Phosphoric Acid

Phosphoric acid is a colorless, odorless, inorganic compound. It is mainly found as an aqueous solution with a concentration of about 85%. It is represented by the chemical formula H₃PO₄. It is extensively used in distinct fields. Its applications include its usage in manufacturing fertilizers, various pharmaceutical products, skin care products, and much more.

In this article, we will do an in-depth study of Phosphoric acids, their properties, their usage, and others in detail.

Phosphoric Acid Overview
IUPAC Name	Ortho Phosphoric Acid
Chemical Formula	H3PO4
Molar Mass	97.99 g/mol
Melting Point	42.4 °C OR 108.3 °F OR 315.5 K
Boiling Point	407 °C OR 765 °F OR 680 K

What is Phosphoric Acid?

Phosphoric acid is a weak inorganic acid which is colorless and odorless. It is generally found in a liquid state with a concentration of not more than 85%. It is found as a white solid crystal solid in its pure form. The IUPAC name of Phosphoric Acid is orthophosphoric acid.

The conjugate base of phosphoric acid is Dihydrogen phosphate. The structure of its crystal is monoclinic and its molecular shape is tetrahedral. It is a non-poisonous and non-toxic acid in low concentrations but can be dangerous in high concentrations. Various properties of Phosphoric Acid are,

Formula of Phosphoric Acid

Phosphoric Acid consists of three hydrogen atoms, one phosphorous atom along with four oxygen atoms.

Chemical Formula of Phosphoric Acid is H₃PO₄

Phosphoric Acid Chemical Name

The chemical name of phosphoric acid is orthophosphoric acid.

Phosphoric Acid Molecular Weight

The molecular weight of phosphoric acid is 97.99 g/mol.

Calculation of Molecular Weight

• Molecular Weight of Hydrogen(H) is 1 g
• Molecular Weight of Phosphorus(P) is 30.99 g
• Molecular Weight of Oxygen(O) is 16 g
• Molecular Weight of H₃PO₄ is (1 × 3 + 30.99 × 1 + 16 × 4) = 97.99 g/mol

Phosphoric Acid Equivalent Weight

We can easily calculate the equivalent weight of phosphoric acid with the help of the following formula:

Equivalent Weight = Molecular Weight/Number of electrons

Since the molecular weight of Phosphoric acid is 97.99 g/mol and number of replaceable hydrogen ions in the acid is 2. By using the equivalent weight formula for phosphoric acid we get:

97.99/2 = 48.99 ≈ 49 g/mol

The equivalent weight of phosphoric acid is 49 g/mol.

Phosphoric Acid Structure

Phosphoric acid is represented as H₃PO₄ or PO-(OH)₃. The structure of Phosphoric Acid consists of three hydroxyl groups (-OH), each single-bonded with the central phosphorus atom (P) and one oxygen atom which is in double-bond with the central available phosphorus atom (P). The structure of Phosphoric acid is illustrated below:

Phosphoric Acid Structure

Lewis Structure of Phosphoric Acid

In the Lewis structure, the bonded electrons are shown with a solid line, and non-bonded electrons are shown with the dots. The structure of phosphoric acid consists of one oxygen atom, one phosphorus atom, and three hydroxyl group (-OH). The Lewis structure of Phosphoric Acid is shown below:

Here, each oxygen atom has 2 lone pairs, and the rest of the other electrons of each atom are bonded.

Preparation of Phosphoric Acid

Phosphoric Acid has many applications and it is generally prepared by the two processes namely:

• Wet process
• Thermal process

Wet Process

In wet process, phosphoric acid is produced from a naturally occurring crystal rock Fluorapatite. This crystal contains the phosphate mineral. Fluorapatite is reacted with concentrated sulphuric acid (H₂SO₄) and water (H₂O). This reaction results in the formation of phosphoric acid and calcium sulfate (gypsum) along with some insoluble impurities. The reaction occuring in wet process is as follows:

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

The side products and impurities formed during this reaction can be removed by the process of filtration and evaporation. Then by vacuum distillation this acid is concentrated to ca 56-70% P₂O₅ (super phosphoric acid). The impure product formed by the wet process can be used as fertilizer manufacture without any further purification.

Thermal Process

Thermal process is another method used to obtain phosphoric acid. This process is helpful in obtaining a purer form of phosphoric acid. In this process,a phosphorus rock along with coke is heated or burnt at high temperature (about 2000 ℃ ) in a furnace in the presence of air. This results in the formation of phosphorus pentoxide which is further condensed to form a white powder. It is then hydrated to obtain phosphoric acid. The reaction occuring in wet process is as follows:

Step 1: Formation of phosphorus pentoxide

P₄ + 5O₂ → 2P₂O₅

Step 2: Hydration of phosphorus pentoxide

P₂O₅ + 3H₂O → 2H₃PO₄

We can also obtain a condensed form of polyphosphoric acids by passing the steam to the burner. The products from the burner are then moved into a hydration tower to obtain phosphoric acid as the gaseous phosphorus oxide is absorbed there. The phosphoric acid obtained by thermal process is more pure ranging from 75 – 80%.

Types of Phosphoric Acid

The types of Phosphoric acid is given below:

Orthophosphoric Acid

• Orthophosphoric acid is the other name of phosphoric acid, with oxidation state of +5.
• The prefix ortho is used to distinguish the acid from other related phosphoric acids, called polyphosphoric acids.
• The chemical formula of orthophosphoric acid is H₃PO₄.

Hypophosphoric Acid

• Hypophosphoric acid is a type of phosphoric acid which has a oxidation state of +4.
• It is a mineral and tetra basic acid which is formed due to the slow oxidation of phosphorus in moist air.
• The chemical formula of hypophosphoric acid is H₄P₂O₆. 

Metaphosphoric Acid

• Metaphosphoric acid is a type of phosphoric acid which has a oxidation state of +5.
• It is an inorganic, corrosive, cyclic acid formed from bonded phosphoric acid units.
• The chemical formula of metaphosphoric acid is HPO₃. 

Properties of Phosphoric Acid

The physical and chemical properties of Phosphoric Acid are described below:

Physical Properties of Phosphoric Acid

Some physical properties of the Phosphoric Acid are,

• Appearance: Phosphoric Acid generally appears as a colourless, odorless, and viscous liquid with varying concentrations.
• State: Pure Phosphoric Acid is a white crystalline solid and its concentrated form is a colourless liquid.
• Melting Point: The melting point of Phosphoric Acid is 42.4° Celsius.
• Boiling Point: The boiling point of Phosphoric Acid is around 407° Celsius.
• Nature: Phosphoric acid is a weak acid that is non-toxic and non-volatile.
• Molecular weight: The molecular weight of Phosphoric Acid is 97.99 g/mol.
• Solubility: Phosphoric Acid being a viscous liquid is highly soluble in water and alcohol.
• Density: Phosphoric Acid has a density of around 1.6845 grams per cubic centimeter (at 25° C and 85% concentration).

Chemical Properties of Phosphoric Acid

Some Chemical Properties of Phosphoric Acid are,

• Triprotic Acid: Phosphoric Acid is a triprotic acid that can easily donate three hydrogen ions (H⁺) in a solution.

H₃PO₄ + H₂O ⇋ 3H⁺ + PO4^3-

• Heat Action: On heating, phosphoric acid can form dimers, trimers, and long polymeric chains like polyphosphoric acids and meta-phosphoric acids.
• ​Reducing Agent: Phosphoric Acid can act as a reducing agent that can undergo dehydration reactions to form different phosphorus compounds.
• Reaction with bases: When Phosphoric Acid reacts with the base it undergoes a neutralization reaction which results in the formation of three types of salt.

​Phosphoric Acid reaction with Sodium Hydroxide

Its reaction with sodium hydroxide results in the formation of three types of salts namely Na₂HPO₄, NaH₂PO₄, and Na₃PO₄. One of the reaction equation is given below:

NaOH + H₃PO₄ → NaH₂PO₄ + H₂O​

Phosphoric Acid Acidity

Phosphoric acid, also known as H₃PO₄ is a non-metal phosphate. It is an inorganic weak acid but long exposure to it can result into irritation in skin and eyes.

Phosphoric acid consists of three H⁺ ions. Removal of three H⁺ ions, two H⁺ ions or one H⁺ ion gives out the phosphate ion (PO₄^3-), hydrogen phosphate ion (HPO₄^2-) and dihydrogen phosphate ion (H₂PO₄^–) respectively.

• The pH value of phosphoric acid ranges from 1.08 to 7.00 on pH scale.
• Its pH value and hence acidity changes with change in concentration.
• Higher the concentration of phosphoric acid higher will be its pH value and acidic strength.
• In low concentration it can also behave as a basic.

Hence it can serve properties of both acidic and basic. Its pH value at different concentrations is given below:

Total moles of pure H3PO4 per Litre	Concentration (%)	pH
1	91.7	1.08
10-1	76.1	1.62
10-2	43.1	2.25
10-3	10.6	3.05
10-4	1.30	4.01
10-5	0.133	5.00
10-6	1.34 × 10-2	5.97
10-7	1.80 × 10-3	6.74
10-10	8.24 × 10-4	7.00

Phosphates: Salts of Phosphoric Acid

The salts of phosphoric acid are termed as Phosphates. The important phosphates of Phosphoric Acid are:

• Calcium phosphate
• Ammonium phosphate
• Sodium phosphate

Calcium Phosphate

Calcium phosphate also known as superphosphate is formed by the reaction of phosphate rock with sulphuric acid. It is mainly used as a fertilizer. The chemical reaction involved in the formation of calcium phosphate is:

Ca₃(PO₄)₂ + H₂SO₄→Ca(H₂PO₄)₂ + 2CaSO₄

Ammonium Phosphate

Ammonium phosphate is formed by combining phosphoric acid with anhydrous ammonia in adequate quantity. Diammonium hydrogen phosphate and monoammonium dihydrogen phosphate are types of ammonium phosphate which are used as fertilizers to increase the amount of nitrogen and phosphorus in crops.

Sodium Phosphate

Sodium phosphates are solid crystals that are formed by the reaction of phosphoric acid with concentrated solution of sodium hydroxide. Disodium hydrogen phosphate, Disodium pyrophosphate, Trisodium phosphate, and Monosodium dihydrogen phosphate are some examples of sodium phosphate.

Uses of Phosphoric Acid

Phosphoric Acid has a wide range of applications in diverse industries. A few of its uses are described below:

Phosphoric Acid to Remove Rust

Phosphoric acid act as a good rust remover and therefore it is used as a treatment of rust. When phosphoric acid reacts with iron oxide it forms a protective layer and thus prevents rusting.

Phosphoric Acid in Soft Drinks

Phosphoric Acid is also used in soft drinks and in various other food and beverages as an acidity regulator and to enhance their flavor and taste, it gives a special tart flavour to these products.

Phosphoric Acid Fuel cell

Phosphoric Acid fuel cell (PAFC) uses liquid phosphoric acid as an electrolyte. This liquid phosphoric acid helps in improving stability and performance of the fuel cell. It is also cost efficient and provides an efficiency of about 35% – 45%.

Some other uses of phosphoric acid are:

• Phosphoric Acid is used in the pharmaceutical industry. It is used in various medicines and helps regulate pH levels in drug formation.
• Phosphoric Acid is majorly used in the production of fertilizers to provide an essential nutrient to plants i.e. phosphorus.
• It is used by dentists as an etching solution for tooth cleaning.
• It is also used in the production of vitamins and dietary supplements.
• Phosphoric Acid is also used in the manufacturing of various daily-use products such as mouthwash, detergents, soaps, and various skin care products.

Health Hazards of Phosphoric Acid

In General, phosphoric acid is non-toxic and non-poisonous and is harmless when used in low concentrations. However, it is dangerous at high concentrations and can cause severe damage and irritation to the skin and eyes if exposed. It can even harm the respiratory tract if its vapors are inhaled. Therefore, it should be stored in a cool and ventilated place in a metal container and proper care and precautions must be taken while handling it.

Phosphoric Acid – Summary

IUPAC Name	Ortho Phosphoric Acid
Chemical Formula	H3PO4
Melting Point	42.4 °C
Boiling Point	407 °C
Molecular Weight	97.99 g/mol
Equivalent Weight	49 g/mol
Appearance	Colourless liquid
Odor	Odorless
pKa value	pKa1 = 2.15 pKa2 = 7.20 pKa3 = 12.35
Density	1.6845 g/cm3

Phosphoric Acid-FAQs

1. What happens when H₃PO₄ reacts with NaOH?

When Phosphoric Acid reacts with sodium hydroxide (NaOH) it undergoes neutralization reaction which results in formation of three types of salt namely Na₂HPO₄, NaH₂PO₄, and Na₃PO₄.

2. What is Phosphoric Acid USed for?

Phosphoric acid is used in manufacturing of fertilizers,, detergents, and many household cleaning products.

3. What is the IUPAC Name of Phosphoric Acid?

The IUPAC name of phosphoric acid is Ortho Phosphoric Acid.

4. What happens when Phosphoric Acid is heated?

On heating, phosphoric acid can form dimers, trimers and long polymer chains like polyphosphoric acids and meta-phosphoric acids.

5. What are the Hazards of Phosphoric Acid?

In low concentrations, phosphoric acids are harmless. However, it is dangerous at high concentration and can cause irritation in skin and eyes and can even harm the respiratory tract if inhaled.

6. What is the pH value of Phosphoric Acid?

pH value of Phosphoric acid changes with change in concentration of aqueous solution. Its pH value is 1.62 in a 0.1 M aqueous solution and a 0.01 M solution of phosphoric acid has a pH value of 2.25. Its value ranges from 1.08 – 7 on pH scale.

7. Is Phosphoric Acid a Strong or a Weak Acid?

Phosphoric acid is an inorganic weak acid but long exposure to it can result into irritation in skin and eyes.

8. What is the Structural Formula of Phosphoric Acid?

The structural formula of phosphoric acid is PO-(OH)₃ or H₃PO₄.