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Phosphorus pentasulphide reacts rapidly with moisture or water in the environment to form hydrogen sulphide and phosphoric acid. Phosphoric acid will dissociate to form environmentally ubiquitous ionic species, but may potentially have a secondary effect at very high concentrations due to pH change. At lower concentrations no effects (or even positive, nutritive effects) are predicted. Hydrogen sulphide is considered likely to be the more toxic of the breakdown products and is therefore relevant for the derivation of PNEC values.

The toxicity of the breakdown product hydrogen sulphide was investigated in a study with Daphnia magna. The 48 hour EC50for Daphnia magna was calculated to be 0.036 mM or 0.1224 mg/L (based on measured hydrogen sulphide concentrations); equivalent to 0.16 mg/L phosphorus pentasulphide.

The 24 -hour EC50of hydrogen sulphide in S. vacuolatus was found to be 0.141 mM (based on nominal exposure concentrations) 0.055 mM (based on corrected test concentrations) equivalent to a concentration of 1.87 mg/l hydorgen sulphide. This concentration is calculated to be equivalent to 2.44 mg/l phosphorus pentasulphide.

The 30 minute EC50 forVibrio fisheri is 0.276 mM or 9.51 mg/L, based on measured hydrogen sulphide; equivalent to 12.4 mg/l phosphorus pentasulphide.