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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:
17 Jul - 10 Sep 2014
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
guideline study
Qualifier:
according to guideline
Guideline:
OECD Guideline 112 (Dissociation Constants in Water)
GLP compliance:
yes (incl. QA statement)
Remarks:
Hess, Ministerium für Umwelt, Energie, Landwirtschaft und Verbraucherschutz, Wiesbaden, Germany
Dissociating properties:
yes
No.:
#1
pKa:
9.59
Temp.:
20 °C
No.:
#2
pKa:
6.47
Temp.:
20 °C

Main Test (titration method)

The added volumes of 1 M HCl and the measured pH values were documented.

With the measured values and the concentrations of functional groups (c01and c02), determined from the equivalent points of the main tests, the pKavalues were calculated. The pKavalues were calculated in each case in the linear regions before thee quivalent points of the titration curves. The data points at which the slope of the curve deviated too much from the linear runs were not included in the calculation.

Table 2Table 7 of the report show the added volumes of HCl, the measured pH values and the calculation steps of the three titrations.

For the calculations inTable2-Table7 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 pKais 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 pKavalues are summarized.

Table8: Summary of the calculated pKavalues

Test no.

pKa1

pKa2

1

9.61

6.47

2

9.58

6.52

3

9.57

6.42

mean value of pKa

9.59

6.47

standard deviation

0.02

0.04

 

By means of the titration method from the equivalence points - two dissociation constants of the test item could be inferred at 20 °C: pKa1= 9.59 and pKa2= 6.47.

Conclusions:
An attempt was made to apply the procedures described in OECD Test Guideline 112, Dissociation constants in water (1981). Two equivalence points were observed, which corresponded to pKa values as follows:
pKa1 = 9.59 ± 0.02
pKa2 = 6.47 ± 0.04

However, when considering the structure of the molecule, TETA incorporates four possible ionisable sites (each nitrogen). By using the Marvin calculation software it can be estimated that TETA has a large number of dissociation constants. 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, starting at approximately pH 11 (unprotonated) and finishing at
Executive summary:

The TETA molecule incorporates four possible ionisable sites (each nitrogen). By using the Marvin calculation software it can be estimated that TETA has a large number of dissociation constants.

 

An attempt was made to apply the procedures described inOECD Test Guideline 112, Dissociation constants in water (1981). Two equivalence points were observed, which corresponded to pKavalues as follows:

 

pKa1= 9.59 ± 0.02

pKa2= 6.47 ± 0.04

 

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, starting at approximately pH 11 (unprotonated) and finishing at <pH2 (fully protonated).  It is therefore not possible to identify specific dissociation constants.

Description of key information

pKa = 9.59 ± 0.02 and 6.47 ± 0.04 (OECD Guideline 112)

Key value for chemical safety assessment

pKa at 20°C:
9.59

Additional information

Two dissociation constants of 9.59 ± 0.02 and 6.47 ± 0.04 were established at 20 °C in a test according to OECD 112. However, the test substance incorporates four possible ionisable sites (all nitrogen) and a large number of dissociation constants can be estimated by means of the Marvin calculation software. It is doubtful if the two values given above are meaningful. 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, starting at approximately pH 11 (unprotonated) and finishing at <pH2 (fully protonated).  It is therefore not possible to identify specific dissociation constants.