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Physical & Chemical properties

Dissociation constant

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Description of key information

Study technically not feasible.  

Key value for chemical safety assessment

Additional information

Methods used to determine dissociation constants are the spectrophotometric, titration and calculation methods.

The test substance is a complex mixture of metal hydroxide hydrates present at varying degrees.

Based on a study with Magnesium-aluminium-hydroxide-carbonate, the water solubility of the test substance at neutral pH was determined, to be <0.009 mg/l. Since for the determination of the dissociation constants a test concentration lower than half of the water solubility has to be used, a test concentration of < 0.0045 mg/l should be selected.

The spectrophotometric method relies on the use of pure substances to be able to differentiate between the ionized and non-ionized species of the test substance. Due to the complexity of the test substance in combination with its low water solubility and lack of

UV-activity, the spectrophotometric method cannot be applied.

The use of an alternative method such as titration can also not be considered as this is a less sensitive method than the spectrophotometric method and not generate reliable results given the insolubility of the test substance in distilled water.

The conductometric method cannot be applied either due to the complex nature of the substance in combination with its insolubility in water.

As an alternative, calculation of the pKa values for the major components from their structures using Perrin's calculation method could be considered. However, the molecular structures of the test substance contain metal ions (a.o. coordination complexes) which cannot be computed using the mentioned computer program and would not be supported by the calculation algorithms.

Conclusion:

It was not possible to determine the pKa values of Magnesium-aluminium-hydroxide-carbonate

experimentally. Also, due to the complexity and nature of the test substance, it was not possible to calculate the pKa values using the pKalc version 5.0 computer program. Therefore, pKa values for

[carbonato(2-)]hexadecahydroxybis(aluminium)hexamagnesium cannot be given.