Disulphur dichloride

Endpoint summary

Description of key information

A guideline study according to the OECD TG 111 (Hydrolysis as a Function of pH) showed that S2Cl2 is rapidly (t 1/2 < 2 minutes) and quantitatively (97.7 - 101.7 %) hydrolysed in water at 23 °C and pH ranging from 4 to 9. The hydrolysis products are HCl and different sulphur containing molecules in varying amounts including sulphite, sulphate, thiosulphate, sulphide and precipitable sulphur.

Key value for chemical safety assessment

Bioaccumulation potential:

no bioaccumulation potential

Additional information

S2Cl2 is reported in the literature to undergo rapid hydrolysis in aqueous conditions. A guideline test was performed according OECD TG 111 (Hydrolysis as a Function of pH) to verify fast and quantitative hydrolysis in water. As the hydrolysis was known to occur rapidly, hydrolysis behaviour of the test item in aqueous solutions was investigated in a Tier 1 test at room temperature instead of 50 °C, at different pH (4, 7 and 9). The main hydrolysis products expected are chloride, sulfide, sulfur dioxide, sulfite, sulfate, thiosulfate and sulfur.

The following analytical methods were used to detect different hydrolysis products: chloride, sulphite, sulphate and thiosulphate were quantified via ion chromatography; sufide was determined iodometrically and precipitable sulphur was identified by Raman-Spectroscopy and determined by ICP-OES analysis.

As expected, the hydrolysis is rapid and quantitative. Due to the observed rapid and complete degradation of the test item between sample preparation and injection into ion chromatography apparatus, no precise half-life could be calculated. At 23 °C (room temperature) the half-life was calculated to be < 2 minutes for all pH investigated (4, 7 and 9). Recoveries based on the contents of chloride and total Sulphur were in the range of 97.7 to 101.7 %, indicating quantitative hydrolysis.

The total sulphur fraction was further investigated at different time points and dependent on the pH (4, 7 or 9) contained ca. 9.8 – 25% perceptible sulphur; 6.3 - <4.2% sulphite; < 3.5% sulphate; 15.1 – 60% thiosulfate. Based on the mass balance additional volatile sulphur species as sulphur dioxide are also likely to be present, but were not specifically monitored. The analytical method for determination of sulfide (iodometry) is not a specific method for testing hydrolysis reactions. The performed procedure resulted in an extreme Sulfide concentration (expected influence of equilibrium reaction). Therefore the recovery values reported for sulphide (3.1 – 12.2 %) should be taken as qualitative figures.

Overall, this guideline study according to the OECD TG 111 (Hydrolysis as a Function of pH) showed that S2Cl2 is rapidly (t 1/2 < 2 minutes) and quantitatively (97.7 - 101.7 %) hydrolysed in water at 23 °C and pH ranging from 4 to 9. The hydrolysis products are HCl and different sulphur containing molecules in varying amounts including sulphite, sulphate, thiosulphate, sulphide and precipitable sulphur.

Since S2Cl2 undergoes immediate hydrolysis the hydrolysis products will be evaluated concerning their toxicological properties.

HCl:

There are sufficient data on S2Cl2 and HCl, to conclude that S2Cl2 is corrosive and should be labeled with R35. No systemic toxicity of protons and chloride ion are anticipated at human relevant doses because protons and chloride ions are normal constituents in the body fluid of animal species. There are several evaluations by international expert groups that draw this conclusion:

OECD SIDS assessment on HCl published in 2002 “ Because protons and chloride ions are normal constituents in the body fluid of animal species, low concentrations of hydrogen chloride gas/mist or solution do not seem to cause adverse effects to animals. In fact, the cells of gastric glands secrete hydrochloric acid into the cavity of the stomach and orally administered sulfuric acid, which results in pH change as well, did not cause developmental toxicity to laboratory animals. These facts indicate that hydrogen chloride/ hydrochloric acid is not expected to have developmental toxicity. In addition, no effects on the gonads were observed in a good quality 90-day inhalation study up to 50 ppm” (OECD SIDS 2002).

the German “Commission for the Investigation of Health Hazards of Chemical Compounds in the Work Area” concluded: “Hydrogen chloride at the pH value of biological systems dissociates completely into protons and chloride ions. Provided the MAK value is observed, the amount of chloride ions taken up from 30 mg (assuming 10 m3 respiratory volume, 100 % retention) is equivalent to 1/200 of the daily intake of chloride ions from 6 g (Jungermann and Möhler 1984). In analogy to the estimation for formic acid (MAK value 5 ml/m3) it can be concluded that 2 ml hydrogen chlroide/m3 does not lead to a marked change of the blood pH.” (MAK 2004).

In conclusion, based on the corrosive nature of S2Cl2 or HCl no significant exposure is assumed (high hazard band); and it can be assumed that neither the hydrogen, nor the chloride ion is present at biologically relevant doses after exposure to S2Cl2 and, consequently, no systemic toxicity of the hydrogen or chloride ions is anticipated.

Sulphur containing molecules

Sulphur:

BIBRA Information Department (The British Industrial Biological Research Association) summarized in 1990: in man, sulphur has caused irritation of the skin, eyes and respiratory and gastro-intestinal tracts. There have also been occasional cases of skin allergy. Oral administration caused life-threatening metabolic disturbance or severe anaemia in a few individuals with kidney disease or an enzyme deficiency. Repeated skin applications have resulted in effects on the central nervous system and gastrointestinal tract, and some infants have died. Occupational exposure to sulphur (often in conjunction with other chemicals) has been associated with lung damage and blood effects.

Data on SO2 are considered of particular relevance because 1) SO2 is likely to be a hydrolysis product of S2Cl2 but was not investigated in the hydrolysis study due to the volatile nature and 2) in the aqueous milieu of mammalian bodies the sulfur dioxide will form an equilibrium with the sulfite ion, which is further oxidised to sulfate by sulfite oxidase, which is abundantly available in mitochondria of liver, kidney and heart of eukaryotes. Consequently, SO2 can be used to evaluate the toxicological properties of SO2, sulfite and sulfate.

SO2 was evaluated by the German “Commission for the Investigation of Health Hazards of Chemical Compounds in the Work Area” and the following conclusions were drawn: sulfur dioxide is readily soluble in water and is converted on the moist mucosa to sulfurous acid (H2SO3). In a further oxidation step sulfurous acid is metabolized to sulfuric acid. Excess sulfate is excreted via urine. According to the German “Commission for the Investigation of Health Hazards of Chemical Compounds in the Work Area” (1998) theoretical calculations on the sulfate concentrations demonstrate that no toxicologically relevant increase in the sulfate concentration in the humans can be assumed inhaling SO2 in the occupational setting and, consequently, a developmental or reproductive effect is not anticipated. In their calculation the panel considered quantitative conversion of sulfur dioxyde to sulfate. Exposure level was calculated based on air concentrations of 1.3 mg/m3 over an entire 8 hours shift (=10 m3 air), a volume of distribution of 4.5 l and no increase in urinary sulfate excretion was also considered as worst case assumptions. Based on these assumptions the sulfate concentration in the blood might increase by approx. 3 mg/l. Based on the normal sulfate concentration in blood (8.4 – 19.5 mg/l) and a daily excretion rate of ca. 750 mg sulfate/l urine the authors conclude that no significant increase in sulfate is assumed in humans.

In conclusion, based on the corrosive nature of S2Cl2 no significant exposure is assumed (high hazard band); and it can be assumed that sulphur, sulfur dioxide, sulfite or sulfate is not present at biologically relevant doses after exposure to S2Cl2 and, consequently, no systemic toxicity is anticipated.

Sodium thiosulfate:

Sodium thiosulfate is of low toxicity and authorised as additive or monomer for plastic materials and articles in contact with foods (Regulation (EU) No 10/2011) with a restriction of 7 mg/kg expressed as SO2 (FCM Substance No 519).

Hydrogen Sulphide:

H2S was evaluated by SCOEL in 2007. Nasal lesions found in rats after exposure to H2S are considered the critical

effect. SCOEL indicated: "Considering all these aspects and the preferred value approach in setting OELs, starting from a

NOAEL of 10ppm (14 mg/m3) and using an uncertainty factor of 2, SCOEL proposes an 8-h TWA of 5ppm (7 mg/m3) for H2S.

Given the nature of the acute toxic effects such as eye irritation, unconsciousness and persistent neurological disorders and the fact that short-term exposures do occur in industrial settings, a STEL of 10 ppm is recommended. Moreover, it is strongly advised to avoid exposure to rapid rising high peaks."

In conclusion, based on the corrosive nature of S2Cl2 no significant exposure is assumed (high hazard band); and it can be assumed that hydrogen sulfide is not present at biologically relevant doses after exposure to S2Cl2 and, consequently, no systemic toxicity is anticipated.