Ammonium Ions: Formation, Structure and Bonding, Properties, Applications

"Ammonium has the chemical formula NH4+. It is a polyatomic cation.”

Cations are electron-deficient species with a positive charge. Polyatomic ions, also called molecular ions, are groups of two or more covalently bonded atoms, or can be viewed as a single entity and is a metal complex with a net charge greater than zero. In ammonium, four hydrogen atoms are attached to the central nitrogen atom and the total charge is +1.

Formation of Ammonium Ions- NH4+

In ammonia, nitrogen is the central atom with 5 electrons in the valence shell, 3 of which are shared with the 3-H atom, and 1 lone pair of electrons completes the configuration of the valence shell.

It is an electron-rich species (nucleophile) because it has a lone pair (unshared electron pair) and can donate that electron pair to another atom (electrophile). As a result, ammonia is a donor, and an ammonium ion is formed when an ammonia atom donates its lone pair of electrons to a proton.

Structure and Bonding - NH4+

Since the bond is a polar covalent bond, all four N-H bonds are equivalent. This ion is isoelectronic with methane and borohydride and has a tetrahedral structure.

Properties of Ammonium Ions- NH4+

• The chemical formula for the ammonium ion is [NH4]+.

• Molar mass of ammonium ions is 18.039 g/mol.

• It is a weak acid.

• Its acidity is 9.25.

• The conjugate base of the ammonium ion is ammonia (NH3).

• All ammonium salts are white and water soluble.

• Chemically, ammonium ions behave similarly to ions of alkali metals, especially potassium ions of approximately the same size.

• The ammonium ion acts as a weak acid in aqueous solutions because it decomposes in water to form ammonia and hydrogen ions.

Ammonium Salts- NH4+

• Ammonium salts are ionic compounds of the formula (R)4N+A-. where R is hydrogen, alkyl or aryl and A is an anion. The salts are known as quaternary ammonium salts when R is alkyl or aryl.

• Quaternary ammonium cations are always charged regardless of the pH of the solution. Most ammonium salts are water soluble and highly dissociated. Detergents, food additives, diuretics, surfactants, antistatic agents, and disinfectants are made from ammonium salts. They also show antibacterial properties. Quaternary Ammonium Salts Maintain Cell Osmotic Pressure.

• Long-chain alkyl quaternary ammonium compounds such as benzalkonium chloride, cetylpyridinium chloride, dofanium chloride, benzethonium chloride, methylbenzethonium chloride, cetalkonium chloride, and didecyldimethylammonium chloride have antibacterial, antifungal, and antiviral activity . Anionic detergents deactivate these quaternary ammonium compounds and should not be used in hard water.

• They have their applications as disinfectants. One of the most common uses of quaternary ammonium salts in organic synthesis is as phase transfer catalysts (PTCs) to catalyze reactions between ionic and organic reactants. Chlormequat chloride acts as a plant growth regulator by inhibiting the production of gibberellins.

Applications- NH4+

• Ammonium ion is a by-product of animal metabolism. It is excreted into the water directly by aquatic invertebrates. Urea is less toxic and can be stored more efficiently, so it is converted to urea in mammals, sharks and amphibians during the urea cycle.

• Metabolic ammonium in birds, reptiles and terrestrial snails is converted to uric acid. Because uric acid is a solid, it can be excreted with minimal water loss.

• Ammonium is an important nitrogen source for many plant species, especially those growing in low-oxygen soils. However, it is toxic to most plant species and is rarely used as a sole source of nitrogen.

• Ammonium ions are essential to maintain the acid-base balance in the body.

Detection of Ammonium Ions - NH4+

Ammonium ions in solutions can be detected by adding dilute sodium hydroxide solution and heating slightly. Ammonium ions are converted into ammonia gas, if they are present. Ammonia has a distinctive suffocating odor.