Polyphosphoric acids

Ecotoxicological Summary

Hazard for air

Air

Hazard assessment conclusion:

no hazard identified

Additional information

In accordance with Annex XI, section 1.5 of Regulation (EC) No 1907/2006 (REACH) a read across approach may be used when substances have similarities based on the likelihood of common breakdown products via physical and biological processes, which result in structurally similar chemicals. As mentioned in the hydrolysis summary, polyphosphoric acid is hydrolysed to orthophosphate in environmental conditions. Thus a read across from orthophosphoric acid to pyrophosphoric acid is justified. In addition, studies performed with pyrophosphate salts can also be used. Pyrophosphate salts are ionic in nature and therefore dissociate readily into cations and anions in water. Cations as potassium and sodium are essential micronutrients that are ubiquitous in the enviroment. As such, their uptake is tightly regulated and is therefore not considered to pose a risk for ecotoxicity. In environmental conditions, the pyrophosphate anion is unstable and a number of different processes result to an ultimate breakdown product of orthophosphate.

Phosphorus is an essential nutrient of microorganisms, plants and animals. Phosphoric acid is usually the limiting nutrient in ecosystems: it can be readily utilisable by aquatic/terrestrial plants and microorganisms. Phosphorus compounds are an important component of living matter and all organisms contain a certain quantity; however, excessive bioaccumulation is unlikely due to natural metabolic processes for essential nutrients (Frausto da Silva & Williams, 1991). Thanks to its high water solubility and low vapour pressure, phosphoric acid is mainly found in soil (migrating towards the groundwater table) and water compartments: there, phosphoric acid progressively dissociates affecting the pH of the receiving compartment.

Phosphoric acid progressively dissociates as pH is raised from about 3.0 to above 7.0.

H₃PO₄<=> H⁺+ H₂PO₄^-<=> 2H⁺+ HPO₄^{2 -}<=> 3H⁺+ PO₄^{3 -}; respectively pKa 2.2; pKa 7.2; pKa 12.3.

The third H+ is generally only lost in alkaline conditions. Under relevant environmental pH range (pH5 to pH8), the monovalent and divalent phosphates are dominant (H₂PO₄^-and HPO₄^{2 -}). As reported in "Acid soils of the tropics" (Harder RD) because of this dissolution, when phosphorus fertilizer is placed in a band, very low pH values can be attained. The acidity will then gradually diffuse into the soil surrounding the band. According to Lindsay and Stephenson (1959), pH values as low as 1.5 can be found in a zone immediately around a fertilizer band. Natural pH can vary significantly as well as organisms sensitivity in adapting to such change. A pH range of 6 -9 is generally considered safe for aquatic organisms. Moreover, pH change due to anthropogenic release of phosphoric acid is influenced by the buffer capacity of the receiving water.

References:

Frausto da Silva JJR and Williams RJP, The Biological Chemistry of the Elements: The Inorganic Chemistry of Life; Oxford University Press: Oxford, 1991.

Harder RD (2002) Acid soils of the tropics, An Echo Technical Note

Lindsay WL and Stephenson HF (1959) Nature of the reactions of monocalcium phosphate monohydrate in soils, Soil Sci. Soc. Am. J. 23(1): 12-

Conclusion on classification

Polyphosphoric acid is not considered to pose a risk to the environment and as such are neither classified as harmful nor dangerous to the environment, in accordance with Regulation (EC) No. 1272/2008 (EU CLP).