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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-            pKa= -0.6; 0.74; 0.86

HCrO4- --->H++ CrO42-                 pKa= 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 CrO42-.

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

2 CrO42-+ 2 H+--->Cr2O72-+ H2O

2 HCrO4---->Cr2O72-+ H2O

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

Cr2O72-+ OH---->HCrO4-+ CrO42-

HCrO4-+ OH---->CrO42-+ 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 Cr2O72-will both be present in equilibrium, and at higher pHs (>7) the main species present will be CrO42-. Thus at environmental pHs the species found in solution will be a mixture of Cr2O72-, HCrO4-and CrO42-, irrespective of the form in which the chromium (VI) enters solution.

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