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Dissociation constant

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Reference
Endpoint:
dissociation constant
Type of information:
experimental study
Adequacy of study:
key study
Study period:
28 June 2006
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
study well documented, meets generally accepted scientific principles, acceptable for assessment
Qualifier:
no guideline followed
Principles of method if other than guideline:
Dissociation constant was calculated by the Hammett and Taft Correlations and obtained from the literature.
GLP compliance:
no
Specific details on test material used for the study:
Identity: FAT 40826/A
Batch: TZ 5604 BOP 01/06
Purity: Content of organic part (Na-salt): approx. 78 %
Oligomers: 13 %
Main component: approx. 48 %
Appearance: Solid, orange powder
Expiration date: February 01, 2011
Storage: At room temperature at about 20 °C in a desiccator.
Dissociating properties:
yes
No.:
#1
pKa:
-6.7
Remarks on result:
other: Arenesulfonic acids (3x)
No.:
#2
pKa:
>= 1.65 - <= 1.85
Remarks on result:
other: Diamino substituted chlorotriazine (1x)
No.:
#3
pKa:
>= 1.65 - <= 1.85
Remarks on result:
other: Diamino substituted fluorotriazine (1x)

The molecular structure of the main component of FAT 40826/A was used for the estimation of the dissociation behaviour. The compound has several sites, which can be dissociated (arenesulfonic acids) and two sites, which can be potentially protonated (diaminosubstituted chloro- and fluorotriazine, bases). The estimations are mainly based on the Hammett and Taft equations.


 


Arenesulfonic acids


FAT 40826/A contains three arenesulfonic acid groups (ArSOaH), all attached at naphthalene ring systems, which are known to be very strong acids. The basic pKa value based on the Hammett correlation, independent of the substituents, is about pKa = -6.7. This low pKa is outside of the measurable range of about pKa = 0 to 14. In aqueous solutions, these functional groups will be completely dissociated.


 


Diamino substituted chlorotriazine


It is known that amino substituted triazines will be protonated at the ring nitrogens. The secondary amines attached to the triazine ring are therefore of no relevance. The pKa values of diamino substituted chlorotriazines are well known from the triazine herbicides (pKa approximatively between 1.65 and 1.85. In the environmental relevant pH range, this functional group of FAT 40826/A will be in its neutral form.


 


Diamino substituted fluorotriazine


It is known that amino substituted triazines will be protonated at the ring nitrogens. The two secondary amines attached to the triazine ring are therefore of no relevance. The pKa values of diamino substituted triazines are well known from the triazine herbicides. The pKa values of the fluorotriazines will therefore be comparable to the chlorotriazines (pKa approximatively between 1.65 and 1.85 (Reference 8). The pKa might be slightly higher due to the lower electronegativity of the fluorine atom. Despite of that, this functional group of FAT 40826/A will behave similar to the chlorotriazines as described above and will be in its neutral form in the environmental relevant pH range from about pH 4 to 9.

Conclusions:
The behaviour of the test substance is aqueous solutions is dominated by the strongly acidic ArSO3H-groups. Therefore, the molecule is negatively charged and is present in anionic form over the whole environmentally relevant pH range.
Executive summary:

The dissociation constant in water of the test substance was calculated using the Hammett and Taft Correlation methods, and obtained from the literature. Three pKa's were determined of which the pKa from the strongly acidic ArSO3H-groups is the most important one. The test substance has dissociating properties because the molecule will be negatively charged over the whole environmentally relevant pH region.

Description of key information

The behavior of the test substance is aqueous solutions is dominated by the strongly acidic ArSO3H-groups. Therefore, the molecule is negatively charged and is present in anionic form over the whole environmentally relevant pH range.

Key value for chemical safety assessment

Additional information

The dissociation constant in water of the test substance was calculated using the Hammett and Taft Correlation methods, and obtained from the literature (RCC 2006). Three pKa's were determined of which the pKa from the strongly acidic ArSO3H- groups is the most important one. The test substance has dissociating properties because the molecule will be negatively charged over the whole environmentally relevant pH region.

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