Reaction mass of sodium hydrogensulfide and sodium carbonate

Endpoint summary

Additional information

General read-across concept between sodium sulfide, sodium hydrogensulfide and hydrogen sulfide:

Given that sodium sulfide and sodium hydrogensulfide dissociate in aqueous media (please refer to the Hägg-graph give below), it can safely be assumed that under most physiologically relevant conditions (i.e., neutral pH) sulfide and hydrogen sulfide anions are present at almost equimolar concentrations, thus facilitating unrestricted read-across between both species. Only under extreme conditions such as gastric juice (pH << 2), sulfides will be present predominantly in the form of the non-dissociated hydrogen sulfide. In turn, hydrogen sulfide (H₂S) may be formed from both soluble sulfides, according to the following equilibria:

 

                           Na₂S + H₂O   →  NaOH + NaHS (2Na⁺/ OH^-/ HS^-)

                           NaHS + H₂O  →  NaOH + H₂S (Na⁺/ OH^-/ H₂S)

 

Similarly, hydrogen sulfide dissociates in aqueous solution to form two dissociation states involving the hydrogen sulfide anion and the sulfide anion, according to the following equilibrium:

 

                           H₂S  ↔  H⁺  +  HS^-  ↔  2 H⁺  +  S^2-

 

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.

Inhalation absorption

No data on inhalation absorption are available for the constituents of the Reaction mass, and little reliable information (if at all) is available on hydrogen sulfide absorption and distribution after inhalation.

 

Dermal absorption

In the absence of measured data on dermal absorption, previous guidance primarily directed at organic chemicals with a defined lipophilicity and corresponding percutaneous transfer potential, suggests the assignment of either 10% or 100% default dermal absorption rates. In contrast, the currently available scientific evidence on dermal absorption of metal cations (predominantly based on the experience from previous EU risk assessments) yields substantially lower figures, which can be summarised briefly as follows:

Measured dermal absorption values for metal cations or inorganic metal substances in studies corresponding to the most recent OECD test guidelines are typically 1 % or even less. Therefore, the use of a 10 % default absorption factor is not scientifically supported for such ionic species. This is corroborated by conclusions from previous EU risk assessments (Ni, Cd, Zn), which have derived dermal absorption rates of 2 % or far less (but with considerable methodical deviations from existing OECD methods) from liquid media.

Given that the primary cause between the lack of percutaneous transfer is considered to be the ionic nature, it is proposed to assume similar behaviour for sulfide anions as for metal cations, and to adopt the following dermal absorption factors for sodium sulfide and sodium hydrogensulfide reflective of full-shift exposure, i.e. 8 hours (HERAG fact sheet - assessment of occupational dermal exposure and dermal absorption for metal cations and inorganic metal substances; EBRC Consulting GmbH / Hannover /Germany; August 2007):

From exposure to liquid/wet media: 1.0 %

From dry (dust) exposure: 0.1 %

 

Oral absorption

There are only few publications that allow an assessment of the oral bioavailability of sodium sulfide in rats (no data available regarding the other constituents). The following conclusions can be drawn: after oral administration of sulfide to rats (Curtis et al., 1972) almost 70% are excreted within 48 hrs, 63% via urine and the remainder via faeces. In another study involving i.p.-administration, 90% of the injected dose could be recovered in urine and faeces (Dziewiatkowski, 1945). In conclusion, the assumption appears justified that upon oral uptake the systemic uptake is essentially complete for sulfides and hydrogen sulfides. Therefore, a conservative oral absorption factor of 100% will be taken forward for risk characterisation purposes.

  

Distribution, metabolism and elimination

Following oral administration, sulfides are absorbed rapidly and extensively, and distributed widely throughout all tissues without any particular target tissue (Curtis et al., 1972; Dziewiatkowski, 1945). Upon distribution, sulfide is rapidly excreted as sulfate with thiosulfate having been identified as an intermediate metabolite (Bartholomewet al.,1980). The resulting sulfate is excreted almost quantitatively via urine; experiments with bile duct cannulated rats have shown that biliary excretion is minimal by comparison (Curtis et al., 1972). Methylation with subsequent elimination via exhaled air has been excluded for sulfides (Susman et al., 1978). Sodium, hydroxide and carbonate ions and physiological constituents of the body and are therefore of no toxicokinetic relevance.