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Dissociation constant

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Description of key information

The pKa was measured to be 5.09 at 20 °C

Key value for chemical safety assessment

pKa at 20°C:

Additional information

The pKa of tin bis(2 -ethylhexanoate) was measured according to OECD Guideline 112. The procedure deviated somewhat from the guideline, as the test material is an ionic salt and not in a free-acid or free-base form. Therefore, the measurement could have been confounded by concurrent dissociation of various forms of Sn(II) resulting from dissociation of the salt in the test medium.

Tin bis(2 -ethylhexanoate) is known to readily dissociate upon mixing in water, and various studies have shown that Sn(II) species introduced to aerated water will occur primarily as the poorly soluble Sn(OH)2 species at environmentally-relevant pH. The pKa of 2 -ethylhexanoic acid is reported in several sources to be 4.8; however a specific reference which included methods of determination cannot be found. The results of the Key Study are consistent with the expected dissociation behaviour of 2 -ethylhexanoic acid.

Seby et al. (2001) report a dissociation constant of 1.74 for Sn(II)Cl2 in 3M NaClO4 solution. However, this publication also illustrates the spontaneous oxidation of Sn(II) in oxygenated water to form the insoluble Sn(IV)O2 solid. Therefore, the dissociation constant and water solubility of this insoluble species is not important for the environmental hazard assessment of tin bis(2 -ethylhexanoate).