Bicarbonates

Bicarbonates are the intermediate products formed during the deprotonation of carbonic acid. The chemical formula for bicarbonate is HCO₃⁻, representing a polyatomic anion—a comprehensive understanding of bicarbonates, including the global types.

This topic explores bicarbonates' chemical and physical properties and their diverse applications. Additionally, the relationship between carbonic acid and bicarbonate anion is examined in detail. Specific bicarbonates, such as sodium, potassium, and calcium, are also discussed in detail.

Bicarbonate, called hydrogen carbonate in IUPAC nomenclature, is an intermediate product of carbonic acid deprotonation in inorganic chemistry. Its chemical formula is HCO₃⁻, classified as a polyatomic anion. Bicarbonate is crucial in the physiological pH buffering system, maintaining the acid-base balance in biological systems.

Hydrogen carbonate, or bicarbonate, is a carbon oxoanion that removes a proton from carbonic acid. It has a trigonal planar structure with bond angles of approximately 120 degrees around the carbon atom, which is sp² hybridised.

When a bicarbonate ion's negatively charged oxygen atom combines with a positively charged ion, an ionic compound known as a bicarbonate salt is formed. Most bicarbonates are highly soluble in water under standard temperature and pressure conditions.

1.0Relationship of Carbonic Acid and Bicarbonate Anion

The human body's acid-base balance is maintained through the coordinated action of the respiratory system and kidneys, along with a buffering system based on bicarbonates. The respiratory system eliminates excess carbon dioxide, while the kidneys excrete waste via urine. The bicarbonate buffering system is crucial in neutralising acidic waste, acting as a natural antioxidant. Adequate bicarbonate availability is vital for maintaining the pH balance of blood and other bodily fluids.

Buffers stabilise pH by neutralising acids or bases. For instance, adding a weak acid to unbuffered water causes a significant drop in pH. However, if buffered water is used, the pH remains stable due to the buffer's neutralising action. Bicarbonates form a key component of this buffering system in the human body, comprising carbonic acid (H₂CO₃) as a weak acid and bicarbonate (HCO₃⁻) as a weak base.

When acids are introduced, bicarbonate ions react with hydrogen ions (H⁺) to form carbonic acid, reducing the free hydrogen ion concentration and stabilising pH. The reaction is reversible:

H₂CO₃ ⇌ H⁺ + HCO₃⁻
(Carbonic Acid ⇌ Hydrogen Ion + Bicarbonate Ion)

2.0Physical Properties

1. Bicarbonates typically appear as white crystalline solids, although group 2 bicarbonates in the periodic table are commonly found in solution form.
2. The molecular mass of the bicarbonate ion is 61.017 g/mol.
3. Upon heating, bicarbonates decompose to produce carbon dioxide, water, and the corresponding carbonate.

3.0Chemical Properties

1. The bicarbonate ion has the chemical formula HCO₃⁻. It consists of a central carbon atom surrounded by three oxygen atoms arranged in a trigonal planar structure, with one oxygen atom bonded to a hydrogen atom.
2. The bicarbonate ion has the same number of electrons as nitric acid (HNO₃).
3. Bicarbonate is an amphiprotic species which can act as both an acid and a base. It carries a formal negative one charge and serves as:

• The conjugate acid of the carbonate ion (CO₃²⁻).
• The conjugate base of carbonic acid (H₂CO₃).

4. The bicarbonate ion's pKa is 10.3, and its pKb is 7.7, reflecting its dual acidic and basic behaviour.

4.0Compounds of Bicarbonates

Bicarbonates are essential in biological processes, industrial applications, and everyday life, offering versatility and effectiveness in various fields.

Sodium Bicarbonate (NaHCO₃)

• Preparation: Sodium hydrogen carbonate is made by saturating a sodium carbonate (Na₂CO₃) solution with carbon dioxide (CO₂), producing white crystalline sodium bicarbonate.
  Na₂CO₃ + CO₂ + H₂O → 2NaHCO₃
• A white crystalline solid, sparingly soluble in water. Its aqueous solution is alkaline, turning methyl orange-yellow but showing no colour change with phenolphthalein.

Uses:

• • Mild antiseptic for skin infections.
  • Antacid to neutralise stomach acidity.
  • Baking soda is used to make cakes and pastries light and fluffy.
  • Fire extinguishers.

Potassium Bicarbonate (KHCO₃)

• Preparation: Produced by reacting potassium carbonate (K₂CO₃) with CO₂ in water.
  K₂CO₃ + CO₂ + H₂O → 2KHCO₃
• A white crystalline solid, highly soluble in water.

Uses:

• • Substitute for baking soda for low-sodium diets.
  • Buffering agent in medications.
  • Additive in winemaking.

Ammonium Bicarbonate (NH₄HCO₃)

• Preparation: Formed by combining ammonia (NH₃) and carbon dioxide (CO₂) in water, maintaining a cold reaction environment to prevent decomposition.
  CO₂ + NH₃ + H₂O → NH₄HCO₃
• A white crystalline solid with an ammonia-like odour, soluble in water but insoluble in ethanol.

Uses:

• • Food additives in the baking industry.
  • Fertiliser for agricultural purposes.
  • Ingredient in fire extinguishers.
  • Dye manufacturing.