Phosphorus (P) is a non-metal and is generally a waxy crystalline solid that readily melts and vaporises. It is highly inflammable in the air, forming white vapours of phosphorus pentoxide (P2O5). It combines with oxygen in several proportions, forming various phosphorus oxides. These oxides unite with oases to form compounds termed phosphates. It probably exists in iron and steel as an element but in slags as a phosphate.
Phosphorus possesses a special affinity for oxygen. They form two compounds based on the two valent relations of phosphorus.
- Phosphorus trioxide, P2O3
- Phosphorus pentoxide, P2O5
Phosphorus is a nonmetallic element in Group VA of the periodic table. It exists as several allotropes, the important of which are red phosphorus and the poisonous and spontaneously inflammable white or yellow phosphorus. It occurs in many minerals, particularly phosphates and all living organisms; it is an essential nutrient for plants.
Phosphorus Trioxide – P2O3
Phosphorus trioxide, P2O3, is formed by conducting a slow current of dry air over phosphorus gently warmed when slow combustion takes place, but not ignition. It is a white, voluminous solid that readily sublimes and possesses a garlic-like odour. It unites readily with water, producing phosphorous acid.
P2O3 + 3H2O → 2H3PO3
From moist air, it takes both oxygen and water, forming phosphoric acid, H3PO4.
Phosphorus trioxide (P2O3) is soluble in organic solvents, such as benzene and carbon disulfide, and is made by burning phosphorus in a limited supply of air. At room temperature, phosphorus(III) oxide oxidises to phosphorus(V) oxide in the air; beyond 70°C, it ignites. To produce phosphonic acid or one of its salts, it slowly dissolves in cold water or diluted alkalis. Hot water and phosphoric (III) oxide react violently to produce phosphine and phosphoric (V) acid.
Phosphorus Pentoxide – P2O5
Phosphorus pentoxide, P2O5, is formed when phosphorus is ignited and burned in dry air or oxygen. It is a voluminous, flocculent, white powder without odour; it is not volatile but dissociates into P2O3 and O2 when highly heated. It has a strong attraction for water, uniting with it with a hissing sound and evolution of much heat to form phosphoric acid, or deliquescing in moist air and producing metaphosphoric acid, HPO3.
P2O5 + 3H2O → 2H3PO4
P2O5 + H2O → 2HPO3
The major use of P2O5 in the laboratory results from this affinity. It is convenient for removing the last traces of moisture from gases and is employed for extracting the elements of water from many compounds which contain hydroxyl groups.
Phosphorus is never found uncombined in nature. It occurs in small quantities in granite and older rocks in the form of tricalcic phosphate or phosphate of lime. When these rocks crumble down and form soil, they supply phosphates to plants, storing them in considerable quantity in their seeds. From the seeds, the animals which feed upon them derive a sufficient quantity for their support. Phosphorus collects in the animal system in large quantities and furnishes, as calcic phosphate, the principal earthy component of the bones.
Phosphorus is also an essential ingredient in the brain and nervous tissue, and it passes out of the body constantly in the form of soluble phosphates in the urine and the solid excreta as insoluble earthy phosphates.