Ammonium dihydrogenorthophosphate

Endpoint summary

Description of key information

A short-term toxicity study for fish with ammonium dihydrogenorthophosphate is available. No short-term toxicity studies for aquatic invertebrates, algea and cyanobacteria and microorganisms with ammonium dihydrogenorthophosphate are available. However, reliable studies with read across substances show that there is no aquatic hazard as all toxicity parameters exceeded the highest tested concentration (i.e., 100 mg/L). The read-across rationale can be found in Section 13 and is fully incorporated in the CSR.

Additional information

Table 1 - Summary of results from aquatic toxicity studies

Study	Result
Toxicity to fish (short-term)	96h-LC50 >100 mg/L
Toxicity to aquatic invertebrates (short-term)	48h-LC50 >100 mg/L
Toxicity to aquatic algae	72h-LC50 >100 mg/L
Toxicity to microorganisms	3h-EC50 >100 mg/L

In aqueous solution, ammonium salts are completely dissociated into NH₄⁺and a corresponding anion. This equilibrium depends on temperature, pH and ionic strength of the water in the environment. Un-ionized NH₃species exists in the aquatic environments and the fraction (NH₃/( NH₃+ NH₄⁺)) steeply increases with elevated pH value or temperature. It is well known that toxicity to aquatic organisms has been attributed to un-ionized ammonia (NH₃) species, and NH₄⁺species is considered to be non- or significantly less-toxic (Emerson et al., 1975). However, recent developments in assessing ammonia toxicity clearly show that in contrast to earlier assumptions where un-ionized ammonia was considered to be the toxic component, both the uncharged and charged molecule are toxic. Therefore, a joint toxicity model has been proposed, with ammonia causing most toxicity at high pH values and ammonium ion also contributing to toxicity at lower pH values (U.S. EPA 1999, OECD 2007).

It is generally accepted, that the principal toxic component of ammonium salts such as ammonium nitrate, -sulphate or -chloride is ammonia, rather than the corresponding anion (see also: OECD2004, SIDS ammonium chloride or OECD 2007 ammonium sulphate). Therefore toxicity values for ammonium salts (such as: ammonium -sulphates, phosphates, carbonates, chlorides or nitrates), where the major toxic component is ammonia, can be considered as equivalent. Consequently, this hazard assessment comprises the total topic of ammonia toxicity.

U.S. EPA (1999) published an excellent review on Ammonia toxicity, with special consideration on the pH- and temperature dependence. A huge amount of studies is available on Ammonia toxicity, however a comparability of these toxicity data is difficult, since various temperature- and pH conditions were present during testing. U.S. EPA (1999) re-evaluated existing data on Ammonia toxicity by adjusting toxicity values to definite temperature and pH- conditions and by thereby allowing comparability.

Considering the group of phosphates, ammonium dihydrogenorthophosphate (MAP), diammonium hydrogenorthophosphate (DAP), single superphosphate (SSP) and triple superphosphate (TSP), it is clear that all phosphates show a low or negligible toxicity to aquatic species: for fish LC50 >100 mg/L (nominal concentration, confirmed by chemical analysis with >80% recovery) (MAP), and 1700 mg/L for DAP; for invertebrates EC50 =1790 mg/L (SSP); for algae NOEC≥100 mg/L (for TSP and DAP) (based on nominal conc.).