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EC number: 232-417-0
CAS number: 8017-16-1
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.
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