Electrogravimetry and Coulometry Department of Chemistry

: "Electrogravimetry and coulometry Department of Chemistry"—
2 Factors affecting the nature of the deposit,

ElectrogrvimetryIntroduction,Factors affecting the nature of the deposit,InstrumentationApplications
3 Electrogrvimetry – The product is weighed as a deposit on one of the electrodes (the working
electrode)Involve deposition of the desired metallic element upon a previously weighed cathode,
followed by subsequent reweighing of the electrode plus deposit to obtain by difference the quantity
of the metalCd, Cu, Ni, Ag, Sn, Zn can be determined in this mannerFew substances may be
oxidized at a Pt anode to form an insoluble and adherent precipitate suitable for gravimetric
measurement e.g. oxidation of lead(II) to lead dioxide in HNO3 acid
4 Some Basic concepts
5 electrolytic conductor
metallic conductorelectrolytic conductor
6 Spontaneous redox reaction

CellsElectrochemical Cells : Galvanic: Chemical energy into electrical energy :Spontaneous redox
reactionElectrolytic Cells: Electrical energy into Chemical energynon- Spontaneous redox reaction
7 The anode of an electrolytic cell is positive

Electrolytic CellsThe anode of an electrolytic cell is positivecathode is negative), since the anode
attracts anions from the solution
8 Electrochemical Cells
Potentiometric measurements are made in the absence of current flow. The measured potential is
that of a galvanic or voltaic cell.
9 Electrolytic CellsChemical analyses can also be based on using a cell in an electrolytic fashion
(i.e., driving a reaction with an applied voltage). In these cases, current flows in the cell.
10 Possibility for electrolysis

E APPLIED = E CELL NO CURRENT FLOWS IN CELLElectrolysis does not takes placeE APPLIED
> E CELL CURRENT FLOWS IN CELLELECTROLYSIS TAKES PLACE
11 To drive an endothermic electrochemical reaction, the applied voltage, Applied, must
overcome the
cell potentialIR drop
12 IR dropNote that an electrochemical cell has a resistance to current flow, in analogy with
resistance to current flow in any conductor. The relationship between voltage, current, and
resistance is ohm’s Law:E=I R , where E is potential (voltage), I is current (in amperes) and R is
resistance (in ohms)
13 Porous fritted disk (liquid junction)

A voltaic cell in a circuit consists of :1) a negative electrode -anode2) a positive electrode -
cathode3) an electrolytee V + –Anode ZnCathode CuZnSO4CuSO4Porous fritted disk (liquid
junction)
14 Electrolytic Cells
15 The factors influencing the physical characteristics of deposits are

current densitytemperatureThe effects of temperature are unpredictable and must be determined
empiricallypresence of Complexing agentsMany metals form smoother and more adherent films
when deposited from solutions in which their ions exist primarily as complexes e.g. Cyanide and
ammonia complexes
16 Principles of electrolysis
Electrolysis is the process in which a reaction is driven in its no-spontaneous direction by the
application of an electric current.
17 Std. Oxidation Potential Std. Reduction Potential

Nernst PotentialStd. Oxidation PotentialStd. Reduction Potential
18 Non-nernstian potentials :
OHAMIC (SOLUTION) POTENTIALCONCENTRATION POLARIZATIONOVER POTENTIAL/
Voltage
19 Principles of electrolysis 
Electrolysis is the process in which a reaction is driven in its nonspontaneous direction by the
application of an electric current.Endergonic reaction G>0NOTE: electrolysis is the process of
driving an electrochemical reaction in its non-spontaneous direction through the application of
voltage/current.To accurately assess voltage-current relationships, we must consider some sources
ofnon-Nernstian potentials : Ohmic (solution) potential, Concentration polarization, Overpotential.
20 Electrolysis experiment
Direct current (dc) is current that is always in one direction; it is unidirectional.The direction of
alternating current (ac) reverses periodically.DC voltage sources are often given the battery symbol
with + and – polarities.An arrow through the battery indicates that the source voltage is variable and
can be changed to another dc value.Cathode (working electrode):2 Cu e = Cu(s)Anode (counter
electrode):H2O = ½O2(g) + 2H+ + 2eNet reaction:H2O + Cu2+ = Cu(s) + ½O2(g) + 2H+
21 Ag | AgCl(s), Cl– (0.200M), Cd 2+ (0.00500M) | Cd

An electrolytic cell for determining Cd2+.Current=0.00mA.Schematic of cell in (a) with internal
resistance of cell represented by a 15.0Ω resistor and Eapplied increased to give a current of
2.00mA.Ag | AgCl(s), Cl– (0.200M), Cd 2+ ( M) | CdCd Ag(s) + 2Cl–  Cd(s) + 2
AgCl(s)ReductionWorking electrode operates as a cathode when apply a potential somewhat more
negative than a thermodynamic potential of – 0.734V.
22 Current and potential changes during an electrolysis.

Whenever current flows, three factors act to decrease the output voltage of galvanic cell or to
increase the applied voltage needed for electrolysis.1) Ohmic potential ; Ohmic drop The voltage
needed to force current (ions) to flow through the cell.Eohmic = IRThe output voltage of a galvanic
cell is decreased by IR.Egalvanic = Eequilibrium – IRThe magnitude of the applied voltage for an
electrolysis cell must be more negative than the thermodynamic cell potential by IR in order for
current flow.Eapplied = Ecathode – Eanode – IR= Ecell – IR
23 2) Polarization effects
I = (Ecell – Eapplied) / R = – (Eapplied / R) + (Ecell / R)Overvoltage () is the potential difference
between the theoretical cell potential from Eapplied = Ecell – IR and the actual cell potential at a
given level of current.Eapplied = Ecell – IR – The term polarization refers to the deviation of the
electrode potential from the value predicted by the Nernst equation on the passage of current. Cells
that exhibit nonlinear behavior at higher currents exhibit polarization, and the degree of polarization
is given by an overvoltage or overpotential.
24 Experimental current/voltage curve for operation of the cell shown in Figure Dashed line is the
theoretical curve assuming no polarization. Overvoltage ∏ is the potential difference between the
theoretical curve and the experimental.
25 Factors that influenced polarization

Electrode size, shape, and compositionComposition of the electrolyte solutionTemperature And
Stirring RateCurrent LevelPhysical state of species involved in the cell reactionTwo categories of
polarization phenomena1) Concentration polarization2) Kinetic polarization
26 Concentration Polarization
Concentration polarization occurs because of the finite rate of mass transfer from the solution and an
electrode surface.The electrode potential depends on the concentration of species in the region
immediately surrounding the electrode.When ions are not transported to or from an electrode as
rapidly as they are consumed or created, we say that concentration polarization exists. That is,
concentration polarization means that [X]s  [X]o, , where [X]o is the concentration of X in the bulk
solution and [X]s is concentration of X in the immediate vicinity of the electrode surface.Reactants
are transported to and products away from an electrode by three mechanisms:(1) diffusion, (2)
migration, (3) convection (as a result of stirring, vibration, or temperature gradients)To decrease
concentration polarization :(1) Raise the temperature.(2) Increase stirring(3) Increase electrode
surface area.(4) Change ionic strength
27 Concentration Polarization
Pictorial diagram (a) and concentration vs. distance plot (b) showing concentration change at the
surface of a cadmium electrode. As Cd2+ ions are reduced to Cd atoms at the electrode surface, the
concentration of Cd2+ at the surface becomes smaller than the bulk concentration. Ions then diffuse
from the bulk of the solution to the surface as a result of the concentration gradient. The higher the
current, the larger the concentration gradient until the surface concentration falls to zero, its lowest
possible current, called the limiting current, is obtained.
28 Current-potential curve for electrolysis showing the linear or ohmic region, the onset of
polarization, and the limiting current plateau. In the limiting current region, the electrode is said to be
completely polarized, since its potential can be changed widely without affecting the current.
29 Migration involves the movement of ions through a solution as a result of electrostatic attraction
between the ions and the electrodes.Migration of analyte species can be minimized by having a high
concentration of an inert electrolyte, called a supporting electrolyte, present in the cell.The motion of
ions through a solution because of the electrostatic attraction between the ions and electrodes is
called migration.
30 Electrogravimetric analysis
Electrodeposition analysis in which the quantities of metals deposited may be determined by
weighing a suitable electrode before and after deposition.(a) Electrogravimetric analysis. Analyte is
deposited on the large Pt gauze electrode. If analyte is to be oxidized rather than reduced, the
polarity of power supply is reversed so that deposition still occurs on the large electrode. Apparatus
for electrodeposition of metals without cathode-potential control. Note that this is a two-electrode
cell.(b) Outer Pt gauze electrode. (c) Opened inner Pt Pt gauze electrode designed to be spun by a
mortor in place of magnetic stirring.
31 Tests for completion of the deposition :

1) disappearance of color2) deposition on freshly exposed electrode surface3) qualitative test for
analyte in solutionIn practice, there may be other electroactive species that interfere by codeposition
with the desired analyte.Two general types of electrolytic procedures :1) Electrogravimetry without
potential control2) Controlled-potential ; potentiostatic method
32 Electro gravimetric Analysis

Types ofElectro gravimetric AnalysisConstant Current ElectrolysisConstant Potential Electrolysis
33 Consists of a suitable cell for the purpose of electrolysis

The apparatusConsists of a suitable cellfor the purpose of electrolysisA 6 V STORAGE
BATTERYDirect current sourceAN AMMETERUsed to indicate the currentVoltmeterUsed to indicate
the applied voltageResistor.THE VOLTAGE APPLIED TO THE CELL IS CONTROLLED BY A
RESISTOR.Constant Current Electrolysis
34 Cl-Na+
36 Constant Potential Electrolysis:

By controlled potential electrolysis, it is possible to separate two elements whose deposition
potentials differ sufficiently (by a few tenths of a volt). The potential of the cathode is controlled so
that it never becomes sufficiently negative to allow the deposition of the next element. The potential
of the cathode becomes negative (due to concentration polarisation) and that co-deposition of the
other species begins before the analyte is completely deposited. A large negative drift in the cathode
potential can be avoided by using a three electrode system as shown in Fig.
37 Constant Potential Electrolysis Working electrode :

where the analytical reaction occurs.Auxiliary electrode :the other electrode needed for current
flowReference electrode (SCE) :used to measure the potential of the working electrode
39 A potentiostat maintains the working electrode potential at a constant value relative to a

reference electrode.The electrolysis current passes between the working electrode and a counter
electrode. The counter electrode has no effect on the reaction at the working electrode.Reduction
reactions occur at working electrode potentials ( measured with respect to the reference electrode )
that are more negative than that required to start the reaction. Oxidations occur when the working
electrode is more positive than necessary to start the reaction.Controlled potential means that a
constant potential difference is maintained between the working and reference electrodes.Constant
voltage means that a constant potential difference is maintained between the working and auxiliary
electrodes.Controlled potential affords high selectivity, but the procedure is slower than constant
voltage electrolysis.
40 Charges in cell potential (A) and current (B) during a controlled-potential deposition of copper.
The cathode is maintained at –0.36V (vs. Lingane, Anal. Chem.. Acta, 1948, 2, 590.)
41 A mercury cathode for the electrolytic removal of metal ions from solution

Electrogravimetry and Coulometry Department of Chemistry

Uploaded by

Copyright:

Available Formats

Electrogravimetry and Coulometry Department of Chemistry

Uploaded by

Document Information

Original Description:

Copyright

Available Formats

Share this document

Share or Embed Document

Sharing Options

Did you find this document useful?

Is this content inappropriate?

Copyright:

Available Formats

Electrogravimetry and Coulometry Department of Chemistry

Uploaded by

Copyright:

Available Formats

: "Electrogravimetry and coulometry Department of Chemistry"—

2 Factors affecting the nature of the deposit,

4 Some Basic concepts

6 Spontaneous redox reaction

7 The anode of an electrolytic cell is positive

10 Possibility for electrolysis

13 Porous fritted disk (liquid junction)

15 The factors influencing the physical characteristics of deposits are

17 Std. Oxidation Potential Std. Reduction Potential

21 Ag | AgCl(s), Cl– (0.200M), Cd 2+ (0.00500M) | Cd

22 Current and potential changes during an electrolysis.

25 Factors that influenced polarization

31 Tests for completion of the deposition :

32 Electro gravimetric Analysis

33 Consists of a suitable cell for the purpose of electrolysis

36 Constant Potential Electrolysis:

37 Constant Potential Electrolysis Working electrode :

39 A potentiostat maintains the working electrode potential at a constant value relative to a

You might also like