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Dissociation constant

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dissociation constant
Type of information:
Adequacy of study:
key study
Study period:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: accepted calculattion method
Justification for type of information:
QSAR prediction.
other: REACH guidance on QSARs R.6, May 2008
not specified
Principles of method if other than guideline:
ACD 7.0 software program for estimating the dissociation constant (pKa) in water.The recommended method, “apparent constants, approximated”, was used for the estimation. The program was developed by Advanced Chemistry Development Inc.
90 Adelaide Street West, Toronto, Ontario, M5H 3V9, Canada (; Copyright © 1994-2003; Build: 18 April 2003
GLP compliance:
Dissociating properties:
25 °C
Remarks on result:
other: HL/H+L
25 °C
Remarks on result:
other: H2L/H+HL

1. Defined endpoint: Dissociation constant (pKa) in water

2. Unambiguous algorithm: For the test substance the following fragment descriptors (parent compound) were identified as reaction centres: see report for the structure of the reaction centers.

3. Applicable domain: The identified reaction centers have been found as fragments in the Internal Reaction Centers Database with experimental equations. The substance only contains 78 atoms, which is much less than the limit of ACD/pKa DB of 255. The substance contains two ionization centres with with three fragments each. The substance does not contain the atoms, what are not accepted by ACD/pKa DB. The fragment that occurs in the structure of Macrolex Blau RR applied by the program is verified by the user. The rules applied for the substance appears appropriate. An individual uncertainty for the investigated substance is not available.

4. Statistical characteristics: Each calculation from ACD/pKa DB is provided with its 95 % confidence interval and, when available, literature references with experimental results. The accuracy of calculations for simple structures is usually better than ±0.2 pKa units (for complex structures it is better than ±0.5 pKa units).

5. Mechanistic interpretation: The mechanistic basis of the model is the linear free energy relationship (LFER), which is an empirical correlation between the standard free energies of reaction or activation for two series of reactions, both subjected to the same variations in reactant structures or reaction conditions. As applied to the estimation of acid dissociation constants, the LFER is basically a substituent-effect approach.

6. Adequacy of prediction: The result for Macrolex Blau RR falls within the applicability domain described above and the estimation rules applied for the substance appear appropriate. Therefore the predicted value can be considered reliable yielding a useful result for further assessment.

The dissociation constants for the test substance are estimated to be pKa1 of -0.41 and pka2 of -4.08.
Executive summary:

The dissociation constants of the test substance were estimated by ACD/pKa DB included in ACD/labs 7.00 Release and revealed values of pKa1 = -0.41 and pka2 = -4.08 at 25°C. According to the estimated results, this substance is mainly present in its undissociated form under environmentally relevant pH 5-9.

Description of key information

The dissociation constants for the test substance are estimated to be pKa1 of -0.41 and pka2 of -4.08 at 25°C.

Key value for chemical safety assessment

Additional information

- "should read pKa1 of -0.41 and pka2 of -4.08 at 25°C"

- The substance presents mainly as undissociated form under environmentally relevant pH 5 - 9.