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Physical & Chemical properties

Dissociation constant

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Description of key information

The dissociation constant (pKa) was determined for the two components of the registered substance i.e. the triethanolamine and the carboxylic acid. The pKa for the triethanolamine was found to be 7.97 and the pKa for the carboxylic acid was found to be 4.92.

Key value for chemical safety assessment

pKa at 20°C:
4.92

Additional information

A study was conducted on the registered substance to determine the dissociation constant of the two components of the substance (triethanolamine and carboxylic acid). The dissociation constants were determined in accordance with a test method similar to that of the OECD Testing Guideline 112. The pKa for the triethanolamine was found to be 7.97 and the pKa for the carboxylic acid was found to be 4.92. The pKa of the carboxylic acid component was considered to be the key value as the triethanolamine is known to be highly water soluble and is expected to remain in solution across a wide range of pH's and therefore in normal, acidic and basic environmental conditions. The carboxylic acid component is expected to precipitate in highly acidic conditions. An equivalent study was conducted to determine the pKa values of the components of the structurally similar substance 6 -[(p-Tosyl)amino]hexanoic acid, compound with 2,2',2''-nitrilotriethanol (1:1) (Tosyl salt). The pKa for the triethanolamine component was found to be 7.91 and the pKa for the carboxylic acid component was found to be 4.9. Read-across between the registered substance and the test substance (Tosyl salt) is considered justified owing to the structural similarity of the two substances. Both are salts of a carboxylic acid and triethanolamine. Ionisation of the carboxylic acid group in both isomeric acids will be the primary factor in determining their solubility in water at different pH. The pKas of the carboxylic acid groups in the two isomers are virtually the same: 6-[methyl(phenylsulphonyl)amino]hexanoic acid has a pKa of 4.92 and 6-[(p-tosyl)amino]hexanoic acid has a pKa of 4.90. This, together with the fact that both isomers are the same molecular weight and structurally very close, indicates that they will behave in a similar fashion in aqueous media.