Chromium trioxide

Hydrolysis

Link to relevant study record(s)

Reference

Endpoint:

hydrolysis

Data waiving:

study scientifically not necessary / other information available

Justification for data waiving:

other:

Description of key information

There are no proprietary studies investigating the hydrolysis of the substance in aqueous media. The substance of interest is an inorganic element and sufficient information on the speciation of the substance under various environmental conditions is available in the EU RAR for chromium (VI) compounds (2005). Further details are provided below:

Ionic equilibria for chromium (VI)

Chromium (VI) is a strong oxidising agent and as a result only exists as oxygenated species in the environment. For the chromates, the actual species present in solution depends on the pH according to the following equilibria:

H₂CrO₄---->H⁺+ HCrO₄^-            pK_a= -0.6; 0.74; 0.86

HCrO₄^{- --->}H⁺+ CrO₄^2-                 pK_a= 5.9; 6.49; 6.51

Based on these equilibria alone, at very low pH (e.g. near 0) the dominant species in solution would be the fully protonated form (H₂CrO₄). At pHs between 0 around 6-6.5, the dominant chromate species in solution would be HCrO₄^-, and at pHs above around 6-6.5 the main chromate species in solution would be CrO₄^2-.

In addition to the above equilibria, the chromate ion is in equilibrium with the dichromate ion according to the following equilibria:

2 CrO₄^2-+ 2 H⁺--->Cr₂O₇^2-+ H₂O

2 HCrO₄^---->Cr₂O₇^2-+ H₂O

In addition, there are the following base-hydrolysis equilibria:

Cr₂O₇^2-+ OH^---->HCrO₄^-+ CrO₄^2-

HCrO₄^-+ OH^---->CrO₄^2-+ H₂O

Overall, the available information indicates that at very low pH values (1) the main species in solution is H₂CrO₄, at higher pHs between around 2 and 6, HCrO₄^-and Cr₂O₇^2-will both be present in equilibrium, and at higher pHs (>7) the main species present will be CrO₄^2-. Thus at environmental pHs the species found in solution will be a mixture of Cr₂O₇^2-, HCrO₄^-and CrO₄^2-, irrespective of the form in which the chromium (VI) enters solution.