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Physical & Chemical properties

Dissociation constant

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Endpoint:
dissociation constant
Type of information:
experimental study
Adequacy of study:
supporting study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
study well documented, meets generally accepted scientific principles, acceptable for assessment
Principles of method if other than guideline:
Sodium palmitate was titrated with 0.1 M sodium hydroxide at 25°C to determine the neutralisation endpoint. The dissociation constant was determined from the mean of five titrations of each solution. The pKa was calculated as the solution's pH at half the neutralisation volume.
GLP compliance:
no
Dissociating properties:
yes
No.:
#1
pKa:
8.6 - 8.8
Temp.:
25 °C
Remarks on result:
other: Mean of five determinations
Conclusions:
The dissociation constant, pKa, of sodium palmitate is 8.6 - 8.8 at 25°C.
Executive summary:

Kanicky et al. (2000) titrated sodium palmitate with sodium hydroxide and calculated the mean pKa from the pH at half the neutralisation volume for each of the five replicates. The study is a non-GLP, non-guideline experiment, available in a peer-reviewed published journal article. The experiment follows sound scientific principles, with limitations in design and/or reporting, and is considered adequate for assessment.

Endpoint:
dissociation constant
Type of information:
experimental study
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
study well documented, meets generally accepted scientific principles, acceptable for assessment
Principles of method if other than guideline:
Potassium stearate was titrated with 0.1 M hydrocloric acid at 20°C to determine the neutralisation endpoint. The dissociation constant was determined from the mean of five titrations of each solution. The pKa was calculated as the solution's pH at half the neutralisation volume.
GLP compliance:
no
Dissociating properties:
yes
No.:
#1
pKa:
10.15
Temp.:
20 °C
Remarks on result:
other: Mean of five replicates
Conclusions:
The dissociation constant, pKa, of potassium stearate is 10.15 at 20°C.
Executive summary:

Kanicky and Shah (2002) titrated potassium stearate with hydrochloric acid and calculated the mean pKa from the pH at half the neutralisation volume for each of the five replicates. The study is a non-GLP, non-guideline experiment, available in a peer-reviewed published journal article. The experiment follows sound scientific principles, with limitations in design and/or reporting, and is considered adequate for assessment.

Endpoint:
dissociation constant
Type of information:
read-across from supporting substance (structural analogue or surrogate)
Adequacy of study:
supporting study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
study well documented, meets generally accepted scientific principles, acceptable for assessment
Remarks:
Read across data
Justification for type of information:
REPORTING FORMAT FOR THE ANALOGUE APPROACH

1. HYPOTHESIS FOR THE ANALOGUE APPROACH
At neutral pH, and within the limits of solubility, the substance is expected to dissociate into aluminium species, mainly aluminium hydroxide and fatty acid anions. The fatty acid anions would then be expected to achieve equilibrium with respect to the H+ ions in the water (depending on the pKa of the fatty acids) and at neutral pH there would be ionised acids, unionised acids, and aluminium hydroxide species. As the dissociation constant of the substance is expected to be determined by the equilibrium of the fatty acids with the hydrogen ions in the water, read across between the same fatty acids is considered to be justified.

2. SOURCE AND TARGET CHEMICAL(S) (INCLUDING INFORMATION ON PURITY AND IMPURITIES)
The target substance comprises of a mixture of stearate (C18, saturated) and palmitate (C16, saturated) fatty acids. Therefore, stearate and palmitate salts are considered as read across substances.

3. ANALOGUE APPROACH JUSTIFICATION
The measured dissociation constant for sodium palmitate of 8.8 (Kanicky et al. 2000) and for potassium stearate of 10.15 (Kanicky and Shah 2002) are both relatively high and thus any dissociated fatty acid would be expected to be unionised (protonated) at environmental pH.

4. DATA MATRIX
Kanicky et al. (2000) titrated sodium palmitate with sodium hydroxide and calculated the mean pKa from the pH at half the neutralisation volume for each of the five replicates. Kanicky and Shah (2002) titrated potassium stearate with hydrochloric acid and calculated the mean pKa from the pH at half the neutralisation volume for each of the five replicates. The studies are non-GLP, non-guideline experiments, available in peer-reviewed published journal articles. The experiments follow sound scientific principles and are considered adequate for assessment.
Dissociating properties:
yes
No.:
#1
pKa:
8.6 - 8.8
Temp.:
25 °C
Remarks on result:
other: Mean of five determinations
Conclusions:
The dissociation constant, pKa, of sodium palmitate is 8.6 - 8.8 at 25°C.
Executive summary:

Kanicky et al. (2000) titrated sodium palmitate with sodium hydroxide and calculated the mean pKa from the pH at half the neutralisation volume for each of the five replicates. The study is a non-GLP, non-guideline experiment, available in a peer-reviewed published journal article. The experiment follows sound scientific principles, with limitations in design and/or reporting, and is considered adequate for assessment.

Endpoint:
dissociation constant
Type of information:
read-across from supporting substance (structural analogue or surrogate)
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
study well documented, meets generally accepted scientific principles, acceptable for assessment
Remarks:
Read across data
Justification for type of information:
REPORTING FORMAT FOR THE ANALOGUE APPROACH

1. HYPOTHESIS FOR THE ANALOGUE APPROACH
At neutral pH, and within the limits of solubility, the substance is expected to dissociate into aluminium species, mainly aluminium hydroxide and fatty acid anions. The fatty acid anions would then be expected to achieve equilibrium with respect to the H+ ions in the water (depending on the pKa of the fatty acids) and at neutral pH there would be ionised acids, unionised acids, and aluminium hydroxide species. As the dissociation constant of the substance is expected to be determined by the equilibrium of the fatty acids with the hydrogen ions in the water, read across between the same fatty acids is considered to be justified.

2. SOURCE AND TARGET CHEMICAL(S) (INCLUDING INFORMATION ON PURITY AND IMPURITIES)
The target substance comprises of a mixture of stearate (C18, saturated) and palmitate (C16, saturated) fatty acids. Therefore, stearate and palmitate salts are considered as read across substances.

3. ANALOGUE APPROACH JUSTIFICATION
The measured dissociation constant for sodium palmitate of 8.8 (Kanicky et al. 2000) and for potassium stearate of 10.15 (Kanicky and Shah 2002) are both relatively high and thus any dissociated fatty acid would be expected to be unionised (protonated) at environmental pH.

4. DATA MATRIX
Kanicky et al. (2000) titrated sodium palmitate with sodium hydroxide and calculated the mean pKa from the pH at half the neutralisation volume for each of the five replicates. Kanicky and Shah (2002) titrated potassium stearate with hydrochloric acid and calculated the mean pKa from the pH at half the neutralisation volume for each of the five replicates. The studies are non-GLP, non-guideline experiments, available in peer-reviewed published journal articles. The experiments follow sound scientific principles and are considered adequate for assessment.
Reason / purpose:
read-across source
Dissociating properties:
yes
No.:
#1
pKa:
10.15
Temp.:
20 °C
Remarks on result:
other: Mean of five replicates
Conclusions:
The dissociation constant, pKa, of potassium stearate is 10.15 at 20°C.
Executive summary:

Kanicky and Shah (2002) titrated potassium stearate with hydrochloric acid and calculated the mean pKa from the pH at half the neutralisation volume for each of the five replicates. The study is a non-GLP, non-guideline experiment, available in a peer-reviewed published journal article. The experiment follows sound scientific principles, with limitations in design and/or reporting, and is considered adequate for assessment.

Description of key information

The dissociation constant, pKa, of the substance has been read across from sodium palmitate, with a pKa of 8.6 - 8.8 at 25°C and potassium stearate, with a pKa of 10.15 at 20°C.

Key value for chemical safety assessment

Additional information

The dissociation constants have been taken from published articles, read across from sodium palmitate (Kanicky et al 2000), potassium stearate (Kanicky and Shah 2002).