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.
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
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.
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.
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?
Phosphorus vs Phosphoric Acid
|Phosphorous acid is an acid containing phosphorous and the chemical formula is H3PO3.||Phosphoric acid is a phosphorous containing acid having the chemical formula H3PO4.|
|Phosphorus acid is diprotic||Phosphoric acid is triprotic|
|The molar mass of phosphorous acid is 81.99 g/mol.||The molar mass of phosphoric acid is 97.99 g/mol.|
|The IUPAC name of phosphorous acid is phosphonic acid.||The IUPAC name of phosphoric acid is trihydroxidooxidophosphorus.|
|The melting point of phosphorous acid is 73.6◦C and the boiling point is 200◦C.||The melting point of this compound is 42.35◦C and the boiling point is 213◦C, but at high temperatures, it decomposes.|
|The phosphorus acid is made via hydrolysis of the anhydride of the acid; P4O6 or by phosphorous chloride (PCl3) is hydrolysis by steam||The phosphoric acid is made via wet process or thermal process.|
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.
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