Potassium nitrate

Formula unit Lattice

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5 atoms in one formula unit of an ionic lattice.

The drawing shows one formula unit of an ionic lattice, not a molecule; potassium nitrate has no molecular shape.

IUPAC name: potassium nitrate
Molar mass: 101.1020(50) g/mol
Atoms: 5
SMILES: [N+](=O)([O-])[O-].[K+]
InChIKey: FGIUAXJPYTZDNR-UHFFFAOYSA-N
PubChem CID: 24434

Made of

K N O

Boiling water takes up more than five times as much potassium nitrate as water at about 25 °C. A gram of the salt dissolves in 2.8 mL of water at that temperature and in only 0.5 mL at the boil, according to HSDB, the Hazardous Substances Data Bank. The salt answers to two other names: saltpetre, also spelled saltpeter, and niter for the natural mineral. The US National Cancer Institute’s thesaurus (NCI Thesaurus) calls it a crystalline salt and a strong oxidiser.

Each formula unit pairs a potassium ion, K⁺, with a nitrate ion, NO₃⁻, in which one nitrogen atom holds three oxygen atoms. A mole of it weighs 101.102 g. HSDB describes colourless crystals without any smell. In the European Union’s food-additive specification the compound appears as transparent prisms or a white crystalline powder, with a cooling, salty and pungent taste.

Structure and bonding

K⁺ and the nitrate ion are drawn as two pieces, with no line joining them. Inside the nitrate, nitrogen holds one oxygen by a double bond and the other two by single bonds. The drawing marks nitrogen +1 and each singly bonded oxygen −1, a net −1 for the ion. Potassium nitrate is a lattice of such ions; this pair is one formula unit of it, not a molecule.

That double bond belongs to the drawing, not the ion. In Chem1’s account, a structure with one double bond is just one of three equivalent resonance forms. The real ion has three identical N–O bonds and is flat with trigonal symmetry, the shape Chem1 calls trigonal planar.

The Lattice view draws four unit cells, from a structure measured at 120 K (−153 °C), each holding 4 potassium ions and 4 nitrate ions. Coordination picks out one potassium ion and its 9 nearest oxygen atoms, in 6 nitrate ions, 2.80 to 2.89 Å away. HSDB records rhombic crystals that turn trigonal at 129 °C. Its solution has a pH of about 7, HSDB states, and the article on acids and bases explains why such a salt stays near neutral.

Where it turns up

Mammoth Cave once supplied saltpetre. Bats had left guano in the soil of that cave and its neighbours, making the cave dirt rich in calcium nitrate, the raw material for saltpetre. The Eye Draught map, the cave’s earliest known map, predates 1809 and stresses its worth for saltpetre. Mining paid handsomely in the early 1800s, and more so once the War of 1812 brought wartime demand; the miners were mostly enslaved African American men. Demand fell steeply after the war ended in 1815, and the owners abandoned their vats and the wooden piping that carried water.

In nature, potassium nitrate is the mineral niter. Closer to home it turns up in toothpastes sold for sensitive teeth and in cured red meat and poultry. It also appears in personal care goods, landscaping and yard products, and pesticides sold to consumers.

Why it matters

In the United States the salt is made commercially from potassium chloride and nitric acid, HSDB states. From the mid-19th century to the 1950s, on HSDB’s account, the leading process was a different one: converting Chile saltpetre with potassium chloride. The US Geological Survey (USGS) describes that saltpetre as a mixed sodium–potassium nitrate. Reports to the US Environmental Protection Agency put the national volume for 2023 at 250 million to under 400 million pounds.

USGS classes potassium nitrate as a manufactured potash fertiliser, and fertiliser took about 86 per cent of use in 1975, on HSDB’s breakdown. Heat-transfer salts took another 6 per cent. Glass and ceramics took 4 per cent. Haz-Map, a database of occupational hazards, lists the tempering of steel and baths for plating silver and copper among its uses. In the NCI Thesaurus its chief uses are gunpowder making, fertiliser and medicine.

Properties

Properties of potassium nitrate Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 24434.
Property	Value	Unit	Source
Formula (Hill order)	KNO₃
Molar mass	101.1020(50) g/mol	g/mol	IUPAC/CIAAW 2024
Potassium atoms	1
Nitrogen atoms	1
Oxygen atoms	3
PubChem molecular weight	101.103 g/mol	g/mol	PubChem

Safety

Potassium nitrate does not burn, but it makes other materials burn more fiercely, and on heating it decomposes to nitrogen oxides and oxygen, which raises the fire hazard. Under the signal word Danger, notifications to the European Chemicals Agency class it as an oxidiser that may intensify fire and as an irritant to skin, eyes and airways. Swallowed, it could affect the blood by forming methaemoglobin, the International Chemical Safety Card warns. The PubChem Laboratory Chemical Safety Summary for potassium nitrate records these classifications and its exposure limits.

Questions

Is saltpetre the same as potassium nitrate?

Yes. Potassium nitrate is more commonly known as saltpetre, the US National Park Service notes, and the US Geological Survey spells it saltpeter, with the formula KNO₃, when listing it among manufactured fertilisers. As a natural mineral the same compound is niter, which the RRUFF Project, a database of minerals, writes as K(NO₃).

Why is potassium nitrate in toothpaste for sensitive teeth?

It is the active ingredient of a tooth desensitiser. The US Food and Drug Administration's monograph M022, on over-the-counter oral health care products, defines that kind of ingredient as one acting on the dentin of the tooth to block the perception of stimuli that people with dental hypersensitivity feel. Toothpastes made to prevent decay and ease painful sensitivity fall under US rules for over-the-counter anticaries, or anti-decay, drugs, HSDB notes.

Why is potassium nitrate used in cured meat?

It serves as a source of nitrite. US federal regulation 21 CFR 181.33 records prior sanctions, approvals the US Department of Agriculture issued, for sodium nitrate and potassium nitrate as sources of nitrite in making cured red meat and cured poultry products. JECFA, an expert committee on food additives that the UN's food and health agencies run together, classes potassium nitrate as a colour retention agent and as a preservative.

Can potassium nitrate affect the blood?

Yes, through nitrite. Bacteria working without oxygen can reduce nitrate to nitrite, chiefly in the mouth and stomach and, on HSDB's account, in the lower intestine and bladder too. Nitrite oxidises haemoglobin to methaemoglobin, which cannot carry oxygen to the tissues. Acute poisoning from nitrate-rich well water or vegetables is almost always seen in infants. Their stomachs can be less acidic, which favours nitrate-reducing bacteria, and fetal haemoglobin is oxidised twice as fast as the adult kind.