POTENTIOMETRICTITRATION

Siham AbdounMsc., PhD

1. Introduction:

Potentiometric method include two type of

measurement these are ;

1. Direct measurement of an electrode potential

from which concentration of an active ion may

be found

2. Change of E.M.F. of an electrode cell brought

about by the addition of titrant

Both methods are based on quantitative

measurement of E.M.F. of cell is given by

E cell = E reference + E indicator +E junction

As the reference electrode potential is

independent of solution and junction potential is

constant so the cell potential is measure of

indicator electrode potential and can give

information on the nature and concentration of

substance under test.

In the potentiometric titration the titration

reaction is followed by measurement of

concentration of one or more species

potentiometrically. The beaker or flask

becomes one of the half cells and the

reference electrode is the other half cell.

There is a different between titration reaction

and cell reaction; in titration reaction the

reactant and products be in the same half cell

and the titration reaction always at

equilibrium while in cell reaction is not at

equilibrium.

In potentiometric titration the change in

electrode potential upon the addition of titrant

are noted by the volume of titrant added. At the

end point the rate of change of potential is

maximum

The potentiometric end point has been applied

to all types of chemical reaction. It can be used

with colored or opaque solution

2. Instrumentations:

There are three types of instrumentations

which are used for measurement of

potentials these are:

1. Non – electronic instruments

2. Electronic instruments

3. Automatic instruments

1. Non – electronic instruments

A potentiometer for titrations can be made from

simple instrument of battery for current supply,

two dry cell, a resistance and voltmeter; the

instrument can be operated by dipping the

electrodes in the sample solution and record the

voltmeter reading.

2. Electronic instruments

These instruments have many advantages over

non- electronic instruments

3. Automatic instruments

The use of manual instrument to locate the

end point and to draw a titration curve is

time consuming and boring job; so

automatic instrument for recording and

performing titration curve provides a logical

solution

3. Types of Potentiometric titration:

Potentiometric titrations may be applied to

different type of reactions of these are;

acid – base, oxidation reduction,

precipitation and complexmetirc

1. Acid – Base titrations:

The neutralization of acid or base is always

accompanied by the changes the

concentration of H+ and OH- ions.

In these reactions hydrogen electrode is used

as indicator electrode and N- calomel

electrode as a reference electrode.

A known volume of the acid titrant is kept in a

beaker with continuous stir; the hydrogen and

N- calomel electrode are connected by the salt

bridges and connected to a potentiometer which

record the EMF of the solution, into the beaker

the after the addition of base from burette the

values of EMF are plotted against volume of

titrant added and a curve are obtained.

The potential of a hydrogen electrode is given by :

E+ E⁰-0.0591 log a H+

Where E⁰ is standard electrode potential and pH is - logaH+

E+ E⁰+ 0.0591 pH

As the standard electrode potential is constant so the

cell potential or EMF is proportional to the change of

pH during the reaction.

The point where E.M.F increased rapidly is the end

point.

Amore sensitive and precise method for measure

end point is to plot the slope of curve against

volume as the slope is maximum at equivalence

point, the maximum value give the end point

2. Complexmetric titration

A metal electrode is used whose ions are

involved in the complex formation, example

silver electrode is used to measure cyanide ion

with standard solution of silver

Ag++ 2CN- (Ag (CN)2)-

K= (Ag+)(CN-)2

(Ag(CN)-2)

In this case, solid silver cyanide begins to get

precipitated soon after the equivalence point.

The further addition of silver neither changes

the concentration of the complex nor changes

the silver ion to any extent, so that the curve

has an almost horizontal portion shortly after

the equivalence point

In many Complexmetric reactions the situation

cannot be handled so easily because more than

one complex is formed. Thus, the reactions in

the case of mercuric ion with cyanide are:

Hg2++3CN- Hg (CN)3-

Hg2++4CN- Hg (CN)42-

However, these situations have became widely

used because of the discovery of the metal

chelating agents such as EDTA

3. Oxidation-reduction titrations

Redox reactions can be followed by an inert

indicator electrode. The electrode assumes a

potential proportional to the logarithm of the

concentration ratio of two oxidation states of

the reactant or the titrant whichever is

capable of properly poising the substance

being oxidized to substance being reduced.

For example ,

Ca4+ + Fe2+ Ce3+ + Fe3+

It is generally considered that such a

reaction consists essentially of two half

reactions whose standard potentials may be

used to calculate the standard potential of

the reactions.

Fe+ Fe3+ + e, Eo = -0.67 V ..……. (i)

Ce4+ e Ce3+ , Eo = +1.61 V …….. (ii)

Ce4+ + Fe2+ Fe3++ Ce3+, Eo = +0.85 V… (iii)

The equilibrium constant , K, of any reaction

may be calculated from the following formula :

Eo = log10 K ……. (iv)

Where Eo is the number of equivalent of

electricity associated with one molar unit of

reaction .

If an acidic ferrous solution is titrated with a

standard ceric solution at 25o C, the potential of a

platinum electrode in contact with the solution will

be given by either of the following equations:

E=Eo ce4+/ ce3+ - 0.0591 log10 [ce3+] …. (v)

1 [ce4+]

E=Eo Fe3+/ Fe2+ - 0.0591 log10 [Fe3+] … (vi)

1 [Fe2+]

It would be more convenient to use the

equation (vi) before the equivalence point, as

the right hand term of this equation could be

easily found from the known extent of the

titration . if equation (v) is used , then the

[Ce3+] / Ce4 ) ratio has to be calculated by

means of the equilibrium constant, After the

equivalence point, calculations are done by

means of equation (v).

From equation (vi) it is evident that the

potential that the potential at the start of the

titration should be—co, because Fe3+ ions are

the only ions present and there are no Fe3+

ions.

At the mid-point of the titrations, where

[Fe2+]=[Fe3+], equation (vi) because :

E=Eo Fe3+/ Fe2+ ……(vii)

At the equivalence point, the concentration of

unchanged ferrous ions will be equal the

concentration of the unchanged ceric ions.

Similarly , the concentration of cerous ions will

be equal to the concentration of ferric ions.

Thus, it can be concluded that;

[Fe2+] =[Ce4+] ……(viii)

[Fe3+] [Ce3+]

Now, k = [Fe3+][Ce3+] ……(ix)

[Fe2+][Ce4+]

At equivalence point,

[Fe3+][Ce3+]= k……(x)

[Fe2+][Ce4+]

On combining equations (v) and (vi) with

equation (x) , we get

Eep = EoCe4+ / Ce3+ - log10 K …..(xi)

Eep = EoFe

3+ / Fe2+ + log10 K ……(xii)

On adding, equations (xii), we get

Eep = EoFe

3+/ce3++EoFe

3+/Fe2+

2

where Eep= End point potential .

Oxidation-reduction titration may be used in

procedures such as monitoring of cyanide

wasters from metal plating industries or

chlorine compounds in bleach compounds

manufacturing, and the used of these bleach

compounds in paper man fracturing. They are

also extensively used in water pollution,

sewage treatment, agricultural and biochemical

studies.

4. Precipitation titrations. Any precipitation

titration that involves insoluble salts of metals

such as mercury, silver, lead and copper may be

followed potentiometrically . The indicator

electrode may be made of the metal involved in

the reaction or may be an electrode whose

potential is governed by the concentration of the

anion being precipitated

The magnitude of the potential change at the

end point depends on the solubility of the

substance being precipitated as well as the

concentration involved. The titration of

chloride ions with a standard solution of silver

nitrate using a silver metal indicator electrode

is an example of a precipitation titration.

The other electrode to complete the cell is

unimportant, provided that it is a true reference

electrode, i.e., it maintains a constant potential.

In the above case it will be assumed that the

normal hydrogen electrode (N H E.) is used and

this assumption is convenient because standard

potential may be used directly. The potential of

silver electrode will be governed by the

appropriate Nernst equation :

EAg+/ Ag = Eo

Ag+/ Ag + log10 [Ag+] …..(i)

As soon as enough silver nitrate to precipitate Cl

as AgCl has been added, the following equilibrium

is established,

AgCl Ag+ + Cl- ………….(ii)

The equilibrium constant for the above reaction is

KAgCl = [Ag+][Cl-]=10-10 …………..(iii)

If 0.1 N sodium chloride is titrated against 0.1 N

silver nitrate, the silver ion concentration may

be considered to be 10-9 N as soon as few drops

of silver nitrate have been added. Equation (i)

can be used to calculate the indicator electrode

potential.

EAg+/Ag=0.08 V + 0.0551 log10 10-9 = 0.2681

…….. (iv)

Similarly, half wave through the titration will be

when the chloride ions concentration has been

reduced to 0.033 N.

EAg+/Ag=0.08 V+0.0591 log 10 (3 10-9) = 0.30 V

At the equivalence point, [Ag+]=[Cl-] = 10-5N

EAg+/Ag=0.08 V+0.0591 log 10 10-5 = 0.50 V

4. Non – aqueous titration

The potentiometric method has been found to

be useful for carrying out titrations in non-

aqueous solvents. The ordinary glass- calomel

electrode system can be used

Generally the millivolt scale of the

potentiometer rather than the pH scale should

be employed because the potential in non-

aqueous titration may exceed the pH scale.

Advantages of Potentiometric titrations

over 'classical' visual indicator methods are:

1. Can be used for coloured, turbid or

fluorescent analyte solution.

2. Can be used if there is no suitable indicator

or the colour change is difficult to ascertain.

3. Can be used in the titration of polyprotic

acids, mixtures of acids, mixtures of bases or

mixtures of halides. loured, turbid or

fluorescent analyte solution

4. The apparatus required is inexpensive, reliable

and readily available.

5. It is easy to interpret the titration curve.