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EC number: 235-487-0 | CAS number: 12251-53-5
Hydrolysis
Administrative data
Link to relevant study record(s)
Description of key information
When sodium aluminate is diluted by water, it will hydrolysed readily and precipitation of Al(OH)3 and release of free sodium ion will occur. Hydrolysis of aluminium ions has two possible “directions” towards a neutral pH, i.e. base hydrolysis and acid hydrolysis. Both acid and base hydrolysis of aluminum results in precipitation of aluminium hydroxide. Sodium aluminate decomposes to aluminum hydroxide when added to water and can therefore not be considered as water soluble.
Key value for chemical safety assessment
Additional information
Commercial sodium aluminate have an excess of sodium hydroxide stabilising the solubility of sodium aluminate liquid. When sodium aluminate is diluted by water, the stability will change and precipitation of Al(OH)3will occur.Hydrolysis of aluminium ions has two possible “directions” towards a neutral pH, i.e. base hydrolysis and acid hydrolysis. Both acid and base hydrolysis of aluminum result in precipitation of aluminium hydroxide. This hydrolysis reaction is very rapid, and is completed within 1-7 seconds (Lettermann et al. 1973).
Zhao et al. (2009) investigated the effect of pH in the range of 4.0 to 6.4 on the aluminium chloride hydrolysis at low concentration level. As coagulant aluminium chloride was diluted with deionised water. At pH 4.0, mono- and dimeric aluminum species [Al(OH)2(H2O)2-3]+and [Al2O2(OH)(H2O)0-5]+were detected as main products. With increasing pH, hydrolysis and polymerization increased. At pH 5.0, aluminum species mainly aggregated and assemblied to median polymeric species (Al6-Al10 species) and these further to large polymeric species (Al11- Al21). At pH 5.8 metastable median and large polymers decomposed into small alluminum species and disaggregated into dimeric species. With pH 6.4 the majority of aluminum formed to Al(OH)3 amorphous flocs. In alkaline solution, depends on the concentration of aluminate, the soluble aluminate ions, Al(OH)4 ‾ and Al2O(OH)62 - ions are considered to be the totally dominating species (Szabo et al. 1978).
Aluminium hydroxide, Al(OH)3, is amphoteric. It is soluble in both strong acids and strong bases. Nevertheless,the transformation/dissolution study according to OECD 29 showed that at around neutral pH (pH 6 and pH 8), the solubility of aluminium hydroxide is minimal. In all tested conditions, the amounts of aluminium released, both at high loading (100 mg/L) and low loading (1 mg/L), by the samples and the blanks were undistinguishable (CIMM, 2007). Sodium aluminate decomposes to aluminum hydroxide when adds to water and can therefore not be considered as water soluble.
Letterman, R.D., Quon, J.E., and Gemmell, R.S., "Influence of Rapid Mix Parameters on Flocculation,"Jour. of Amer. Water Works Association, 63:11:716-722 (1973).
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