occur in which the E.M.F. or current strength yielded by the
thermo-battery is insufficient to effect the desired electro-
lysis, one can use the thermo-battery and accumulators
together, coupled in parallel or in series. l
As already noted at the commencement of this chapter,
a second method of increasing the current strength in any
circuit is^ that dependent upon a decrease of the total
resistance. This is made up of the internal resistance of
1 Elbs, Chem. Zeitg. 1893, 17, 66, 97.
FIG. 21. Accumulator Battery with Switch-
board.
INCREASING THE CURRENT STRENGTH 73
the source of electrical energy and of the resistance of the
external circuit, the latter of which equals the resistance of
the metallic and fluid conductors, and in addition that pro-
duced by polarisation.
The reduction of the internal resistance of the source of
electrical energy is only feasible with galvanic elements. It
is effected by coupling a large number of the cells in
parallel that is, by greatly increasing the superficial area of
the electrodes.
In the case of accumulators at least, as regards the
larger patterns this has already been carried out, and the
internal resistance of these is extremely small. In the case
of dynamos and thermo-batteries the internal resistance
cannot be reduced at will in this manner. The resistance
of the external circuit may be diminished in various ways.
It is known that the resistance of the metallic conductors
or * leads,' apart from the specific resistance of the metal
used, is directly proportional to their length, and inversely
proportional to their sectional area. For copper wire
conductors it is customary to allow 2 or 3 amperes per
sq. mm. ; and since in electrolytic experiments currents
of more than 2 amperes are comparatively rarely used, it
follows that for this class of work copper wire 1 sq. mm. in
sectional area is ample. The resistance offered by the
electrolytic cell may be diminished, as in the case of the
battery cell, by increase of the superficial area of the
electrodes. A second method is to increase the conduc-
tivity of the electrolyte. 1 This method, in most cases of
electrolytic deposition, cannot be made use of, since a
definite composition and degree of concentration of the
solution have to be maintained in order to effect the desired
separations. As a general rule it is not advantageous in
electrolytic analysis to utilise the electrical energy in the
most economical manner.
1 [In the majority of in stances heating an electrolyte increases its
conductivity, and this method of reducing the resistance of the
electrolytic cell is very often used. Translator's note.']
74 MEASURING AND REGULATING THE CURRENT
CHAPTER XI
REDUCING THE CURRENT STRENGTH
THE reduction of the current strength is of course effected
by means exactly the reverse of those adopted in order to
increase it. The reduction of the E.M.F. of the source of
electrical energy cannot be effected in the case of primary
or secondary cells ; and it is only in special cases as, for
example, in separations based upon the different ' decom-
position values ' of metallic salts that need for a reduction
of the E.M.F. exists. A reduced E.M.F. can be obtained
from a thermo-battery when one reduces the number of
single elements concerned in the production of the current.
The manner in which this is achieved has already been
described (see p. 72). The E.M.F. produced by dynamos
can be reduced by inserting resistances in the shunt circuit
that excites the magnets. The strength of the field is thus
reduced and the E.M.F. diminished.
In spite of this arrangement the E.M.F. produced by
dynamos is usually too high for many purposes, and the
E.M.F. in the circuit is still further reduced by inserting
resistances in it made from wire of high specific resistance,
or by using only a fraction of the main current for the
electrolysis. The alloys used for these resistances are
the following: 'German silver,' ' Rheotan,' ' Nickel in,'
' Manganin/ arid ' Konstantan.' Of these the alloys of
manganese and copper, and of nickel, manganese, and
copper, are the most satisfactory, as their resistance does
not vary much with the temperature. These resistance
REDUCING THE CURRENT STRENGTH 75
coils of wire are employed with all forms of current
producer, they are extremely convenient in use, and
they enable one to obtain the desired current strength
within extremely narrow limits. In order to reduce the
current from any source of electrical energy to the desired
strength, one might stretch out a length of wire made from
one of these poorly conducting alloys, and by means of a
sliding contact take the current from any desired point
FIG. 22. Resistance Box with Plug Contacts.
upon it. Such a simple arrangement is in most cases, how-
ever, not sufficient.
The customary form is shown in fig. 22. The insulated
wire is wound into rolls or coils, the ends of these are
soldered to brass connections arranged in order upon a
wooden or slate base, and the well-insulated coils- are
enclosed in a box having this base as its cover. The brass
pieces that represent the terminals of the coils are so
76 MEASURING AND REGULATING THE CURRENT
arranged that by the use of plugs they can be put in or
out of circuit as desired. The resistance of the coils is
expressed in ohms. They are arranged in the box in
order of value, in a similar manner to that found in a set
of weights, so that any resistance between 1 and 100 ohms
can be produced. If all the plugs be inserted as shown in
the illustration, the current passes entirely through the
pieces of brass upon the cover of the box, and does not
travel by any of the coils. Another form of resistance box
FIG. 23. Resistance Box with Mercury Contacts.
is shown in fig. 23. In this case the ends of the coils are
brought into small depressions, or cups, filled with mercury
when the box is in use, and bent pieces of stout copper
wire are used instead of plugs, to place the coils out of
circuit.
One can make a similar variable resistance of less
range by forming spirals of * Manganin ' or ' Nickelin '
wire of different thicknesses, and by fixing these upon
a frame, as shown in fig. 24. The ends of the wire
REDUCING THE CURRENT STRENGTH
77
are soldered to stout copper wires which dip into mercury
contact cups. If small binding screws be used with this
resistance, the whole or part of any division of it may be
cut out by binding two neighbouring wires together.
Since it is not necessary in the greater number of elec-
trolytic separations to use resistances of any definite and
fixed value, one frequently finds resistances in use which
have not been standardised, and in which the contacts are
made not by means of plugs or of mercury cups, but by
means of the so-called ' sliding contacts.' Figs. 25 and 26
FIG. 24. Frame Resistance.
are illustrations of this type of resistance ; many other forms
of it exist.
The wire for the separate coils can be roughly measured
according to the resistance data supplied by the manufac-
turer, and the arrangement of the coils upon the frame
does not follow any order of value. When very strong
currents are being employed it is preferable to use strips
of thin sheets of the different alloys, in place of wire, in
resistance frame.
If one desires to employ the second method for the pro-
duction of feeble currents that in which only portion of
the main current is utilised the following arrangement is
78 MEASURING AND REGULATING THE CURRENT
adopted. A strip of sheet brass, or a strip of one of the
above-named alloys, is fastened in zigzag manner upon a
small board, and the bends of the strip of brass or other
alloy are soldered to brass pins or to the usual form of bind-
FIG. 25. Adjustable Resistance. FIG. 26. Adjustable Resistance.
ing screws, so that the current may be led from any of
these points at will. This instrument is inserted in the
main current circuit, and by use of these numerous contacts
any desired fraction of the main current is switched off
through a shunt circuit containing the electrolytic cell.
PAET III
THE ELECTROLYTIC PROCEDURE
A. INTRODUCTORY
IN order that the current produced by any source of elec-
trical energy may exert its dissociating influence upon a
salt solution, it is necessary that suitable conductors should
be chosen whereby the current may be led to and from the
liquid. These conductors or 'electrodes' may be formed
of different metals or of carbon, but for analytical work
only those of platinum are employed. Platinised sheets of
other metal have not proved serviceable j and although
gold has proved suitable in certain instances, yet platinum
is to be preferred.
The electrode material must not only be proof against
the acids used to dissolve the deposited metal, when the
electrolysis has been completed and the deposit weighed,
but must be also proof against the action of the anions
liberated during the electrolysis.
For example, gold would be useless as an electrode
material for the electrolysis of chloride or alkali-sulphide
solutions. Even platinum is slowly attacked by chlorine,
and on this account the electrolysis of solutions of chlorides
is undertaken in as few cases as possible. If a solution of a
zinc salt solution be electrolysed with a platinum kathode, a
black deposit will remain upon the latter especially notice-
able at that part of the electrode which cut the level of the
electrolyte when the zinc has been dissolved off'.
80 THE ELECTEOLYTIC PROCEDURE
This deposit is insoluble in acids. According to Vort-
mann and others, it consists of finely divided platinum, and
its removal from the electrode is difficult and is harmful to
the latter.
In order to avoid this cause of injury to the platinum
electrode, it is customary when zinc is to be deposited to
coat the electrode previously with silver, copper, or tin.
The surface of the electrodes is generally smooth, but it
has been found that in certain cases it is more advan-
tageous to use electrodes that have become roughened by
frequent use, or that have been artificially made dull and
dead by means of the sand-blast.
Peroxide deposits and also metallic deposits of anti-
mony adhere better to such a roughened surface. The
strength of the sheet metal used for the electrodes should
not be too low ; the metal ought to be sufficiently thick to
resist any mechanical strains to which it may be subjected.
Electrodes made from an alloy of platinum and iridium are
found to resist both the chemical action and mechanical
wear and tear of use, better than those made of platinum
alone.
As regards the form of the electrodes, great differences
may exist, and practically all the possible forms are in actual
use. They may be divided into two broad classes. Under
the one come all those electrodes used in pairs for the
electrolysis of liquids contained in beakers or other non-
conducting vessels. Under the other are grouped those
forms in which one electrode acts as a basin or vessel for
holding the electrolyte.
Neither class possesses decisive advantages in all cases ;
but each is found to be especially convenient in particular
separations.
The 'basin electrode' is used in two forms. That
recommended by Classen is shown in fig. 27, and has no
HP.
The form shown in fig. 28 is recommended by N. v.
Klobukow. It possesses a spherical cup -shaped bottom, per-
INTRODUCTOEY 81
pendicular sides, and a lip. The latter form may be slightly
more convenient for use when it is necessary to calculate
the electrode surface covered by a definite volume of the
liquid, but apart from this it has no advantage over the
former. It is of great convenience to have marks upon
the inner surface of the walls of the basin which denote
the superficial area covered by the liquid when standing at
any particular height in the basin ; since this simplifies the
work when carrying out electrolytic depositions by means
of definite current densities.
The basin shown in fig. 27 is about 9 cm. in diameter
and about 4 cm. in depth ; its capacity is between 200 and
250 c.cms. liquid.
FIG. 27. Basin for Electrolysis. FIG. 28. Basin for Electrolysis.
The other electrode for use with this basin can have
many forms. The form shown in fig. 29 corresponds best
to the shape of the basin, and secures a uniform current
density ; it suffers from the disadvantage that, in spite
of the openings in its sides, the volume of liquid enclosed
within it passes but slightly into circulation. The ' saucer
electrode,' shown in fig. 30, possesses a round hole at its
deepest part ; while the ' disc electrode,' shown in fig. 31,
is pierced with many round openings. These two forms
are more satisfactory than the first, since they hinder the
circulation less, and the holes permit the escape of the
gases liberated at their under surface.
The separate parts of these electrodes should be riveted
together ; if they should be soldered even with gold, the
G
82
THE ELECTROLYTIC PROCEDURE
solder will be speedily dissolved away when they are used as
anodes.
FIG. 29.
Basin Electrode.
FIG. 30.
Saucer Electrode.
FIG. 31.
Disc Electrode.
Turning to the other group of
electrodes, many forms are found to
have been proposed and used. The
simplest arrangement is that of two
sheets of platinum opposed to one
another in the vessel containing the
electrolyte ; but this is seldom adopted.
In some cases a fork-shaped anode
is used with a single sheet of metal
as kathode, in order to obtain a
uniform coating upon each side of tho
latter. The forms most in use, how-
ever, are a cylindrical or conical sheet
electrode, enclosing a similarly shaped
FIG. 32^The Mans- S P iral f thick P latinum wire ' One of
feld Electrodes. the oldest types of this arrangement
is the so-called Mansfeld electrode,
shown in fig. 32. This consists of a closed platinum
cylinder, and of a platinum wire spiral used within the
INTRODUCTORY
83
cylinder. The first practical attempts to effect electrolytic
determinations of metals were made with this form of elec-
trode.
If one decides to make use of this form, it will be
advisable to cut the cylinder through in the direction o.
its axis, and also to bore some holes at other points in it,
in order to facilitate the circulation of the electrolyte,
and also to allow the deposition to occur to some extent
upon the outer side of the cylinder. The conical jacket
electrode shown in fig. 33 is also a form much used, and
FIG. 33. FIG. 34. FIG. 35.
FIGS. 33-35. Cone-shaped and Spiral Electrodes.
in this case the electrode is better when provided with
openings to promote the circulation of the electrolyte.
Figs. 34 and 35 show the forms of spiral used with this
jacket sheet electrode. The form shown in fig. 36 is,
however, to be preferred if a uniform current density at
all parts of the surface of the cone is required. The cone
is generally used about 8 cm. in height, and about 6 cm.
in diameter at its base. If these jacket electrodes be pro-
vided with an opening parallel to their axis, they possess
84 THE ELECTROLYTIC PEOCEDUEE
the advantage that when the electrolysis is completed
they can be lifted directly out of the solution. The gas
bubbles that are given off from the lower parts of the wire
electrode also aid the thorough mixing of the electrolyte
during the electrolysis. The disadvantage of these forms
lies in the difficulty of obtaining a uniform current density
at all parts of the sheet electrode, and on this account
these electrodes cannot be used in performing some of the
more particular electrolytic depositions.
As holder for the electrodes, a stand having a heavy
cast-iron base into which a strong glass rod is fixed will be
found to answer best. Upon this glass rod slides an arm
of brass, copper, or aluminium, capable of being fixed at
various heights, and bearing two binding screws, one for
connection with the current supply, the other for connec-
tion with the electrode. If the jacket electrode is to be
employed, either two such stands are used, or a compound
holder such as that illustrated in fig. 36 is used, in which
the conducting parts a and b are separated by an insulating
piece x.
If the basin electrode be used, one of the arms must be
bent into a ring, and this is provided on its upper side with
three platinum points, which make the electrical contact
between the ring and the basin. When the column of the
stand is of glass, both arms may be fixed upon it, as shown
in fig. 37.
Another form suitable for use with the basin electrode
consists of a turned wooden foot having a thick metal wire
bent into a circle fixed upon it. The basin rests firmly
upon this, while a bent metal arm, fixed at one side of this
wooden base, serves to hold the disc electrode over the
middle point of the basin. This form of holder has not
come much into use, one of its disadvantages being that it
is impossible to heat the electrolyte during the electrolysis
by means of a small burner placed beneath the basin.
In order to avoid loss of the liquid during the electrolysis,
the basin or beaker must be covered with a large clock-
INTRODUCTOKY
85
When the former is used to hold the electrolyte,
this glass will answer if it have only a hole through its
centre ; but when a beaker is used, it is necessary to have
a narrow slot in the glass cover, extending from the cir-
cumference to the centre.
It is especially necessary in electrolytic work to see that
the electrodes are perfectly clean ; with dirty electrodes it
is impossible to obtain a uniform and adherent deposit.
Films of grease, which can be produced merely by passing
FIG. 36. Stand for Electrolysis. FIG. 37. Stand for Electrolysis.
the fingers over the electrode surface, are especially detri-
mental.
If an electrode that has been soiled in this way
cannot be freed from its impurity by heating to redness,
or by treatment with acids, it will be necessary to heat it
with fused acid potassium sulphate or with borax, or to
clean it by mechanical means with fine sea-sand. The
methods first named are least harmful to the electrode, and
are to be preferred, when effective. The electrode when
perfectly clean is well washed, and is dried by. heating.
In order to carry out the electrolysis, the prepared
86 THE ELECTEOLYTIC PROCEDURE
solution of the metallic salt containing the necessary
additions of other chemicals is placed in the platinum
basin, or in the beaker designed for use with one of the
sheet electrodes. For electrolytic analyses, the volume of
liquid should lie between 150 c.cms. and 200 c.cms. ; the
volume of liquid to be dealt with must therefore be brought
within these limits by concentration or dilution, as the
necessities of the case demand. If the jacket electrode,
either in cylinder or cone form, is being used, the distance
apart of the two electrodes will be alike at every point if
care has been taken to centre the inner electrode properly
when fixing it in the holder. Care must also be given
to guard against any displacement or contact of the two
electrodes when placing them in, or when taking them out
of, the liquid in the beaker. When the basin electrode is
used, attention must be paid not only to the central position
of the disc electrode, but also to its height above the bottom
of the basin. This should lie between 1'5 and 2*0 cms. in all
cases. The clock-glass used for covering the vessel in which
the electrolysis is performed must be laid on at the com-
mencement. When a metallic deposit is to be obtained,
the basin or the sheet electrode is used as kathode ; but
when a deposit of the metal as peroxide is desired, the
current direction is reversed, and the basin or sheet electrode
is used as anode.
The current density given in the directions for the
different depositions in the following pages is always
calculated upon the superficial areas of these electrodes,
and not upon those of the wire ones. When the current
connections are first made for such an electrolytic cell as
that described above, a very large resistance is always used
in the circuit ; and the amperes having been calculated
that will yield the desired current density with the electrode
surface that is immersed, the resistance is reduced until
this current strength is obtained. Since the interior sur-
face area of the form of platinum basin customarily used
(see fig. 27), having a diameter of about 90 mm. and a
INTRODUCTORY 87
depth of about 40 mm., is only determined with difficulty
mathematically, one uses the following data for the current
density calculation : 125 c.cms. liquid cover 100 sq. centi-
metres, and 180 c.cms. cover 150 sq. centimetres of the
interior surface of the basin.
These ratios are sufficiently exact for use in most cases,
since the maintenance of the current density within more
accurate limits is unnecessary.
The concentration of the electrolyte undergoes change
during the electrolysis, and in most instances this occa-
sions an increase in the resistance it offers to the passage
of the current. If the external conditions remain the
same i.e. if the E.M.F. of the battery or dynamo, and
resistance of the external circuit, remain unaltered the
current strength, and therefore the current density at the
electrodes of the electrolytic cell, will be diminished. In
order to maintain the current density at the kathode (when
peroxide depositions are in progress at the anode) at a
definite value, many measurements must be made during
the course of the electrolysis, and the strength of the
current must be increased proportionately to the growth of
the resistance of the electrolytic cell, by diminishing the
resistance that has been inserted in the external circuit.
The E.M.F. measured at the terminals of the electrolytic
cell is always a guide to the resistance of the cell, and by fre-
quent measurements one can observe the increase in this as
the electrolysis proceeds. As already mentioned, it is now
customary to calculate the current density that is, the
strength of the current in amperes per unit of surface
upon a superficial area of 100 sq. centimetres.
This ' normal density,' as it is called, js used both for
the kathode, and for the anode when peroxide deposits are
being obtained. For electrodes of other size the calcula-
tion is simple. In the following special part of this work
all the current strengths given are calculated for a super-
ficial area of 100 sq. centimetres, even when this is not
particularly noted.
88 THE ELECTROLYTIC PROCEDURE
As regards temperature, in most cases the electrolysis
can be conveniently carried out at the normal indoor
temperature of 20 C. (68 F.) The electrolyte frequently
increases in temperature when strong currents are employed,
but it is often advisable to heat the liquid by the aid of some
external source of heat, in order to reduce the resistance it
offers to the current, and to lessen the -time necessary for
the complete deposition of the metal. A high temperature-
is also often requisite in order to obtain a metallic deposit
possessing the desired physical characteristics. If then it
be desired to electrolyse a warm or hot solution, a burner
with a small easily regulated flame is placed under the
basin or beaker containing the liquid. One may use for
this purpose either an ordinary Bunsen burner from which
the vertical tube has been removed by unscrewing, or one
may make use of a small Bunsen burner especially designed
for this purpose, possessing a horizontal mixing tube turned
upwards at its end. In order to obtain an equal and
regular temperature, a piece of asbestos paper or board is
loosely placed under the basin or other vessel, so that the
latter is heated by the intervening air rather than by
direct contact with the hot asbestos. In the majority
of instances it is sufficient to lead strong currents 1-0
to 1 '5 amperes through the warmed electrolyte ; the tem-
perature then remains about the same. When this is not
feasible the temperature of the electrolyte will fall from
60 C. to about 40 C. during the electrolysis, if no ex-
ternal heating be used. Such a fall in temperature ought
not to prove detrimental to the results obtained with
any really trustworthy method, and therefore, t in the
conduct of technical electrolytic analyses, the use of a
burner for maintaining the temperature has been dis-
continued.
The completion of the deposition and end of the electro-
lysis can be determined in many ways. When the deposit