







39 03

39 03


Silver . .

107 Q2

107 92


Copper (cupric)




" (cuprous)
Mercury (mercuric)
" (mercurous) ....


















Chlorine .





126 85

126 85


21O. The Silver Voltameter. If a neutral 15 per
cent solution of silver nitrate (AyNO^ is electrolyzed
between a silver anode and a platinum cathode, or between
two silver electrodes, silver is transferred with the current,
and is deposited on the cathode as adherent crystals if the
electrode be of sufficient size. Silver is removed from the
anode by the acid radical NO., as fast as it is deposited on
the cathode. The uniform results obtained in the electrol-



ysis of silver nitrate have led to its adoption as a standard
method for the measurement of a current. When applied
to this purpose an electrolytic apparatus is called a

A convenient form of
silver voltameter is
shown in Fig. 104. The
middle silver plate is the
cathode, and the two
outer ones together con-
stitute the anode. They
are attached by spring
clamps to terminals af-
fixed to an insulating
support ; the whole can
be removed from the
sol ution by loosening the
screw B. A rack and
pinion, worked by means

of the milled head P, allows the plates to be adjusted
to a greater or less depth of immersion. The anode
plates must be of pure silver.

The practical unit of current strength is the ampere.
Its electro-magnetic definition will be given later. A cur-
rent has the strength of one ampere when it deposits silver
at the rate of 0.001118 gm. per second, or 4.025 gins, per
hour. The mass of silver deposited in t seconds by a
current of / amperes is

m = Izt,
where z is the electro-chemical equivalent. Hence

Fig. 104.



The divisor in this expression may be 4.025 multiplied by
the time of deposit expressed in hours.

211. The Copper Voltameter. The silver voltameter
is not employed for currents much larger than one ampere ;
for larger currents the copper voltameter is used. It con-
sists of smooth copper plates immersed in a solution of
copper sulphate acidulated with a few drops of sulphuric
acid. The electro-chemical equivalent of copper (cupric)
is less than one-third that of silver; for the same current
the weight of copper deposited in a given time is therefore
correspondingly less. The results are not so uniform as
those secured by silver nitrate, the practical electro-chemical
equivalent being a function of the temperature and the
density of the current at the cathode. By density of cur-
rent is meant the fraction of an ampere per square centi-
metre of cathode surface. It is commonly expressed
reciprocally as the number of square centimetres per

212. Reversibility of the Daniell Cell. When a cur-
rent flows from zinc to copper through a Daniell cell, zinc
is dissolved and copper is deposited. The E.M.F. of the
cell operating in this way as a generator is about 1.1 volts.
Suppose now an opposing E.M.F. greater than 1.1 volts be
applied to the terminals of the cell. The copper then
becomes the anode and the zinc the cathode, or the cell is
worked backwards. When the cell is worked forwards
as a generator, the electro-positive ions travel toward the
copper plate, as represented in the upper diagram of Fig.
105 ; the cell is then giving out energy in the form of an ;
electric current, with a corresponding loss in its store of
potential energy. Suppose this process to continue till



one gramme-equivalent (>5 gms.) of zinc has been dis-
solved, and one gramme-equivalent (63.4 gms.) of copper
has been deposited. Then let the cell be worked back-
wards with the reactions of the lower diagram of Fig. 105 ;
the cell is then receiving energy, and storing it up in the
increase of zinc and CuS0 4 at the expense of copper and
ZnS0 4 . When a gramme-equivalent of copper has been
removed from the copper plate
and a gramme-equivalent of
zinc has been deposited on the
zinc plate, the cell is in its in-
itial state. There has been no
loss of materials, and they are
in the same chemical condition
as at the outset. Except for
the small loss by heat due to
resistance, the energy given out
by the cell during the direct
action equals the energy stored
up during the reversal of its
functions. Hence during the direct action as a generator
there can be no counter E.M.F. to work against to prevent
the conversion of the potential energy of the cell into the
energy of an electric current. This cell is therefore com-
pletely reversible and does not polarize.

The simple voltaic element belongs to another class.
Suppose it to work forwards till an equivalent of hydrogen
has been given off at the copper and an equivalent of zinc
has gone into solution. Then let it be worked backwards
till an equivalent of copper has gone into solution and an
equivalent of hydrogen has been given off at the zinc.
At the end of the experiment an equivalent of both zinc
and copper has gone into solution and two equivalents of

Fig. 105.



hydrogen have been set free. The cell does not return
to its initial state at the end of the experiment, and there
must be a compensation for the net chemical changes. 1
This compensation is found in the counter E.M.F. of
polarization when the cell works forward as a generator.
The simple voltaic element is an example of a non-revers-
ible or polarizable cell. Only reversible elements work
with maximum efficiency.

213. Polarization of an Electrolytic Cell. If the
two platinum electrodes of Hofmann's apparatus (Fig.
102) be connected to a sensitive gal-
vanometer immediately after they
have been used for the electrolysis of
sulphuric acid, it will be found that
energy has been stored up to some
extent and the cell will furnish a
current. The chemical and electrical
functions are now reversed; the hy-
drogen and oxygen on the electrodes
unite to form water, and a reverse
current flows through the cell. The
apparatus may be set up as in Fig.
106. B is the battery to furnish the
current to decompose the sulphuric
acid. Hydrogen accumulates in the
tube H and oxygen in the tube 0.
Let the two-point switch /S be now turned so as to cut
off the battery and to join the electrolytic cell to the
galvanometer Cr. The needle will be sharply deflected by
the current from the Hofmann's apparatus. To determine
its direction, a thermal couple, consisting merely of a

1 Nernst's Theoretical Chemistry, Trans, by Palmer, p. 597.

Fig. 106.


copper and an iron wire soldered together and placed in
the circuit of the galvanometer at T, is convenient. When
such a couple is slightly heated a current passes across
from On to Fe. It may be tried before charging the
electrolytic cell, and the direction of the deflection of the
galvanometer may be noted. It will then be found that
the current produced by the electrolytic cell will flow out
from A and in at C, or in the reverse direction to the
current which separates the gases, oxygen and hydrogen.
The E.M.F. of polarization is therefore a back or resisting

214. Electrolysis with and without Polarization.
When electrolysis takes place between two metallic plates
of the same kind, immersed in a salt of the same metal,
the polarization of the electrodes is very small. Thus,
with copper in copper sulphate, or zinc in zinc sulphate, or
silver in silver nitrate, the polarization is slight; the small
counter E.M.F. exhibited is probably due to a difference
in the surface of the anode and cathode, and to a difference
in the density of the solutions in immediate proximity to
the plates. Zinc in zinc sulphate shows no appreciable polar-

But when the electrolysis effects a change in the chemical
composition of the electrolyte, polarization results. The
ions set free, such as hydrogen and oxygen, have a ten-
dency to reunite by means of a reverse current and a
ivvt-rse chain of molecular interchanges. In such cases
work is done during electrolysis, and potential energy is
stored up in the form of chemical separations.

In the first kind, where the metal is simply transferred
from one electrode to the other, a very weak E.M.F. is suf-
ficient to produce electrolysis; in the second, the applied




E.M.F. must exceed the counter E.M.F. of polarization
before visible separation of the ions is accomplished.

215. Grove's Gas Battery. Grove's gas battery is

constructed on the basis of
the facts just described. The
platinum strips are fused
into the tops of the two
tubes (Fig. 107), which are
fitted into two necks of a
Woulffs bottle filled with
dilute sulphuric acid. After
the tube H has been nearly
filled with hydrogen by elec-
trolysis, the terminals P and
N become respectively the
positive and negative of a
voltaic element. The sur-
faces of the platinum plates
are covered with platinum
black for the purpose of in-
creasing the surface of the

ng. 107.


liquid in contact with plati-
The action may be represented thus:

H.80, | 0.

After the first exchange of atoms this becomes

Water is re-formed at the expense of the oxygen and hydro-
gen. The water or sulphuric acid voltameter is therefore


reversible element. Similar results may be obtained with
other gases, notably hydrogen and chlorine.

216. Plant's Storage Cell. If the platinum elec-
trodes of the sulphuric acid voltameter be replaced In-
lead. we have the Plante storage cell, which is the basis of
all modern storage batteries. Take two pieces of sheet
lead and solder to each a short length of copper wire as
a terminal. Attach the lead strips to opposite sides of a
block of dry wood, and immerse the r> /

plates in dilute sulphuric acid (Fig.
108). Pass a current through the
cell for a few minutes. The oxygen
liberated at the anode will oxidize the !
lead, forming a dark-brown coating
of the peroxide of lead. An ordinary
electric house-bell may be connected
with the cell by a switch, as in Fig.
106. When the switch is turned,
cutting off the charging battery and
connecting the lead electrolytic cell
with the bell, the latter will ring
vigorously for a few seconds. The operation mav be
repeated, showing that energy is stored up in the cell by
the process of electrolysis. The E.M.F. of polarization
in this case is somewhat over two volts. Plante subjected
his cells to repeated charging in opposite directions, so
that both plates should be modified to an appreciable
depth by alternate oxidation and reduction. This process
was called " forming " the plates.

In most modern storage cells the plates, cast or rolled in
the form of grids, are provided with lead oxides which
compose the "active material." These oxides are changed

Fig. 108.


into peroxide at the anode, and reduced by hydrogen to
spongy lead at the cathode during the operation of charg-
ing. The chemical reactions of
the storage cell are very complex,
and are to some extent undeter-
mined. Sulphuric acid is formed
during the charging of the cell,
and disappears during the dis-
charge. Some sulphate of lead is
also formed during the discharge,
and is reduced by hydrogen with
slow charging. The electrode
which is the anode when charging
and the cathode when discharging
is called the positive pole of the
cell. Fig. 109 represents a cell
of the "chloride accumulator."

217. Theory of Electrolysis.
Many reasons have been ad-
duced which go to show that dissociation of acids and
salts takes place when they are dissolved in water. Hy-
drochloric acid,, for example, is dissociated into positive
hydrogen and negative chlorine; sulphuric acid into two
positive hydrogen atoms and the negative acid radical
S0. This dissociation, if it actually occurs, is intimately
connected with the conduction of electricity by electro-
lytes. Clausius proposed the theory that momentary dis-
sociations occur with succeeding recombinations, a process
of intermolecular exchanges ; and that the electric current
determines only the direction in which such exchanges
shall take place. Such transient dissociation would suffice
to account for the observed conduction of very small cur-


rents by electrolytes without any visible separation of free
ions; but it is incompetent to explain other facts of physi-
cal chemistry. This phenomenon of partial electrolysis
von Helmholtz called electrolytic convection, and assumed
that it takes place by the agency of the uncombined atoms
in the liquid. The modern theory makes all electrolytic
conduction depend upon these dissociated atoms.

If gaseous hydrochloric acid be introduced between
platinum electrodes connected with a voltaic battery, no
appreciable transfer of electricity occurs; neither does
pure water conduct electricity ; but if the hydrochloric
acid be dissolved in water, the solution becomes conduct-
ing, with the electrolytic separation of hydrogen and
chlorine. The inference is justifiable that the acid must
have undergone an important molecular change by solution
in water, because after solution it conducts electricity, and
before solution it does not. The same inference does not
apply to the solvent, because it does not suffer electrolytic
decomposition. The molecular change which the acid
undergoes by solution is dissociation into electro-positive
and electro-negative ions, thus :


The capacity of a dissolved substance to conduct electricity
therefore presupposes a molecular cleavage into positively
and negatively charged atoms. The larger the number of
such dissociated molecules in a solution, the better it con-
ducts. It is not necessary that all the molecules of the
substance be dissociated by the solvent. Those that are
not decomposed remain electrically neutral and take no
part in the transfer of electricity.

Let the cell in Fig. 110 contain a water solution of
hydrochloric acid with platinum electrodes. These elec-



Fig. 110.

trodes are charged as shown by connection with a battery,
which maintains a constant potential difference between

them. The solution contains pos-
itively charged hydrogen atoms
and negatively charged chlorine
atoms, besides neutral molecules
which have not been decomposed.
Then the positive charge on the
anode attracts the negative chlo-
rine atoms and repels the positive
hydrogen atoms, while the reverse
actions occur at the negatively
charged cathode. All these forces combine to produce a
simultaneous and equal procession of hydrogen atoms from
anode to cathode, and of chlorine atoms from cathode to
anode. This double procession of free ions with their
electric charges represents the passage of an electric cur-
rent through an electrolyte.

218. Electrolysis in the Arts. Electrolysis is now
employed on a large scale for a number of distinct pur-
poses in the arts and _indus tries. These may be classed
under four heads, viz.,Jthe reduction of metals from their
ores or solutions /[the copying of types, casts, woodcuts,
and metal workythe covering of objects in base metals
with gold, silver, or nickel j'and the manufacture of various
chemicals, such as caustic soda, bleaching liquors, and
chlorate of potassium. The first three of these are in-
cluded under the general term of electro-metallurgy.

Pure copper is now produced on an enormous scale by
electro-deposition. After a second process of reduction in
a blast furnace the " blister " copper, containing small
quantities of gold, silver, oxide of iron, and sulphides, is


cast into slabs which serve as the anode plates in the elec-
trolytic bath of copper sulphate. Several plants are now
in operation in the United States, with a capacity of from
50 to 100 tons of pure copper daily.

Aluminium is reduced in large quantities from a fused
mixture of electrolytes. Cryolite, a double fluoride of
aluminium and sodium, is first fused by the passage of a
very large current between huge carbon electrodes. To
this fused mass is added bauxite, a ferruginous hydrate of
aluminium, and this is dissolved by the fused cryolite.
The cryolite serves as the bath and the aluminium oxide
is electrolized. Its solution produces a marked reduction
in the resistance of the bath. Only a small per cent of the
cryolite is decomposed.

If copper is deposited on any surface, such as coins,
ornaments, and stereotype plates, an exact impression is
obtained in reverse relief. If a mould in plaster or wax
be taken of any object, and be covered with a conducting
film of plumbago or finely powdered bronze, the mould
can be coated with a deposit of copper. When this is
filled with type metal, an exact reproduction of the origi-
nal is obtained. This process is largely employed to
reproduce repousse and other works of art in facsimile,
and to multiply copies of woodcuts or other engravings
for printing. The electrolytic solution is acidulated
copper sulphate.

The art of electro-plating was invented early in the
present century. The objects to be covered with a thin
deposit of gold, silver, or nickel must first be made chemi-
cally clean ; they are then hung in the bath as the cathode.
For gold and silver plating the solution is cyanide of gold
or silver dissolved in cyanide of potassium ; for nickel
it is a double sulphate of nickel and ammonium. The


anode in each case must be a plate of the same metal as
the one to be deposited at the cathode. The solution
then continues to have the same density.


1. The weight of a cathode silver plate was 30.3726 gms. before
the deposit on it and 30.4C85 gms. after deposition, which lasted
half an hour. Find the average current in amperes.

2. The following data are taken from a copper voltameter meas-
urement :

Weight of cathode before deposit .... 83.4925 gms.

" after " .... 84.4475 "
Time of deposit, 30 min.
Find the mean current.-

3. The silver deposited in a silver voltameter in 45 min. was
2.8095 gms. Find the average current.

4. A current of 1 ampere is sent through three electrolytic cells
in series for 30 min. The first contains cyanide of silver dissolved
in cyanide of potassium ; the second, zinc sulphate ; the third, nickel
sulphate. Find the weight of metal deposited in each.

5. If one litre of hydrogen under standard conditions weighs
0.08987 gm., how many amperes will liberate 250 c.c. of hydrogen
in 10 m. 22 s. ?




219. Ohm's Law. The relation between the electro-

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Online LibraryHenry S. (Henry Smith) CarhartPhysics for university students (Volume 2) → online text (page 17 of 28)