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Water solubility

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Reference
Endpoint:
water solubility
Type of information:
experimental study
Adequacy of study:
key study
Study period:
25 FEB 2010 - 23 JUL 2010
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
guideline study
Qualifier:
according to guideline
Guideline:
OECD Guideline 105 (Water Solubility)
Deviations:
yes
Remarks:
solubility determined as function of pH at 2, 4, 7, and 9
Principles of method if other than guideline:
Test substance loaded to dilute buffers prepared in distilled water at 10g/L in duplicate test vessels for each pH. pH adjusted to target values of 2, 4, 7, and 9.
GLP compliance:
no
Type of method:
flask method
Key result
Water solubility:
4 585 mg/L
Temp.:
20 °C
pH:
ca. 7
Remarks on result:
other: Solubility of 2-ethyl hexanoate at pH 7, based on dissolved organic carbon concentration and carbon content of 67.1%
Key result
Water solubility:
4 555 mg/L
Temp.:
20 °C
pH:
ca. 9
Remarks on result:
other: Solubility of 2-ethyl hexanoate at pH 9, based on dissolved organic carbon concentration and carbon content of 67.1%
Key result
Water solubility:
238 mg/L
Temp.:
20 °C
pH:
ca. 4
Remarks on result:
other: Solubility of 2-ethyl hexanoate at pH 4, based on dissolved organic carbon concentration and carbon content of 67.1%
Details on results:
results were determined also for pH 2, however, the speciation of test subsance and associated carbon content at this pH could not be reliably assumed. The results for pH 2 are therefore not reported.
Conclusions:
Interpretation of result: soluble (1000-10000 mg/L)
The pKa of 2-ethylhexanoic acid is approximately 4.8, and therefore the substance is expected to be fully ionized at pH > 6.8. The solubility limit of this dissociation product of stannous octoate was confirmed to be approximately 4,500 mg/L at pH 7 and greater. The expected decreased water solubility with decreased pH was observed, as solubility limit at pH 4 was 238 mg/L. An attempt was made to determine solubility of the Sn component of this salt as a function of pH. While the expected trend of decreasing solubility with increased pH was observed, the speciation of Sn under these conditions, and the associated extent of oxidation to Sn(IV) species could not be determined. Other sufficient evidence exists to indicate that Sn(II) introduced to oxygenated water will be oxidized to Sn(IV) species, which will occur as the insoluble solid Sn(IV)O2. Therefore, further assessment of water solubility of this Sn component of Stannous Octoate is not warranted.
Executive summary:

The water solubility of the dissociated components of the stannous octoate salt were determined as a function of pH, according to OECD guideline 105. The pKa of 2-ethylhexanoic acid is approximately 4.8, and therefore the substance is expected to be fully ionized at pH > 6.8. The solubility limit of this dissociation product of stannous octoate was confirmed to be approximately 4,500 mg/L at pH 7 and greater. The expected decreased water solubility with decreased pH was observed, as solubility limit at pH 4 was 238 mg/L. An attempt was made to determine solubility of the Sn component of this salt as a function of pH. While the expected trend of decreasing solubility with increased pH was observed, the speciation of Sn under these conditions, and the associated extent of oxidation to Sn(IV) species could not be determined. Other sufficient evidence exists to indicate that Sn(II) introduced to oxygenated water will be oxidized to Sn(IV) species, which will occur as the insoluble solid Sn(IV)O2. Therefore, further assessment of water solubility of this Sn component of Stannous Octoate is not warranted.

Description of key information

The water solubility of the dissociated components of the stannous octoate salt were determined as a function of pH, according to OECD Guideline 105. The pKa of 2-ethylhexanoic acid is approximately 4.8, and therefore the substance is expected to be fully ionized at pH > 6.8. The solubility limit of this dissociation product of stannous octoate was confirmed to be approximately 4,500 mg/L at pH 7 and greater. The expected decreased water solubility with decreased pH was observed, as solubility limit at pH 4 was 238 mg/L. An attempt was made to determine solubility of the Sn component of this salt as a function of pH. While the expected trend of decreasing solubility with increased pH was observed, the speciation of Sn under these conditions, and the associated extent of oxidation to Sn(IV) species could not be determined. Other sufficient evidence exists to indicate that Sn(II) introduced to oxygenated water will be oxidized to Sn(IV) species, which will occur as the insoluble solid Sn(IV)O2. Therefore, further assessment of water solubility of this Sn component of Stannous Octoate is not warranted.

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.