Reaction mass of chromium (+ 3) hydroxide sulfate and sodium sulfate

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 Cr(VI) compounds.

Typically salts (ionic bonding) are electrovalent substance. Electrovalent substances are made up of ions in the solid state. The oppositely charged ions are held together by strong electrostatic (coulombic) force of attraction. Due to these forces the ions cannot move. When these substances are dissolved in water, the ions free themselves from this binding. Thus the break up of an electrovalent compound into free mobile ions when dissolved in water or when melted, is called electrolytic dissociation. In the liquid state the ions become free and mobile. But the oppositely charged ions always remain in close proximity of each other.

Cr(OH)(SO₄)(s) ---> Cr³⁺(aq) + SO₄^2- (aq) + OH^- (aq). This is a 100% dissociation.

This multi-constituent substance contains numerous ionic species that form complex equilibria in water which change over an environmentally-relevant pH range. Consequently it is not possible to perform testing to measure simple hydrolysis events so testing is waived.

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; Palmer and Wittbrodt, 1991).

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 (Losi et al., 1994): Cr³⁺ + 3 H₂O ⇔ Cr(OH)₃ (s) + 3 H⁺ (Keq = 1 x 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.