Phosphoric Acid – Properties, Structure and Uses

The H₃PO₄ compound name is Phosphoric acid or orthophosphoric acid or phosphoric(V) acid. It is one of the most commonly used acids. The phosphoric acid formula is H₃PO₄ Its uses extend to various industries, including agriculture, personal care, pharma, beverages, and more. However, before we move on to the uses of this, let’s learn some important details about its structure and properties that ultimately influence its uses. 

Occurrence of Phosphoric Acid

The raw form of Phosphoric acid is extracted from the phosphate rocks. A more pure form is made industrially using white phosphorus. The pure form of this acid is mostly found in a less condensed form and a crystalline solid state. Generally, it is odourless, colourless, syrupy, and non-volatile.

This is a mineral acid, and its formula is H₃PO₄. This acid’s formula comprises one atom of phosphorus, four oxygen & three hydrogen atoms.

Phosphoric Acid Structure

We know that the valency of phosphorus is five. Therefore, its structure of it has a centrally placed phosphorus atom. It is bonded together via a double bond with an oxygen atom. The phosphorus also stays connected to three hydroxyl groups via single bonds.

Lewis Structure of Phosphoric Acid

The Lewis structure of phosphoric acid shows the bonded and non-bonded electrons as follows in the following image:

It contains the following pairs of electrons:

• Eight bonded pairs of electrons
• Eight non-bonded pairs of electrons

Phosphoric Acid Uses 

It is an important chemical with a myriad of uses in several industries and even in our daily lives. The following sections will explore some of the most popular uses of phosphoric acid, including its use in 

• Rust removal
• Food and Beverage
• Agriculture
• Personal care
• Dentistry
• Pharma
• Other Uses

Rust Removal

The reddish brown deposition on iron and steel requires strong chemicals for its removal. Among the various acids, It is the most commonly used in the removal of rust.

The reaction usually involves the conversion of reddish ferric oxide to a black-coloured compound called ferric phosphate. This black substance can be easily removed, and the metal stays protected. 

Food And Beverage

Since This is not harmful, it is often used in the food and beverage industry. It finds application in the following processes:

• It is used as a food additive.
• It serves as an acidity regulator in cheese, processed meat, jams, cereal bars, and more.
• In beverages, the acid is used as an acidulant. 
• It enables a thorough check of bacterial growth and fungi growth.

Personal Care

It is widely used in the production of various personal care products. For instance, cleansing products, fragrances, bath products, hair care items, dyes, nail products, skincare products, and makeup are some of the most popular uses of phosphoric acid. It helps regulate the pH and bacterial activity in personal care products. 

Dentistry 

It also finds usage in dentistry. Dentists often use it as an etching solution and for teeth cleansing. If you read the labels of various mouth cleaning products, you will come across phosphoric acid in the ingredients list. This is also used in anti-nausea medicines.

Agriculture

A major portion of phosphoric acid goes to the agricultural industry. Almost 80% of the phosphoric acid produced is employed in fertiliser production. It is also used as a flavouring agent in poultry feed.

Pharmaceutical Industry

As a commonly used intermediate in pharmaceuticals, phosphoric acid is used in dentistry, teeth whiteners, anti-nausea medicines, and more. It serves as an acidifying agent in various pharmaceuticals. Other uses include

• For preparing albumin derivatives
• In dental cement
• In mouth washing liquids
• For removing necrotic debris
• For acidifying urine

Other Uses of Phosphoric Acid

There are some other important uses of Phosphoric acid. This acid is also used as an electrolyte in oxyhydrogen generators and fuel cells. Synthetic detergents are prepared by making use of phosphoric acid. This is also used in the treatment of metals and water.

In the construction industry, it is prevalent in the removal of cement smears, mineral deposits, and hard water stains. In the production of activated carbon products, it acts as a chemical oxidising agent.

Properties of Phosphoric Acid

It is a tribasic acid as it has three replaceable hydrogen atoms. It is an extremely useful substance. This white crystalline solid forms polymeric chains at high temperatures. 

The main properties influencing its use are enumerated below:

• Form: white crystalline solid
• Phosphoric acid molecular mass = 97.994 g/mol
• Boiling point = 158 °C 
• Melting point = 42.35 °C
• IUPAC Name: Ortho Phosphoric Acid
• Chemical Formula: H₃PO₄
• Density: 2.030 g.cm-3

Salts of Phosphoric Acid

Phosphates are the salts of phosphoric acid. The three most important phosphates are stated below: 

(a) Ammonium phosphates

Mono ammonium dihydrogen phosphate is a common phosphate salt of phosphoric acid. Diammonium hydrogen phosphate is also popular. These phosphates are produced when anhydrous ammonia combines with phosphoric acid. The most common use of these phosphates is in the agricultural industry as fertilisers.

(b) Calcium phosphates

Calcium phosphates are produced by the action of sulphuric acid on the phosphate rock. It is also called superphosphate. This type of phosphate also finds applications in the agricultural industry. It is responsible for the following activities:

• Encouraging strong root growth
• Giving rise to healthier healthy plants
• Ensures higher yields as it serves as a quick-acting fertiliser 
• It improves ripening. 
• It helps support root, fruit, and seed crops. 
• It is for transplanting flowers, trees, cuttings, and shrubs.

(c) Sodium phosphates

The fraction of phosphoric acid with a concentrated solution of sodium hydroxide in specific proportions gives rise to sodium phosphates. The product is in the form of solid crystals. Monosodium dihydrogen phosphate, Trisodium phosphate, Disodium hydrogen phosphate, and Disodium pyrophosphate are some commonly found sodium phosphates. 

Phosphoric Acid Hazards

Phosphoric acid, when in low concentration, is usually non-toxic. It does not cause much harm to the skin or any other body part. It is reportedly dangerous at much higher concentrations. When it comes in contact with the skin in higher concentration, it can cause severe irritation or skin burns. It can even damage the eyes when in high concentration. If the vapours of H₃PO₄ are accidentally inhaled, they are also known to cause irritation in the human respiratory tract. A coated fibreboard container (with polyethene inner lining) or a metallic container is best suited for the storage of this acid. It is advisable to be kept in a well-ventilated, cool place.

Important Safety Concerns with Phosphoric Acid

It can cause acute or even chronic health effects. Acute or short-term health effects can occur either immediately or shortly after being exposed to phosphoric acid:

• When it comes in contact with the eyes, it tends to irritate and burn them.
• Inhalation of phosphoric acid can irritate the nose and throat, leading to coughing and wheezing.

 Chronic or Long-term effects of Phosphoric acid exposure include:

• Inflammation of the lungs.
• Repeated exposure can cause coughing, phlegm, bronchitis, and shortness of breath.  
• The skin turns dry, resulting in the appearance of cracks over it.
• Chronic effects last for months or even years.

Phosphoric Acid Safety Measures 

On exposure to a harmful dose of phosphoric acid, you must opt for the following first-aid measures before seeking medical help. 

• When this comes in contact with the eye or skin, you must immediately flush the area with lots of water. Wash thoroughly for at least 30 minutes.
• If you swallow or sip phosphoric acid, you must rinse your mouth with normal water. If you begin vomiting, then you should stop rinsing for the meanwhile because it may cause airway blockage. Also, try to lean forward and reduce the accidental entry of solids or liquids into the air passage. 
• If you inhale phosphoric acid fumes, immediately get some fresh air.

Conclusion

This is a weak acid, and thus it finds application in various food and agricultural industries. It is important to note that phosphoric acid is different from phosphorous acid. The phosphorous acid formula is H₃PO₄ while the phosphoric acid formula is H₃PO_4.

Frequently Asked Questions

1. What are some uses of phosphoric acid? 

It is found in fertilizers (80% of total use), detergents, and many household cleaning products. Because dilute solutions have a pleasant acid taste, it is also used as a food additive, lending acidic properties to soft drinks and other prepared foods, as well as in water treatment products.

2. Is phosphoric acid poisonous? 

It can cause lung irritation. Repeated exposure may result in bronchitis with cough, phlegm, and/or shortness of breath. Long-term exposure to the liquid may cause skin drying and cracking. 

3. What happens if someone touches Phosphoric acid?

A. Some short-term (acute) health effects usually occur shortly after exposure to phosphoric acid: It can irritate and burn our eyes; it can cause nose and throat coughing & wheezing.

4. Is phosphoric acid stronger than phosphorous acid? 

A. No, phosphoric acid (H₃PO₄) is a weaker acid than phosphorous acid (H₃PO₃). However, This is a stronger oxidising agent than phosphorous acid.

5. Is phosphoric acid natural?

A. Phosphoric acid is produced from the mineral phosphorus. It occurs naturally in many foods. According to the National Institutes of Health, It plays a role in forming strong bones and teeth. It helps to support kidney function and food energy storage.