Tetrasodium pyrophosphate

Toxicity to aquatic algae and cyanobacteria

Description of key information

One key study to assess the toxicity of tetrasodium pyrophosphate to aquatic algae exists, this study is conducted on an analogous substance (see justification below). On this basis sodium and potassium pyrophosphates are not considered to be toxic to algae. 
In addition, sodium, potassium and phosphates are essential nutrients for both terrestrial and aquatic plants and are an integral component of the growth media for the algal inhibition test (OECD 201, sodium as NaHCO3 and phosphate as KH2PO4). Phosphates are known to increase the growth of aquatic plants and aquatic toxicity is unlikely to occur therefore testing for this endpoint is not justified.
The additional supporting data provided were not considered to be adequate or reliable to fulfil neither the guideline nor the regulatory requirements. In addition, there is no evidence within this data to suggest that essential validity criteria are met.

Key value for chemical safety assessment

EC50 for freshwater algae:

100 mg/L

EC10 or NOEC for freshwater algae:

100 mg/L

Additional information

Rationale for read across:

 

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.

 

Sodium and potassium pyrophosphates are ionic in nature and therefore dissociate readily into cations and anions in water the toxicity of the both the cation and the anion must be addressed. Potassium and sodium cations are essential micronutrients that are ubiquitous in the environment. As such, their uptake is tightly regulated and is therefore not considered to pose a risk for ecotoxicity. The pyrophosphate anion is unstable in aqueous solutions with the degree of instability varying according to pH. In distilled water pyrophosphates will hydrolyse slowly via abiotic mechanisms to inorganic phosphate. In natural waters a number of different processes can occur; abiotic hydrolysis, biotic degradation (as a result of the action of phosphatases which cleave pyrophosphate into orthophosphate subunits) and assimilation by organisms in the water all resulting in an ultimate breakdown product of orthophosphate.

It is therefore deemed scientifically justified to avoid any further vertebrate testing and use the data from a study conducted on an orthophosphate (with either a potassium or sodium cation) for hazard assessment purposes.

The substance tested is essentially a mixture of two potassium orthophosphates: potassium dihydrogenorthophosphate and dipotassium hydrogenorthophosphate.

The substance has the following properties:

Chemical name: Phosphoric acid, potassium salt (2:3), dihydrate.

Synonyms (IUPAC name): Tripotassium trihydrogen diphosphate dihydrate.

Chemical formula: H3 O4 P . H2 O . 3/2 K

CAS number: 66922-99-4 

Molecular weight: 346.29

Melting range: No complete melting up to 573 K

Solubility in water ( g/L, at 20 °C): 705 (pH 7)

 

 

It is therefore considered appropriate; due to the physicochemical nature of the substance tested and its identity, for this data to be used for read-across purposes for the following substances:

 - disodium dihydrogenpyrophosphate

 - trisodium hydrogen diphosphate

 - tetrasodium pyrophosphate

 - tetrapotassium pyrophosphate

Any further testing would not be scientifically justified as all substances would ultimately dissociate to their anionic and cationic forms in natural waters and these ions (Na+, K+ and PO₄^3-(from P₂O₇^4-) are all ubiquitous and are not considered to pose a risk of ecotoxicity.