1. Electrogravimetry and coulometry Department of Chemistry-
Electrogravimetry and coulometry
Dr.Bhagure G.R.
Department of Chemistry

2. Factors affecting the nature of the deposit,
Electrogrvimetry
Introduction,
Factors affecting the nature of the deposit,
Instrumentation
Applications

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 metal
Cd, Cu, Ni, Ag, Sn, Zn can be determined in this manner
Few 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 conductor
electrolytic conductor

6. Spontaneous redox reaction
Cells
Electrochemical Cells : Galvanic: Chemical energy into electrical energy :
Spontaneous redox reaction
Electrolytic Cells: Electrical energy into Chemical energy
non- Spontaneous redox reaction

7. The anode of an electrolytic cell is positive
Electrolytic Cells
The anode of an electrolytic cell is positive
cathode 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 Cells
Chemical 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 CELL
Electrolysis does not takes place
E APPLIED > E CELL CURRENT FLOWS IN CELL
ELECTROLYSIS TAKES PLACE

11. To drive an endothermic electrochemical reaction, the applied voltage, Applied, must overcome the
cell potential
IR drop

12. IR drop
Note 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 -anode
2) a positive electrode - cathode
3) an electrolyte e V + –
Anode Zn
Cathode Cu
ZnSO4
CuSO4
Porous fritted disk (liquid junction)

14. Electrolytic Cells

15. The factors influencing the physical characteristics of deposits are
current density
temperature
The effects of temperature are unpredictable and must be determined empirically
presence of Complexing agents
Many 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 Potential
Std. Oxidation Potential
Std. Reduction Potential

18. Non-nernstian potentials :
OHAMIC (SOLUTION) POTENTIAL
CONCENTRATION POLARIZATION
OVER 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>0
NOTE: 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 of
non-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+ + 2e
Net 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) | Cd
Cd Ag(s) + 2Cl–  Cd(s) + 2 AgCl(s)
Reduction
Working 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 = IR
The output voltage of a galvanic cell is decreased by IR.
Egalvanic = Eequilibrium – IR
The 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 composition
Composition of the electrolyte solution
Temperature And Stirring Rate
Current Level
Physical state of species involved in the cell reaction
Two categories of polarization phenomena
1) Concentration polarization
2) 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 color
2) deposition on freshly exposed electrode surface
3) qualitative test for analyte in solution
In 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 control
2) Controlled-potential ; potentiostatic method

32. Electro gravimetric Analysis
Types of
Electro gravimetric Analysis
Constant Current Electrolysis
Constant Potential Electrolysis

33. Consists of a suitable cell for the purpose of electrolysis
The apparatus
Consists of a suitable cell
for the purpose of electrolysis
A 6 V STORAGE BATTERY
Direct current source
AN AMMETER
Used to indicate the current
Voltmeter
Used to indicate the applied voltage
Resistor.
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 flow
Reference 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.

42. Thank you