Question

Indicator electrode is (a) SHE (b) Calomel electrode (c) \(\mathrm{Ag} / \mathrm{AgCl}\) electrode (d) Quinhydrone electrode

Short answer

The indicator electrodes from the given options are (b) Calomel electrode, (c) Ag/AgCl electrode, and (d) Quinhydrone electrode.

Step-by-step solution

Understanding Indicator Electrodes

Indicator electrodes are used in electrochemical cells to measure the voltage of the cell, which correlates to the concentration of a particular ion in solution. These electrodes respond to changes in the electrochemical potential of the reaction.

Identifying the Indicator Electrodes

Among the given options, identify which electrodes are commonly used as indicator electrodes. Standard Hydrogen Electrode (SHE) is typically used as a reference electrode, while Calomel, Silver/Silver Chloride (Ag/AgCl), and Quinhydrone electrodes can be used as indicator electrodes.

Choosing the Correct Answer

Considering the purpose of indicator electrodes, we exclude SHE as it is known as a reference electrode. The remaining three options, Calomel electrode, Ag/AgCl electrode, and Quinhydrone electrode, can function as indicator electrodes. Therefore, the correct answers are (b) Calomel electrode, (c) Ag/AgCl electrode, and (d) Quinhydrone electrode.

Key concepts

Electrochemical Cells

An electrochemical cell is a device capable of either generating electrical energy from chemical reactions or facilitating chemical reactions through the introduction of electrical energy. Electrochemical cells are composed of two electrodes (an anode and a cathode) immersed in an electrolyte solution. The flow of electrons from one electrode to another through an external circuit is what we experience as electrical current.

There are two main types of electrochemical cells: galvanic cells, which convert chemical energy into electrical energy spontaneously, and electrolytic cells, which require electrical energy to drive non-spontaneous chemical reactions. These cells are fundamental to batteries, electroplating, and various types of sensors and are key in the fields of electrochemistry and energy storage.

Standard Hydrogen Electrode (SHE)

The Standard Hydrogen Electrode, often abbreviated as SHE, acts as a reference point for measuring electrode potentials. It is considered to have a potential of 0.00 volts by definition. The SHE consists of a platinum electrode coated with platinum black, in contact with a solution containing 1M hydrogen ions, and is immersed in an atmosphere of hydrogen gas at one atmosphere pressure.

When using SHE, it is connected with another electrode, the potential of which is to be measured; the voltage difference between the two gives us the potential of the unknown electrode. Although SHE is widely used due to its universality, it is not practical for routine laboratory measurements because it requires strict conditions to maintain its standard state.

Calomel Electrode

The Calomel electrode is a type of reference electrode commonly used in electrochemical measurements. It contains mercury, mercurous chloride (calomel), and a saturated potassium chloride solution. The calomel electrode's potential is well-established, making it stable and reliable, but it is temperature-dependent.

Calomel electrodes are often used in potentiometric titrations or pH measurements. They provide a stable voltage reference against which changes in an indicator electrode's potential can be measured. Despite their usefulness, the presence of mercury limits their use due to toxicity and environmental concerns.

Silver/Silver Chloride (Ag/AgCl) Electrode

The Silver/Silver Chloride electrode is another well-known reference electrode. It is made by coating a silver wire with silver chloride and immersing it in a solution of potassium chloride. The Ag/AgCl electrode is known for its stable and reproducible potential and simple construction.

This electrode is durable and less hazardous compared to the calomel electrode and is used in many practical applications such as sea water analysis and biological systems. Since Ag/AgCl electrodes are less sensitive to temperature changes than Calomel electrodes, they are often preferred for fieldwork and practical laboratory setups.

Quinhydrone Electrode

The Quinhydrone electrode is a type of pH indicator electrode used to measure the pH of a solution. It comprises a platinum or gold electrode immersed in a solution alongside quinhydrone, which is a compound formed by the chemical combination of hydroquinone and benzoquinone.

At the electrode surface, quinhydrone dissociates into quinone and hydroquinone, creating a potential that depends on the hydrogen ion concentration of the solution. This potential can be measured against a reference electrode, thus allowing for the determination of the solution's pH. Quinhydrone electrodes are useful for measuring the pH in solutions where glass electrodes may be problematic, though they are not as commonly used as other pH electrodes.