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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:
OECD Guideline 112 (Dissociation Constants in Water)
Version / remarks:
1981
Deviations:
no
GLP compliance:
yes (incl. certificate)
Dissociating properties:
yes
No.:
#1
pKa:
2.69
Temp.:
20 °C
No.:
#2
pKa:
2.71
Temp.:
20 °C
No.:
#3
pKa:
2.7
Temp.:
20 °C
No.:
#4
pKa:
2.71
Temp.:
20 °C
No.:
#5
pKa:
2.7
Temp.:
20 °C

The dissociation constant of the test item was determined to be 2.70 at 20.0 ± 0.5 °C. The dissociation constant was attributed to the carboxylic acid functional group. This dissociation constant therefore describes the protonation and deprotonation of the test item.

In basic or neutral pH aqueous solutions, the ionized (deprotonated) form will be predominantly present. As sample solutions are strongly acidified, the neutral, unionized form will dominate.

On assessment of the test item structure for environmentally relevant dissociating functional groups, only a strongly acidic carboxylic acid functional group was identified. Although an amide functional group was also present, this chemical class does not allow for significant ionization within the environmentally relevant pH range.

Titration of the water blanks showed rapid basification of the sample solution pH, with solution pH values within the range 10.94 to 10.96 being obtained following the first addition of 0.5 mL of 0.1 M potassium hydroxide solution.

Conclusions:
The dissociation constant of the test item was determined to be 2.70 at 20.0 ± 0.5 °C.
Executive summary:

The dissociation constant of the substance in water was determined under GLP in accordance with OECD TG 112, using a titration method. Sample solutions were prepared to give a nominal concentration of 0.01 M at half titration (50 mL). The samples were then stored under a nitrogen headspace while equilibrating to the test temperature of 20 ± 0.5 °C. Each sample solution was then titrated by the addition of 10 aliquots of 0.5 mL of 0.1 M potassium hydroxide solution. Three

sample blanks, each comprising of 47.5 mL of high purity water, were also treated as detailed for the sample solutions.

The sample solution pH was recorded initially and after the addition of each aliquot of base. The pH probe was calibrated at pH 4, pH 7 and pH 10, using certified reference buffer solutions, before use. Five individual determinations of the dissociation constant were done, resulting in a mean of 2.70 at 20 ± 0.5 °C. The dissociation constant was attributed to the carboxylic acid functional group. This dissociation constant therefore describes the protonation and deprotonation of the test item.

Description of key information

Dissociation constant pKa = 2.70 at 20 °C (GLP, OECD TG 112)

Key value for chemical safety assessment

Additional information