Dichloride titanium oxide

Hydrolysis

Description of key information

Target compound titanium oxychloride (TiOCl2) is an unstable chemical compound which occurs as intermediate compound during the process of the hydrolysis of parent compound titanium tetrachloride (TiCl4). The conduct of a study on hydrolysis of target compound titanium oxychloride itself is being waived due to technical difficulty of measurement of the expected very short half-life time of titanium oyxchloride in water when reacting to TiO2 and HCl, particularly in the relevant pH range pH 4 - 9. An estimate half-life of below 0.1 min is stated in an expert report.

Key value for chemical safety assessment

Half-life for hydrolysis:

0.1 min

at the temperature of:

4 °C

Additional information

The conduct of a study on hydrolysis of target compound titanium oxychloride itself is being waived due to technical difficulty of measurement of the expected short half-life of titanium oyxchloride in water when reacting to TiO2 and HCl, particularly in the relevant pH range pH 4 - 9. The evidence presented therefore is being based on chemical handbook entries on parent compound titanium tetrachloride (read across approach), as well as on experience from handling and use of target compound titanium oxychloride itself.

Target compound titanium oxychloride (TiOCl2) is an unstable chemical compound which occurs as intermediate compound during the process of the hydrolysis of parent compound titanium tetrachloride (TiCl4).

1. step: TiCl4 + H2O↔TiOCl2 + 2 HCl

2. step: TiOCl2 + H2O↔TiO2 + 2 HCl

Summary reaction: TiCl4 + 2 H2O↔TiO2 + 4 HCl

Addition of HCl to the reaction medium, or lack of H2O, lead to a partial hydrolysis only, which results in the formation of titanium oxide chlorides. The Raman spectrum of a yellowish TiCl4 solution in aqueous hydrochloric acid show the occurrence of [TiO2Cl4]4- and [TiOCl5]3- species, as TiOCl2 in aqueous hydrochloric acid reveal the same spectrum.

As shown by these chemical reactions, parent compound titanium tetrachloride hydrolyses via target compound titanium oxychloride to final hydrolysis products hydrogen chloride and titanium dioxide. Intermediate formation of hydroxides, e.g. titanium dihydroxide oxide (CAS 12026-28-7) and/or titanium hydroxide (CAS 12651-23-9 or 20338-08-3) is probable, however rapid hydrolysis to titanium dioxide is deemed probable. The titanium dioxide formed is amorphous or have weak crystallinity but corresponds finally to the naturally occurring mineral rutile albeit in micro-disperse form.

As both target compound titanium oxychloride as well as parent compound titanium tetrachloride hydrolyse in water rapidly with an assumed half-life of below 0.1 min (Fisk et al 2010), the hazard assessment bases accordingly on the hydrolysis product titanium dioxide, which is considered stable under environmental conditions (Fisk et al 2010). The pH effects caused by the other final hydrolysis product hydrochloric acid are not of relevance for hazard assessment. According to the OECD SIDS report on hydrogen chloride (SIAR 2002) hydrogen chloride gets readily dissociated in contact with water into chlorides and into hydronium ions.

In aqueous solution the intermediate compound titanium oxychloride only can be stabilised via shifting the chemical equilibrium as shown above towards the side of the target compound in presence of hydrochloric acid. When being diluted with water (e.g. accidental discharge into a natural water body), again the hydrolysis reaction would proceed to the final hydrolysis products titanium dioxide and hydrochloric acid. Therefore, based on the information given it is to be concluded that in aqueous conditions in the environmentally relevant pH range pH 4 - 9, target compound TiOCl2 is an intermediate so short-lived that cannot be isolated and therefore has to be considered as highly unstable in water.

When aqueous solution containing target compound titanium oxychloride (stabilised with hydrochloric acid) is exposed to the atmosphere, to a certain extent the evaporation of hydrochloric acid might be observed. No relevant evaporation of titanium oxychloride itself, as the molecule only can exist in aqueous medium in its hydrated form, in equilibrium with hydrochloric acid.

Even though not relevant for the hazard assessment of titanium oxychloride (which only occurs in aqueous medium), it shall be finally be mentioned that when parent compound titanium tetrachloride is getting exposed to air dense white fumes evolve (white colour due to the white hydrolysis product titanium dioxide). In case water is deficient in the air, for a certain period of time a partial hydrolysis of the TiCl4 molecule and thus the occurrence of target compound titanium oxychloride might be observed in the atmosphere. Under normal air humidity conditions full hydrolysis eventually will be observed, though. Rigo et al (1998) quantified the half-life time of titanium oxchloride under special laboratory reactor conditions to ca. 6 h. The particle size of the resulting titanium dioxide ranges about 400 to 700 nm (Watson & Kibler 1931).

• Rigo M, Canu P, Angelin L, Della Valle G (1998) Kinetics of TiCl4 Hydrolysis in a Moist Atmosphere, Industrial & Engineering Chemistry Research 37(4):1189-95

• Watson PD, Kibler AL (1931) The Relation between Obscuring Power and Particle, Number and Size of Screening Smoke. DOI: 10.1021/j150322a013 J. Phys. Chem. 35(4):1074–90