The calomel electrode is a

Reference electrode

The correct answer is option 2. Reference electrode.

The calomel electrode is a specific type of electrode used in electrochemistry, particularly as a reference electrode. Let's break down the options to understand why:

1. Standard hydrogen electrode (SHE):

The standard hydrogen electrode is a reference electrode used to define the standard hydrogen electrode potential (which is 0 V by definition). It consists of a platinum electrode immersed in a solution with \( H^+ \) ions at 1 M concentration and a hydrogen gas pressure of 1 atm.

Calomel electrode is not SHE: The calomel electrode is not the same as the standard hydrogen electrode.

2. Reference electrode:

A reference electrode is an electrode that has a stable and well-known electrode potential. It is used as a reference against which the electrode potential of other electrodes can be measured.

Calomel electrode is a reference electrode: The calomel electrode, specifically the saturated calomel electrode (SCE), is a widely used reference electrode in electrochemical measurements. It consists of a mercury electrode in contact with a saturated potassium chloride (KCl) solution containing mercury(I) chloride (\( Hg_2Cl_2 \)).

3. Platinum electrode:

Platinum electrodes are commonly used in electrochemistry due to their inertness and conductivity. They are often used as working electrodes rather than reference electrodes.

Calomel electrode is not platinum electrode: While platinum electrodes are important in many electrochemical applications, the calomel electrode is distinct in its composition and purpose.

4. Mercury electrode:

Mercury electrodes can be used in electrochemistry, including as working electrodes or parts of certain reference electrodes.

Calomel electrode includes mercury: The calomel electrode does indeed involve mercury, specifically in the form of mercury(I) chloride, which is an integral part of its construction.

Given the above explanations, the calomel electrode fits best into the category of a reference electrode, specifically the saturated calomel electrode (SCE). Therefore, the correct answer is: (2) Reference electrode