Chemistry of Calomel

Preparation:

1. When mercuric chloride is heated with mercury in an iron pot, calomel is obtained.
$$\ce{HgCl2 + Hg->[\Delta]Hg2Cl2}$$

2. When mercuric chloride is treated with less amount of stannous chloride, calomel is obtained.
$$\ce{HgCl2 + SnCl2->Hg2Cl2 + SnCl4}$$

Note: When excess amount of mercuric chloride is added, compounds other than calomel are formed.

It can also be prepared by grinding the mixture of mercury with conc. H2SO4 together with it, small amount of NaCl as well as mercury is added to get calomel.

Physical properties:

• It is white amorphous solid, sparingly soluble in water.
• It is insoluble in most of the acids and soluble in aqua regia.
• It’s melting point is 302oC and boiling point is 324oC.

Chemical properties:

1. Action of heat: Calomel decomposes on heating to give corrosive sublimate.
$$\ce{Hg2CL2 ->[\Delta]HgCl2 + Hg}$$

2. Action with ammonia:  When calomel is treated with ammonia gas, we get black colored compound due to the formation of amino mercuric chloride and mercury.
$$\ce{Hg2Cl2 + 2NH3->\underset{\text{Black}}{\ce{[Hg + Hg(NH2)Cl v ] + Hg}} + NH4Cl}$$

3. Action with aqua-regia: Calomel gets dissolved in aquaregia to give corrosive sublimate.
$$\ce{\underset{\text{Calomel}}{\ce{Hg2CL2}} + HNO3 + 3HCl->\underset{\text{Corrosive sublimate}}{\ce{HgCl2}} + NOCl + H2O}$$

4. Action with NaOH solution: When calomel is treated with NaOH solution, dark grey ppt of mercurous oxide is formed.
$$\ce{Hg2CL2 + 2NaOH->\underset{\text{Mercurous oxide}}{\ce{ Hg2O v }}+ NaCl + H2O}$$

Uses:

•  It is used as calomel electrode.
•  It is used as purgative to remove constipation as medicine.
•  It is used as bed pill along with sodium bicarbonate.

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