Polyatomic ions are charged species that contain more than one atom held together by covalent bonds. The polyatomic ion examples we will cover in this article are:
- Ammonium
- Acetate
- Carbonate
- Chromate
- Cyanide
- Hydroxide
- Nitrate
- Nitrite
- Oxalate
- Phosphate
- Thiocyanate
Ammonium ion
It is a positively charged polyatomic ion with the chemical formula NH4+. It has a tetrahedral geometry(sp3 hybridization).It is formed by the reaction of aqueous ammonia and acids.
NH3 + H+ → NH4+
If we add concentrated hydroxide to an aqueous ammonium solution, then red litmus paper(moistened) turns blue; this is used to detect ammonium ions. It is an important nitrogen source for many plant species. The lewis structure of ammonium ion is given below:
Acetate ion
It is a negatively charged ion with the chemical formula CH3COO–. It is formed by the removal of a proton from the carboxy group of acetic acid.
To detect acetate ions, we add ethanol and conc. H2SO4 to a salt of acetic acid and heat it, the presence of fruity odor confirms the presence of acetate ions. It is used to line fabrics in robes and loungewear. It is also used as a precursor of acetyl-CoA for synthesizing fatty cells. The lewis structure of acetate ions is given below:
Carbonate ion
It is a negatively charged ion with one chemical formula CO32- arranged in trigonal planar geometry.
Carbonate ions soften water and are used in the production of paper and glass industry. When we add dilute sulfuric acid to a salt containing carbonate ion, there is an effervescence with the evolution of carbon dioxide (a colorless and odorless gas). The lewis structures of carbonate ion are shown below:
Chromate ion
These are the anions present in chromate salts. Its chemical formula is CrO42- and is a powerful oxidising agent and is bright yellow in colour.
They have tetrahedral geometry. Chromates are used in inks and dyes as pigments, in chrome plating (to get rid of corrosion), leather finishing and metal rust proofing. Pb2+, Ag+ and Ba2+ forms sparingly soluble precipitate with chromate ions. PbCrO4 and BaCrO4 form a yellow precipitate while Ag2CrO4 forms a brick red precipitate. This is used to detect chromate ions.
Cyanide ion
It is an anion containing the C≡N group (carbon triple-bonded to nitrogen) and is conjugate base of hydrogen cyanide. It has a linear geometry with a negative charge on the carbon atom.
The major use of cyanide is in mining gold and silver. Cyanide ions are also used to produce CN-containing compounds (usually nitriles). We add conc. H2SO4 to cyanide-containing solution; hydrocyanic gas is formed and the reaction color changes from pale green to blue.
Hydroxide ion
It is an anion with the chemical formula OH– derived from base (NaOH, KOH, Ca(OH)2) or water by loss of a proton.
Hydroxides are used in manufacturing paper, pulp, soaps and detergents. If we add an ammonium salt to a base containing hydroxide ions, ammonia gas will be evolved (which can be detected by bringing a glass rod dipped in concentrated HCl, ammonia gas will give solid white fumes).
Nitrate ion
It is an anion with the chemical formula NO32- , formed by the loss of a proton from nitric acid.
Nitrates are used as oxidizing agents, fertilizers, and explosives. To detect nitrate ions, we add concentrated H2SO4 to a salt containing nitrate ions, cool it, and then add FeSO4 to it. A dark brown ring confirms the presence of nitrate ions.
Nitrite ion
Nitrite is an anion with the chemical formula NO2– and is derived from nitrous acid. It has a bent shape with sp2 hybridization.
They are used to prepare azo dyes and other colorants. When dilute H2SO4 is added to a salt containing nitrite, reddish-brown fumes of NO2 gas are observed.
Oxalate ion
It is an anion of a dicarboxylic acid with the formula C2O42- which is used as a human and plant metabolite.
They are used as reducing agents (strontium and barium oxalate), in photography and to remove ink stains. The oxalate anion exists in planar geometry (like potassium oxalate) and non-planar in other cases (like caesium oxalate). Free oxalate anion has an orthogonal D2h structure.
The addition of concentrated H2SO4 to an oxalate salt produces colorless, odorless gas (CO2), which turns lime water milky (detection of oxalate ion).
Phosphate ion
It is an anion with the chemical formula PO43- and is derived by the loss of three protons from phosphoric acid. It has a tetrahedral geometry.
It is used in fertilizers. Phosphate ions provide energy to cells and are important for bone and teeth formation.
For detection of phosphate ions, we add conc. HNO3 and ammonium molybdate to the solution containing phosphate ions and heat it. A canary yellow precipitate of ammonium-phosphomolybdate is observed, which confirms the presence of phosphate ions.
Sulfate ion
It is a sulfur oxoanion with the chemical formula SO42- and is derived from sulphuric acid by deprotonation of both OH groups. It has a tetrahedral geometry.
It has various industrial uses. It is used to produce plaster, as an electrolyte in galvanic cells, and as a detergent in shampoo formulations. An aqueous solution of salt-containing sulfate ions(acidified with acetic acid) reacts with BaCl2 to form a white precipitate of BaSO4.
Sulfite ion
This is also a sulfur oxoanion but with the chemical formula SO32- and is a conjugate base of bisulfite.
They are used as preservatives, bleaching agents and dechlorinating agents. It has trigonal pyramidal geometry. When a solution containing sulfite ions reacts with dilute H2SO4, SO2 gas evolves, which has a smell of burning sulfur. This gas turns potassium dichromate paper(acidified with dil. H2SO4) green confirming the presence of sulfite ions.
Thiocyanate ion
It is an anion with the chemical formula SCN– where the negative charge is shared by nitrogen and sulfur equally(ambidentate ligand). It is derived from thiocyanic acid.
It is utilized in bleach and disinfectants and to prepare silver thiosulphate, which inhibits corrosion in steel. It is an important spectrophotometric reagent. A blood-red color is observed when iron nitrate is added to a solution containing thiocyanate ions.
Conclusion
We discussed thirteen important polyatomic ion examples, their uses, molecular geometry, lewis structure and qualitative tests for their detection.
Hello, I am Sakshi Anand completed my Master’s in Chemistry. My area of specialization is Inorganic Chemistry. I am here to make chemistry easy and enjoyable to read. Complex ideas do not require complex language. I am an avid reader and enjoy researching intensively.
Hi Fellow Reader,
We're a small team at Techiescience, working hard among the big players. If you like what you see, please share our content on social media. Your support makes a big difference. Thank you!