Potentiometric Titration, Slides of Pharmaceutical Analysis

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Potentiometric

Titrations

Reference Book:

**1. Pharmaceutical Chemistry by Leslie G. Chatten

2. Instrumental Analysis by Skoog**

• Determination of the amount or concentration of an

compound based upon the determination of potential

value of the titration solution.

• During titration process, the active ingredient being

determined decrease in concentration; this decrease in

concentration results in change of the potential value of

the solution measured by an electrode (indicating)

immersed in the titration solution.

• At the end point of the titration, when the titrant is no

longer being used up, the addition of small amount of

the titrant results in sudden sharp change in the

potential developed at the electrode.

Potentiometric titration

An arrangement in which transformation of chemical energy into electrical takes place is known as Galvanic cell or electrochemical cell, e.g Daniell cell.

• Daniell cell consists of a zinc rod dipped into a solution of zinc sulphate and a copper rod dipped into a solution of copper sulfate.
• The two solutions are separated by a porous partition which permits the solution to come in contact with one another.
• When two electrodes are connected by a wire, electric current passes through the wire. Electrochemical Cells

Reaction at zinc electrode (oxidation): Zn Zn ++

• 2 e Reaction at copper electrode (reduction): Cu ++
• 2 e Cu Overall reaction takes place: Zn + CuSO 4 = ZnSO 4
• Cu Or Zn + Cu ++ = Zn ++
• Cu

• As long as the electrodes are connected by a wire electrons will flow and current will pass until either Zn or Cu ++ is exhausted. Half-cell: A cell consists of two half-cells. Electrode: It is a material (metallic rod/bar/strips) which conducts electrons into and out of a solution. Indicating electrode & Reference electrode

Half- cells and its classification:

• A cell consists of two half-cells.
• Electrode + solution Classification:

1. A Metal in equilibrium with its ions
2. A metal in equilibrium with a saturated solution of a slightly soluble salt
3. Two soluble species in equilibrium at an inert electrode Half- cell Half- cell

1. Half-cell: Metal in equilibrium with its ions: M | M a + ; Zn | Zn 2+ ; Cu 2+ | Cu etc. Calculate the potential developed at an electrode consisting of a cadmium electrode in a solution of cadmium ions at an activity of 0. 05 gram-ion/liter. (E 0 of Cadmium is - 0. 403 V). Cd

(0.05)

• 2e

= Cd E = E 0

• log a M

n = - 0.441 V

Calculate the potential of two half-cells of the following system: Sn | Sn 2+ (a=4x

- 2 ) || Cu 2+ (a=2x - 3 ) | Cu [Std red Sn= 0.136V, Cu = - 0.337 V]

• 0.51 V

2. Half-cell: A metal in equilibrium with a saturated solution of a slightly soluble salt:

• This type of half-cell is widely used as reference electrode.
• Silver/silver chloride electrode, calomel electrode.
• A silver-silver chloride is prepared in such a way that silver and silver chloride are in intimate contact with each other and with chloride ion. Ag | AgCl | Cl -
• Reaction at the electrode is written as- AgCl + e = Ag + Cl - (a=1)

• A reference is an electrode that has the half-cell potential known, constant, and completely insensitive to the composition of the solution under study.
• In conjunction with this reference is the indicating or working electrode , whose response depends upon the analyte concentration. Electrodes

 Is reversible and obeys the Nernst equation  Exhibits a potential that is constant with time  Returns to its original potential after being subjected to small currents Ideal Reference Electrode

• A number of holes around the glass tube just below the

surface of the solution permit the hydrogen gas to bubble

out.

• The hydrogen gas bubbling out disturbs the surface of the

liquid so that the platinum surface is alternately covered

with hydrogen gas and solution containing H

• The SHE develops a potential which is considered

arbitrarily as zero.

• The standard hydrogen electrode is written

diagrammatically:

Pt | H

2 (p= 1 atm)

| H

(a= 1 )

• The reduction reaction at the electrode is written:

2 H

(a=1)

+ 2e = H

2 (p=1 atm)

Advantages of SHE:

• It has no error in strongly alkaline solution.
• Can be used for the determination of electrode potential of metal electrode system.
• Also can be used for the determination of pH of the solution. Disadvantages of SHE:
• It requires that the platinum black surface of the electrode be renewed daily.
• It requires a long time to come to equilibrium.
• Dissolved gas such as ammonia, carbon dioxide etc. interferes with its use.
• The surface of the platinum is usually poisoned with colloidal material or a variety of other substances which might be present.

• The standard calomel electrode is written diagrammatically: Hg | Hg 2 Cl 2 | Cl -
• The reduction reaction at the electrode is written as- Hg 2 Cl 2 + 2 e = 2 Hg + 2 Cl -
• The Nernst equation for the electrode can be written as:
• The potential developed by the calomel electrode depends upon the concentration of the potassium chloride solution. (as shown in the equation above)
• The most common concentration employed is a saturated solution.
• Potential of a saturated calomel electrode containing saturated KCl is + 0. 244 V.

E = E

0

• log

n a 2 Cl-

Advantages of SCE:

• Relatively cheap.
• Can be readily prepared.
• Gives a potential which is reproducible in relation to the SHE.