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Dissociation constant

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Reference
Endpoint:
dissociation constant
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
guideline study
Qualifier:
according to guideline
Guideline:
OECD Guideline 112 (Dissociation Constants in Water)
Version / remarks:
and OPPTS 830.7370
Deviations:
not specified
GLP compliance:
yes
Dissociating properties:
yes
pKa:
5.09
Temp.:
20 °C

The pKa values for phosphoric acid were 2.52 and 7.10 at 20 °C

The  pKa value for 4-nitrophenol was 7.16.  The pKa values for the reference substances were in good agreement with published literature values. The mean (n = 3) pKa value for tin (II) ethylhexanoate was determined to be  5.09 (SD = 0.0337, CV - 0.662%) at 20°C. These results are consistent with a reported pKa value of 4.8 for 2 -ethyl hexanoic acid, and indicate that essentially complete dissociation of the test substance will occur at  environmentally-relevant pH (i.e., 99% dissociated at pH > 6.8).

Conclusions:
The pKa value of tin bis(2 -ethylhexanoate) was determined to be 5.09 (SD 0.0337, CV = 0.662%) at 20 °C. The pKa values for phosphoric acid were 2.52 and 7.10 at 20 deg C. The pKa value for 4-nitrophenol was 7.16. The reference substance data were in good agreemant with the literature values.
Experimental Termination date- June 25, 2002
Executive summary:

The objective of this study was to determine the dissociation constant(s) of tin bis(2 -ethylhexanoate) in water at 20 °C according to OECD Guideline 112 and GLP. The titration method was used to determine the dissiciation constant K (expressed as its log value, pK) of Stannous 2-ethylhaxanoate in water. Phosphoric acid and 4 nitrophenol were used as reference substances. The pKa value of tin bis(2 -ethylhexanoate) was determined to be 5.09 (SD 0.0337, CV = 0.662%) at 20 °C. This indicates that essentially complete dissociation of the test substance will occur at  environmentally-relevant pH (i.e., 99% dissociated at pH > 6.8)

Description of key information

The objective of this study was to determine the dissociation constant(s) of tin bis(2 -ethylhexanoate) in water at 20 °C according to OECD Guideline 112 and GLP. The titration method was used to determine the dissiciation constant K (expressed as its log value, pK) of Stannous 2-ethylhaxanoate in water. Phosphoric acid and 4 nitrophenol were used as reference substances. The pKa value of tin bis(2 -ethylhexanoate) was determined to be 5.09 (SD 0.0337, CV = 0.662%) at 20 °C. This indicates that essentially complete dissociation of the test substance will occur at  environmentally-relevant pH (i.e., 99% dissociated at pH > 6.8)

Key value for chemical safety assessment

pKa at 20°C:
5.09

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).