Phosphoric Acid

Phosphoric acid is also called orthophosphoric acid. It is a weak, inorganic acid that is obtained from the mineral phosphorus. In raw form, it can be obtained from phosphate rocks, whereas, in pure, it is produced in the industries from white phosphorus. In pure form, it exists in the solid crystalline state. In the liquid state, it is colorless, odorless and non-volatile and is less concentrated. Further, it can be found in various foods and beverages such as soft drinks, dairy products, cereal bars, flavored waters, and processed meats.

Chemical Structure of Phosphoric Acid

The chemical or molecular formula of phosphoric acid is H₃PO₄. So, it is made of one phosphorus atom, four oxygen atoms and three atoms of hydrogen.

It is a phosphorus oxoacid that is made of one oxo and three hydroxy groups bonded to phosphorus atom by covalent bonds. The phosphorus at center is connected to the oxygen atom by a double bond, whereas, it is connected to three hydroxyl groups (-OH) through single bonds. So, it can donate up to three H⁺ ions.

The three hydrogen atoms are acidic in nature and can separate themselves from the molecule as H⁺ ions called protons. When it happens or all hydrogen atoms leave the molecule, the orthophosphate ion (PO₄^3-) is formed. This ion is known as phosphate. If one proton is removed, it gives dihydrogen phosphate ion H₂PO- ₄, similarly, if two protons are removed, it forms hydrogen phosphate ion HPO₄^2-.

Phosphoric acid is safe for children and it does not contain genetically modified organisms (GMOs). It has been used in food since 1885. It was used first in the preparation of formulation of beverage that later known as Coca-Cola.

It was discovered independently by two Swedish chemists as a part of bone ash in 1770. After four years, Scheele found that this acid can be made by adding nitric acid to phosphorus.

Physical Properties of Phosphoric acid

• Its molecular weight is 98.00 g/mol.
• Its melting point is 42.4 degrees Celsius.
• Its boiling point is 158 degrees Celsius.
• It is highly soluble in water and ethyl alcohol.
• Its density is 1.88 g/cm³.
• It exists as a clear colorless liquid as well as a transparent crystalline solid.
• It changes into a transparent brittle glass when it is exposed to red heat and then cooled.

Chemical Properties of Phosphoric acid

• It is a phosphorus-containing inorganic acid.
• It is a weak acid with three acidic and replaceable H atoms.
• When the temperature is high, phosphoric acid molecules can react with each other to form dimers, trimers, and even long polymeric chains
• Its salts are solid and most of them are relatively insoluble in water unless a strong mineral acid is present.

Preparation of Phosphoric Acid

There are two different processes or methods for the preparation of phosphoric acid as described below;

1. Wet Process

In this method, it is prepared from a crystal rock fluorapatite, which is found naturally and contains the phosphate mineral. The fluorapatite is reacted with concentrated sulphuric acid H₂SO₄ and water H₂O to form phosphoric acid and calcium sulfate or gypsum with some amount of insoluble impurities. The unwanted chemical compounds and impurities are separated through filtration and evaporation.

Later the acid is concentrated to 56% to 70% P₂O₅ through vacuum distillation. The reaction takes place as shown below;

Ca₅ (PO₄)3Cl + 5H₂SO₄ + 10H₂O → 3H₃PO₄ + 5CaSO₄.2H₂O + HCl

The acid formed in this method is an impure acid. It can be used in the manufacturing of fertilizers without purifying it further.

2. Thermal Process

It is another method to produce phosphoric acid. In this method, phosphorus is heated or burnt at high temperature in the presence of atmospheric oxygen. It leads to the formation of phosphorus pentoxide, which is condensed to form a white powdery substance. Then it is hydrated by a separate process to get phosphoric acid.

Furthermore, in some cases, steam is also supplied to the burner to form a condensed form of polyphosphoric acid. This acid is then made to pass through a hydration tower where the gaseous phosphorus oxide is absorbed and phosphoric acid is obtained.

Phosphoric Acid Salts

They are called phosphates, some of the important phosphates are described below;

1. Ammonium phosphates

Diammonium hydrogen and monoammonium dihydrogen phosphates are commonly used as fertilizers. These phosphates are obtained by mixing a specified quantity of phosphoric acid with anhydrous ammonia in a revolving drum.

2. Calcium Phosphates

They are widely used as fertilizers. They are produced by reacting sulfuric acid with phosphate rock. The reaction that takes place is shown below:

Ca₃(PO₄)₂ + H₂SO₄ →Ca(H₂PO₄)2 + 2CaSO₄

This phosphate is superphosphate with 20% P₂O₅. When phosphate rock is reacted with phosphoric acid other than sulfuric acid, a more concentrated form of calcium dihydrogen phosphate is produced that is with 55% P₂O₅. The reaction is given below:

Ca₃(PO4)2 + 4H₃PO₄ → 3Ca(H₂PO₂)2

Triple superphosphate is produced in the above reaction. This phosphate level is obtained as the product is not diluted with calcium sulfate in this case.

3. Sodium Phosphates

This type of phosphates is formed when phosphoric acid is reacted with a concentrated solution of sodium hydroxide. The phosphates are formed in the form of solid crystals. Some examples are Disodium hydrogen phosphate, Trisodium phosphate, Monosodium dihydrogen phosphate, etc.

Phosphoric Acid Uses

Phosphoric acid offers lots of uses. It is used in several industries, agriculture and products that we use. Here are some of its major uses:

Removal of Rust

It is used for removing rust from metals like iron, steel, and more. When it is applied to the rust, it reacts with the rust and changes the reddish-brown iron into a black-colored ferric phosphate, which can be removed easily.

Food and beverage industry

It is widely used as a food additive. It regulates the acidic character of foods like jams, cheese, processed meats, and more. Further, in the beverage industry, it is used as an acidulant, which gives specific flavors to foods. It is also used to check the growth of fungi and bacteria.

Personal care

It is widely used in the preparation of a large variety of personal care products such as cleaning products, bath products, hair care products, makeup items, etc. further, it is also used to control the pH of these products.

In Agriculture:

It is also used in the production of fertilizers. Further, it is also used as a flavoring agent in animal and poultry foods.

Pharmaceutical:

In the pharma industry, it is used as an intermediate. It is also used in dentistry to clean teeth. It is also found in teeth whiteners or mouth washes. Further, it is also an ingredient in anti-nausea medicines.