Potentiometric Electrodes: Types, Advantages, Disadvantages

Potentiometric electrodes are used in potentiometry. The necessary equipment for potentiometry consists of a potential measurement instrument, a reference electrode, and an indication electrode.

Table of Contents

Interesting Science Videos

Types of Potentiometric Electrodes

different types of electrodes used in potentiometry are:

Indicator electrodes

The ideal indicator electrode should react quickly to changes in ion concentration as well as when other ions present in the sample matrix are present. Consequently, a perfect indicator electrode is selective, repeatable, and functional for a long period. The ion-selective electrode is the most effective indicator electrode in potentiometry. There are mainly two types of indicator electrodes. They are:

Metallic indicator electrode

These electrodes develop electric potential in response to redox reactions on the metal surface. It responds to a redox reaction at the metal surface and does not participate in many chemical reactions (Inert). It simply transmits e- to or from a reactive species in solution and works best when its surface is large and clean. Au, Pt, Ag, Cu, Zn, Cd, and Hg can be used as indicator electrodes.

These are mainly classified into four types of electrodes used in potentiometry.

1. The first type of electrode

A simple metal electrode of the first type is submerged in a solution that contains its ion. These electrodes are reversible concerning their metal ion, implying that they are in direct equilibrium with the solution.

e.g., if we place a copper metal in a solution containing metal ion Cu2+ then,

Half cell reaction is: Cu²⁺ + 2e- → Cu (s)

Electrode potential by the activity Of Cu²⁺ is

E _ind= E⁰ _Cu–(0.0592/2) log a _Cu(s) / a _Cu2+

The first type of electrode is not common because…

Many metal electrodes dissolve in the presence of acids, therefore they can only be used in neutral or basic solutions.
Easily oxidized; only usable after deaeration to eliminate oxygen from analyte solutions
Some metals don’t offer reproducible potentials.
Metallic indicator electrodes are not very selective and react to both their cations and other cations that can be decreased more quickly.

Limitation:

Interferes with cu+2 in a less selective manner than Ag+.
May depend on pH.
Quickly oxidized; deaeration necessary.
An inconsistent response.

2. Second kind electrode

The activity of the metal ion or anion employed in the coating can be directly determined using this kind of electrode.e.g., if we use a silver electrode for chloride ion determination.

Half cell reaction is

AgCl(s) + e- Ag(s) + Cl-

Electrode potential is E, nd = E^O — (0.0592/1) log a _{Ag (s)}a_cr/a_AgCI(s)

3. Third kind electrode:

It is a metal electrode assembly, and the equilibrium potential depends on the concentration of a cation present in the solution that is distinct from the cation of the electrode metal.

a mercury electrode, for instance, is used to measure the pH of calcium-containing liquids. A new equilibrium is created if a little amount of a solution containing the calcium EDTA complex is also added.

Equilibrium reaction is

CaY^2- Ca²⁺ + Y ^4-

The potential of a mercury electrode in this solution is

E_ind = K – (0 05920) log . K_fa _CaY^2- /a_ca²⁺

4. Metallic redox indicator

Inert metal electrodes (Pt, Au, Pd) are frequently used as indicator electrodes in oxidation-reduction systems. The electrode serves as an e-source/sink for electrons moved from the solution’s redox system.

e.g., Detection Of Ce³⁺ with Pt electrode

Half-cell reaction is

Ce⁴⁺ + e- Ce³⁺

Potential is

E _ind = E^O – (0.0592/1) log a _Ce3+/a _Ce4+

Limitation:

Processes of electron transport at inert electrodes are typically irreversible.
It does not react to table reactions predictably.

Ion selective electrode

An ion-selective electrode (ISE) is an electrochemical sensor that operates on the idea of potentiometry, or measuring the cell potential (i.e., ISE versus a standard reference electrode) at near-zero current. In these circumstances, the boundary potential at the ISE-solution interface is controlled by electrochemical thermodynamic rules or by the well-known Nernst equation.

Potentiometric sensors known as ion-selective electrodes (ISEs) have a selective membrane to reduce matrix interferences. The pH electrode, which has a thin glass membrane that reacts to the H+ content in a solution, is the most used ISE. Fluoride, bromide, nitrate, cadmium, and gases in solution such as ammonia, carbon dioxide, nitrogen oxide, and oxygen are other characteristics that can be assessed.

Glass electrode

In a glass electrode, the sensing material is a thin glass membrane in the form of a bulb. Hydrogen ions in the fluid can make it sensitive. A potential is created in a thin glass barrier that separates two electrolytes.

A pH-sensitive glass electrode is a carbon dioxide electrode. It is used for the determination of dissolved carbon dioxide in water samples and is highly helpful in environmental chemistry.

Ion exchange membrane

Ion exchange electrodes are built on a membrane that conducts electricity. A form of organic polymer or ion-exchange resin called an ion-exchange membrane, carries particular dissolved ions out of the solution while blocking other ions or neutral molecules.

To move H+ ions, proton-exchange membranes are employed.

In some alkaline fuel cells, OH ions from dissolved solutions are transported using anion exchange membranes.

Enzyme electrode

Enzyme electrodes are based on the idea that substrates and enzymes can interact during biological processes. Clark and Lyons were the first to develop it.

By keeping β-glucosides in a gel layer and connecting a cyanide-sensitive membrane electrode to it, it is possible to create an amygdalin-sensitive electrode. Enzyme electrodes can monitor glucose and urea. Patients who have diabetes can benefit from it.

Crystalline electrode

Electrodes made of crystalline or solid-state membranes include an insoluble inorganic salt. Electrodes made of crystal offer high selectivity. The electrode response may be affected by ions that can enter the crystal structure. The potential is created at the membrane of crystalline electrodes through an ion-exchange mechanism.

Reference electrode

A reference electrode is stable and has a known electrode potential. Its remarkable stability is accomplished by using the redox system, which requires saturated concentrations in each of the participating solutions of the reaction.

Reference electrodes need to have a repeatable and steady voltage. Preferably, reversible-type electrodes are employed as reference electrodes. A tiny cathodic current in a reversible electrode causes the reduction reaction, whereas a small anodic current causes the oxidation reaction.

Silver/Silver Chloride Electrode

The silver/silver chloride reference electrode is made of a silver wire (Ag) coated with a coating of solid silver chloride (AgCl), which is then submerged in a solution that is saturated with KCI and AgCl.

The reaction in the half cell is AgCl (s) + e – Ag (s) + CI (satd).

Advantages

Easy to produce
2. Rapidly demonstrates potential and achieves reproducible equilibrium between -30⁰C and 135⁰C.
Stable & accurate despite large temperature variations

Disadvantages

It reacts to samples.

Saturated calomel electrode

A reference electrode based on the reaction between elemental mercury and mercury (I) chloride is known as a saturated calomel electrode (SCE). A saturated solution of potassium chloride in water serves as the aqueous phase in contact with the mercury and mercury (I) chloride. The electrode is generally connected to the fluid in which the other electrode is immersed via a porous frit ie., salt bridge.