Phosphoric Acid: Preparation, Properties and Phosphates

Phosphoric acid (H₃PO₄) is an odorless and colourless phosphorus containing inorganic acid. It has four atoms of oxygen, one atom of phosphorus and three atoms of hydrogen. It appears as a clear colourless liquid or as transparent crystalline solid. The melting point of the pure solid is 42.35C and has a density of 1.834gm/cm3. The liquid is 85% aqueous solution. It is also known as Phosphoric (V) acid or Orthophosphoric acid. 

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Structure of Phosphoric Acid

The structure of the Phosphoric Acid consists of a central Phosphorus atom which is bonded together with an oxygen atom through a double bond. It is also connected to three hydroxyls through a single bond. 

Phosphoric Acid Structure

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Properties

Given below are the physical and chemical properties of phosphoric acid:

Physical Properties

The Physical properties of Phosphoric acid are;

• It is colourless, odourless, viscous liquid and contains the property of reddening litmus in high degree, with a density of 1.834gm/cm3.
• When solid, it is white and crystal like with a melting point of 42.35C.
• It is non-toxic and non-volatile in nature. 
• It forms a transparent brittle glass when exposed to red heat and is later cooled.
• When exposed to heat, the acid retains the same composition with new acquired properties. 
• The most commonly used Phosphoric acid concentration is 85% in H?0 water.

Chemical Properties

The Chemical Properties of Phosphoric acid are;

• It can release upto 3 H⁺ ions and thus, it can react differently than the mineral acids.
• Reaction with bases forms three classes of salts by substitution of one, two or three hydrogen atoms. Na?HPO?, NaH?PO?, Na?PO? are formed separately.
• When the molecules are exposed to high temperature, it forms dimers, trimers and long polymeric chains as seen in Polyphosphoric and Meta Phosphoric acids. 

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Phosphates

Phosphates are salts of phosphoric acid. The salts of the Phosphoric acid are compounds that are broadly used in agriculture, industry and in domestic use. The three important phosphate of the Phosphoric acid are;

Ammonium Phosphate

The most common Phosphate salts are monoammonium dihydrogen phosphate and diammonium hydrogen phosphate. These are used in fertilisers and are prepared by combining the correct quantity of Phosphoric acid with anhydrous ammonia in a revolving drum. The selection of the ammonium phosphate depends on the amount of nitrogen and phosphorus required for the crop.

Calcium Phosphates

Calcium Phosphates are also widely used in fertilizer manufacturing. These are also known as superphosphate. The calcium dihydrogen phosphate is made by reaction of sulphuric acid with phosphate rock;

Ca₃(PO₄)₂ + 2H₂SO₄Ca(H₂PO₄)₂ + 2CaSO₄

This  contains 20% of P₂O₅. If the phosphate crystal reacts with phosphoric acid rather than sulfuric acid, then a more intense form of calcium dihydrogen is made with 55% of P?O?. The resultant is known as triple superphosphate.

Ca₃(PO₄)₂ + 4H₃PO₄3Ca(H₄PO₄)₂

Sodium Phosphates

When phosphoric acid reacts with a concentrated solution of sodium hydroxide under controlled conditions and proportions, sodium phosphates are formed. The generated product is in solid crystal form. Some of the examples of sodium phosphates are, 

• Monosodium dihydrogen phosphate: usually used in metal washing and surface formulation. Also used as pH control agent in toothpaste, enamel coating for sanitary ware. It can also be used as coating on lead pipes to protect it from dissolution from the acids present in the water.
• Disodium hydrogen phosphate: it is used as a softening agent in cheese, in enamels and ceramic glazes, for dye production, etc.
• Trisodium phosphate: it is used in heavy duty cleaners, degreasing steel, etc.
• Disodium pyrophosphate: used in production of food items like bread and cakes. Also used as dispersant in oil-well boring mud. 
• Food grade phosphoric acid: used to acidify foods and drinks.

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Preparing Phosphoric Acid

Phosphoric acid can be manufactured using two processes, wet process and thermal process.

Wet Process to Prepare Phosphoric Acid

In the wet process, the phosphoric acid is made from the crystal rock known as fluorapatite, which contains the phosphate mineral. The compound is reacted with concentrated sulfuric acid and water. The reaction leads to formation of phosphoric acid and sulfate along with some other impurities. The excess chemical compounds and the impurities are removed through filtration and evaporation. The acid produced from this process is impure and is often used in fertilizer manufacturing without any further purification.

Thermal Process to Prepare Phosphoric Acid

In this chemical reaction, the phosphorus is heated or burnt at high temperature in the presence of air. This burning or heating leads to generation of phosphorous pentoxide, which is condensed to form a white powder. This white powder is hydrated in a separate process to obtain phosphoric acid.

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Uses of Phosphoric Acid

Phosphoric acid is one of the popular chemical compounds that is used in several industries and even in food processing. Some of the famous uses of phosphoric acid are found in the following industries;

• Agriculture: It is used for manufacturing fertilizers and as a flavouring agent in animal and poultry feed. 
• Dentistry: In dentistry, it is used as an etching solution and for teeth cleaning. It is also used in several mouth cleaning products. It is also found to be used in anti-nausea medicines. 
• Treating Rust: Phosphoric acid is used in corrosion prevention when used in phosphate conversion coating. It is also used in treating and removal of rusts from the metal compounds.
• Skincare Products: It is used as an pH controlling agent in skin care products. It is also used in toothpaste, soaps and detergents. 
• Food Processing: It is used as a food additive and also to acidify the food and beverages. It is also used in dairy industries. 
• Other Uses: It is also used for phosphoric acid fuel cells, for producing activated carbon, for compound semiconductor processing, etc. 

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Hazards of Phosphoric Acid

The concentrated solutions of phosphoric acid are irritants to the skin and the mucous membrane. The fumes from the phosphoric acid may be fatal if inhaled. The fumes generated when in high temperature can be irritating to the respiratory passage and the eyes and skin. 

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Things to Remember

• The formula for phosphoric acid: (H₃PO₄) 
• Phosphoric acid is a colourless, odorless phosphorus which is either a clear liquid or a clear crystalline solid. 
• It is used for various purposes such as agriculture, dentistry, skin care products, rust removal, processing food, etc. 
• There are two ways to prepare phosphoric acid which includes the wet process and thermal process.
• The three types of phosphates are ammonium, calcium and sodium. 

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