Reference Electrodes: To make a potential measurement, you’ll need at least two electrodes. Those two electrodes are known as the indicator and reference electrodes in potentiometry. The reference electrode must be built so that its composition is fixed and its response is stable over time, with observed variations in measured potential owing only to changes in analyte concentration for the measured potential to be meaningful in this context.

In this article, we will learn about a primary reference electrode and its type, i.e., standard hydrogen electrode and calomel electrode. You will also understand the construction and working of standard hydrogen electrodes and calomel electrodes, the advantages of the reference electrode, etc.

What is Reference Electrode?

The electrode potential of any other electrode that can be measured is called a reference electrode. In other words, the electrode whose half-cell potential is known and is constant and completely insensitive to the composition of the solution is called a reference electrode. The reference electrode can act as both anode or cathode depending upon the nature of other electrodes.

Types of Reference Electrodes

The reference electrodes are classified into two types:

1. Primary reference electrode: The standard hydrogen electrode is called a primary reference electrode.
2. Secondary reference electrode: The electrode whose potential is determined by connecting to the standard hydrogen electrode is called a secondary reference electrode.
   Example: Calomel electrode

What is Electrode Potential?

The electrical difference (electromotive force) set up between the metal and its ion in the solution is electrode potential. The electrode potential is also defined as the tendency of an electrode to lose or gain electrons when it’s in contact with the solution of an ion. The electrode potential is termed oxidation potential. Suppose the oxidation occurs at the electrode concerning the reference electrode. In that case, it is called reduction potentials if the reduction occurs at the electrode concerning the reference electrode like a standard hydrogen electrode, calomel electrode, etc.

In the half cell, the metal electrode is suspended in a solution of one molar concentration at \({\rm{298}}\,{\rm{K}},\) and the electrode potential is called standard electrode potential. It is represented by the symbol \({\rm{E}}^\circ .\)

Standard Hydrogen Electrode

A standard hydrogen electrode is a primary reference electrode used to determine the electrode potential of a cell. The absolute value of the electrode potential of a single electrode cannot be determined because oxidation half-reaction or reduction half-reaction cannot take place alone. It is measured using reference electrodes like standard hydrogen electrodes, calomel electrodes, etc.

Standard hydrogen electrode is set up by passing pure hydrogen gas at one atmospheric pressure in a solution of \({{\rm{H}}^{\rm{ + }}}\) ions of concentration of \({\rm{1 mol/L}}\) in contact with a platinum foil.

The standard hydrogen electrode is represented as: \({\rm{Pt}},\,{{\rm{H}}_2}(\;{\rm{g}})(1\;{\rm{atm}})\mid {{\rm{H}}^ + }(1\;{\rm{mol}}/{\rm{L}})\)

Standard hydrogen electrodes can work both as an anode and as a cathode.

When a standard hydrogen electrode undergoes oxidation in a cell, it acts as an anode. During this process, the hydrogen atom changes into \({{\rm{H}}^{\rm{ + }}}\) ions which go into the solution.

\({{\rm{H}}_{\rm{2}}} \to {\rm{2}}{{\rm{H}}^{\rm{ + }}}{\rm{ + 2}}{{\rm{e}}^{\rm{ – }}}\)

When a standard hydrogen electrode undergoes a reduction in a cell, it acts as a cathode. During this process, H⁺ ions from the solution change into hydrogen gas.

\({\rm{2}}{{\rm{H}}^{\rm{ + }}}{\rm{ + 2}}{{\rm{e}}^{\rm{ – }}} \to {{\rm{H}}_{\rm{2}}}\)

Thus, the electrode is reversible with respect to \({{\rm{H}}^{\rm{ + }}}\) ions.

The value of electrode potential of the standard hydrogen electrode is assumed to be zero at \({\rm{298}}\,{\rm{K}}.\)

Secondary Electrode: Calomel Electrode

Calomel is the most common secondary electrode because its potential remains constant over a long period of time. It contains mercury in contact with calomel, i.e., \({\rm{H}}{{\rm{g}}_2}{\rm{C}}{{\rm{l}}_2}\). A solution containing chloride ions, like a solution of \({\rm{KCl}}\), is poured over.

Calomel electrode is written as,

When is acts as a cathode: \({\rm{Cl}}({\rm{aq}})\left| {{\rm{H}}{{\rm{g}}_2}{\rm{C}}{{\rm{l}}_2}(\;{\rm{s}})} \right|{\rm{Hg}}({\rm{l}})\)

When is acts as an anode: \(\left. {{\rm{Hg}}\left( {\rm{I}} \right)} \right|\left. {{\rm{H}}{{\rm{g}}_2}{\rm{C}}{{\rm{l}}_2}\left( {\rm{s}} \right)} \right|{\rm{Cl}}\left( {{\rm{aq}}} \right)\)

Electrode reaction of calomel electrode is

Saturated Calomel Electrode (SCE)

Generally, \(0.1{\rm{M}},\,1{\rm{M}}\), or a saturated solution of \({\rm{KCl}}\) is used in a calomel electrode. The calomel electrode in which the saturated solution of \({\rm{KCl}}\) is used is called the saturated calomel electrode. The emf of the calomel electrode depends upon the concentration of \({\rm{C}}{{\rm{l}}^ – }\) ions in the solution.

Measurement of Electrode Potential

To determine the electrode potential of an electrode, a cell is set up using the electrode as one of the electrodes and the second electrode as the reference electrode. The emf of the cell is measured. The emf of the cell is the sum of the oxidation potential and reduction potential of the cell.

The direction of the flow of current indicates whether the oxidation takes place or reduction takes place on the electrode under the investigation with respect to the reference electrode. According to this, the electrode potential is termed oxidation potential or reduction potential.

Reading on the voltmeter will be obtained only if the positive terminal of the voltmeter has been connected to the positive electrode, i.e., on which reduction occurs and the negative terminal to the negative electrode, i.e., on which oxidation occurs.

Reference Electrodes in Potentiometry

Potentiometry is used to find the concentration of a solute in a solution. In this method, the potential between two electrodes is measured using a high impedance voltmeter. One electrode is the reference electrode, and the other is the test electrode. The most common reference electrode used in potentiometry is the saturated calomel electrode \(\left( {{\rm{SCE}}} \right)\).  

The concentration of the test solution is calculated using the Nernst equation.

\({{\rm{E}}_{{\rm{cell}}}}{\rm{ = }}{{\rm{E}}^{\rm{o}}}_{{\rm{cell}}}{\rm{ – }}\frac{{{\rm{RT}}}}{{{\rm{nF}}}}{\rm{log}}\left( {\rm{Q}} \right)\)

Applications of Reference Electrodes

Application of standard hydrogen electrode

1. It can be used to determine a range of \({\rm{pH}}\) values of solutions.
2. The electrode potential of the standard hydrogen electrode is zero. Therefore, the electrode potential of another electrode can be easily calculated.
   Application of calomel electrode
3. It is used to measure \({\rm{pH}}\) and electrical potential of the solution.

Summary

You can recall the definition of the reference electrode and its types by reading this article. You can also explain the construction and working of standard hydrogen electrodes and calomel electrodes. Apart from this, you will be able to grasp the significance of reference electrodes too.

FAQs on Reference Electrodes

Q.1: Why do we need reference electrodes?
Ans: A reference electrode is needed to find the electrode potential of any other electrode as the potential of the reference electrode is known.

Q.2: What is a reference electrode in potentiometry?
Ans: In potentiometry, the potential between two electrodes is measured using a high impedance voltmeter, in which one of the electrodes is a reference electrode and the other one is the test electrode. The most common reference electrode used in potentiometry is the saturated calomel electrode \(\left( {{\rm{SCE}}} \right)\).  

Q.3: What is an \({\rm{SCE}}\) reference electrode?
Ans: Calomel electrode contain mercury in contact with calomel, i.e., \({\rm{H}}{{\rm{g}}_2}{\rm{C}}{{\rm{l}}_2}\). A solution containing chloride ions, like a solution of \({\rm{KCl}}\), is poured over. A Calomel electrode in which a saturated solution of \({\rm{KCl}}\) is used is called \({\rm{SCE}}\) (saturated calomel electrode).

Q.4: What are secondary reference electrodes?
Ans: The electrode whose potential is determined by connecting to the standard hydrogen electrode is called a secondary reference electrode.
Example: Calomel electrode

Q.5: What is the function of reference electrodes?
Ans: Reference electrode gives constant potential, which is used to calculate the potential of another half-cell.

Q.6: Which reference electrode is used to measure the electrode potential of other electrodes?
Ans: Standard Hydrogen Electrode is used to measure the electrode potential of other electrodes.

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