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Physical & Chemical properties

Water solubility

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Description of key information

Aluminium sulphate is a very soluble salt (>> 100 g/L), however aluminium speciation rapidly leads to salting out to various degrees which are highly pH dependent. Solubility of monomeric Al can descend as low as 16 µg Al/L within a 90 minute experiment in pure water, and 12 µg/L after 28 days in reconstituted natural medium.

Key value for chemical safety assessment

Additional information

Numerous literature values are available, showing high solubility (between 300 and 600 g/L as eq. anhydrous form). However, while some of the selected data sources are considered reliable, experimental details are not available.

An experimental study, conducted with an analogue hydrated form, is also provided, and consistent with the above range.

Additionally, two valid transformation/dissolution studies were conducted, the first a 24 h study at nominal pH 6 and 8, in ISO 6341 reconstituted test medium 10x diluted (pH6) or undiluted (pH8), and aluminium concentrations dropped from 8.8 to 7.5 mg Al/L and 0.36 mg Al/L, respectively. This drop was less than expected due to the considerable reduction from the target pH (especially at pH6, which descended to 4.75 within a couple of minutes) caused by the capacity of aluminium to consume the alkalinity of the test medium (approximately 0.5 mg/L of alkalinity as CaCO3 per mg/L of alum according to Pernitsky and Edzwald, 2003). For this reason a 28 day TDp study was performed at a more environmentally realistic concentration of 1 mg/L at pH 6 (in diluted ISO 6341). In this case pH 6 was selected as the solubility was found to be the higher of the 2 pHs tested. In this study the original nominal concentration of 1 mg/L as aluminium sulphate (88 µg as elemental aluminium) dropped to 16 µg/L as elemental aluminium within 7 days and just 12 µg/L after 28 days and can therefore be considered as insoluble when added to environmental media within a few hours

A supporting publication reports consistent information on significant pH- and speciation-dependency, in deionised water, with a minimum solubility of 16 µg Al/L (pH = 6.0).

Moreover, in both experiments, solutions were filtered (0.2 µm) before analysis, in order to remove polymeric and colloidal species, as it is generally assumed that only dissolved metal ions will pass through such a filter.

This explains the variability between experiments observed.

Speciation is detailed under Section 5 on Environmental fate and pathways.