Boron trichloride

Endpoint summary

Additional information

A degradation category approach is applied, based on the very rapid and complete degradation (hydrolysis) of boron trichloride in water to form hydrochloric acid and boric acid. The description and justification of the category is presented in Section 13.

The endpoint results of the degradation products are transcribed to boron trichloride.

 

Relevant information for the 2 degradation products:

Hydrogen chloride:

Boron trichloride, to react with terrestrial organisms, has first to pass the water in the soil. BCl3 degrades rapidly within 1 minute in water to hydrogen chloride and boric acid. The main effect of hydrochloric acid is to lower the pH. The constituents of HCl in water are the naturally occurring ions H⁺ and Cl^-. No other than pH effects are expected. As discussed in the Guidance Document on Info and CSA, Part B, it can be assumed at first that the sensitivity of pelagic and sediment or soil living organisms is comparable. For BCl3, respectively the degradation product HCl, the decrease of the pH, caused by HCl, will be decisive for toxicity not only for the aquatic but also for the sediment and soil organisms. It can further be assumed that soil and sediment will buffer the H+ ions, so that a disordered pH in water will have less impact on the pH within soil.

Relevant aquatic organisms need a pH in the range of 6 to 9. If the pH is lowered by the test substance to <6, a toxicity at least to some of the organisms is foreseen. This is one of the basic assumptions in the EC directive 2006/44/EC on the quality of fresh waters needing protection or improvement in order to support fish life, which state that the pH of the fish water has to be in the range of 6 to 9. It seems reasonable to adopt this range also for other aquatic and terrestrial organisms.

A pH of 6 corresponds theoretically to NOECwater = 0.036 mg HCl/L or 0.039 mg BCl3/L in pure, unbuffered water. In natural water a NOECwater >0.039 mg BCl3/kg would be obtained, depending on the buffer capacity and the species concerned. Three times this value is estimated to reflect more practical conditions of natural water and could be used for classification: NOECwater = 0.12 mg BCl3/L.

Applying the equation for the equilibrium partitioning, see Section B.7.2.4 of the ECHA Guidance Document on Information and CSA, for a standard soil with 60 % solids and 20 % water, and assuming a low adsorption of Koc = 10, a NOECsoil of ca. 0.03 mg BCl3/kg ww is estimated from the NOECwater.

 

Boric acid:

Tests with macroorganisms, including arthropods, and plants and microorganisms were reported in the IUCLID on boric acid. The lowest NOEC range after transcribing to BCl3 from 33 to 237 mg BCl3/kg dw.

Endpoint conclusion:

Although the NOECsoil of ca. 0.03 mg BCl3/kg ww for HCl is only a rough estimate it indicates that the toxicity of HCl is more relevant to soil organisms than that of boric acid.

Toxic effects can be predicted if the pH is <6, caused by HCl. For a risk assessment a NOECsoil based on pH = 6 and not on a mg/kg basis is considered to be the first choice for each of the terrestrial organisms, as not a systemic, species specific effect is the underlying mechanism of toxicity but the acidity of the medium.