Polyphosphoric acids

Hydrolysis

Additional information

Polyphosphoric acid is a mixture of the corresponding acids to phosphate anion and its condensed phosphates as follows:

- orthophosphoric acid or phosphoric acid (17 -76%),

- pyrophosphoric acid (23 -50%),

- triphosphoric acid (1.5 -25%),

- tetraphosphoric acid (0 -12%)

- and pentaphosphoric acid (0 -7%).

All linear condensed phosphates are known to be hydrolyzed in aqueous solutions to yield less condensed phosphates and ultimately orthophosphate (see reference to the Kirk-Othmer Encyclopedia of Chemical Technology). The reaction is called hydrolysis because the water is an active participant in the reaction. During the reaction, bonds in both the phosphate and water are broken. The resulting hydrogen atom unites with the oxygen in the broken phosphate couple, forming P-OH. The OH portion of water reacts with the phosphorous in the broken phosphate couple, forming P-OH. The result is one molecule of condensed phosphate has reformed into two new phosphates.

When the simplest condensed phosphate, pyrophosphate, undergoes hydrolysis, it is converted into two orthophosphate ions. When tripolyphosphate, the next simplest condensed phosphate, undergoes hydrolysis one orthophosphate ion and one pyrophosphate ion is formed. In both cases, the hydrolysis reaction leads directly to the formation of orthophosphate. The low-molecular-weight phosphates, including pyrophosphates, tripolyphosphates and tetrapolyphosphates, break down primarily through “end group” hydrolysis (clipping) in which a terminal phosphate tetrahedron is cleaved from the tail.

Much literature on the hydrolytic degradation of phosphates shows that several important factors affect the rate of such reactions (see publications cited in the chapter 5.1.2. of the IUCLID). The rate of hydrolysis is dependent upon the temperature, pH, concentration of phosphate and ionic environment. Hydrolysis of condensed phosphates is also affected by which phosphate specie is present. It is generally accepted that pyrophosphate is the most stable linear condensed phosphate. During hydrolysis of the longer chained phosphates, shorter chains as well as orthophosphate are formed. Among the shorter chains formed is pyrophosphate.

Research suggests that when the pyrophosphate concentration reaches a certain level due to hydrolysis of higher condensed phosphates, the rates of reversion diminish. It may be that equilibrium is set up between the higher condensed phosphate and its molecular fragments after hydrolysis. Once a certain amount of the fragments are produced, the equilibrium is satisfied and hydrolysis diminishes.