Polyphosphate - Structure & Formation

Structure & Formation

• Structure of triphosphoric acid

• Polyphosphoric acid

• Cyclic trimetaphosphate

• Adenosine diphosphate (ADP)

The structure of tripolyphosphoric acid illustrates the principles which define the structures of polyphosphates. It consists of three tetrahedral PO₄ units linked together by sharing oxygen atoms. Structurally, the outer tetrahedra share one vertex with the central tetrahedron; the central tetrahedron shares two corners with the other tetrahedra. The corresponding phosphates are related to the acids by loss of the acidic protons. In the case of the cyclic trimer each tetrahedron shares two vertices with adjacent tetrahedra.

Sharing of three corners is possible as in the sheet-structure Phyllosilicates, but such structures occur only under extreme conditions. Three-corner sharing also occurs in phosphorus pentoxide, P₄O₁₀, which has a 3-dimensional structure.

Chemically, the polymerization reaction can be seen as a condensation reaction. The process begins with two phosphate units coming together.

2 PO₄3− + 2 H+ P₂O₇4− + H₂O

It is shown as an equilibrium reaction because it can go in the reverse direction, when it is known as an hydrolysis reaction because a water molecule is split (Lysed). The process may continue in steps; at each step another PO₃ unit is added to the chain, as indicated by the part in brackets in the illustration of polyphosphoric acid. P₃O₁₀ can be seen as the end product of condensation reactions, where each tetrahedron shares three corners with the others. Conversely, a complex mix of polymers is produces when a small amount of water is added to phosphorus pentoxide.