Tetrasodium pyrophosphate

Toxicity to microorganisms

Description of key information

One key study to assess the toxicity of tetrasodium pyrophosphate to STP microorganisms 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 STP microorganisms. 
In addition, according to the Guidance on Information Requirements and Chemical Safety Assessment (chapter R.7b section R7.8.14), toxicity to STP microorganisms is best assessed using an “Activated Sludge Respiration Inhibition Test” according to OECD Guideline 209 as it is the most widely accepted indicator of the combined activity of sludge microorganism. According to the OECD 209 Guideline, the STP microorganism should be fed a synthetic sewage feed made by dissolving a number of inorganic compounds, including 2.8g K2HPO4 in IL of water which is fed to the microorganisms at a rate of 50 ml/day (containing 140 mg K2HPO4) if the sewage sludge is not to be used immediately. This indicates that phosphates are necessary for the survival of the sewage microorganism and therefore no further testing for this endpoint is required. 

Key value for chemical safety assessment

EC50 for microorganisms:

1 000 mg/L

EC10 or NOEC for microorganisms:

1 000 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 risk assessment purposes (appropriate assessment factors to be considered).

The substance has the following properties:

Chemical name: dipotassium hydrogenorthophosphate

Chemical formula: H3O4P.2K

CAS number: 7758-11-4

Molecular weight: 175

Melting range: >450°C

Solubility in water considered to be very soluble (63.0 - 65.0% w/w at 20.0 ± 0.5°C) pH of solution: 10.1-10.7

It is therefore considered appropriate; due to the physicochemical nature of the substance tested and the read across arguments detailed, 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 PO43-(from P2O74-) are all ubiquitous and are not considered to pose a risk of ecotoxicity.