Tin bis(2-ethylhexanoate)

Water solubility

Description of key information

The Tin bis(2-ethylhexanoate) substance is a salt, which dissociates upon introduction to water.  The Sn(II) component is dissociated and oxidized in oxygenated water to become the insoluble Sn(IV)O2 species.  The 2-ethylhexanoate component has pH-dependant solubility, which is approximately 4,500 mg/L at pH 7 and above.  

Key value for chemical safety assessment

Water solubility:

4 500 mg/L

at the temperature of:

20 °C

Additional information

The Tin bis(2-ethylhexanoate) substance is a salt, which dissociates into its Sn(II) and 2-ethyl hexanoate component ions upon introduction to water. Therefore, the water solubility of this substance must be evaluated on the basis of solubility of these dissociated components and undissociated salt as a function of pH. The 2-ethylhexanoate component has pKa of approximately 4.8, and therefore, will occur as the fully ionized species (> 99%) at pH > 6.8 which is representative of pH occurring in the environment. Testing of water solubility as a function of pH, using tin bis(2 -ethylhexanoate) loadings of 10g/L to water, indicate a solubility limit of approximately 4,500 mg/L for the 2 -ethylhexanoate component at pH of 7 and higher. At pH 4, water solubility is significantly reduced to approximately 200 mg/L. The Sn(II) component of Tin bis(2-ethylhexanoate) is known to occur primarily as the Sn(OH)₂ species in water of pH between 6 and 8 (Seby et al., 2001). The reported dissociation constant for Sn(II)Cl₂ is 1.74 (Seby et al., 2001), indicating that the Sn portion of tin bis(2 -ethylhexanoate) would be fully dissociated at pH > 3.7. Based on the standard reduction potential determined by Seby et al. (2001), this species is expected to spontaneously oxidize (coupled with reduction of molecular oxygen present in water equilibrated with the atmosphere), where the resulting Sn(IV) is speciated as the insoluble SnO₂ solid. Attempts have been made to measure the water solubility of total Sn species as a function of pH, using tin bis(2 -ethylhexanoate) loadings of 10g/L to distilled water adjusted to pH 2, 4, 7, and 9 with dilute buffers. While these results have indicated the expected trend of decreased solubility with increased pH, the speciation of Sn and degree of completeness of its oxidation/reduction reactions, cannot be discerned. Sufficient evidence exists to conclude that any Sn(II) species which may be introduced to the aqueous environment as a result of tin bis(2-ethylhexanoate)emissions will ultimately be speciated as the insoluble Sn(IV)O₂ solid. Because this species is an insoluble inorganic solid, hazard properties such as aquatic toxicity, bioaccumulation, and inter-media environmental transport are of lessened concern. Considering the pKa values for Sn(II)Cl₂ (1.75) and 2-ethylhexanoic acid (4.8), it is possible for a small fraction of this substance to have transient existence as the undissociated salt at very acidic pH. Under such conditions, the 2-ethylhexanoate groups (each having log Kow = 2.64) will impart a very hydrophobic property to the substance, which is expected to be essentially insoluble in water. The properties of the moderately soluble and mobile 2-ethylhexanoate component of this substance will therefore be the primary determinant of hazard and associated risk for the Tin bis(2-ethylhexanoate) substance in the environment.