Phosphoric Acid                  

Phosphoric acid is one of the most famous acids that are in use in many industries, especially in the manufacturing of fertilizers. The salts of phosphoric acid which are phosphates are mainly in use in agriculture and even at home. Students might even hear this term “phosphoric acid” often in their chemistry classes.

What is Phosphoric Acid?

Phosphoric acid falls into a group of weak acids. Phosphoric acid is also known as orthophosphoric acid which helps us to easily distinguish it from other phosphoric acids such as polyphosphoric acid. Another name for phosphoric acid is phosphoric(V) acid. The chemical name of phosphoric acid’s formula \(H_3PO_4\). Phosphoric acid is a colourless syrup of 85% concentration in water \(H_2O\). The pure compound of phosphoric acid is a colourless solid.

All three hydrogens in phosphoric acid are acidic in nature to varying degrees and can be lost from the molecule as \(H^+\) ions known as protons. When all the three \(H^+\) ions of phosphoric acid are removed, the result is an orthophosphate ion i.e., \(PO_{4}^{3-}\). It is commonly called “phosphate”. Removal of one or two protons from phosphoric acid gives dihydrogen phosphate ion \(H_2PO_4^-\), and the hydrogen phosphate ion \(HPO_4^{2-}\), respectively. Orthophosphoric acid also forms esters i.e., organophosphates.

Phosphoric acid is commonly heard in chemical laboratories as an 85% aqueous solution. Phosphoric acid is a colourless, odourless, and non-volatile syrupy liquid. Thou, phosphoric acid does not meet the strict definition of a strong acid but the 85% solution of it can still severely irritate the skin and damage the eyes.

The name orthophosphoric acid is in use to distinguish this acid from other phosphoric acids like pyrophosphoric acid. The name phosphoric acid is actually the current IUPAC nomenclature.

Preparation of Phosphoric Acid

Phosphoric acid is usually manufactured or prepared using two different types of processes. These processes are the wet process and the thermal process.

1. Wet Process

During the wet process of preparing phosphoric acid, it is produced from a naturally occurring crystal rock fluorapatite. Fluorapatite contains the phosphate mineral within it. This compound is then reacted with concentrated sulphuric acid \(H_2SO_4\) and water \(H_2O\). When this reaction takes place, it results in the formation of phosphoric acid and calcium sulfate i.e., gypsum with some amount of insoluble impurities. The extra chemical compounds and impurities are later removed by the process of filtration and evaporation. The acid is then concentrated to ca 56% – 70% \(P_2O_5\) I.E., super phosphoric acid using vacuum distillation. The reaction can be written as:

\(Ca_5(PO_4)3Cl + 5H_2SO_4 +10H_20 \rightarrow 3H_3PO_4 + 5CaSO_4.2H_2O + HCl\)

The product made from the wet process is an impure acid. This impure acid can be in use without further purification, for fertilizer manufacture.

2. Thermal process

Another method that is in use to produce phosphoric acid is the thermal process. In this process, phosphorus is given sufficient heat or burnt at high temperature in the presence of air. The results of burning in the generation of phosphorus pentoxide are then condensed to form a white powdery substance. Later It is hydrated in a separate process to obtain phosphoric acid.

Sometimes sufficient amount of steam is also given to the burner where a condensed form of polyphosphoric acids is produced. The products are then directly allowed to pass through a hydration tower. In the hydration tower, the gaseous phosphorus oxide is absorbed and phosphoric acid is obtained.

Salts of Phosphoric Acid

The salts of phosphoric acid are Phosphates. The important phosphates are –

1. Ammonium phosphates

Diammonium hydrogen phosphate and monoammonium dihydrogen phosphate and are commonly in use as fertilizers. They are produced by mixing the correct quantity of phosphoric acid with anhydrous ammonia in a revolving drum. The selection of which ammonium phosphate to use for the crop relies on the amount of nitrogen and phosphorus.

2. Calcium Phosphates

Calcium phosphates are broadly in use as fertilizers. It is made by the reaction of sulfuric acid with phosphate rock

\(Ca_3(PO_4)_2 + H_2SO_4 \rightarrow Ca(H_2PO_4)_2 + 2 CaSO_4\)

This is the superphosphate. It contains about 20% \(P_2O_5\). On the reaction of phosphate, crystal is with phosphoric acid, other than sulfuric acid, a more intense form of calcium dihydrogen phosphate is produced with a general higher \(P_2O_5\) level i.e., 55%

\(Ca_3(PO_4)_2 + 4H_3PO_4 \rightarrow 3Ca(H_2PO_2)_2\)

This is triple superphosphate. This developed level of phosphate is attained because the yield is no longer diluted with calcium sulfate.

3. Sodium Phosphates

On the reaction of phosphoric acid with a concentrated solution of sodium hydroxide under controlled conditions proportions of sodium, phosphates are formed. The product formed is in the form of solid crystals. Examples of sodium phosphates are Disodium hydrogen phosphate, Trisodium phosphate, Disodium pyrophosphate and Monosodium dihydrogen phosphate.

FAQs on Phosphoric Acid

Question 1: Write a short note on the hazards of phosphoric acid.

Answer: Phosphoric acid is non-toxic in nature. Phosphoric acid does not cause harm to the skin or any part of the body in low concentration. Phosphoric acid is dangerous only at higher concentrations and can cause severe skin irritation or burns and even damage to the eyes. It can also cause irritation in the respiratory tract if vapours are inhaled in the body. Phosphoric acid should be kept in a metallic or coated fibreboard container with polyethylene inner package and in a cool and well-ventilated place.

Question 2: What are the physical and chemical properties of phosphoric acid?

Answer: Physical properties: A pure phosphoric acid is normally in the form of a white crystalline solid. It has a melting point of \(42.4^oC\). The acid is colourless, odourless and a viscous liquid with a density of 2.030 g.cm. Phosphoric acid is non-toxic and non-volatile in nature.

Chemical properties: Phosphoric acid or \(H_3PO_4\) release up to three \(H^+\) ions. Due to this property, phosphorous acid can react differently in comparison to other mineral acids. On reaction with bases, usually results in the formation of three classes of salts. When the molecules of phosphoric acid are exposed to high temperatures it forms dimers, trimers also long polymeric chains as seen in polyphosphoric acids and meta-phosphoric acids.

Question 3: What are the uses of phosphoric acid?

Answer: The common uses of phosphoric acid are –

In Agriculture

The most common use of phosphoric acid is in the agriculture domain. Phosphoric acid is broadly in use in the production of fertilizer and as a flavouring agent in animal or poultry feed.

In Dentistry

Phosphoric acid is in use in dentistry where dentists often use the chemical compound as an etching solution and for cleaning the teeth. Phosphoric acid is also present in mouth cleaning products. Other than this phosphoric acid is found in anti-nausea medicines.

In the treatment of Rust

Phosphoric acid is in use in treating rusts and removing them from metal components. It is useful in the process of the phosphate conversion coating. Phosphoric acid helps in corrosion resistance.

Skincare Products

Phosphoric acid widely in use in adjusting or controlling the pH level in skincare products. Phosphoric acid is present in toothpaste, soaps, and detergents as well.

Food and Beverage Industry

Phosphoric acid is in use as a food additive and to acidify foods and beverages. Phosphoric acid helps in creating a certain taste.