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Overall, based on available reviews and handbooks, an experimental hydrolysis study in the gas phase and a recent guideline hydrolysis study according to OECD TG 111 “Hydrolysis as a Function of pH” it can be concluded that thionyl dichloride undergoes violent decomposition in aqueous milieu and decomposes quantitatively to HCl (CAS n° 7647-01-0) and SO2 (CAS n° 7446-09-5) with half-lives between 1.4 minutes and 5.9 minutes in the gas phase and < 2 minutes in water. SOCl2 is classified as R14 (Reacts violently with water). 
Since SOCl2 reacts vigorously and completely with water within minutes with formation of HCl and SO2 these hydrolysis products are considered relevant for the potential toxicity of thionyl dichloride.
Local effects: Thionyl dichloride and both hydrolysis products are considered to be corrosive; thionyl dichloride is labeled with R35 (causes severe burns).
Potential systemic effects: In the aqueous milieu of mammalian bodies the sulfur dioxide will form an equilibrium with the sulfite ion, which is further oxidized to sulfate by sulfite oxidase, which is abundantly available in mitochondria of liver, kidney and heart of eukaryotes. Endogenously occurring inorganic sulfate plays important roles in physiology, a sufficient supply with sulfate is even required for normal fetal development, excess sulfate will be excreted. Hydrogen chloride will dissociate in an aqueous milieu, as well, the resulting chloride ion also plays an important role in physiology, is abundantly available in the body anyway, and has not to be regarded as toxicologically relevant. As the mammalian physiology is maintained at well buffered conditions, no pH-related effects due to the formation of hydronium ions have to be expected, either.

Key value for chemical safety assessment

Additional information

Thionyl dichloride undergoes immediate disintegration upon contact with water, as cleavage products sulfur dioxide and hydrogen cloride are formed. Rapid, spontaneous, violent and exothermic hydrolysis in water is described in different reviews on physico-chemical properties and in toxicological evaluations.

Ullmann’s Encyclopedia of Industrial Chemistry, 2012 (Ref), indicates: “Thionyl chloride is hydrolyzed rapidly and completely by water in an exothermic reaction (∆H = -136 kJ/mol) with excess water. SOCl2 reacts vigorously and completely [with water] with formation of HCl and SO2. It fumes in moist air due to HCl formation”.

ACGIH (2010) evaluated the toxicity of thionyl dichloride and defined a TLV “There are limited toxicological data available for thionylcloride; however, its vapors have long been recognized as highly irritating to skin, eyes, and mucous membranes due to the formation of sulfur dioxide and chloride in reaction with water. Thus, the TLV is derived from the TLVs of those irritant gases. “

Thionyl dichloride is also evaluated by the EU and labeled according to DSD and GHS with R35 and Skin. Corr. 1A, respectively (ESIS). In a Classification and Labelling Justification prepared by Austria the physic-chemical properties are described as follows: “violent decomposition in water to hydrochloric, sulfurous, and sulfuric acid (1 ppm produces a total irritant gas concentration of 3 ppm); when exposed to moist air it produces fumes,…” (C&L 1997).

In a recent experimental study the kinetics of the hydrolysis of thionyl dichloride in the gas phase was investigated by Johnson et al, 2003. The atmospheric fate of SOCl2 has been investigated in a sealed mixing chamber by simulating the effects of varied temperature and humidity. It was determined that humidity plays an important role, as water reacts rapidly with the parent compound, forming HCl(g) and SO2(g) in a nearly stoichiometric ratio of 2:1. The rate constant for hydrolysis at 297 K was determined to be kh = (6.3+3.5) x E10-21 cm³/molecules, resulting in a tropospheric lifetime due to hydrolysis on the order of only a few minutes for this volatile inorganic species. Derived from the decay constants, half-lives for thionyl dichloride of 1.4 minutes (74% relative humidity) to 5.9 minutes (28% relative humidity) are calculated at 20°C.

Subsequent to a draft decision on a compliance check of the dossier by ECHA submitted to the registrant on 11 June 2013 the registrant decided to initiate a guideline hydrolysis study in order to confirm rapid and quantitative hydrolysis of thionyl dichloride in a guideline hydrolysis study according to OECD TG 111 “Hydrolysis as a Function of pH”.

The test was performed according to the OECD Guidelines for Testing of Chemicals, Section 1 – Physical-Chemical Properties, OECD TG 111 (2004). As the hydrolysis was known to occur rapidly, hydrolysis behavior of the test item in aqueous solutions was investigated in a Tier 1 test at room temperature instead of 50 °C, at pH 4, pH 7 and pH 9. The hydrolysis behavior was monitored by ion chromatography analysis with conductivity detection. Chloride, Sulfite and Sulfate as the only by Ion Chromatography hydrolysis detectable products were identified by comparison with the retention time of the calibration substances. The concentrations of the formed Chloride in the hydrolysis test solutions were determined by external calibration method and correspond to the theoretical values assuming a complete degradation of the test item (recovery 99.1 to 102.4 %). Sulfite and Sulfate were quantified accordingly. Recoveries were in the range of 80 to 95 %. Losses of Sulfur might be explained by the evaporation of gaseous Sulfur Dioxide. For further details on this study see study endpoint summary in IUCLID chapter 5.1.2.: hydrolysis.

Due to the observed immediately and complete degradation (100 %) of the test item between sample preparation time and injection time by ion chromatography, the half-life time at 23 °C (room temperature) was calculated to by < 2 minutes at all pH (4, 7 and 9).

Overall, based on available reviews and handbooks, an experimental hydrolysis study in the gas phase and a recent guideline hydrolysis study according to OECD TG 111 “Hydrolysis as a Function of pH” it can be concluded that thionyl dichloride undergoes violent decomposition in aqueous milieu and decomposes quantitatively to HCl (CAS n° 7647-01-0) and SO2 (CAS n° 7446-09-5) with half-lives between 1.4 minutes and 5.9 minutes in the gas phase and < 2 minutes in water. SOCl2 is classified as R14 (Reacts violently with water).

Based on the available information on the physicochemical properties of thionyl dichloride it is considered appropriate by the registrant to follow other thionyl dichloride evaluations and to conclude that due to spontaneous and exothermic hydrolysis thionly dichloride undergoes immediate disintegration. The data of the hydrolysis products, HCl and SO2, should be considered as appropriate and sufficient to characterize the toxicity of thionyl dichloride for endpoints where no data on the instable parent compound are available.

HCl (corrosive - comprehensive data available, e.g. MAK value documentation, OECD SIDS for HCl):

Hydrogen chloride or its aqueous solution, hydrochloric acid is corrosive and irritating and causes direct local effects on the skin, eyes and gastro-intestinal tract after direct exposure to sufficiently high concentrations. Vapour of hydrogen chloride or small droplets (aerosol /mist) of hydrochloric acid can also be inhaled and cause direct local effects on the respiratory tract. Hydrogen chloride will rapidly dissociate and the anion will enter the body electrolyte pool. That is, hydrogen chloride per se is not expected to cause effects to the body. The local effects of the aqueous solution of hydrogen chloride are the effects of the H+ ion (local deposition of protons, pH change) rather than effects of the chloride ion. Inhaled hydrogen chloride is partially neutralized before it reaches the lower respiratory tract by naturally occurring ammonia gas in the respiratory system (Soskolne et al., 1989). Immediate defence against pH changes is provided by body buffers that can take up or release protons instantaneously in response to changes in acidity of body fluids. Regulation of pH ultimately depends on the lungs (carbon dioxide excretion) and kidneys (bicarbonate regeneration through proton secretion in urine).

Chloride is a normal constituent of the blood and the excess is expected to be excreted into the urine (Ganong, 2001). The uptake of sodium chloride via food is about 3.5-9 gram per person per day (Battarbee and Meneely, 1978; FASEB, 1979). It means 2.1-5.5 g chloride is taken into the body via food. The daily intake through inhalation during an 8-hour work shift is estimated to be only

108 mg/day under the worst case at the working place. The body pool of this anion (Cl-) is large, and the uptake of chloride via exposure to hydrogen chloride/ hydrochloric acid is much less than the uptake of chloride via food, it is therefore unlikely that occupational aerosol exposures significantly alter the normal body load.

The uptake of protons via exposure to hydrogen chloride/ hydrochloric acid is not expected to change the pH in the blood under normal handling and use conditions (non-irritating). The pH of the extracellular fluid is regulated within a narrow range to maintain homeostasis. Via urinary excretion and exhalation of carbon dioxide, the pH is maintained at the normal pH of 7.4 (from OECD SIDS for HCl).

SO2: (corrosive - comprehensive data available, e.g. MAK value documentation)

Sulfur dioxide rapidly dissolves extra- or intracellularly in contact with the moist surfaces of the mucous membranes of the respiratory tract forming sulfite and bisulfite as hydration products. Reaction with disulfide structures resulting in formation of S-sulfonates as well as radical intermediates can be involved in the biochemistry of such subsequently formed sulfites. In addition sulfite is further metabolized by oxidation to sulfate by sulfite oxidase. Most of the sulfur dioxide and its metabolites are distributed widely in the body and stay there for long time. Elimination occurs as unchanged sulfur dioxide from the lung and as sulfate in the 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. 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.

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