Amines, polyethylenepoly-, triethylenetetramine fraction

Dissociation constant

Link to relevant study record(s)

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 pK_avalues were calculated. The pK_avalues 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 2–Table 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 pK_ais 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 pK_avalues are summarized.

Table8: Summary of the calculated pK_avalues

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: pK_a1= 9.59 and pK_a2= 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 pK_avalues as follows:

 

pK_a1= 9.59 ± 0.02

pK_a2= 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.