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Dissociation constant

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Reference
Endpoint:
dissociation constant
Type of information:
(Q)SAR
Adequacy of study:
key study
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
results derived from a valid (Q)SAR model and falling into its applicability domain, with adequate and reliable documentation / justification
Justification for type of information:
1. SOFTWARE
QSAR Toolbox 4.4

2. MODEL (incl. version number)
Acidic pKa (OASIS Consensus) model

3. SMILES OR OTHER IDENTIFIERS USED AS INPUT FOR THE MODEL
C[N+](C)(C)CCO of choline (CAS 62 -49 -7) as best approximation for choline chloride (CAS 67-48-1), because of restrictions of pKa calculations of ionized structures.

4. SCIENTIFIC VALIDITY OF THE (Q)SAR MODEL
Please refer to attached QMRF

5. APPLICABILITY DOMAIN
Please refer to attached QPRF

6. ADEQUACY OF THE RESULT
- The model is scientifically valid (see attached QMRF).
- See attached QPRF for reliability assessment.
Guideline:
other: REACH Guidance on QSARs R.6
Principles of method if other than guideline:
- Software tool(s) used including version: OECD QSAR Toolbox v4.4 (Developer: LMC, University "Prof. As. Zlatarov", Bourgas, Bulgaria, November 2018)
- Model(s) used: Acidic pKA model (OASIS Consensus)
- Model description: see field 'Justification for type of information', 'Attached justification'
- Justification of QSAR prediction: see field 'Justification for type of information', 'Attached justification'
Specific details on test material used for the study:
C[N+](C)(C)CCO
Dissociating properties:
yes
pKa:
10.6
Temp.:
25 °C
Remarks on result:
other: QSAR calculation
Conclusions:
pKa value = 10.06
Executive summary:

Using the OASIS Consensus calculator implemented in the OECD QSAR Toolbox v4.4 a pKa value of 10.6 was predicted for choline. The prediction was in the applicability domain of the model.

Description of key information

Dissociation constant: 10.6 at 25 °C (OASIS Consensus) of choline (CAS 62 -49 -7) as best approximation of choline chloride (because of restrictions for pKa calculation of ionized structures.

Key value for chemical safety assessment

pKa at 20°C:
10.06

Additional information

Choline chloride has an ionic substance and one of the ions contains an alcohol group which typically is able to dissociate. To cover this both facts as best approximation for the calculation of the dissociation constant of choline chloride, Choline (CAS 62 -49 -7) was used as input molecule for calculation model because of restrictions for pKa calculation of ionized structures. Using the OASIS Consensus a pKa value of 10.6 at 25 °C was predicted, indicating dissociating properties.


The calculation was classified as Klimisch 2, the estimation result is in the scientifically expected range and consequently and the available information is sufficient to cover the tonnage-driven data requirement under REACH.


Supporting information to confirm dissociation properties of choline chloride is given by OECD SIDS Report. Choline chloride is a quaternary amine salt, it can be easily concluded that it dissociates in water into the corresponding positively charged quaternary hydroxyl alkylammonium ion and the negatively charged chloride ion. The data was gained from secondary literature (OECD SIDS, 2004).


This is because typically salts (ionic bonding) are electrovalent substance. Electrovalent substances are made up of ions in the solid state. The oppositely charged ions are held together by strong electrostatic (coulombic) force of attraction. Due to these forces the ions cannot move. When these substances are dissolved in water, the ions free themselves from this binding. Thus the break up of an electrovalent compound into free mobile ions when dissolved in water or when melted, is called electrolytic dissociation. This is a 100 % dissociation. Nevertheless the quaternary ammonium salt has an alcohol group which is from a chemically point of view able to dissociate. This dissociation from to alcohol into a proton and the hydroxyl-molecule is calculated as mentioned above and can be considered as the dissociation constant beside the electrolytic dissociation.