Sodium sulfide (Na2(Sx))

Additional physico-chemical information

Administrative data

Type of information:

other: expert statement

Adequacy of study:

key study

Applicant's summary and conclusion

Conclusions:

Aqueous Na2Sx solutions are only stable at pH > 10. At lower pH values they are decomposed to H2S and S. At physiological pH values, hydrogen sulfide in the non-dissociated form (H2S) and the hydrogen sulfide anion (HS-) will be present in almost equimolar proportion, whereas only very small amounts of the sulfide anion (S2-) will be present. In conclusion, under physiological conditions, inorganic sulfides or hydrogen sulfides as well as H2S will dissociate to the respective species relevant to the pH of the physiological medium, irrespective the nature of the “sulfide”, which is why read-across between these substances and H2S is considered to be feasible without any restrictions.

Executive summary:

Justification for read across

The derivation of the DNELs is based on read across to other sulfur based substances. Toxicological data specifically for Sodium sulfide (Na₂(S_x)) from animal studies are not available. Therefore, because of the lack of appropriate experimental data, read-across from studies with H₂S is proposed based on the following reasoning:

                    

Unrestricted read-across between the substances Sodium sulfide (Na₂(S_x)), sodium hydrogensulfide and dihydrogen sulfide is considered feasible, in view of the potential systemic toxicity being driven by the sulfide ion as the only relevant species released from any of the sulfide substances under physiological conditions. In this context, it is further considered to be very unlikely that the sodium ions add any toxicological concern.

 

Aqueous Na2Sx solutions are only stable at pH > 10. At lower pH values they are decomposed to H2S and S ([1, 2, 3, 4, 5])

 

The soluble compound Sodium sulfide (Na₂(S_x)) can safely be assumed to be present dissociated in water and relevant biological media([6]). From Sodium sulfide (Na₂(S_x)), hydrogen sulfide (H₂S) may be formed according to the following equilibria:

Na₂S_x+ H₂O → 2 Na⁺+ HS^-_x+ OH^-

Na₂S_x+ H₂O → NaOH + NaHS_x(2 Na⁺+ HS_x^-+ OH^-)

NaHS_{x +}H₂O → (x-1)S + NaOH + H₂S (Na⁺+ OH^-+ H₂S)

The toxic effects resulting from the sodium ion is negligible. Hydrogen sulfide dissociates in aqueous solution to form two dissociation states involving the hydrogen sulfide anion and the sulfide anion:

H₂S ↔ H⁺+ HS^-↔ 2 H⁺+ S^2-

The pKa values for the first and second dissociation steps of H₂S are 7.04 and 11.96, respectively. Therefore, at physiological pH values, hydrogen sulfide in the non-dissociated form (H₂S) and the hydrogen sulfide anion (HS^-) will be present in almost equimolar proportion, whereas only very small amounts of the sulfide anion (S^2-) will be present. In conclusion, under physiological conditions, inorganic sulfides or hydrogen sulfides as well as H₂S will dissociate to the respective species relevant to the pH of the physiological medium, irrespective the nature of the “sulfide”, which is why read-across between these substances and H₂S is considered to be feasible without any restrictions.

 

[1] E. Dachselt, „Thioplaste“, Deutscher Verlag für Grundstoffindustrie, Leipzig 1971, pp. 35

[2] M.B. Berenbaum, “Polysulfide Polymers” in N. G. Gaylord, ”Polyethers”, Interscience Publishers, 1962, 49-51

[3] D. Peschanski; G. Valensi, J. chim.Phys. 46(1949), pp. 602

[4] M. Menzel, Expert statement “Investigation of the reaction of sodium polysulfide solution with diluted hydrochlorioc acid”, AkzoNobel, Greiz (March 2010) (attached)

[5] Hagg-graph (attached)

[6]Beauchamp et al. (1984): A critical review of the literature on hydrogen sulfide toxicity; CRC Crit. Rev. Toxicol. 13, 25-97.