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Dissociation constant

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Reference
Endpoint:
dissociation constant
Type of information:
(Q)SAR
Adequacy of study:
key study
Study period:
28 Oct - 10 Nov 1999
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
accepted calculation method
Justification for type of information:
Since the potentiometric method outlined in the OECD Guideline 112 was found unsuitable for the determination of the pKa, modeling software ACD pKa version 4.5 was used.
Qualifier:
according to guideline
Guideline:
EPA OPPTS 830.7370 (Dissociation constants in water)
Principles of method if other than guideline:
calculated with ACD pKa software version 4.5
GLP compliance:
yes
Remarks:
GLP claimed for an unsuccesful experiment; QSAR calculation is not GLP compliant
Dissociating properties:
yes
No.:
#1
pKa:
-3.9
Remarks on result:
other: calculated

The UV spectra of both solutions s at pH=1.25 and at pH=12.5 were recorded immediately after their preparation and did not significantly differ from each other over the detection range (200- 800 nm). Therefore the spectrophotometric method did not allow distinguishing the ionized and the molecular species, if they existed over the range of pH studied.

The examination of the chemical structure showed that the test item should be very difficult to protonate in very acidic medium.

Description of key information

pKa = -3.9 (calculated)

Key value for chemical safety assessment

Additional information

The dissociation constant of the test item was not determinable by spectrophotometric method according to OECD 112 due to no significant differences within the UV spectra at pH=1.25 and pH=12.5. However, the dissociation constant was calculated by ACD pKa software (v4.5).