REFERENCEELECTRODES

PRESENTED BY- TAKEEN KHURSHIDB.Sc Chemistry 6TH SEMESTER

REFERENCE ELECTRODE

An ideal reference electrode is one that maintains a constant potential irrespective of the amount of current (if any) that is passed through it.

Voltammetric methods are those in which current passing in an electrochemical cell is measured as a function of the potential applied to the working electrode. Potential, by definition, is not something that can be directly measured. Rather, the measurement of applied potential requires that a reference point first be established, and individual potentials be measured relative to that reference point. This is accomplished by placing a second electrode, called the reference electrode, in the cell and measuring potential as the energy difference between the two electrodes, called the reference electrode.

Reference electrodes should be constructed using half-cell components that are stable over time and with changing temperature, present at well-defined values of activity. They should possess fixed, reproducible electrode potentials.

The reference half-cell with which most of us are familiar is the standard hydrogen electrode (SHE), composed of an inert solid like platinum on which hydrogen gas is adsorbed, immersed in a solution containing hydrogen ions at unit activity. The half-cell reaction for the SHE is given by

H+ + e- → ½ H2 (g) ( 1atm)

The potential of the standard hydrogen electrode is arbitrarily assigned the value of zero (0.000) volts at 250 C. When standard hydrogen electrode is coupled with another electrode, the EMF of the cell thus formed will evidently be equal to the potential of that electrode to which it is coupled. The potentials obtained in this way are called relative potentials, or electrode potentials on the hydrogen scale.

Practical application of the SHE is limited by the difficulties in preparing solutions containing H+ at unit activity and maintaining unit activity for H2 (g) in the half-cell. Most experiments carried out in aqueous solutions utilize one of two other common reference half cells – the saturated calomel electrode (SCE) or the silver-silver chloride electrode (Ag/AgCl).

The Calomel ElectrodeCalomel electrode is most widely used as secondary reference electrode. The standard calomel electrode (SCE) comprises of wide glass – tube fitted with narrow side tubes. Pure mercury is placed at the bottom of the wide tube and covered with a paste of mercurous chloride (calomel). A solution of KCl is introduced above the paste. A saturated KCl solution may also be used. A Pt wire sealed into a glass tube is used to make electrical contact. The side tube containing KCl solution provides the salt bridge, which connects the electrode to any other electrode. The calomel electrode is represented as

Cl– (1 M) Hg2Cl2 , Hg And the half-cell reaction is written as

Hg2Cl2 + 2e– 2Hg + 2Cl–

Saturated Calomel ElectrodeElectrode is mercury coated with calomel (Hg2Cl2)

Electrolyte is a solution of potassium chloride.

Silver/Silver Chloride Electrode (Ag/AgCl)

The silver/silver chloride reference electrode is composed of a silver wire (Ag) that has been coated with a layer of solid silver chloride (AgCl), immersed in a solution that is saturated with KCl and AgCl. The half cell reaction is

AgCl (s) + e - ↔Ag (s) + Cl- (sat’d)

Both the SCE and the Ag/AgCl reference electrodes offer stable half-cell potentials that do not change over time

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