Phosphoric Acid [H3PO4]: Structure | Properties | Uses

Phosphoric acid is a corrosive inorganic acid with the chemical formula H3PO4. It’s a weak acid, available in a range of quantities, purities and reagent grades.

In its pure form, phosphoric acid is a colorless solid. In less concentrated form, it is odorless, viscous liquid with a density of 1.885 g/mL. It is usually non-volatile and non-toxic, but 85 percent solution can still damage the eyes and severely irritate the skin.

The IUPAC term for this compound is orthophosphoric acid. Ortho is a Greek term for ‘true,’ so the prefix ‘ortho’ refers to the true form of phosphoric acid, i.e., H3PO4.

Let’s dig deeper and find out how phosphoric acid is produced, what it looks like on molecular scales, what are its physical and chemical properties, and where is it used.

Phosphoric Acid Profile

Molar mass: 97.994 g/mol
Appearance: Thick, colorless liquid

Boiling point: 485 K or 212 °C
Melting point: 315.5 K or 42.35 °C (of crystalline solid)

Dynamic viscosity: 2.4 to 9.4 centipoise (85% aqueous solution)
Density: 1.83 g/cm3 (solid);  1.68 g/cm3 (85% aqueous solution at 25 °C)

Structure

The H3PO4 Lewis structure

The H3PO4 compound has a total of 32 valence electrons. Since Phosphorus is the least electronegative, it is placed in the center. Hydrogen atoms are attached to the outside of the Oxygen atoms to form OH groups.

In this structure, each atom has a full outer shell: Phosphorus and Oxygen atoms have eight valence electrons, and Hydrogen atoms have two valence electrons around them.

Phosphorus can have an expanded octet (more than eight valence electrons), so it obtains a formal charge of +1 while Oxygen (the one that is not attached to Hydrogen atom) obtains a formal charge of -1. By forming a double bond, Phosphorous and Oxygen lose their charges.

There are still 32 valence electrons in the structure, and each atom has a formal charge of zero. Thus, it is the most likely or plausible Lewis structure for H3PO4.

The compound contains 3 O-H single bonds, 3 P-O single bonds, and 1 P=O double bond. Every single bond is a sigma bond, while the double bond consists of a sigma and a pi bond.

h3po4 structurePhosphoric acid: ball-and-stick model and dimensions of structural formula  

The molecular geometry of PO(OH)is tetrahedral, and the crystal structure is monoclinic.  And since there is asymmetric charge distribution on the central atoms, the molecule is said to be polar.

Recent advances in nanotechnology, including X-ray absorption near edge structure and large angle X-ray scattering techniques, have allowed scientists to precisely determine the structure of phosphoric acid.  The P=O bond distance in the structure is close to 152 picometers, and the P=O-O distances have been refined to 309 picometers, with a bond angle close to tetrahedral.

How Is It Produced?

Phosphoric acid can be synthesized through either a wet process or a thermal process.

In the latter, air, water, and elemental Phosphorus are used as raw materials. It involves three crucial steps: combustion, hydration, and demisting. This is how it goes:

First, the liquid elemental Phosphorus is burned in a combustion chamber at high temperatures ranging between 1650 and 2760 °C. This oxidation reaction takes place in the ambient air and produces phosphorus pentoxide.

4P  +  5O->  2 P2O5

The product is then hydrated with water to form strong phosphoric acid (in the liquid state).

P2O+  6 H2O  ->  2 P2O5

In the final step, high-pressure-drop demisters are used to remove the phosphoric acid mist from the combustion gas stream.

The concentration of the phosphoric acid synthesized in this process usually ranges between 75 and 85 percent. Such concentration levels are needed for nonfertilizer product manufacturing and some high-grade chemical production. A few efficient plants recover extremely concentred phosphoric acid (as high as 99.9%) using the same thermal process.

However, almost 80% of the phosphoric acid is manufactured via the wet-process. In this method, the sulfuric acid is treated with a naturally occurring, phosphate-containing mineral such as hydroxyapatite.

Ca5(PO4)3OH  +  5 H2SO ->  3 H3PO+  5 CaSO+  H2O

The mineral is dried, crumbled, and then fed into the reactor along with sulfuric acid. The reaction merges sulfate with calcium (from the mineral), producing calcium sulfate (gypsum).

Processed water is then added, and gypsum, along with other insoluble impurities, is eliminated through a filter. Phosphoric acid produced in this wet-process contains 25-30% phosphorus pentoxide.

Usually, the acid is further concentrated to meet the requirements for fertilizer production. In most cases, phosphoric acid is concentrated to 40-55% phosphorus pentoxide through two/three vacuum evaporators.

Properties

The pure phosphoric acid in the lab looks like a thick, colorless, orderless, crystalline solid that melts at 42.35 °C. Its physical property depends on the purity, concentration of the acid, as well as the temperature at which you are viewing it.

Phosphoric acid is less corrosive and less hazardous compared to nitric and sulfuric acid. It is a non-combustible substance in both liquid and solid forms.

When the acid is exposed to red heat and then cooled to room temperature, it forms a transparent, brittle glass. The composition of acid remains the same after this exposure to heat.

Generally, phosphoric acid is available as a colorless syrup of 85 percent concentration in water, with a density of 1.885 grams per milliliter.

In aqueous solution, phosphoric acid acts like a triprotic acid, having three ionizable hydrogen atoms. The compound can lose these atoms as protons (H+ ions).

  • When one proton is removed, the result is a dihydrogen phosphate ion H2PO4
  • When two protons are removed, the result is a hydrogen phosphate ion HPO42−
  • Removal of all three protons gives an orthophosphate ion PO43−.

Read: 10 Strongest Bases Ever Synthesized

The acid is soluble in both water and ethanol. However, most of its salts are water-insoluble unless a strong mineral acid is present.

Applications

Phosphoric acid has several essential applications. Its salts, such as ammonium phosphates and calcium phosphates, are widely used as fertilizers. The acid is used to produce electrolytes, rust removal agents, pH modifiers, etching agents, household cleaning agents, and sanitizing agents.

Since the substance is non-toxic and mildly acidic in nature, it is also used in beverages, food flavoring, cosmetics, and dental and skincare products. Let’s elaborate on how useful is phosphoric acid.

In a less concentrated form, it is used to manufacture various types of safer phosphate fertilizers to help plants grow. In fact, almost 90% of the synthesized phosphoric acid is used for fertilizers.

Many soft drinks, including Coca-Cola, contain phosphoric acid. It gives the drink a slightly sour flavor. Although its concentration is quite low, the extreme consumption of such drinks can cause dental erosion and even lead to the formation of kidney stones.

Phosphoric acid has advantageous characteristics as an electrolyte, such as good ionic conductivity, low volatility, stability at relatively high temperatures, and carbon dioxide and carbon monoxide tolerance.

Since pure phosphoric acid has high proton conductivity and excellent thermal stability, polybenzimidazole (PBI) doped with phosphoric acid is the most promising among PBI-based membranes, which can deliver good fuel cell performance even at low humidification levels and 200 °C.

Specific uses of phosphoric acid: 

  • Sodium triphosphate is used in soaps and detergents.
  • Dicalcium phosphate is used in toothpaste as a polishing agent.
  • Phosphate coating is applied to steel parts for corrosion resistance and lubrication.
  • It is used to control the pH of cosmetic and personal care products.
  • In the construction industry, it is used to remove mineral deposits and clean hard water stains,

Global Market

The global phosphoric acid market is expected to reach $61 billion by 2027

Many developing countries have started focusing on scaling up their phosphate mining and phosphate production. Some national governments have already collaborated with different vendors globally to set up plants for obtaining minerals for phosphoric acid production.

In 2019, the global phosphoric acid market size was valued at $45.85 billion. It is expected to reach over $61 billion by 2027, with a compound annual growth rate of 3.7%.

Read: 8 Strongest Acids Ever Known To Us

The market is categorized into Asia Pacific, Europe, North America, Latin America, and the Middle East and Africa. The Asia Pacific region is expected to dominate the global market in the near future. The growth of the phosphoric acid market in those regions will be further fueled by the developed agriculture sector of India and China.

Written by
Varun Kumar

Varun Kumar is a professional science and technology journalist and a big fan of AI, machines, and space exploration. He received a Master's degree in computer science from Indraprastha University. To find out about his latest projects, feel free to directly email him at [email protected] 

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