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Dissociation constant

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Description of key information

Not applicable for Fatty acids, C16-18 (even numbered), aluminum salts

Palmitic acid (57-10-3): pKa = 4.75 (est.)
Stearic acid (57-11-4): pKa = 4.75 (est.)

Behaviour of aluminium in water is very complex. Aluminium speciation depends strongly on pH:
pH < 5.7: Al3+ and [Al(OH)2]+
pH 5.7 to 6.7: [Al(OH)]2+, [Al(OH)2]+ and Al(OH)3
pH > 6.7: [Al(OH)4]-, polymerization

Key value for chemical safety assessment

Additional information

The test is scientifically not possible to perform: Fatty acids, C16-18 (even numbered), aluminum salts is not soluble in water. Furthermore for complex mixtures (UVCB) with ionisable components the assessment of pKa is clearly complicated.

pKa values of the two main fatty acid components from literature are given instead.

The behaviour and dissociation of aluminum is common knowledge, as described in the assessment report for the three aluminum salts (chloride, nitrate and sulphate) under the Canadian Environmental Protection Act: At low pH values, dissolved aluminum is present mainly in the aquo form (Al3+). Hydrolysis occurs as pH rises, resulting in a series of less soluble hydroxide complexes (e.g. Al(OH)2+, Al(OH)2+). Aluminum solubility is at a minimum near pH 6.5 at 20 °C (Al(OH)3) and then increases as the anion, Al(OH)4-, begins to form at higher pH.