1. Potentiometry
Dr Hisham E Abdellatef
2010

2. It is a method of analysis in which we determine the concentration of an ion or substance by measuring the potential developed when a sensitive electrode is immersed in the solution of the species to be determined.

3. Mo = Mn++ ne Applying Nernest equation.
Determination of the substances by potentiometric
technique can be carried out by two ways:
Direct potentiometry and
potentiometric titrations

4. The potential of the indicator electrode cannot be measured alone;
For any potentiometric measurement we must have:
Reference electrode
Indicator electrode.
Potentiometer
Salt bridge to connect the two electrode solutions and complete the circuit.

6. A- Reference electrode
Reference electrode must:
Have a constant potential
Its potential must be definite
To express any electrode we have to mention:
Redox reaction at the electrode surface.
Half cell and Nernst equation.
Sketch of its design.
Any necessary conditions for its preparation.
Any necessary precautions for its use.

7. Standard Hydrogen Electrode
It’s a primary reference electrode. Its potential is considered to be zero.
Electrode reaction:
half cell: pt/ H2 , H+ (1N) 
Eo = zero
d-Limitation
It is difficult to be used and to keep H2­ gas at one atmosphere during all determinations.
It needs periodical replating of Pt. Sheet with Pt. Black

8. Hg | Hg2Cl2 (sat’d), KCl (sat’d) | |
Saturated calomel electrode (S.C.E.)
Hg | Hg2Cl2 (sat’d), KCl (sat’d) | |
electrode reaction in calomel hal-cell
Hg2Cl2 + 2e = 2Hg + 2Cl–
Eo = V
E = Eo – ( /2) log[Cl–]2 = V
Temperature dependent

9. Hg2Cl2 ⇌ 2 Hg22+ + 2Cl- Sp Hg2Cl2 = [Hg22+]2 [Cl-]2
Potential of the electrode depends on the chloride ion
Hg2Cl2 ⇌ 2 Hg Cl-
Sp Hg2Cl2 = [Hg22+]2 [Cl-]2
Ksp = 1.8 ×10–18
E = Eo – (0.0591/2) log[Cl–]2 = V

10. The crystal structure of calomel(Hg2Cl2), which has limited solubility in water (Ksp = 1.8 ×10–18).
KCl
E volt
Saturated
0.241 1M
0.280
0.1 M
0.334
Hg2Cl2  Hg Cl– Ksp = 1.8 ×10–18
Saturated KCl = 4.6 M KCl

11. E (saturated KCl) = + 0.199V (25oC)
Silver-silver chloride electrode
Ag(s) | AgCl (sat’d), KCl (xM) | |
AgCl(s) + e = Ag(s) + Cl–
Eo = V
E = Eo – ( /1) log [Cl–]
E (saturated KCl) = V (25oC)

12. SpAgCl = [Ag+] [Cl-]
E = Eo – ( /1) log [Cl–]

13. Disadvantage of silver-silver chloride electrode
It is more difficult to prepare than SCE.
AgCI in the electrode has large solubility in saturated KCl
Advantage of Ag/AgCI electrodes over SCE.
It has better thermal stability.
Less toxicity and environmental problems with consequent cleanup and disposal difficulties.

15. B- Indicator electrode
its potential is sensitive to the concentration of analyte
Ecell=Eindicator-Ereference
It must be:
(a) give a rapid response and
(b) its response must be reproducible.
Metallic electrodes: where the redox reaction takes place at the electrode surface.
Membrane (specific or ion selective) electrodes: where charge exchange takes place at a specific surfaces and as a result a potential is developed.

16. 1. Electrodes for precipitemetry and complexometry
a- First-order electrodes for cations:
e.g. in determination of Ag+ a rode or wire of silver metal is the indicator electrode, it is potential is:
It is used for determination of Ag+ with Cl-, Br- and CN-. Copper, lead, cadmium, and mercury

17. Example of First-order electrode
Ag+ + e = Ag(s) Eo = V
E = – ( /1) log {1/[Ag+]}

18. b) Second order electrodes for anions A metal electrode can sometimes be indirectly responsive to the concentration of an anion that forms a precipitate or complex ion with cations of the metal. Ex. 1. Silver electrode The potential of a silver electrode will accurately reflect the concentration of iodide ion in a solution that is saturated with silver iodide. AgI(s) + e = Ag(s) + I– Eo = – 0.151V E = – – ( /1) log [I–] = – ( /1)pI

19. 2. Mercury electrode for measuring the concentration of the EDTA anion Y4–.
Mercury electrode responds in the presence of a small concentration of the
stable EDTA complex of mercury(II).
HgY2– + 2e = Hg(l) + Y4– Eo = 0.21V
E = – ( /2) log ([Y4–] /[HgY2–])
K = 0.21 – ( /2) log (1 /[HgY2–])
E = K – ( /2) log [Y4–] = K +( / 2) pY

20. 2. Inert electrodes (Indicators electrodes for redox reaction)
Chemically inert conductors such as gold, platinum, or carbon that do not participate, directly, in the redox process are called inert electrodes. The potential developed at an inert electrode depends on the nature and concentration of the various redox reagents in the solution.
Examples:
Ag(s) | AgCl[sat’d], KCl[xM] | | Fe2+,Fe3+) | Pt
Fe3++e = Fe Eo = V
Ecell = Eindicator – Ereference
= {0.770 – ( /1) log [Fe2+]/[Fe3+]} – {0.222 – ( /1) log [Cl–]}

21. 2) Membrane indicator electrodes
The potential developed at this type of electrode results from an unequal charge buildup at opposing surface of a special membrane. The charge at each surface is governed by the position of an equilibrium involving analyte ions, which, in turn, depends on the concentration of those ions in the solution.
The electrodes are categorized according to the type of membrane they employ :
glass,
polymer,
crystalline,
gas sensor.
The first practical glass electrode. (Haber and Klemensiewcz, Z. Phys. Chem, 1909, 65, 385.

22. 3. indicator electrodes for neutralization reaction
Glass Membrane Electrode

24. Composition of glass membranes 70% SiO2 30% CaO, BaO, Li2O, Na2O,
and/or Al2O3
Ion exchange process at glass membrane-solution interface:
Gl– + H+ = H+Gl–
(a) Cross-sectional view of a silicate glass struture. In addition to the three Si│O bonds shown, each silicon is bonded to an additional oxygen atom, either above or below the plane of the paper. (b) Model showing three-dimensional structure of amorphous silica with Na+ ion (large dark blue) and several H+ ions small dark blue incorporated.

25. Glass Membrane Electrode
E = K (pH1 - pH2)
K= constant known by the asymmetry potential.
PH1 = pH of the internal solution 1.
PH2 = pH of the external solution 2.
The final equation is:
E = K pH
Standardization
at pH=7.00 , E = 0 V.
pH 4.00, E= mV/pH unit
Asymmetry potential
E of the 2 reference electrodes
pH of the internal solution
Liquid junction potential

26. Measurement of pH (cont.)
Ecell = E°cell - (0.0591)log[H+] + constant
• Ecell is directly proportional to log [H+]
electrode

28. pH Meters

29. Glass Membrane Electrode
Advantages of glass electrode:
It can be used in presence of oxidizing, reducing, complexing
Disadvantage:
Delicate, it can’t be used in presence of dehydrating agent e.g. conc. H2SO4, ethyl alcohol….
Interference from Na+ occurs above pH 12 i.e Na+ excghange together with H+  above pH 12 and higher results are obtained.
It takes certain time to come to equilibrium due to resistance of glass to electricity.

30. Mobilties of ions in water at 25oC:
Junction potential :
a small potential that exists at the interface between two electrolyte solutions that differ in composition.
Development of the junction potential caused by unequal mobilities of ions.
Mobilties of ions in water at 25oC:
Na+ : 5.19 × 10 –8 m2/sV K+ : 7.62 × 10 –8 Cl– : 7.91× 10 –8

31. To reduce the liquid junction potential to only few millivolts one has to:
Use a sat for preparation of the junction which its cation and anion have very near mobilities, so that they move by the same rate e.g. KCl and KNO3. (K+ =74, Cl- = 73 and NO3- = 76)
Use high concentration of the salt for preparation of the bridge, to reduce the effect of difference in rates of migration of other ions in the electrode solutions.

32. 2. Standard Hydrogen Electrode
electrode reaction:
Nernst equation
E = pH
When it is connected with NHE as reference electrode the e.m.f. of the cell :
Ecell = zero –(–0.059 pH)
= pH
pH = E / 0.059

33. Disadvantages:-
It cannot be used in solution containing oxidising agent which will oxidiose [ ½ H­2 = H+ + e ] or reducing substances which will reduce [ H+ + e = ½ H­2 ] especially in presence of platinum black
It cannot be used in reactions involving volatile constituent’s e.g. CO2, as it will be bubbled out by the H2 gas.
It cannot be used in presence of catalytic poisons which will affect Pt black which catalyses the electrode reaction.
It needs repletion with Pt black.
It is not easy to keep H2 gas at one atmospheric pressure during all measurements.

34. 3. Antimony electrode Sb/Sb2O3
Electrode reaction:
Sb2O H+ + 6 e = 2 Sb H2O
Nernst equation
E25 = E0 – pH

35. Advantages Easy to use, cheep and durable.
Disadvantages
can only be used within pH range 2 – 8 at lower pH Sb2O3 dissolves and at higher pH Sbo dissolves.
It cannot be used in presence of oxidizing agents, reducing agents, complexing agents and noble metals

36. Quinhydrone electrode

37. Advantages
It is not affected by catalytic poisons.
Easy to prepare and use.
It comes to equilibrium rapidly.
Disadvantages:
It cannot be used in presence of oxidising agents and reducing agents
The upper limit of the electrode use is pH 8
It needs to be used freshly.

38. Application of potentiometry
Direct potentiometric measurements
Eobs = Eref + Eja - Eind

39. 2. Potentiometric titration

40. It is used for all types of volumetric analysis: acid base, precipitimetry, complexometry and redox
It is used when it is not easy or impossible to detect the end point by ordinary visual methods i.e:
For highly coloured or turbid solutions.
For very dilute solutions 10-3, 10-6 M.
When there is no available indicator

41. Potentiometric titration

42. Titration of 2.433mmol of chloride ion with 0.1000M silver nitrate.
Titration curve.
First-derivative curve.
Second-derivative curve.

43. Application of potentiometric titration in
Neutralization reactions: glass / calomel electrode for determination of Ph
b) Precipitation reactions: Membrane electrodes for the determination of the halogens using silver nitrate reagent
c) Complex formation titration: metal and membrane electrodes for determination of many cations (mixture of Bi3+, Cd2+ and Ca2+ using EDTA)
d) Redox titration: platinum electrode For example for reaction of Fe3+/ Fe2+ with Ce4+/Ce3+

44. Neutralization reactions:
glass / calomel electrode for determination of pH

45. Precipitation reactions: Membrane electrodes for the determination of the halogens using silver nitrate reagent

46. Complex formation titration: metal and membrane electrodes for determination of many cations (mixture of Bi3+, Cd2+ and Ca2+ using EDTA)

47. Redox titration: platinum electrode For example for reaction of Fe3+/ Fe2+ with Ce4+/Ce3+