Potentiometry - principle, theory, Slides of Pharmaceutical Analysis

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POTENTIOMETRY

Introduction to Potentiometry

• (^) 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.

ELECTRIC POTENTIAL

• (^) The amount of work needed to move a unit charge from a reference point to a specific point against an electric field.

NERNST EQUATION

• (^) The Nernst Equation allows us to calculate the voltage produced by any electrochemical cell given Eo values for its electrodes and the concentrations of reactants and products.
• (^) The potential of a metal electrode at 25°C immersed into a solution of its ions if given by
• (^) E = E 0 + (0.0592 / n) logc where
• (^) E 0 - std potential of the metal
• (^) n – valency of ions
• (^) c – concentration of ions

• (^) For any Potentiometric measurement
• (^) We must have:

1. Reference electrode

2. Indicator electrode.

3. Potentiometer

4. Salt bridge to connect the two electrode

solutions and complete the circuit.

Electrochemical cell

• (^) An electrochemical cell is an apparatus consisting of electrodes that dip into an electrolyte and in which a chemical reaction either produces electricity or is caused by an electric current.

Three distinct charge transfer processes are described

1. Electrons move in electrodes and wires from zinc electrode to

copper electrode.

2. Ions move in solutions:

a. In solution on the left, zinc ions move away from the electrode

and sulfate ions move towards it.

b. In solution on the right, copper ions move towards the

electrode and negatively charged ions (sulfate) away from it.

c. In salt bridge positive ions move right and negative ions left.

3. On the surfaces of electrodes electrons are transferred to ions

or vice versa:

a. Zinc electrode dissolves: + − Zn → Zn2 + 2e

b. Metallic copper is deposited on the electrode surface:

Cu2+ 2e + → Cu ↓

• (^) In a potentiometric measurement an indicator electrode responds to changes in emf or pH of a solution or effective concentration of the analyte.
• (^) A potential or voltage develops at the interface of electrode and analyte solution is measured with relative to a reference electrode.
• (^) This measured potential will be proportional to concentration of analyte in the sample.

Saturated calomel electrode

• (^) Outer sleeve filled with sat.KCl
• (^) Inner jacket contain a wire contact with paste of Hg, Hg 2 Cl 2 and KCl.
• (^) Space b/w inner and outer tube filled with sat. or 1N OR 0.1N KCl soln.
• (^) Potential of electrode depends upon the concn of KCl and temp.
• (^) Electrode reaction in calomel half-cell
• Hg 2 Cl 2 (s) + 2e = 2Hg(l) + 2Cl - 20°C 25°C S.KCl 250 mV 246 mV 1N 286 mV 285 mV 0.1N 338 mV 338 mV

Hydrogen electrode

• (^) Used as reference as well as indicator electrode.
• (^) Reference-when dipped in standard acid soln.
• (^) Indicator-when dipped in sample soln.
• (^) Purity of H2 and pressure of hydrogen affect the potential of the electrode.
• (^) Potential of other electrodes can be measured.

• (^) Defined as the potential that is developed

between H2 gas adsorbed on the Pt metal and

H+ of the solution.

• (^) It is used for determination of electrode

potential of metal electrode system

• (^) determination of pH of the solution
• (^) Pt,H2 (g, 1atm) | H+ (aq, a = 1.00) |
• (^) |2 H+ (aq) + 2 e ─ ↔ H2 (g)