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Physical & Chemical properties

Dissociation constant

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Reference
Endpoint:
dissociation constant
Type of information:
experimental study
Adequacy of study:
key study
Study period:
17 November 2014 - 26 November 2014
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
guideline study
Qualifier:
equivalent or similar to guideline
Guideline:
OECD Guideline 112 (Dissociation Constants in Water)
Deviations:
no
GLP compliance:
yes (incl. QA statement)
Dissociating properties:
yes
No.:
#1
pKa:
9.21
Temp.:
20 °C

The pKa values were calculated in each case in the linear regions before the equivalent points of the titration curves.The data points at which the slope of thecurve deviated too much from the linear runs were not included in the calculation.

For the calculations the following definitions were used:

·        c01: concentration of functional groups, which are titrated up to the first EP (mol/L)

·        c02: concentration of functional groups, which are titrated up to the second EP (mol/L)

·        [HCl]: concentration of the added HCl (mol/L)

·        [A-] = c0-[HCl]: matches the concentration of not reacted functional groups (mol/L)

·        [HA] = c0-[A-]

·        A = [HA]/[A-]

The pKa is calculated by pH+log A. The values represented in the tables are rounded values. The calculation was carried out with the original not rounded values.

In the following table the calculated pKa values are summarized.

Table 7: Summary of the calculated pKavalues

Test no.

pKa1

1

9.15

2

9.17

3

9.22

4

9.40

5

9.12

mean value of pKa

9.21

standard deviation

0.08

 

By means of the titration method from the equivalence points - one dissociation constant of the test item could be inferred at 20 °C: pKa1= 9.21.

Conclusions:
An attempt was made to apply the procedures described in OECD Test Guideline 112, Dissociation constants in water (1981). One equivalence point was observed, which corresponded to a pKa value as follows:

pKa1 = 9.21 ± 0.08

However, when considering the structure of the molecule, the test substance incorporates a lot of possible ionisable sites (each nitrogen). The experimental titration curves obtained in this study are consistent with a complex ionisation profile that would be expected for this type of molecule, indicating that protonation takes place over a wide pH range. It is therefore not possible to identify and assign specific dissociation constants.

Description of key information

Dissociation constant determination was carried out using a titration method. 

Key value for chemical safety assessment

pKa at 20°C:
9.21

Additional information

An attempt was made to apply the procedures described in OECD 112, Dissociation constants in water (1981). One equivalence point was observed, which corresponded to a pKavalue as follows:

 

pKa1= 9.21 ± 0.08

 

However, the experimental titration curves obtained in this study are consistent with a complex ionisation profile that would be expected for this type of molecule, indicating that protonation takes place over a wide pH range. It is therefore not possible to identify and assign specific dissociation constants.