Mercuric Chloride – Structure, Properties, Effects

Mercuric Chloride

Mercury and chloride combine to give several compounds, including HgCl (Mercurous Chloride), HgCl₂ (Mercuric Chloride), and HgCl₃ (Trichloromercurate). When mercury (II) combines with chlorine, it gives mercuric chloride. The formula is HgCl₂. It is also called bichloride of mercury, mercury (II) chloride, mercury dichloride, or dichloromercury. The compound is highly toxic and corrosive to humans. If consumed, it can lead to several complications. The triatomic molecule finds application in different industries owing to its unique physical and chemical properties.

Structure of Mercuric Chloride

Let us learn about its structure in detail.It has a linear coordination geometry and a crystal structure. Its molecular shape is also linear, and the bond angle is 180 degrees. The following diagram shows the electronic distribution of a molecule:

Thus, HgCl₂  has  2 + 7 * 2 = 16 valence electrons. Where Hg contributes two electrons, each Cl has seven electrons in its valence. The number of bonding pairs is two, and there are no nonbonding pairs.

Synthesis of Mercuric Chloride

It is prepared by the action of chlorine on mercury.

It can also be obtained by adding hydrochloric acid (HCl) to hot, concentrated mercury compounds like nitrate. The following equation represents the reactions:

Hg₂(NO₃)₂ + 4 HCl → 2 HgCl₂ + 2 H₂O + 2 NO_2.

Another common method for the preparation of mercuric chloride involves heating a mixture of solid mercury(II) sulfate and NaCl (sodium chloride). The process gives volatile HgCl₂ that can be separated by sublimation.

Physical Properties of Mercuric Chloride

The key physical properties of mercuric chloride are as follows:

• Chemical Formula: HgCl₂
• Melting Point: 529°F or 276 °C
• Boiling Point: 576°F at 760 mmHg or 304 °C
• Vapor Pressure: 1 mmHg at 277.16°F
• Specific Gravity: 5.44 at 77°F
• Molecular Weight: 271.52 g/mol
• Water Solubility: 5 to 10 mg/mL at 72°F
• Density: 5.43 g/cm³
• Color: Colorless crystals or white powder
• Odor: Odorless
• Volatile Nature: Slightly volatile at room temperature
• Taste: Metallic taste
• Solubility: Soluble in ethanol, methanol, ethyl acetate, diethyl ester, and acetone

Chemical Properties of Mercuric Chloride

It is slightly soluble in pyridine, acetic acid, and carbon disulfide. The compound is unstable in the presence of alkalies. It readily reduces to mercurous chloride and mercury.

1. Reaction to Sunlight

It decomposes into metallic mercury in the presence of organic matter when sunlight exposes it.

2. Reaction with NaOH

It reacts with sodium hydroxide and gives a yellow precipitate. The reaction is as follows:

HgCl₂ + 2 NaOH → 2 NaCl + Hg(OH)₂

3. Reaction with KI

I reacts with KI to give orange ppt. It dissolves in excess of the same to give  K₂HgI₄ or Nessler’s reagent. The series of reactions can be represented as follows:

HgCl₂ +2KI⟶ HgI₂ ↓

HgI₂ is the orange precipitate. Now, this mercuric iodide reacts with KI, forming Nessler’s reagent.

HgI₂ +2KI⟶ soluble K₂​HgI₄

​Nessler’s reagent is widely used to identify ammonium ions (NH⁴⁺).

4. Oxidizing Ability

Mercuric chloride reacts with stannous chloride and oxidizes it to stannic chloride. In the reaction, mercuric chloride gets reduced to mercurous chloride.

2 HgCl₂ + SnCl₂ → Hg₂Cl₂ + SnCl₄

Mercurous chloride then reacts with excess stannous chloride and gets reduced to metallic mercury (gray). The reaction can be written as follows:

Hg₂Cl₂ + SnCl₂ → 2 Hg(l) + SnCl₄

Gray-colored mercury is obtained as the product of this reaction.

5. Reaction with Ammonia

It reacts with gaseous ammonia to give a white precipitate of [Hg(NH_2​)Cl]. The balanced reaction can be represented as follows:

HgCl_2​ +2NH₃ → Hg(NH₂)Cl + NH₄Cl

6. Reaction with Copper

It gets converted to mercury by placing copper metal in the aqueous solution of HgCl₂. The equation for the reaction is as follows:

2 Cu + HgCl₂ → 2 CuCl + Hg

​Effects of Mercuric Chloride on Humans

Mercuric chloride is highly toxic to humans. It serves as a poison. Acute mercuric chloride exposure can cause the following issues:

• Increased salivation
• Foul breath
• Ulceration of mucous membranes
• Inflammation
• Abdominal pain
• Bloody diarrhea

Dermal exposure can result in:

• Dermatitis (red and inflamed skin)
• Burns and blisters

Significant mercuric chloride exposure can also cause:

• Anuria or suppression of urine formation
• Oliguria or scanty urination)
• Acute renal failure
• Weak pulse
• Psychic disturbances
• Circulatory collapse
• Seizures
• Dyspnea or shortness of breath

Large amounts of mercuric chloride can lead to death in 24 hours. The ultimate cause of death is found to be kidney failure or gastrointestinal tract damage. Victims of acute exposure usually take up to two to three weeks to die.

Did you know: Mercuric chloride is not just harmful to humans but also to animals. The compound leads to long-term effects in the aquatic environment. It is highly toxic to aquatic organisms. Moreover, bioaccumulation of this chemical can take place along the food chain, i.e., from aquatic organisms to humans when they have seafood.

Historical Use of Mercuric Chloride

Earlier mercuric chloride was widely used for a range of purposes. However, owing to its toxic nature, its use has been curtailed to quite an extent.

In Preservation

During the late 19th century, mercuric chloride was used for the preservation of biological specimens. The objects were dipped or painted with the mercuric chloride solution to prevent them from moths, mold, and mites. The specimens placed in drawers were protected by the addition of crystalline mercuric chloride. It was also used in tanning and wood treatment.

However, research on its poisonous nature led to a decline in its use. Alternative treatment processes involving copper sulfate and zinc chloride emerged and proved to be less toxic.

In Medicine

Mercuric chloride was a popular disinfectant in the early twentieth century. It was used for fighting measles germs, protecting fur coats, removing red ants, and more.

Mercuric chloride was also used to treat syphilis before the advent of antibiotics. It was ingested, inhaled, and applied topically. But its frequent accidental poisonings and deliberate consumption for suicide due to easy availability led to a significant decline.

Applications of Mercuric Chloride

The various applications of mercuric chloride are as follows:

To form an amalgam with metals: Mercuric chloride forms an amalgam with aluminum. The aluminum strips get covered by a thin layer of this amalgam when treated with an aqueous solution of mercuric chloride. The amalgamated aluminum undergoes various reactions. For instance, in Barbier’s reaction, halocarbons react with amalgamated aluminum. It is also used as a reducing agent in organic synthesis. Another commonly amalgamated metal using mercuric chloride is zinc.

Other uses of mercuric chloride include:

• It is used as a catalyst in the conversion reaction of acetylene to vinyl chloride. Vinyl chloride serves as a precursor to polyvinyl chloride.
• As a depolarizer in batteries
• As a reagent in organic synthesis
• It is used for surface sterilization of explants like a leaf or stem nodes in plant tissue culture.
• Mercuric chloride is used in the preservation of anatomical specimens.
• Leather tanning
• Manufacturing of ink for mercurography
• It is used in disinfectants and antiseptics.
• As an intensifier in photography
• Its solution is used as a dip for bulbs.
• It is used as an ant repellent.

Conclusion

Mercury chloride is a toxic yet useful compound. Therefore, care should be taken while handling it in real-life applications. It is a precursor for many reactions and helps formulate important reagents like the Nessler reagent.

Frequently Asked Questions

Q1. What measures should be taken in case of dermal exposure to mercuric chloride?

If the skin is exposed to mercuric chloride, the following steps should be taken immediately to avoid harmful effects:

• Immediately remove the person from exposure while avoiding self-exposure.
• Remove contaminated clothing.
• Check for vital signs, such as pulse and respiratory rate. If the pulse is disturbed, provide CPR.
• In case the person is not breathing, give artificial respiration.
• Wash the exposed skin area with soap and water for 15 minutes.
• If an eye is exposed to mercuric chloride, it must be flushed using lukewarm water for 15 minutes.

Q2. What steps should be taken if a person ingests mercuric chloride?

Ingestion can cause severe problems. You must take the following measures to deal with the toxicity of the substance:

• Evaluate vital signs and provide CPR if no pulse is detected.
• If the person is not breathing, provide artificial respiration.
• Obtain further instructions on an antidote from the local hospital.
• If the ingested amount is small and the victim is conscious and alert, give them water or milk: 125 mL for children up to 1 year old, 200 mL for 1 to 12-year-olds, and 250 mL for adults.
• Promote excretion by giving a saline cathartic or sorbitol This step is again only for conscious and alert victims.

Q3. What precautions should be taken while storing mercuric chloride?

It should be separated from food and feed. One must store mercuric chloride in an area without drain/sewer access. It should be placed in well-closed bottles and kept in the dark. It is a severe water pollutant, so care should be taken while transporting and discarding it.