Potentiometry and potentiometric measurements

potentiometer• A device for measuring the potential of an

electrochemical cell without drawing a current or altering the cell’s composition.

Potentiometric measurements:Potentiometric measurements are made using a

potentiometer to determine the difference in potential between a working (an indicator) electrode and a counter (a reference) electrode.

- Cathode is the working/indicator electrode. (right half-cell)

- Anode is the counter/reference electrode. (left half-cell)

Ecell = Ec ─ EaWhere : Ec is the reduction potential at the

cathode. : Ea is the reduction potential at the

anode.The role of the counter electrode is reduced to

that supplying a reference potential thus the counter electrode is called the reference electrode.

Indicator Electrode: electrode that responds to analyte and donates/accepts electrons

Reference Electrode: second ½ cell at a constant potential

Cell voltage is difference between the indicator and reference electrode

PotentiometryUse of Electrodes to Measure Voltages that Provide

Chemical Information.– Various electrodes have been designed to respond

selectively to specific analytes

Indicator Electrodes

1.) Two Broad Classes of Indicator ElectrodesMetal Electrodes

Develop an electric potential in response to a redox reaction at the metal surface

Ion-selective ElectrodesSelectively bind one type of ion to a membrane to generate an electric potential

Metallic indicator electrodes:

1- Electrodes of the first kind:An electrode of this type is a metal in contact with a solution containing

its cation. The most common ones:a- Silver electrode (dipping in a solution of AgNO3) Ag+ + e ↔ Agb- Copper electrode: Cu+2 + 2e ↔ Cuc- Zn electrode: Zn+2 + 2e ↔ Zn

2- Electrode of the second kind:Electrode of this kind is a metal wire that coated with one of its salts

precipitate. A common example is silver electrode and AgCl as its salt precipitate.

3- Redox electrode:An inert electrode that serves as a source of sink for electrons for

redox half reaction, or in another words; an inert metal is in contact with a solution containing the soluble oxidized and reduced forms of the redox half-reaction. The inert metal is usually is platinum (Pt).

Ion-selective ElectrodespH Measurement with a Glass Electrode

• Glass electrode is most common ion-selective electrode• Combination electrode incorporates both glass and reference electrode in

one body

Ag(s)|AgCl(s)|Cl-(aq)||H+(aq,outside) H+(aq,inside),Cl-(aq)|AgCl(s)|Ag(s)

Outer referenceelectrode

[H+] outside(analyte solution)

[H+] inside Inner referenceelectrode

Glass membraneSelectively binds H+

Electric potential is generated by [H+] difference across glass membrane

The glass pH electrode:Advantages over other electrodes for pH measurements:• Its potential is essentially not affected by the presence of

oxidizing or reducing agents.• It operates over a wide pH range.• It responds fast and functions well in physiological

systems. Principle: For measurement, only the bulb needs to be submerged.

There is an internal reference electrode and electrolyte (Ag| AgC||Cl─) for making electrical contact with the glass membrane, its potential is necessarily constant and is set by the concentration of HCl.

Theory of the glass membrane potential:The pH electrode functions as a result of ion exchange on the surface

of a hydrated layer. The membrane of a pH glass electrode consists of chemically bonded Na2O and SiO2. For the electrode to become operative, it must be soaked in water. During this process, the outer surface of the membrane becomes hydrated. When it is so, the sodium ions are exchanged for protons in the solution:

SiO─ Na+ (solid) + H+ (solution) ↔ SiO─H+ (solid) + Na+ (solution)

The protons are free to move and exchange with other ions.

Charge is slowly carried by migration of Na+

across glass membrane

Potential is determined by external [H+]