The Structure, Chemical Formula, and Properties of Phosphoric Acid

Properties of phosphoric acid
Phosphoric acid is a commonly used chemical in a wide variety of applications. Here's more about the structure, chemical formula, and uses of phosphoric acid.
Did You Know?
Along with carbonic acid, phosphoric acid is the inorganic acid most ingested by humans. It is used in various foods to add a sour flavor, including in some carbonated drinks.
Phosphoric acid is an inorganic or mineral acid with the chemical formula H3PO4. It is also known as orthophosphoric acid. Its IUPAC name is actually trihydroxidooxidophosphorus phosphoric acid, but thankfully, it is virtually never called that.
It can be formed by two processes; wet and thermal. The former is more common, and gives a less pure form of phosphoric acid that is acceptable for industrial uses. For applications in the food or cosmetic industry, purer forms are obtained by the thermal method. The most commonly employed reaction for the wet process is given below. The P2O5 content in the product of this method may vary from 25% to 70%.

Ca5(PO4)3X + 5H2SO4 + 10H2O ➙ 3H3PO4 + 5CaSO4.2H2O + HX

The 'X' in this reaction can be a halide (F-, Cl-, Br-) or a hydroxide (OH-) ion.

The thermal process of procuring phosphoric acid consists of dissolving phosphorus pentoxide in dilute phosphroic acid. This gives a very pure form of phosphoric acid that can be used for more sensitive applications, but requires elemental white phosphorus, which can be hard to obtain.
The phosphorus atom in a phosphoric acid molecule is bonded with 4 oxygen atoms, one with a double bond and the other three with single bonds. The oxygen atoms with a single bond are further connected to one hydrogen atom each, thus satisfying their valency. The single-bonded oxygen atoms are bunched much closer to each other than any of them is to the double-bonded oxygen atom. The 3D structure of phosphoric acid is given below.
Structure of Phosphoric acid
Here's a 2D version of the structure, for easier comprehension.
2D Structure of Phosphoric acid
  • As mentioned before, pure anhydrous phosphoric acid is a white crystalline solid in normal conditions. It melts at 108.2°F (42.35°C) and boils (decomposes) at 316.4°F (158°C).

  • It has a density of 2.03 g/mL in the solid state, and 1.685 g/mL in the most commonly used form of an 85% solution.

  • It has the rare property of being an inorganic acid that can form chains of its own molecules. These reactions, typically occurring at high temperature and accounting for the loss of water molecules, form classes of phosphoric acids known as pyrophosphoric and metaphosphoric acids. The general formula of these compounds is, respectively, H(HPO3)nOH and (HPO3)n, where n = 3,4,5....

  • Metaphosphoric acid is a solid, glassy substance and is the most dehydrated form of phosphoric acid achievable by just heating. It is often used as a dehydrating agent itself, since it becomes very hygroscopic (attracted to water). Only an extremely strong desiccant will change metaphosphoric acid to phosphorus pentoxide.
Phosphate Fertilizers
► Phosphoric acid is arguably the most used mineral acid. Most of it (about 90%) is used in the production of phosphate fertilizers.
Carbonate Drinks
► It is nontoxic in dilute form, and is thus used in numerous food products. It is primarily used in many carbonated drinks for its sour taste. It is widely available and cheaper to produce than organic substitutes such as citric or malic acid.
Dentist use
► It is used in dentistry to clean and roughen the surface of a tooth that is about to receive a filling.
► In one of the less widespread uses, it can also be used to remove and to a degree counteract corrosion. It reacts with iron oxides to form ferric phosphate. This can be easily scrubbed off or even left on the surface as a moderate protection against further corrosion.
Industries uses
► It is used as a pH-adjuster in some industries. Hydroponics employs phosphoric acid to lower the pH of the solution, with the dual benefit that phosphorus aids plant growth, particularly flowering. It is also used in cosmetic products to counteract alkaline components.