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Phosphoric Acid

Phosphoric Acid – Formula, Structure, Preparation, Physical and Chemical Properties, Uses and Health Hazards

How many of you can say you haven't had a cola drink at least once in your life?

Some of you may be completely in love with such drinks, while others may have a strong aversion to them. When it comes to the cola-craze, some of you may even fall into the balancing zone!

Whatever the case may be, did you know that the special tanginess and delicacy in any of your cola drinks is owed to one special component- "Phosphoric acid"? 

Yes, indeed. Phosphoric acid is responsible for the flavour of cola drinks.


Citric acid replaces phosphoric acid in other non-alcoholic beverages except for cola drinks. Phosphoric acid is also found in numerous sports drinks, bottled teas, and fruit-flavoured drinks, in addition to cola drinks.

In a 2006 study published in the American Journal of Clinical Nutrition, researchers discovered that cola, not carbonated beverages, causes osteoporosis in women and that phosphoric acid can harm bone health. Some epidemiological studies, including one published in the American Journal of Clinical Nutrition, have connected phosphoric acid to reduced bone density. 

On the contrary, other research has found that a low phosphorus intake causes decreased bone density. Guess who paid for the research? PepsiCo.


Phosphoric Acid - Introduction

Phosphoric acid is an oxoacid of phosphorus with four oxygen atoms, one phosphorus atom, and three hydrogen atoms. It is also known as phosphoric(V) acid or orthophosphoric acid. It is found in teeth and bones, and it helps in metabolic processes. It appears as a clear, colourless solution in liquid form and as a transparent, crystalline solid in solid form.

Orthophosphoric acid (H3PO4) is one of the most popular acids that is used in many industries, especially in the manufacturing of fertilisers. The salts of this acid are known as phosphates. 

Phosphoric acid is a triprotic acid in nature with three donatable protons. It aids in the binding of divalent cations as a sequestering agent. It is extensively used in orthodontics and dentistry. Phosphoric acid has an important role in biogeochemistry and biochemistry.

It ionises to give three H+ ions. Hence, its basicity is 3. All three hydrogens are acidic, with pKa1=2.14, pKa2=7.20, and pKa3=12.37 for the first, second, and third hydrogens, respectively.

Phosphoric Acid - Structure

The structural formula is PO-(OH)3 or H3PO4 is used to characterise it and the hybridisation of phosphorus in orthophosphoric acid is sp3. The geometry of orthophosphoric acid is tetrahedral. A double bond holds the core phosphorus atom and an oxygen atom together. Through single bonds, it is additionally attached to three hydroxyl (-OH) groups.


Phosphoric Acid - Preparation

Fluorapatite contains phosphate minerals, which are used to make phosphoric acid (a naturally occurring crystal rock). Fluorapatite reacts with concentrated sulphuric acid and water. As a result, phosphoric acid (H3PO4) and calcium sulphate, as well as other contaminants, are generated. The excess chemical compounds and contaminants are removed using the filtration and evaporation processes.

  • Red phosphorus on being heated with conc. HNO3 produces phosphoric acid.

P4 + 5HNO3 → H3PO4 + H2O + 5NO2

  • Wet Process: Phosphoric acid is prepared by adding sulphuric acid to calcium phosphate rock.

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

Where X = F, Cl, Br, -OH

  • Thermal Process: Phosphorus is heated at a high temperature with air in this procedure. This produces phosphorus pentoxide, which is then condensed to form a white powder. The white powder is then hydrated in a separate process to produce phosphoric acid. We can make pure phosphoric acid through this process. Another method of preparing orthophosphoric acid is by bubbling phosphorus pentoxide with water.

P4O10 + 6H2O→ 4H3PO4

Phosphoric Acid - Physical Properties 

  • Phosphoric acid (H3PO4) is a white crystalline substance in its pure state.
  • It is non-toxic and in its pure form, it is solid at room temperature.
  • It has a molar mass of 97.99 g mol-1.
  • It is mostly available in the form of an aqueous solution (almost 85%) and is an odourless, colourless, and non-volatile liquid. 
  • Orthophosphoric acid is a sticky liquid. It is treated as a weak acid but it still can cause irritation on the skin and can damage the eyes. It can also damage the membranes in the nose.
  • Phosphoric acid (H3PO4) has a melting point of 42.4°C.
  • H3PO4 is non-volatile, colourless, and odourless.
  • It is viscous in liquid form and has a density of 2.030 g cm-3.

Phosphoric acid - Chemical properties

  • The oxidation state of P in H3PO4 is +5. 
  • Phosphoric acid or H3PO4 can release up to three H+ ions. It is a tribasic acid. 

H3PO4 ⇌ H2PO4- + H+, pKa1 = 2.14

H2PO4-⇌ HPO42- + H+, pKa2 = 7.20

HPO42-⇌PO43- + H+, pKa3 =12.37

  • The reaction of phosphoric acid with bases usually results in the formation of three categories or classes of salts.
  • Phosphoric acid molecules form dimers, trimers, and even lengthy polymeric chains when exposed to high temperatures, as observed in polyphosphoric acids and metaphosphoric acids.
  • Cyclo triphosphate acid, trimetaphosphoric acid, and trimetaphosphate are all names for cyclic meta-phosphoric acid (H3O9P3). It belongs to the non-metal phosphates class of inorganic chemicals.


Phosphoric Acid - Uses

Orthophosphoric acid is an important chemical with a wide range of applications in a variety of industries, agriculture, and everyday products. In any event, we will look at some of the most common applications of phosphoric acid.

Rust Removal

  • Phosphoric acid is a chemical that is commonly used to eliminate rust from metals like iron and steel.
  • When this acid is added to rust, it reacts with the rust and transforms the reddish-brown iron, which is often ferric oxide (iron oxide), into a black-coloured molecule called ferric phosphate. This black ferric phosphate material is simple to get rid of.

In Food and Beverage

  • Food additives containing phosphoric acid are common. Jams, cereal bars, processed meats, cheese, and other foods contain this acid, which works as an acidity regulator.
  • Phosphoric acid is used as an acidulant in the beverage industry.
  • It also enhances the flavour of these beverages. It is also used in the dairy, food, and brewing industries as a sanitising agent.
  • It aids in the prevention of fungal and bacterium growth.

In Agriculture

  • According to some statistics, about 80% of the phosphoric acid produced is utilised to make fertiliser, especially phosphate fertilisers like ammonium phosphate.
  • Phosphoric acid is also utilised in animal and poultry feed as an additive and flavouring ingredient.

Personal Care

  • Phosphoric acid is utilised in the production of a variety of personal care products.
  • Cleaning products, bath products, fragrances, hair care and colour products, nail products, makeup, and other skin care products are just a few examples.
  • Phosphoric acid, on the other hand, is utilised to regulate the pH of these skincare products.

Pharmaceutical Industry

  • The majority of the time, this acid is used as a pharmaceutical intermediate.
  • Dentistry is one of the most common uses for phosphoric acid. 
  • It is commonly used to clean teeth and is used as an etching solution.
  • Phosphoric acid is also used in teeth whiteners and mouthwash liquids. Anti-nausea medications frequently contain phosphoric acid.

Other Applications

  • It is used as an external standard in analytical techniques such as NMR. Phosphorus-31 is used in nuclear magnetic resonance (NMR).
  • In fuel cells and oxyhydrogen generators, phosphoric acid is utilised as an electrolyte. Phosphoric acid is also used to make synthetic detergents and cleanse water and metals.
  • It is also used in construction to get rid of mineral deposits, cement streaks, and hard water stains.
  • It forms activated carbon compounds by acting as a chemical oxidising agent.
  • Other uses include phosphoric acid fuel cells and the generation of activated carbon.
  • To selectively etch indium, gallium, and arsenide with respect to indium phosphide in compound semiconductor manufacturing.
  • It is also used to selectively etch silicon nitride with respect to silicon dioxide in microfabrication.

Phosphoric Acid - Health Hazards

The phosphoric acid in soft drinks has the potential to cause tooth erosion. Phosphoric acid may also contribute to the production of kidney stones, especially in people who have previously experienced kidney stones.

Concentrated sulphuric and nitric acid are more corrosive and dangerous than phosphoric acid. But concentrated solutions of phosphoric acid irritate the skin and mucous membranes. P2O5 fumes in the vapours can irritate the throat and induce coughing. However, at 10 mgm-3 they might be tolerated.

Phosphoric acid, in both solid and liquid form, is non-combustible. If inhaled, it can be lethal. Phosphine, phosphoric acid, and hydrogen chloride, which are found in fire smoke, can irritate respiratory tracts, eyes, and skin.

At low quantities, phosphoric acid is non-poisonous and does not harm the skin or any other part of the body. Even in higher dosages, it's harmful since it irritates the skin and can even harm the eyes.

If the vapours of H3PO4 are breathed in, it can also disrupt the respiratory system. Hence, this acid should be stored in a metallic or covered fibreboard compartment (with a polyethene internal bundle) and kept cool and well aired.

Practice Problems 

Q1. Phosphoric acid is used as an electrolyte in which type of cell?

A. Voltaic Cell
B. Hydrogen Cell
C. Diesel Engine
D. Fuel Cell

Answer: Phosphoric acid is used in fuel cells. Phosphoric Acid Fuel Cells (PAFCs) operate at temperatures ranging from 150 to 200 degrees Celsius (about 300 to 400 degrees F). The electrolyte in PAFCs is phosphoric acid, as the name implies. From the anode to the cathode, positively charged hydrogen ions migrate through the electrolyte.

So, option D) is the correct answer.

Q2. Phosphoric Acid serves as a

A. Food Additive
B. Leavening Agent
C. Emulsifier
D. Food Colourant

Answer: Acidifying foods and beverages, such as colas and jams, with food-grade phosphoric acid gives them a tangy or sour taste. The phosphoric acid acts as a preservative as well. Soft drinks that include phosphoric acid, such as Coca-Cola, are known as phosphate sodas or phosphates. As a result, it is a food additive. 

So, option A) is the correct answer.

Q3. Orthophosphoric acid has the basicity of

A. 1
B. 3
C. 0
D. 2

Answer: Orthophosphoric acid (H3PO4) is a tribasic acid with three donatable hydrogen ions (Phosphoric acid or H3PO4 can release up to three H+ ions). So, its basicity is 3. 

H3PO4 ⇌ H2PO4- + H+, pKa1 = 2.14

H2PO4-⇌ HPO42- + H+, pKa2 = 7.20

HPO42-⇌PO43- + H+, pKa3 =12.37

Q4. Phosphoric acid is an efficient

A. Oxidising agent
B. Dehydrating agent
C. Reducing agent
D. Hydrating agent

Answer: Phosphoric acid acts as a dehydrating agent. Phosphoric acid, most commonly in the form of polyphosphoric acid, is a potent dehydrator. It is the anhydride of phosphoric acid that captures OH through the creation of a HO-P bond.

Frequently Asked Questions - FAQ

Question 1. What happens on heating phosphoric acid?
Answer: The orthophosphoric anhydrous acid is a crystalline white solid with a melting point of 42.3 °C. There will be a loss of component oxygen when phosphoric acid is heated to temperatures of roughly 200 °C. Dehydration creates a series of acids, from pyrophosphoric acid (H4P2O7) through metaphosphoric acid (H4P2O8).

Question 2. Why is phosphoric acid added to cola?
Answer: Phosphoric acid is a colourless and odourless crystalline liquid. It delivers soft drinks with a tart flavour while also preventing the growth of mould and germs, which may easily flourish in a sugar solution. The phosphoric acid in soda contributes a significant amount of its acidity. It is then treated again to become phosphoric acid.

Question 3. Is phosphoric acid capable of dissolving rust?
Answer: Few stronger acids will completely eradicate rust, but they will etch and pit the surface of any remaining ferrous metal, weakening it. One of the few acids that may eliminate rust without oxidising or damaging the iron beneath it is phosphoric acid.

Question 4. Can phosphoric acid be used as a disinfectant?
Answer: Bowl cleanse is a phosphoric acid solution that disinfects toilet bowls and urinals by removing dangerous microorganisms. The 25 % phosphoric acid solution removes mineral particles from toilets rapidly and without causing damage to the pipes.

Related Topics

Allotropes of Phosphorus


Phosphorus halides



Alkali Metals

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