Aluminium sulphate

Dissociation constant

Administrative data

Endpoint:

dissociation constant

Data waiving:

study scientifically not necessary / other information available

Justification for data waiving:

the study does not need to be conducted because the substance is hydrolytically unstable (half-life less than 12 hours)

the study does not need to be conducted because the substance has no ionic structure

Justification for type of information:

JUSTIFICATION FOR DATA WAIVING
In accordance with Section I of REACH Annex XI, the dissociation constant (required in Section 7.16) does not need to be measured as the registration substance itself does not have ionisable structural features.
The substance will totally dissociate into ions, and form different species, the pKa of the parent substance cannot be determined, and this study is not scientifically necessary:

Based on the basic knowledge of soluble aluminium slats, including aluminium sulphate, the form of the element in water varies from mononuclear to polynuclear hydroxo complex depending on the pH of solution:
Al3+→ [Al(OH) ]2+ → [Al(OH)2]+ → Al(OH)3(↓) → [Al(OH)4]- → [Al2O(OH)6]2-
pH < 4   →                 →                 →                 →                     → pH > 10
Due to physical-chemical properties (see section 4.8 water solubility), the substance dissolving in water results a strong acidic solution. Using the flask method , the water solubility of 202030/A at 20°C was ≥ 1000 g/l. The pH of the aqueous sample was 2.4. As a consequence in this aqueous solution, the Aluminium compound exists mainly as Aluminium ion and complexes of aluminium species [e. g. Al(OH)4-, the predominant species at higher pH], which are not expected to dissociate under naturally relevant pH (4-9) conditions.
In addition, in aqueous solution Al3+ does not occur as the free ion but is surrounded by six molecules of water to form (Al(H2O)6)3+. As pH increases, protons are removed from the co-ordinated waters to give various hydrolysis products. Progressive hydrolysis leads to the formation of aluminium hydroxide. Concentrated commercial products are stable over months and years. When applied AS hydrolyses within minutes to Al(OH)3.
Based on the study of hydrolysis of aluminium salts (see section 5.1.2 Hydrolysis), the process of polymeric aluminium species formation and decomposition was demonstrated.
Therefore the study of dissociation constant does not need to be conducted.