Difference Between Phosphorus and Phosphoric Acid

Key Difference – Phosphorus vs Phosphoric Acid
 

Phosphorus and phosphoric acid are two forms of acids containing the chemical element phosphorous (P). The chemical structures of the two molecules are nearly similar but the chemical and physical properties are different from each other. The key difference between Phosphorus and phosphoric acid is that Phosphorus acid (IUPAC name: Phosphonic acid) is diprotic whereas phosphoric acid (IUPAC name: Trihydroxidooxidophosphorus) is triprotic.

CONTENTS

1. Overview and Key Difference
2. What is Phosphorus Acid
3. What is Phosphoric Acid
4. Similarities Between Phosphorus and Phosphoric Acid
5. Side by Side Comparison – Phosphorus vs Phosphoric Acid in Tabular Form
6. Summary

What is Phosphorus Acid?

Phosphorous acid is an acid containing phosphorous and the chemical formula is H₃PO₃. The IUPAC name of phosphorous acid is phosphonic acid. Although this chemical structure contains three hydrogen atoms, it is a diprotic acid. A diprotic acid is an acid that is capable of releasing two hydrogen ions (protons) to an aqueous medium. Phosphorous acid is also called orthophosphorous acid.

The molar mass of phosphorous acid is 81.99 g/mol. At room temperature, it is a white solid that is deliquescent (absorb water from the air when exposed and dissolve). The melting point of phosphorous acid is 73.6◦C and the boiling point is 200◦C. At temperatures above the boiling point, the compounds tend to decompose. When considering the chemical structure of the phosphorous acid, it has a phosphorous atom as the central atom bonded with two –OH groups and one oxygen atom bonded via a double bond and a hydrogen atom bonded via a single bond. This structure is known as a Pseudo-tetrahedral structure.

Figure 01: Chemical Structure of Phosphorous Acid

The phosphorus acid is made via hydrolysis of the anhydride of the acid; P₄O₆.

P₄O₆ + 6 H₂O → 4 H₃PO₃

But in industrial scale productions,phosphorous chloride (PCl₃) is hydrolyzed by steam.

PCl₃ + 3 H₂O → H₃PO₃ + 3 HCl

Phosphorous acid is used as a reducing agent in chemical analysis. This acid readily converts into phosphoric acid when heated to about 180◦C. the salts formed by phosphorous acid are known as phosphites. The most common application of phosphorus acid is that; it is used in the production of basic lead phosphite (a stabilizer in PVC).

What is Phosphoric Acid?

Phosphoric acid is a phosphorous containing acid having the chemical formula H₃PO₄. The IUPAC name of this compound is trihydroxidooxidophosphorus. It is a triprotic acid because it can release three protons (hydrogen ions) in an aqueous medium.

The molar mass of phosphoric acid is 97.99 g/mol. Phosphoric acid is available as a white solid that is deliquescent or as a syrupy liquid that has a high viscosity. However, this compound odorless. The melting point of this compound is 42.35◦C and the boiling point is 213◦C, but at high temperatures, it decomposes.

Figure 02: Chemical Structure of Phosphoric Acid

The production of phosphoric acid is done via two main ways; wet process and thermal process. The wet process involves the production of phosphoric acid from fluorapatite. It is known as phosphate rock and the chemical composition is 3Ca₃(PO₄)₂.CaF₂. This phosphate rock is finely ground to increase the surface area and is reacted with concentrated sulfuric acid that gives phosphoric acid and gypsum (CaSO₄.2H₂O) as products.

Ca₅(PO₄)₃F + 5H₂SO₄ + 10H₂O → 3H₃PO₄+ 5CaSO₄·2H₂O + HF

The thermal process of phosphoric acid production includes the burning elemental phosphorous to obtain very pure phosphoric acid. The burning of elemental phosphorous gives phosphorous pentoxide (P₂O₅). This compound is then hydrated to produce phosphoric acid.

P₄  +  5O₂→ 2P₂O₅

P₂O₅  +  3H₂O  →     2H₃PO₄

Major applications of phosphoric acid are in fertilizer production. Phosphoric acid is used to produce three types of phosphorus fertilizers; triple superphosphate, diammonium hydrogen phosphate, and monoammonium dihydrogen phosphate.

What are the Similarities Between Phosphorus and Phosphoric Acid?

• Both Phosphorus and Phosphoric Acid are acids containing phosphorous.
• Both Phosphorus and Phosphoric Acid are able to release protons when in aqueous solutions.

What is the Difference Between Phosphorus and Phosphoric Acid?

Summary – Phosphorus vs Phosphoric Acid

Phosphorus acid and phosphoric acid are phosphorous containing acids that have many industrial applications such as in the production of fertilizers. The difference between Phosphorus and phosphoric acid is that Phosphorus acid is diprotic whereas phosphoric acid is triprotic.

Reference:

1.“Phosphorous acid.” Wikipedia, Wikimedia Foundation, 28 Feb. 2018. Available here
2.The Editors of Encyclopædia Britannica. “Phosphorous acid.” Encyclopædia Britannica, Encyclopædia Britannica, inc., 23 Feb. 2018. Available here
3.Lazonby, John. “Phosphoric Acid.” The Essential Chemical Industry online. Available here

Image Courtesy:

1.’Phosphonsäure – Phosphorous acid’ By NEUROtiker – Own work, (Public Domain) via Commons Wikimedia
2.’Phosphorsäure – Phosphoric acid’ By NEUROtiker – Own work, (Public Domain) via Commons Wikimedia

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