Tin bis(2-ethylhexanoate)

Dissociation constant

Link to relevant study record(s)

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).