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Environmental fate & pathways

Hydrolysis

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Description of key information

Diphosphorus pentaoxide is extremely hygroscopic and reacts violently with water to form an aqueous solution of different phosphorus oxyacids. The proportion of the oxyacids as well their further hydrolysis depends on the way of preparation of the sample and on several physical parameter (van Wazer page 453) like temperature, pH value and concentration.
The end product of the hydrolysis is the orthophosphoric acid, whereby the hydrolysis and the main product of the rapid hydrolysis is tetrametaphosphoric acid which is quite stable under conditions relevant for ecotoxicological studies like low concentration and quite neutral pH value.
Hazardous reaction:
Especially fine powder of the hexagonal form can react explosively with water, forming a mixture of acidly phosphorus oxyacids. The intensity of the reaction leads to the proposal of a supplemental hazard classification EUH014 (reacts violently with water) for completion of the Annex VI part 3 classification (see chapter 2).
Environment:
Hydrolysis of P2O5 in water under conditions relevant for ecotoxicological studies (100mg/L, 25°C) leads to slightly acidic solutions (pH 3.6) with the main product oftetrametaphosphoricacid. Further hydrolysis within the time frame of ecotoxicological studies is insignificant.
Oral exposure:
Hydrolysis of P2O5in dilute hydrochloric acid (0.5%) under conditions relevant for toxicological studies (37°C) leads to solutions that undergo further hydrolysis. After about an hour the main products are mono- and diphosphoric acid.

Key value for chemical safety assessment

Half-life for hydrolysis:
0.01 min
at the temperature of:
25 °C

Additional information

Diphosphorus pentaoxide is hygroscopic forming with water an aqueous solution of phosphorus oxyacids (primary hydrolysis) which are subject to further (secondary) hydrolysis to the end product phosphoric acid, H3PO4.

The reactivity of the primary hydrolysis differs with the crystalline modification, whereby the hexagonal form reacts immediately or almost explosively to a clear solution, the orthorhombic(R-)form swells slowly to a milky sol or a suspension and the orthorhombic (S-)form builds with a limited amount of water a gel changing with time or dilution into a clear solution.

The less soluble parts consist of high molecular phosphoric acids, which can also be achieved by improper circumstances in the solution process, like a small surface or insufficient stirring,

The hydrolytically scission of the P-O-P linkages depends in general from the following factors, and the given magnitudes (by van Wazer) are representative for the transformation products of the secondary hydrolysis. 

Factor 

Approximate effect on rate 

1. Temperature

105-106faster from freezing to boiling

2. pH

103-104slower from strong acid to base

3. Enzymes

As much as 105-106 faster

4. Colloidal gels

As much as 104-105 faster

5. Complexing cations

Very many-fold faster in most cases

6. Concentration

Roughly proportional

7. Ionic environment in the solution

Several-fold change

These factors lead to a fluctuation of the half live time of different oxyacids from several minutes to some years.

Usually the commercial grade of phosphorus pentaoxide belong to the M form, consisting of P4O10 molecules forming with water immediately a clear aqueous solution of different phosphorus oxyacids according to a well investigated reaction scheme.

In the only possible first step of the hydrolysis an ultraphosphate ( H2P4O11) is produced. This unstable ultraphosphate break down to give either tetrametaphosphoric acid (H4P4O12) , a four unit cyclic molecule, or the iso tetrametaphosphoric acid (H2P3O9)with an end group attached on a three unit cyclic molecule.

The tetrameta­phosphoric acid is the main product of the rapid primary hydrolysis and quite stable under relative mild conditions whereby the branching point on the substituted trimetaphosphoric acid undergoes further rapid degradation eiher in tetrapolyphosphoric acid ( H6P4O13) or trimetaphosphoric acid ( H3P3O9) and isotetrapolyphosphoric acid ( H6P4O13). The later one also has a branching point and should as well as the trimetaphosphoric acid rapidly undergo further degradation into tri polyphosphoric acid . Further reactions of the remaining chain polyphosphoric acids are leading quite slowly to pyrophosphoric acid ( H4P2O7) and finally orthophosphoric acid ( H3PO4).