Phosphate-Oxide

One of the most important phosphorus compounds of a commercial nature are those of acids and oxides. Most of the phosphate oxides produced commercially is that of phosphorus pentoxide that is obtained by burning white phosphate.

Phosphorus is contained in some rocks, or from deposits created on the earth many years ago. Phosphate rock that produces commercial quantities of phosphate is called apatite. Other deposits are usually formed as a result of bone fossilising, or from bird droppings called guano. Apatite in rock occurs as a purple to pale green mineral, it contains chlorine, fluorine and other elements as well as phosphate. When rocks containing phosphate are exposed to weathering and erosion begins, they gradually start releasing phosphorus ions which are soluble in water. This creates an important nutrient, or fertiliser, for plants.

Phosphorus forms two common oxides; phosphorus (III) oxide and phosphorus (V) oxide. Both have a structure based on that of elemental white phosphorus:

- Phosphorus (III) oxide, is a white crystalline solid that has a smell similar to that of garlic along with a poisonous vapour. It oxidises slowly when exposed to the air and burns when the temperature reaches 70 degrees Celsius. It dissolves slowly in cold water and forms phosphoric acid.

- Phosphorus (V) oxide is a white flocculent powder that can be made by heating elemental phosphorus in an over abundance of oxygen. Phosphorus (V) oxide is quite stable and a poor oxidising agent. When dropped into water is emits a hissing sound as heat is liberated and it forms an acid. Because of its affinity with water it is widely used to remove water from various compounds and as a drying agent.

The chemical compound phosphorus pentoxide was used as a dehydrating agent and powerful desiccant for most of last century. It was used as concentrated phosphoric acid. It was generally obtained through the 'thermal process' of burning white phosphorus that was dissolved in dilute phosphoric acid, in order to produce a more concentrated acid. This process is being replaced by the 'wet phosphorous acid process' as a result of filter technology improvements. It also removes the need to produce white phosphorus as a starter material.

Pure phosphorus trioxide is a crystalline monoclinic solid that is colourless. It has a melting point of 23.8 degrees Celcius and a boiling point of 173.1 degrees Celsius. It is a waxy solid substance when impure. If it is not wholly pure phosphorus trioxide slowly turns a red colour when exposed to light. This happens as the white phosphorus converts into red phosphorus because of the impurities. It is possible to purify phosphorus trioxide by repeatedly exposing it to light and sublimation (the transformation of solid to gas without having to pass through an intermediate liquid phase). When it becomes pure phosphorus trioxide in no longer glows in the air.Pure phosphorus trioxide is very poisonous and emits a pungent acid like smell. However, it will oxidise in oxygen, or in the air, to become phosphorus pentoxide. If pure phosphorus trioxide is shaken energetically along with cold water it can be hydrolysed to form phosphorus acid.