Barium carbonate (BaCO₃): Structure, Properties & Uses

Barium carbonate is the inorganic compound with the formula BaCO3. It is a salt composed of barium cations (Ba²⁺) and carbonate anions (CO₃²⁻). Like most alkaline earth metal carbonates, it is a white salt that is poorly soluble in water. It occurs as the mineral known as witherite. It is commonly used in the manufacture of other barium compounds, as a flux in metallurgy, and as a filler in plastics and rubber.

It is a basic carbonate of barium and is used in the manufacture of glass, ceramics, and other inorganic chemicals. The compound has a density of 4.286 g/cm³, a melting point of 811°C (polymorphic transformation), and a boiling point of 1450°C, decomposing from 1360°C. It is insoluble in methanol and partially insoluble in water, and decomposes in acids.

Structure of Barium Carbonate

Barium carbonate crystals in the solid state have an orthorhombic crystal structure. The orthorhombic structure is characterized by three mutually perpendicular axes of unequal lengths. Within this unit cell, the barium ions (Ba²⁺) are arranged in a specific pattern. Each barium ion is surrounded by nine oxygen atoms (O²⁻) from neighboring carbonate ions. These oxygen atoms don’t belong to a single carbonate ion but rather come from six different carbonate ions surrounding the barium. This gives the barium a nine-coordinate geometry.

Anion & Cation

Barium Ions (Ba²⁺): These large cations are surrounded by a coordination number of 9, meaning each barium ion is typically surrounded by nine oxygen atoms from the carbonate ions. The arrangement of these ions in the crystal lattice is such that it minimizes the repulsive forces between the positively charged barium ions.

Carbonate Ions (CO₃²⁻): The carbonate ion is a planar, trigonal molecule with a central carbon atom covalently bonded to three oxygen atoms. The structure of the carbonate ion can be represented as a triangle with the carbon atom at the center and the oxygen atoms at the corners. Because of resonance, the bond lengths between carbon and oxygen are equal, this gives it an ion a symmetric shape.

Bonding and Geometry

  • Ionic Bonding: The primary interaction in barium carbonate is the ionic bond between the barium cations and the carbonate anions. The electrostatic attraction between these oppositely charged ions holds the crystal lattice together.
  • Coordination Geometry: In the crystal lattice, each barium ion coordinates with oxygen atoms from multiple carbonate ions, resulting in a complex three-dimensional network. The exact geometry around the barium ions can be described as a distorted tricapped trigonal prism, a typical feature for such large cations in complex ionic compounds.

Preparation of Barium carbonate (BaCO₃)

Barium carbonate (BaCO₃) is prepared commercially from barium sulfide by two main methods: the soda ash method and the carbonation method.

Carbonation Process

This is the more common method and involves reacting a soluble barium compound with carbon dioxide (CO₂) to form the insoluble barium carbonate precipitate. Here are two variations:

Barium Sulfide (BaS) Reaction:

  • An aqueous solution of barium sulfide (BaS) is reacted with carbon dioxide gas (CO₂) at temperatures ranging from 40 °C to 90 °C.
Reaction: BaS (aq) + H₂O (l) + CO₂ (g) → BaCO₃ (s) + H₂S (g)

  • The product is a slurry of barium carbonate (BaCO₃) and hydrogen sulfide gas (H₂S) as a byproduct. The H₂S is removed due to its toxicity and because it can interfere with the precipitation process.

Barium Chloride (BaCl₂) Reaction (Less Common):

  • This method involves reacting a solution of barium chloride (BaCl₂) with ammonium bicarbonate (NH₄HCO₃) or a mixture of ammonia (NH₃) and carbon dioxide (CO₂) at controlled conditions.
Reaction (using ammonium bicarbonate):

BaCl₂ (aq) + 2NH₄HCO₃ (aq) → BaCO₃ (s) + 2NH₄Cl (aq) + CO₂ (g) + H₂O (l)

  • This method is less common but offers advantages like producing a purer barium carbonate and avoiding the toxic hydrogen sulfide byproduct.

Soda Ash Process (Less Common)

This method involves reacting an aqueous solution of barium sulfide (BaS) with sodium carbonate (Na₂CO₃), also known as soda ash, at temperatures around 60 °C to 70 °C.

Reaction: BaS (aq) + Na₂CO₃ (aq) → BaCO₃ (s) + Na₂S (aq)

  • The product is a mixture of barium carbonate (BaCO₃) and sodium sulfide (Na₂S) in solution. The sodium sulfide needs to be separated and disposed of properly.

Following Steps (Common to Both Methods):

After the initial reaction, regardless of the method used, these common steps follow:

  • Solid-Liquid Separation: The barium carbonate precipitate (BaCO₃) is separated from the liquid solution using techniques like filtration.
  • Washing: The separated barium carbonate is washed to remove any impurities.
  • Drying: The washed barium carbonate is dried at temperatures around 300 °C to remove any remaining water.
  • Grinding (Optional): The dried barium carbonate may be further ground to achieve a desired particle size for specific applications.

Physical Properties of Barium carbonate (BaCO₃)

  • Barium carbonate is a white solid that precipitates from a solution of barium hydroxide and urea.
  • It is also known as barium monocarbonate and has a chemical formula of BaCO₃.
  • The density of barium carbonate is 4.286 g/cm³.
  • The melting point is 811°C, with a polymorphic transformation occurring at this temperature.
  • The boiling point is 1,450°C, and it starts to decompose from 1,360°C.
  • Barium carbonate is insoluble in water and methanol, but it is soluble in most acids.
  • The specific heat capacity is 0.14483 J/mol.K.
  • The complexity of barium carbonate is 18.8.
  • The odor of barium carbonate is odorless.
  • The refractive index is 1.676.

Chemical Properties of Barium carbonate (BaCO₃)

  • Barium carbonate reacts with acids such as hydrochloric acid to form soluble barium salts, such as barium chloride: BaCO₃ + 2HCl → BaCl₂ + H₂O + CO₂
  • When soluble calcium salts react with barium carbonate, they form barium sulfate, which remains in the solution, and calcium carbonate: BaCO₃ + CaSO₄ → BaSO₄ + CaCO₃
  • The pyrolysis of barium carbonate gives barium oxide: BaCO₃ → BaO + CO₂
  • PH: Alkaline in water due to slight dissolution and hydrolysis, forming barium hydroxide and carbonic acid.
  • Stability: Stable under normal conditions of use and storage.
  • Incompatibilities: Incompatible with acids and strong oxidizing agents.
  • Toxicity: Toxic if ingested or inhaled.

Uses of Barium carbonate (BaCO₃)

  • It is used in the manufacture of other barium compounds, as a flux in metallurgy, and as a filler in plastics and rubber.
  • It is used as an antacid and as a source of barium in radiographic studies.
  • It is used in the production of glass, ceramics, and other inorganic chemicals.
  • It is used in the manufacture of ceramic products.
  • It is used in the oil and gas drilling industry to increase the density of drilling muds, stabilize the borehole, and lubricate the drill bit.
  • It is used to remove impurities and enhance the optical qualities in glass manufacturing.
  • Commonly used in the production of barium compounds, which are used in the manufacture of fireworks, flares, and other explosives.

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