Phosphoric acid

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8 atoms, 7 bonds.

IUPAC name: phosphoric acid
Molar mass: 97.9940(56) g/mol
Atoms: 8
SMILES: OP(=O)(O)O
InChIKey: NBIIXXVUZAFLBC-UHFFFAOYSA-N
PubChem CID: 1004

Made of

Structural formula of phosphoric acid. Every atom in the molecule is drawn. It shows which atoms are joined to which, not the shape. The model above shows the shape.

Most of the phosphate rock dug from US mines, more than 95 per cent by US Geological Survey figures, is turned into wet-process phosphoric acid and superphosphoric acid. The wet process digests the rock with sulfuric acid. From there it becomes ammonium phosphate fertilisers and animal feed supplements. HSDB, the Hazardous Substances Data Bank, ranks it as the second largest volume mineral acid produced, behind sulfuric acid.

Each molecule holds one phosphorus atom, four oxygen atoms and three hydrogen atoms, for a molar mass of 97.994 g/mol. The pure compound is a solid that melts at 42.35 °C, but what industry ships is usually an 85 per cent solution in water. HSDB describes that liquid as syrupy and the 100 per cent acid as crystals. The acid has no smell, tastes sour, does not burn and mixes with water in any proportion.

Structure and bonding

The viewer shows one phosphorus atom at the centre with four oxygen atoms around it, three of them each carrying a hydrogen. The model draws the bare oxygen with a double bond and the other three with single bonds. In Ball-and-stick that double bond is visibly the shortest link to phosphorus: 1.50 Å in the model, against 1.62 Å for each P–O bond that carries a hydrogen.

The angles around phosphorus are not all equal. Each O=P–O angle in the model is about 114°, while the angle between any two hydroxyl oxygens (the oxygens carrying a hydrogen) is about 104 to 105°. At every hydroxyl oxygen the P–O–H angle measures 119.0°. In Space-filling the phosphorus is almost hidden behind its four oxygens, with the three small hydrogens capping the outside, and Labels marks each sphere P, O or H.

What the viewer shows is the neutral molecule. In water the acid gives up its hydrogens one at a time, as Chem1 sets out: first to dihydrogen phosphate, H₂PO₄⁻, then hydrogen phosphate, HPO₄²⁻, then phosphate, PO₄³⁻. Near neutral pH, most of it is present as dihydrogen phosphate and hydrogen phosphate ions rather than as the drawn molecule.

Where it turns up

In fizzy soft drinks the acid supplies the sourness. HSDB calls it the most economical acidulant, or souring ingredient, for carbonated drinks. The US Food and Drug Administration (FDA) lists its technical effects in food as including pH control agent, sequestrant, a substance that binds metal ions, and flavour enhancer.

Teeth meet it a second way. In dentistry and orthodontics it serves as an etching solution, according to the NCI Thesaurus of the US National Cancer Institute. The etch cleans and roughens the tooth surface where fillings or dental appliances will be placed. The same acid at 85 per cent is a standard wet etchant in semiconductor manufacturing.

Metalworking takes it for rustproofing metals before painting and for brightening aluminium. The US Consumer Product Information Database records it in 334 consumer products, from car-care goods to pet-care items. The USGS also notes a Quebec company planning high-purity acid for lithium-iron-phosphate battery cathodes as well as for food and industry.

Why it matters

Phosphoric acid is the step between a phosphate mine and a fertilised field. In 1983, 89 per cent of it went into fertilisers, on HSDB’s figures, with animal feeds, detergents and water treatment, and food and drink sharing most of the rest.

In the laboratory the acid and its three ions form what Chem1 calls a triprotic system, one that can give up three protons. It is widely used for buffers, solutions that keep their pH nearly steady when acid or base is added, in biochemistry. The pK_a of an acid-base pair is the pH at which its acid and base forms are equal in concentration. A buffer works best when that value lies close to the target pH. Chem1 puts the pK_a of the dihydrogen phosphate and hydrogen phosphate pair at 7.21, close to neutral.

Phosphate is also a constituent of bone and teeth. Parathyroid hormone keeps blood levels of inorganic phosphate steady by exchange with the mineral in the skeleton. Vitamin D, for its part, stimulates the gut’s absorption of phosphate.

Properties

Properties of phosphoric acid Molar mass summed from IUPAC/CIAAW 2024 standard atomic weights; formula, atom and bond counts and PubChem's molecular weight from PubChem CID 1004.
Property	Value	Unit	Source
Molecular formula (Hill order)	H₃O₄P
Molar mass	97.9940(56) g/mol	g/mol	IUPAC/CIAAW 2024
Hydrogen atoms	3
Phosphorus atoms	1
Oxygen atoms	4
Bonds	7
Single bonds	6
Double bonds	1
PubChem molecular weight	97.995 g/mol	g/mol	PubChem

Safety

Concentrated phosphoric acid is corrosive to skin and eyes. In the Globally Harmonized System (GHS), 99.9% of 4293 European Chemicals Agency reports give it the signal word Danger and the statement “causes severe skin burns and eye damage”. The acid reacts with metals to release flammable hydrogen. The PubChem Laboratory Chemical Safety Summary and the NIOSH Pocket Guide card for phosphoric acid give the full hazard information.

Questions

What shape is a phosphoric acid molecule?

ChEBI describes it as one oxo oxygen, bonded to phosphorus alone, and three hydroxy (O–H) groups joined to a central phosphorus atom, so the phosphorus has four oxygen neighbours. PubChem's computed model draws the oxo oxygen with a double bond, a little closer to phosphorus than the others, and the angles around phosphorus are not all equal. In water the molecule loses its hydrogens one at a time.

Is phosphoric acid a strong or weak acid?

Weak: CAMEO Chemicals, the US hazardous-materials database, files it among the weak acids. It releases its three protons in separate steps, each with its own pK value, the pH at which that step's acid and base forms are equal in concentration. HSDB lists 2.15, 7.09 and 12.32, and Chem1's buffer table gives 2.16, 7.21 and 12.3. HSDB reports a pH of 1.5 for a 0.1 normal solution in water.

Why is phosphoric acid in soft drinks?

It makes the drink sour and holds its acidity steady. The US Food and Drug Administration (FDA) lists pH control agent, flavour enhancer and sequestrant among its technical effects in food; a sequestrant binds metal ions, and the NCI Thesaurus names iron, copper, calcium and magnesium ions among those it binds. HSDB rates it the most economical souring agent on the market for carbonated drinks.

Is phosphoric acid dangerous?

Concentrated phosphoric acid is corrosive to metals and to living tissue, CAMEO Chemicals notes. HSDB describes its mist as an irritant to the eyes, upper airways and skin. The dilute acid in soft drinks is different, a food additive that the US Food and Drug Administration lists. The liquid sold for industry is usually an 85 per cent solution, and the PubChem Laboratory Chemical Safety Summary sets out its hazards.

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An acid gives a proton and a base takes it: Arrhenius, Brønsted–Lowry and Lewis Hydrogen chloride gives its proton to water effectively completely; acetic acid gives only a small fraction.