Sodium dichromate

Hydrolysis

Description of key information

The substance forms reversible equilibria and the proportion of the species present depends on various environmental factors.  An overview of the expected behaviour of the substance in sterile aqueous media is provided in the EU RAR, furthermore the speciation of the substance is affected by various other reducing or oxidising species present:

Additional 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 dichromates, the actual species present in solution depends on the pH according to the following equilibria:

H₂CrO₄⇔ H+ + HCrO₄^-             pKa₁ = -0.6; 0.74; 0.86

HCrO₄^-⇔ H+ + CrO₄^2-                  pKa₂ = 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:

2CrO₄^2-+ 2H+ ⇔ Cr₂O₇^2- + H₂O

2HCrO₄^-⇔ 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.

Ionic equilibria for chromium (III)

The predominant forms of chromium (III) present in solution are Cr³⁺ at very low pH, then with increasing pH, Cr(OH)²⁺, Cr(OH)₂⁺, Cr(OH)₃ and finally Cr(OH)^4- at very high pH. The species Cr(OH)₂⁺ is thought to occur only over a very narrow pH range (approximately pH 6.27-6.84).

In solution, the equilibrium between chromium (III) ions and chromium hydroxide lays well over to the side of the relatively insoluble hydroxide at pH>5 according to the following equilibrium:

Cr³⁺+ 3H₂O ⇔ Cr(OH)₃ (s) + 3H⁺                 Keq = 1.10-12

This means that at pHs >5 chromium (III) can be expected to precipitate out of solution as the insoluble hydroxide, often in conjunction with iron. However, complexation of chromium (III) ions with organic matter (such as citric acid, diethylenetriaminepentaacetic acid (DTPA), fulvic acid) can result in increased solubilisation of chromium (III) at higher pHs.