electrogravimetry-211216084524.pdf

Prof. Santosh S. Dipke
(Assistant Professor)
Department of Chemistry
Pratap College, Amalner
ELECTROGRAVIMETRY

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 Theory of electrogravimetric analysis
 Terms used in electrogravimetric analysis
 Completeness of deposition
 Electrolytic separation of metals
 Character of the deposit
 Electrolytic separation of metals with controlled cathode potential
 Apparatus, Determination of copper ( constant current procedure )
 Determination of antimony, copper, lead and tin in bearing metal (controlled
current procedure)
Content

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 Element to be determined is deposited electrolytically upon a suitable electrode
 Filtration is not required
 Co-deposition of two metals can often be avoided with controlled experimental conditions
 Electro-deposition is governed by Ohm's Law and by Faraday's two Laws of Electrolysis
1. The amounts of substances liberated (or dissolved) at the electrodes of a cell are directly
proportional to the quantity of electricity which passes through the solution. i.e.
(mass of chemical deposition) 𝑚 𝑄 𝑞𝑢𝑎𝑛𝑡𝑖𝑡𝑦 𝑜𝑓 𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐𝑖𝑡𝑦

𝑚
𝑄
= 𝑍 = 𝑃𝑟𝑜𝑝𝑜𝑟𝑡𝑖𝑜𝑛𝑎𝑙𝑖𝑡𝑦 𝑐𝑜𝑛𝑠𝑡𝑎𝑛𝑡,
𝑒𝑙𝑒𝑐𝑡𝑟𝑜𝑐ℎ𝑒𝑚𝑖𝑐𝑎𝑙 𝑒𝑞𝑢𝑖𝑣𝑎𝑙𝑒𝑛𝑡 𝑜𝑓 𝑡ℎ𝑒 𝑠𝑢𝑏𝑠𝑡𝑎𝑛𝑐𝑒 (𝑒. 𝑐. 𝑒)
Introduction

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2. The amounts of different substances liberated or dissolved by the same quantity of electricity
are proportional to their relative atomic (or molar) masses divided by the number of electrons
involved in the respective electrode processes.
𝑚 𝐸
𝐸 = 𝑚𝑜𝑙𝑎𝑟 𝑚𝑎𝑠𝑠/𝑉𝑎𝑙𝑒𝑛𝑐𝑒
Introduction
 It follows from the Second Law that when a
given current is passed in series through
solutions containing copper(II) sulphate and
silver nitrate respectively, then the weights of copper and silver deposited in a given time will
be in the ratio of 63.55/2 to 107.87
 Ohm's Law expresses the relation between the three fundamental quantities, current,
electromotive force, and resistance: The current I is directly proportional to the electromotive
force E and indirectly proportional to the resistance R, i.e. I = E/R

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 Ampere (A): The fundamental SI unit of current, and which is defined as the constant current
which, if maintained in two parallel rectilinear conductors of negligible cross-section and of
infinite length and placed one meter apart in a vacuum, would produce between these
conductors a force equal to 2 x 10-7 newton per meter(N/m) length.
 Volt (V): The unit of electrical potential, which is the difference of potential between two
points of a conducting wire which carries a constant current of one ampere, when the power
dissipated between these two points is one joule per second
 Ohm (Ω): The unit of electrical resistance, which is the resistance between two points of a
conductor when a constant difference of potential of one volt applied between these two points
produces a current of one ampere
 Coulomb (C): The unit of quantity of electricity, and is defined as the quantity of electricity
passing when a current of one ampere flows for one second.
 To liberate one mole of electrons, or of a singly charged ion, will require L x e coulombs,
where L is the Avogadro constant (6.022 x 1023 mol-1) and e is the elementary charge (1.602 x
10-l0C); the resultant quantity (9.647 x 104 C mol-1 ) is termed the Faraday constant (F)
Electrical Units

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 Voltaic (Galvanic) and Electrolytic Cells:
• A cell consists of two electrodes and one or more solutions in an
appropriate container. If the cell can furnish electrical energy to
an external system it is called a voltaic (or galvanic) cell.
• The chemical energy is converted more or less completely into
electrical energy, but some of the energy may be dissipated as
heat.
TERMS USED IN ELECTRO-GRAVIMETRIC ANALYSIS
• If the electrical energy is supplied from an external
source the cell through which it flows is termed an
electrolytic cell and Faraday's Laws account for the
material changes at the electrodes.
• A given cell may function at one time as a galvanic cell
and at another as an electrolytic cell: a typical example is
the lead accumulator or storage cell
Electrolytic Cell

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 Cathode: The cathode is the electrode at which reduction occurs.
 In an electrolytic cell it is the electrode attached to the negative terminal of the source, since
electrons leave the source and enter the electrolysis cell at that terminal.
 The cathode is the positive terminal of a galvanic cell, because such a cell accepts electrons at
this terminal
 Anode: The anode is the electrode at which oxidation occurs.
 It is the positive terminal of an electrolysis cell or the negative terminal of a voltaic cell
 Polarised electrode: An electrode is polarised if its potential deviates from the reversible or
equilibrium value.
 An electrode is said to be 'depolarised ' by a substance if that substance lowers the amount of
polarisation

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 Current density: The current density is defined as the current per unit area of electrode
surface. It is generally expressed in amperes per square centimeter (or per square decimeter) of
the electrode surface.
 Current efficiency: By measuring the amount of a particular substance that is deposited and
comparing this with the theoretical quantity (calculated by Faraday's Laws), the actual current
efficiency may be obtained.
 Decomposition potential: If a small potential of, say, 0.5 volt is applied to two smooth
platinum electrodes immersed in a solution of 1M sulphuric acid, then an ammeter placed in
the circuit will at first show that an appreciable current is flowing, but its strength decreases
rapidly, and after a short time it becomes virtually equal to zero.

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 If the applied potential is gradually increased, there is a slight increase in the current until,
when the applied potential reaches a certain value, the current suddenly increases rapidly with
increase in the e.m.f.
 It will be observed, in general, that at the point at which
there is a sudden increase in current, bubbles of gas
commence to be freely evolved at the electrodes.
 Upon plotting the current against the applied potential a
curve similar to that shown in Fig.
 The point at which the current suddenly increases is
evident, and in the instance under consideration is at
about 1.7 volts
 The potential at this point is termed the 'decomposition potential' and it is at this point that the
evolution of both hydrogen and oxygen in the form of bubbles is first observed
 We may define the decomposition potential of an electrolyte as the minimum external potential
that must be applied in order to bring about continuous electrolysis

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 If the circuit is broken after the e.m.f has been applied, it will be observed that the reading on
the voltmeter is at first fairly steady, and then decreases, more or less rapidly, to zero.
 The cell is now clearly behaving as a source of current, and is said to exert a 'back' or 'counter'
or 'polarisation' e.m.f., since the latter acts in a direction opposite to that of the applied e.m.f
 This back e.m.f. arises from the accumulation of oxygen and hydrogen at the anode and
cathode respectively; two gas electrodes are consequently formed, and the potential difference
between them opposes the applied e.m.f.
 When the primary current from the battery is shut off, the cell produces a moderately steady
current until the gases at the electrodes are either used up or have diffused away; the voltage
then falls to zero.
 This back e.m.f. is present even when the current from the battery passes through the cell and
accounts for the shape of the curve in above Fig.
 The back e.m.f is usually regarded as being made up of three components:
a) the reversible back e.m.f.
b) a concentration polarisation e.m.f.; and
c) an activation overpotential

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a) Reversible back e.m.f. : This is the e.m.f of the voltaic cell set up by the passage of the
electrolysis current. Consider the electrolysis of a molar solution * of zinc bromide between
smooth platinum electrodes.
 The application of a potential will result in the deposition of zinc on the cathode (thus
producing a zinc electrode) and liberation of bromine at the anode (thus producing a bromine
electrode).
The reaction at the cathode is:
And at the cathode is:
 The potential of the cathode at 25°C can be calculated from the expression

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b) Concentration Polarisation e.m.f. (Concentration Overpotential): In the electrolysis of an
acidic solution of copper(II) sulphate between platinum electrodes, concentration changes take
place in the solution in the neighborhood of the electrodes.
• At the cathode depletion of copper ions occurs near the surface; the reversible potential of the
copper electrode therefore shifts in the negative direction.
• At the anode accumulation of hydrogen ions (2H2O  O2 + 4H+ + 4e-) and perhaps of oxygen
(if the solution is not already saturated with it) causes the reversible potential of the oxygen
electrode to shift in the positive direction.
• Both effects tend to increase the back e.m.f.
• The concentration overpotential is increased by increased current density and decreased by
stirring.
c) Activation Overpotential: This is due to the effect of the potential applied to the electrode on
the activation energy of the electrode reaction
• The effect is most marked when gases are liberated at the electrode, and varies according to
the nature of the electrode material

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• Overpotential: It has been found by experiment that the decomposition voltage of an
electrolyte varies with the nature of the electrodes employed for the electrolysis and is, in many
instances, higher than that calculated from the difference of the reversible electrode potentials.
• The excess voltage over the calculated back e.m.f is termed the overpotential.
• Overpotential may occur at the anode as well as at the cathode.
• The decomposition voltage ED is therefore:
• where Eo.c. and Eo.a. are the overpotentials at the cathode and anode respectively. The
overpotential at the anode or cathode is a function of the following variables
1. The nature and the physical state of the metal employed for the electrodes. The fact that
reactions involving gas evolution usually require less overpotential at platinised than at
polished platinum electrodes is due to the much larger effective area of the platinised electrode
and thus the smaller current density at a given electrolysis current.
2. The physical state of the substance deposited. Ifit is a metal, the overpotential is usually small;
if it is a gas, such as oxygen or hydrogen, the overpotential is relatively great.

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3. The current density employed. For current densities up to 0.01 A cm-2, the increase in
overpotential is very rapid; above this figure the increase in overpotential continues, but less
rapidly.
4. The change in concentration, or the concentration gradient, existing in the immediate vicinity
of the electrodes; as this increases, the overpotential rises
• The overpotential of hydrogen is of great importance in electrolytic determinations and
separations.
• It is greatest with the relatively soft metals, such as bismuth (0.4 V), lead (0.4 V), tin (0.5 V),
zinc (0.7 V), cadmium (1.1 V) and mercury (1.2 V)

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• The overvoltage values given refer to the electrolysis of 0.05M sulphuric acid with a current
density of 0.01 A em -2, and can be compared with the value (0.09 V) for a bright platinum
electrode under similar conditions.
• The existence of hydrogen overpotential renders possible the electro-gravimetric
determination of metals, such as cadmium and zinc, which otherwise would not be deposited
before the reduction of hydrogen ion.
• In alkaline solution, the hydrogen overpotential is slightly higher (0.05-0.03 volt) than in acid
solution.
• Oxygen overpotential is about 0.4-0.5 volt at a polished platinum anode in acid solution, and
is of the order of 1 volt in alkaline solution with current densities of 0.02-0.03 A em - 2.
• As a rule the overpotential associated with the deposition of metals on the cathode is quite
small (about 0.1-0.3 volt) because the depositions proceed nearly reversibly

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• For the electrolysis of a solution to be maintained, the potential applied to the electrodes of the
cell (Eapp) must overcome the decomposition potential of the electrolyte (ED) (which as shown
above includes the back e.m.f. and also any overpotential effects), as well as the electrical
resistance of the solution.
• Thus, Eapp. must be equal to or greater than (ED + IR), where I is the electrolysis current, and
R the cell resistance.
• As electrolysis proceeds, the concentration of the cation which is being deposited decreases,
and consequently the cathode potential changes.
• If the relevant ionic concentration in the solution is ci , and the ion concerned has a charge
number of 2, then at a temperature of 25°C, the cathode potential will have a value given by:
COMPLETENESS OF DEPOSITION

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• If the ionic concentration is reduced by deposition to one ten-thousandth of its original value,
thus giving an accuracy of 0.01 per cent in the determination, the new cathode potential will
be:
• It follows that if the original solution contains two cations whose deposition potentials differ
by about 0.25 V, then the cation of higher deposition potential should be deposited without any
contamination by the ion of lower deposition potential.
• In practice, it may be necessary to take steps to ensure that the cathode potential is unable to
fall to a level where deposition of the second ion may occur
COMPLETENESS OF DEPOSITION

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• If a current is passed through a solution containing copper(II), hydrogen and cadmium(II)
ions, copper will be deposited first at the cathode.
• As the copper deposits, the electrode potential decreases, and when the potential becomes
equal to that given by the hydrogen ions, hydrogen gas will form at the cathode.
• The potential at the cathode will remain virtually constant as long as hydrogen is evolved,
which would mean as long as any water remains, and it is therefore unable to become
sufficiently negative to permit the deposition of cadmium.
• Thus, metal ions with positive reduction potentials may be separated, without external control
of the cathode potential, from metal ions having negative reduction potentials.
ELECTROLYTIC SEPARATION OF METALS

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• Silver can be readily separated from copper, even though they both have positive reduction
potentials, because the difference between the two values is large (silver, + 0.779 V; copper, +
0.337 V), but as indicated above, when the standard potentials of the two metals differ only
slightly, the electro-separation is more difficult.
• An obvious solution to this problem is to decrease the concentration of one of the ions being
discharged by incorporating it in a complex ion of large stability constant (Section 2.23).
• As an illustration of the kind of result achieved, the deposition potentials for 0.1M solutions of
ions M2+ of the following metals have the values indicated: zinc, + 0.79 V; cadmium, +0.44
V; copper, +0.34 V.
• When 0.1 mole of the corresponding cyanides are dissolved in potassium cyanide to give an
excess concentration of potassium cyanide of O.4M, the deposition potentials become: zinc, +
1.18 V; cadmium, + 0.87 V; copper, + 0.96 V
• An interesting application of these results is to the direct quantitative separation of copper and
cadmium. The copper is first deposited in acid solution; the solution is then made slightly
alkaline with pure aqueous sodium hydroxide, potassium cyanide is added until the initial
precipitate just re-dissolves, and the cadmium is deposited electrolytically
ELECTROLYTIC SEPARATION OF METALS

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• The ideal deposit for analytical purposes is adherent, dense, and smooth; in this form it is
readily washed without loss.
• Flaky, spongy, powdery, or granular deposits adhere only loosely to the electrode, and for this
and other reasons should be avoided.
• As a rule, more satisfactory deposits are obtained when the metal is deposited from a solution
in which it is present as complex ions rather than as simple ions.
• Thus silver is obtained in a more adherent form from a solution containing the [Ag(CN z )] -
ion than from silver nitrate solution.
• Nickel when deposited from solutions containing the complex ion [Ni(NH3)6] 2+ is in a very
satisfactory state for drying and weighing.
• Mechanical stirring often improves the character of the deposit, since large changes of
concentration at the electrode are reduced, i.e. concentration polarisation is brought to a
minimum
• Increased current density up to a certain critical value leads to a diminution of grain size of the
deposit
CHARACTER OF THE DEPOSIT

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• Beyond this value, which depends upon the nature of the electrolyte, the rate of stirring, and
the temperature, the deposits tend to become unsatisfactory.
• At sufficiently high values of the current density, evolution of hydrogen may occur owing to
the depletion of metal ions near the cathode.
• If appreciable evolution of hydrogen occurs, the deposit will usually become broken up and
irregular; spongy and poorly adherent deposits are generally obtained under such conditions.
• For this reason the addition of nitric acid or ammonium nitrate is often recommended in the
determination of certain metals, such as copper; bubble formation is thus considerably
reduced.
• Action of NO3
- ion at the copper cathode is represented by:
• The NO3
- ion is reduced to ammonium ion at a lower (i.e. less negative) cathode potential than
that at which hydrogen ion is discharged, and, therefore, acts to decrease hydrogen evolution.
• The nitrate ion acts as a cathodic depolariser. Raising the temperature, say to between 70 and
80°C, often improves the physical properties of the deposit. This is due to several factors,
which include the decrease in resistance of the solution, increased rate of stirring and of
• diffusion.
CHARACTER OF THE DEPOSIT

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