Potentiometry: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 electrodeindicator electrode).

The potential of the indicator electrode is measured relative to reference electrode reference electrode possessing constant potential .

The concentration of the ion is determined from Nernest Nernest equationequation.E25°C= Eo - 0.059/n log 1/ [Mn+]

The two electrodes form the two half cells of electrochemical cell , the electromotive force (e.m.f.) 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.

1‑Galvanic Cell (voltaic cell):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: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.

ClaClassification of the electrodesssification of the electrodesThey are classified according toThey are classified according to

Type Function

First type

Second type

Third type

Redox type

IndicatorReference

1-Normal hydrogen

2-Ag/AgCl

3-Calomel

1- Redox reaction

2- Complexometric

reaction3-

Preceptimetric reaction

4- Acid base reaction

Mode of action

Electron transfer Ion- Selective

Electrodes used in potentiometric measurements:Electrodes used in potentiometric measurements:Classification of electrodes:Classification of electrodes:

A) According to the type:A) According to the type:1) First type (kind or order):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+ + neE25

oC = Eo - 0.059/n log 1 / [Mn+]

From Nernest equation E α [Mn+]e.g. Titration of Cu2+ with EDTAAt 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 EDTAIt is used for any metal except Fe, Cr, Co, Ni WHY?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

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E.p.

2) Electrode of Second type: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.

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 ‑ 0.059 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

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3) Electrode of Third type: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 / CaCl2Represented by the following equations:

PbC2O4 Pb2+ + C2O42-

CaC2O4 Ca2+ + C2O42-

CaCl2 Ca2+ + 2Cl-

Increase in [Ca2+] will supress dissociation of CaC2O4, leading to lower availability of C2O4

2- .

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

4) Redox electrode: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

E25o

C = Eo - 0.059/n log Fe2+ /Fe3+

E25o

C α 1/ [red] / [oxid]

E.g. Determination of Fe2+ with Ce4+

E

mls of titrant

Fe3+/Fe2+

Ce4+/Ce3+

E.p.

B) According to the Function:B) According to the Function:Electrodes can be classified according to the function into:

1) reference electrodes and 2) indicator electrodes

Reference electrodesReference electrodesIn 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.

1-Half cell presentation: Pt, H2 / H+ (1.0 N) // 2-Type :1st type3-Electrode reaction: 2H+ + 2e ↔ H2 4- Nernest equation:

E25o

C = Eo - 0.059/n log H2 / [H+]2

E25oC = Eo + 0.059 log [H+]

E α [H+] in N.H.E [H+] = 1 , log 1 is zeroSo E25

oC = 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.

A)A) Normal hydrogen electrode (N.H.E.)Normal hydrogen electrode (N.H.E.)

platinum coated with Pt black

Salt bridge

[H+] = 1 M

(gas its conc is considered 1)

The Secondary Reference Electrodes:The Secondary Reference Electrodes:1‑ The Calomel Electrode:1‑ The Calomel Electrode:1‑Half cell presentation: Hg0/Hg2Cl2 / KCl (saturated, 1 N or 0.1N) //

2- Type :2nd type

3‑ Electrode reaction: Hg22+ + 2e ↔ 2Hg0

4‑Nernst Equation :E25o

C = Eo - 0.059/2 log Hg0 / [Hg22+]2

From the Nernst equation it is clear that the electrode potential depends on the [Hg2

2+]; which is produced from the dissociation of the very small amount of the sparingly soluble Hg2Cl2.

Hg2Cl2 ↔ 2Hg22+ + 2 Cl

The mercurous ion concentration is controlled

by the solubility product of the salt KspHg2Cl2KspHg2Cl2 = [Hg2

2+]2 [Cl‑]2

[Hg22+]2 = KspHg2Cl2 / [Cl‑]2 

E25o

C = Eo - 0.059/2 log [Cl‑]2 / KspHg2Cl2

E α 1/ [Cl-] electrode potential depends on the [Cl-] **Although maximum potential is obtained onusing 0.1N KCl; however, we usually use Sat. KCl WHY?

2- Silver Electrode:2- Silver Electrode:1‑Half cell presentation: Ag0/AgCl /,KCl (saturated, 1 N or 0.1N) //2- Type :2nd type3‑ Electrode reaction: Ag+ + e ↔ Ag0

4‑Nernst Equation : E25o

C = Eo - 0.059/1 log Ag0 / [Ag+]

As explained under calomel electrode, the Ag+ ion concentration is controlled the solubility product of the salt KspAgClKsp AgCl = [Ag+] [Cl‑][Ag+] = Ksp AgCl / [Cl‑] E25

oC = Eo - 0.059/1 log [Cl‑] / Ksp AgCl  

E α 1/ [Cl-] electrode potential depends on the [Cl-] **

Sat KCl solu

AgClAg

Indicator electrodesIndicator electrodesAn 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.

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.

N.B: 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:

EEobsobs = E = Erefref + E + Ejj – E – Eindind

where Eref , Ej and Eind are the potentials of reference electrode, liquid junction and indicator electrode.

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

Liquid junction potential

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

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.

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: E25o

C = Eo - 0.059/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.p.

E

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E.p.

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

mls of titrant

E

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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: E25o

C = Eo - 0.059/n log 1/ [Ag+] so E α [Ag+]

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 increasesSo E increases sharply

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