HgCl₂ — Mercury(II) Chloride

Mercury(II) Chloride (HgCl2), commonly known as corrosive sublimate, is a highly toxic inorganic compound composed of mercury and chlorine. It appears as a white crystalline solid that sublimates easily and dissolves moderately in water. Due to its strong oxidizing and antimicrobial properties, it was once widely used as a disinfectant, preservative, and antiseptic. However, because of its severe toxicity and environmental persistence, its medical and industrial use has been largely discontinued or tightly regulated.

Despite its dangers, mercury(II) chloride remains an important laboratory reagent and is used in certain chemical syntheses, catalysis, and metallurgical processes. Its name ‘corrosive sublimate’ originates from its ability to corrode organic tissue and its tendency to sublime upon heating.

The molecular structure of Mercury(II) Chloride is linear, represented as \(Cl–Hg–Cl\). The mercury atom is in a +2 oxidation state and forms covalent bonds with two chlorine atoms. The molecule exhibits a linear geometry due to sp hybridization of mercury’s orbitals. In the solid state, HgCl2 forms an extended network stabilized by weak van der Waals forces.

Although the Hg–Cl bonds are primarily covalent, they exhibit partial ionic character due to the electronegativity difference between mercury and chlorine. This hybrid bonding contributes to its solubility in both polar solvents (like water) and non-polar solvents (like ether).

Mercury(II) Chloride can be prepared by direct combination of mercury and chlorine gas, or by the oxidation of mercury(I) chloride (calomel). The most common laboratory and industrial methods include:

• Direct Chlorination:

  \( Hg + Cl_2 \rightarrow HgCl_2 \)

• Oxidation of Mercury(I) Chloride:

  \( Hg_2Cl_2 + Cl_2 \rightarrow 2HgCl_2 \)

• Reaction with Nitric Acid and Sodium Chloride:

  \( Hg(NO_3)_2 + 2NaCl \rightarrow HgCl_2 + 2NaNO_3 \)

After synthesis, the compound is purified through sublimation, resulting in pure white crystalline HgCl2.

• Appearance: White crystalline or powdery solid.
• Solubility: Soluble in water (6.9 g/100 mL at 20°C), ethanol, and ether.
• Volatility: Sublimes readily when heated, forming mercury vapor and chlorine gas at high temperatures.
• Chemical Stability: Stable in dry air but decomposes in moist air or upon heating.
• Reactivity: Reacts with reducing agents to form metallic mercury; reacts with bases forming insoluble mercuric oxide (HgO).

Due to its high toxicity, all handling of mercury(II) chloride must be done in controlled environments with protective equipment.

Although its applications have decreased significantly due to safety concerns, mercury(II) chloride has historically and scientifically important uses:

1. Disinfectant and Antiseptic: Once used in medicine for sterilization and treatment of syphilis, though discontinued because of mercury poisoning risks.
2. Preservative: Formerly used for preserving biological specimens and wood due to its antibacterial and antifungal properties.
3. Reagent in Laboratories: Used as a catalyst and reagent in organic synthesis, especially in preparing organomercury compounds.
4. Amalgamation and Metallurgy: Involved in refining gold and silver by amalgam formation.
5. Photography and Electroplating: Used in some photographic toning processes and metal plating baths.

Mercury(II) Chloride is one of the most toxic forms of mercury compounds. Exposure through ingestion, inhalation, or skin absorption can lead to severe poisoning. It acts as a protoplasmic poison, disrupting enzyme systems and damaging tissues, especially in the kidneys and gastrointestinal tract.

Symptoms of poisoning include abdominal pain, vomiting, diarrhea, metallic taste, kidney failure, and in severe cases, death. Chronic exposure can cause tremors, memory loss, and neurological disorders. Because of its high toxicity, it has been banned or restricted in consumer products and medicines.

Mercury(II) chloride poses serious environmental hazards. It is highly persistent and bioaccumulates in the food chain. When released into the environment, it can transform into methylmercury by microbial action, a compound even more toxic to aquatic life and humans. Its disposal is regulated under hazardous waste management laws to prevent contamination of soil and water resources.

• Mercury(II) chloride was once called ‘corrosive sublimate’ because it could corrode organic tissues upon contact.
• It sublimes easily, turning directly from solid to vapor without passing through a liquid state.
• Despite being banned in medicine, it is still used in small-scale research for its catalytic properties.
• It is one of the oldest known mercury compounds, used since the 16th century for medicinal and alchemical purposes.