1. POTENTIOMETRY 8th lecture

2. Potentiometry:
Is a method of analysis in which we determine the concentration of an ion or a substance by dipping a suitable sensor in its solution (indicator electrode).
The potential of the indicator electrode is measured relative to reference electrode possessing constant potential .
The concentration of the ion is determined from Nernest equation.E25°C= Eo /n log 1/ [Mn+]
The two electrodes form the two half cells of electrochemical cell , the electromotive force (e.m.f.) of which is the algebraic difference of the two electrode potential (cathode & anode ).
Since the reference electrode potential is constant the change in e.m.f. of the cell is due to the change of the indicator electrode potential which in turn is due to the change of the concentration of the ion to be determined.

3. 1‑Galvanic Cell (voltaic cell): Where the chemical energy is converted to electrical energy which can be supplied to an external circuit i.e. it produces electrical energy. e.g. Daniel cell: Is composed of: 1- A copper electrode dipped in its solution. 2- A zinc electrode dipped in its solution. 3- Both solutions are connected by salt bridge. 4- The electrodes are connected by external circuit. Zn metal dissolves in its solution and transfers to Zn ion while copper ions are deposited on the electrode so a potential difference is produced leading to the production of current.

4. Classification of the electrodes They are classified according to
Type
Function
Reference
Indicator
First type
2- Complexometric reaction
1-Normal hydrogen
1- Redox reaction
Second type
2-Ag/AgCl
3- Preceptimetric reaction
Third type
4- Acid base reaction
3-Calomel
Redox type
Mode of action
Electron transfer
Ion- Selective

5. Electrodes used in potentiometric measurements:
Classification of electrodes:
A) According to the type:
1) First type (kind or order):
In this type electrode potential depends on equilibrium between element and its ions (i.e. two phase electrode).
(Mo / Mn+)
Mo ↔ Mn+ + ne
E25oC = Eo /n log 1 / [Mn+]
From Nernest equation E α [Mn+]
e.g. Titration of Cu2+ with EDTA
At the begining Cu2+ ions conc is very high so E
is high. as E α [Cu2+], then E decreases with the
consumption of copper with EDTA
It is used for any metal except Fe, Cr, Co, Ni WHY?
Because: 1- They are liable to oxidation by air so they give irregular potential.
2- They suffer from crystal deformation and prescence of oxide coating. E
mls of titrant
A strong electrolyte is a solute that completely, or almost completely, ionizes or dissociates in a solution. These ions are good conductors of electric current in the solution.
Originally, a "strong electrolyte" was defined as a chemical that, when in aqueous solution, is a good conductor of electricity. With greater understanding of the properties of ions in solution its definition was gradually changed to the present one.
Examples of Strong Electrolytes
Strong Acid: Perchloric acid HClO4, Hydriodic acid HI, Hydrobromic acid HBr, Hydrochloric acid HCl, Sulfuric acid H2SO4, Nitric acid HNO3, Chloric acid HClO3, Bromic acid HBrO3, Perbromic acid HBrO4, Periodic acid HIO4, Fluoroantimonic acid HSbF6, Magic acid FSO3HSbF5, Carborane superacid H(CHB11Cl11), Fluorosulfuric acid FSO3H, Triflic acid CF3SO3H, Citric Acid
Strong Base: Potassium hydroxide KOH, Barium hydroxide Ba(OH)2, Caesium hydroxide CsOH, Sodium hydroxide NaOH, Strontium hydroxide Sr(OH)2, Calcium hydroxide Ca(OH)2, Rubidium hydroxide RbOH, Magnesium hydroxide Mg(OH)2, Lithium diisopropylamide (LDA) C6H14LiN, Lithium diethylamide (LDEA), Sodium amide NaNH2, Sodium hydride NaH, Lithium bis(trimethylsilyl)amide ((CH3)3Si)2NLi
Ionization is the physical process of converting an atom or molecule into an ion by adding or removing charged particles such as electrons or other ions. This is often confused with dissociation.
Strictly speaking, yes. Dissociation refers to the breaking of a chemical bond without reference to whether the products are ions or neutral fragments. Ionization (ionisation, alternate spelling) refers to breaking of chemical bonds into charged species. The terms are sometimes both used when referring to the ionization reaction HA -----> H(+) + A(-). Dissociation of a molecule can either produce ions or neutral molecules. If it produces ions, then the dissociation is also an ionization. so HCl ---> H+ + Cl- is a dissociation reaction and also an ionization reaction. However, H2 ----> 2H is just a dissociation reaction. However, ionization is a general term which refers to the formation of ions, and not necessarily from dissociation. You can form an ion by removing an electron from it (a cation) or adding an electron (forming an anion, as in F + e ---> F-). If you want to think of cation formation as a dissociation reaction, fine- but the latter is clearly not a dissociation.
The term ionization also has a second meaning when used in discussions of solutions. The way substances behave electrically in water solution is often very different from the way they behave as solids or gases. As an example, let us consider the compound known as acetic acid which is a liquid that does not conduct an electric current. Yet, when acetic acid is added to water, the solution that is formed does conduct an electric current.
In order for a solution to conduct an electric current, ions must be present. Pure acetic acid is made of molecules. It contains no ions. If we pass an electric current into acetic acid, nothing will happen because no ions are present. An important change takes place, however, when acetic acid is added to water. Water molecules have the ability to tear acetic acid molecules apart, breaking them down into hydrogen ions and acetate ions. Now that ions are present, the water solution of acetic acid can conduct an electric current. This process is known as ionization because ions are produced from a substance (acetic acid in this case) that did not contain them originally.
A similar story about the conductivity and nonconductivity of sodium chloride could be told. If the two ends of a battery are attached to a large crystal of sodium chloride, no electric current will flow. One might guess that this result indicates that no ions are present in sodium chloride. However, that is not the case. Indeed, a crystal of sodium chloride is made up entirely of ions, positively charged sodium ions and negatively charged chloride ions. The problem is, however, that these ions are held together very tightly by electrical forces. Sodium ions are bound tightly to chloride ions, and vice versa.
The situation changes, however, when sodium chloride is added to water. Water molecules are able to tear apart sodium ions and chloride ions in much the same way they tear apart acetic acid molecules. Once the sodium ions and chloride ions are no longer bound tightly to each other, they are free to roam through the salt/water solution. The name given to this change is dissociation. The term means that ions already existed in the sodium chloride crystal before it was put into water. Water did not create the ions, it only set them free. It is this difference between creating ions and setting them free that distinguishes ionization from dissociation.
activity (symbol a) is a measure of the “effective concentration” of a species in a mixture Under Creative Commons License: Attribution No Derivatives
E.p.

6. 2) Electrode of Second type:
In this type equilibrium is established between three phases.
The electrode is a metal electrode which is covered with a layer of its sparingly soluble salt immersed in a solution of electrolyte.
This electrode is used for determination of anions (non metal ions).
The electrode potential is indirectly affected by the analyte.
e.g. Ag0 electrode: (Ag0 / AgCl / Cl-) EAg0 α 1/ [Cl-]
i.e. Ag0 is coated with a layer of silver chloride and dipped in chloride solution.
Calomel electrode: (Hg0 / Hg2Cl2 / Cl-) EHg0 α 1/ [Cl-]
Mercury electrode: (Hg0 / Hg2y2- / y4-) EHg0 α 1/ [Y4-]
Similar electrodes for bromide, iodide………can be used.

7. e.g. titration of Cl- with Ag+ Before titration: conc of Cl- is high
In Ag electrode; the potential depends on the chloride concentration in the solution and this is used for its determination. The electrode potential is:
E25 = E0 ‑ log [Cl-]
i.e Potential is inversely proportional to non metal ion concentration.
e.g. titration of Cl- with Ag+
Before titration: conc of Cl- is high
So E is low
During titration:conc of Cl- decreases
So E increases
At e.p. conc of Cl- is zero
So sharp increase in E E
mls of titrant

8. 3) Electrode of Third type:
In this type equilibrium is established between four phases.
E.g. We want to determine Ca 2+ ions in CaCl2 but we have Pb rod
E.g. Pbo / PbC2O4 / CaC2O4 / CaCl2
Represented by the following equations:
PbC2O Pb2+ + C2O42-
CaC2O Ca2+ + C2O42-
CaCl Ca2+ + 2Cl-
Increase in [Ca2+] will supress dissociation of CaC2O4, leading to lower availability of C2O42- .
In this case stress of C2O42- on PbC2O4 decreased leading to more dissociation of PbC2O4 producing more Pb2+ and consequently potential developed on Pbo is increased.
EPbo α [Ca2+]
Lead acetae and calcium acetate
supresses the ionization of each other by common ion

9. 4) Redox electrode:
In this type equilibrium is established between oxidant and reductant in the same solution in presence of inert electrode e.g pt wire (i.e it is one phase system electrode).
e.g. Fe3+ / Fe2+ in presence of pt
E25oC = Eo /n log Fe2+ /Fe3+
E25oC α 1/ [red] / [oxid]
E.g. Determination of Fe2+ with Ce4+
Ce4+/Ce3+ E
Fe3+/Fe2+
E.p.
mls of titrant

10. Reference electrodes B) According to the Function:
Electrodes can be classified according to the function into:
1) reference electrodes and 2) indicator electrodes
Reference electrodes
In this type electrode must have known and constant potential, therefore used as a reference to measure the potential of indicator electrode through galvanic cell.
e.g a)Normal hydrogen electrode (N.H.E.),
b) calomel electrode,
c) silver, silver Chloride electrode.
Notice that as the concentration of ions increases the equivalent conductance decreases but the total conductivity of the solution increases.

11. Normal hydrogen electrode (N.H.E.)
1-Half cell presentation: Pt, H2 / H+ (1.0 N) // 2-Type :1st type 3-Electrode reaction: 2H+ + 2e ↔ H2 4- Nernest equation: E25oC = Eo /n log H2 / [H+]2 E25oC = Eo log [H+] E α [H+] in N.H.E [H+] = 1 , log 1 is zero So E25oC = Eo = zero 5-Advantage: It is a primary reference electrode as its potential is zero. 6-Disadvantages: 1- It is difficult to keep H2 gas at one atmosphere during all determinations. 2- it needs periodical re-plating of pt sheet with pt black.
Salt
bridge
(gas its conc is considered 1)
platinum coated with Pt black
[H+] = 1 M

12. The Secondary Reference Electrodes: 1‑ The Calomel Electrode:
1‑Half cell presentation: Hg0/Hg2Cl2 / KCl (saturated, 1 N or 0.1N) //
2- Type :2nd type
3‑ Electrode reaction: Hg e ↔ 2Hg0
4‑Nernst Equation :E25oC = Eo /2 log Hg0 / [Hg22+]2
From the Nernst equation it is clear that the electrode potential depends on the [Hg22+]; which is produced from the dissociation of the very small amount of the sparingly soluble Hg2Cl2.
Hg2Cl2 ↔ 2Hg Cl­
The mercurous ion concentration is controlled
by the solubility product of the salt KspHg2Cl2
KspHg2Cl2 = [Hg22+]2 [Cl‑]2
[Hg22+]2 = KspHg2Cl2 / [Cl‑]2 
E25oC = Eo /2 log [Cl‑]2 / KspHg2Cl2
E α 1/ [Cl-] electrode potential depends on the [Cl-] **
Although maximum potential is obtained on
using 0.1N KCl; however, we usually use
Sat. KCl WHY?
T K

13. 3‑ Electrode reaction: Ag+ + e ↔ Ag0
2- Silver Electrode:
1‑Half cell presentation: Ag0/AgCl /,KCl (saturated, 1 N or 0.1N) //
2- Type :2nd type
3‑ Electrode reaction: Ag+ + e ↔ Ag0
4‑Nernst Equation : E25oC = Eo /1 log Ag0 / [Ag+]
As explained under calomel electrode, the Ag+ ion concentration is controlled the solubility product of the salt KspAgCl
Ksp AgCl = [Ag+] [Cl‑]
[Ag+] = Ksp AgCl / [Cl‑] 
E25oC = Eo /1 log [Cl‑] / Ksp AgCl  
E α 1/ [Cl-]
electrode potential depends on the [Cl-] **
AgCl Ag
Sat KCl solu

14. Indicator electrodes
An indicator electrode is that which its potential is sensitive to the concentration of one of the participant or products of reaction.
Its potential changes rapidly with the change of the particular ion.
It must:
1‑ Give rapid response.
2‑ Its response is reproducible.
Now when it is connected with a reference electrode, the change in e.m.f. of the formed cell will be due to the change of indicator electrode potential, which in terns reflects the change of the ion under investigation.
A Wheatstone bridge is an electrical circuit used to measure an unknown electrical resistance by balancing two legs of a bridge circuit, one leg of which includes the unknown component.
A galvanometer is a type of ammeter: an instrument for detecting and measuring electric current.

15. Indicator electrodes are classified into two classes: 1‑ Electrodes where electron transfer takes place at the electrode surface. i.e. redox reaction occurs (metallic electrodes). 2‑ Electrodes where (charge) ion exchange takes place at a specific membrane surface. i.e. ion selective electrodes or specific ion electrodes.

16. N.B:
1- We must notice that electrodes potential are related to activity.
2-The most important application of this method is the determination of pH.
3- The observed potential of the cell employed during the measurement is:
Eobs = Eref + Ej – Eind
where Eref , Ej and Eind are the potentials of reference electrode, liquid junction and indicator electrode.

17. Liquid junction potential
The disadvantage of the cell with liquid junction is there is a potential associated with the liquid junction, called the liquid junction potential.
A potential is developed at both boundaries of the junction
This liquid junction potential arises from difference in rate of migration of anion and cations of the bridge salt and the electrolytes in the electrodes solutions. This difference results in unequal charge distribution at the boundaries, thus a potential is developed.
It depends upon:
1- Charge mobilities
2- concentration of the ionic species
3- Nature of the solvent on each side of the boundary
4- pH dependent

18. How to minimize liq.junct. Pot.?
The presence of liquid junction potential limit the accuracy of potentiometric measurements.
How to minimize liq.junct. Pot.?
By using a high concentration of a salt, its ions has nearly equal mobility e.g. KCl or KNO3 (K+ = 74, Cl- = 73, NO3- = 76)
Use high concentration of salt for preparation of the bridge to reduce the effect of difference in migration rates of other ions in the electrode solution.
Ecell = Eind - Eref + Ej

19. Indicator Electrodes Used For Different Reactions:
1‑ Electrodes for Redox Reactions:
These are inert electrodes which acquire the potential of the system in which they are dipped.
They are made of platinum or gold, in the form of foils, coils or plates WHY?
to expose a large surface area to the solution thus attain the equilibrium potential rapidly.
It is one phase system electrode and is connected to Ag/AgCl or calomel electrode to measure the e.m.f. produced.

20. 2‑ Electrodes for complexometric Reactions:
We may use:
A- Electrodes of the First type:
For the determination of a cation (metal ion); where the best electrode used for a cation is its elemental form.
e.g. in determination of Cu2+ with EDTA , a copper rod is the indicator electrode, its potential is directly affected by the analyte concentration as indicated from:
Nernest eq: E25oC = Eo /n log 1 / [Cu2+] so E α [Cu2+]
B- Electrodes of the Second type:
For the determination of a mixture of metals
(Cu2+,Cd2+, Bi3+ by EDTA)
We can not dip three different metal rods,
So we use an electrode that is affected by
the titrant instead of the sample.
We use: (Hgo / Hg2y2- / y4-) EHgo α 1/ [Y4-] E
mls of titrant
E.p.
E.p.

21. 3‑ Electrodes for precipitemetric Reactions:
We may use:
A- Electrodes of the First type:
For the determination of a cation (metal ion); where the best electrode used for a cation is its elemental form.
e.g.1- in determination of Ag+ with chloride , a silver rod is the indicator electrode, its potential is directly affected by the analyte concentration as indicated from:
Nernest eq: E25oC = Eo /n log 1/ [Ag+] so
E α [Ag+] E
mls of titrant
e.g.2- in determination of mix of Cl- & Br- & I- with AgNO3
So better to choose an indicator electrode which
responds to the titrant i.e. silver rod.
Before titration: there is no Ag+ ions so E is low
At e.p. :conc of Ag+ ions increases
So E increases sharply E
mls of titrant

22. 3‑ Electrodes for precipitemetric Reactions: We may use: B- Electrodes of the Second type: For the determination of Cl- or Br- or I- with AgNO3 Det. of anion so use e.g. Ag0 electrode: (Ag0 / AgCl / Cl-) EAg0 α 1/ [Cl-] E
mls of titrant