Amines, polyethylenepoly-, tetraethylenepentamine 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:

16 Jul - 15 Sep 2014

Reliability:

2 (reliable with restrictions)

Rationale for reliability incl. deficiencies:

guideline study with acceptable restrictions

Qualifier:

according to guideline

Guideline:

OECD Guideline 112 (Dissociation Constants in Water)

Deviations:

no

GLP compliance:

yes (incl. QA statement)

Remarks:

Hess, Ministerium für Umwelt, Energie, Landwirtschaft und Verbraucherschutz, Wiesbaden, Germany

Dissociating properties:

yes

No.:

#1

pKa:

9.33

Temp.:

20 °C

Main Test (titration method)

The main tests were performed by titration in 20 incremental steps of c₀₁ up to the first EP, continuing up to the second EP in 20 incremental steps of c₀₂.

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

In the main tests, the existence of two equivalence points could not be confirmed.

With the measured values and the concentration of functional groups (c₀), determined from the equivalent point of the main tests, the pK_a values were calculated.

The pK_a values were calculated in the linear region before the equivalent point of the titration curve.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–Table4 of the report show the added volumes of HCl, the measured pH values and the calculation steps of the three titrations.

For the calculations in Table2 -Table4 the following definitions were used:

·        c₀: concentration of functional groups, which are titrated up to the 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_a 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 5 the calculatedpK_avalues are summarized.

Table5: Summary of the calculated pK_avalues

Test no.	pKa
1	9.29
2	9.33
3	9.38
mean value of pKa	9.33
standard deviation	0.03

 

By means of the titration method from equivalence point - one dissociation constant of the test item could be inferred at 20 °C: pK_a= 9.33.

The standard deviation of the first main test is with 0.11 slightly above the value mentioned in the OECD guideline (±0.1). Due to the very good conformity of the mean pK_avalues of the three main tests the standard deviation of the first main test can be neglected.

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 pKa values as follows:

pKa = 9.33 ± 0.03

However, when considering the structure of the molecule, TEPA incorporates five possible ionisable sites (each nitrogen). By using the Marvin calculation software it can be estimated that TEPA 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 TEPA molecule incorporates 5 possible ionisable sites (each nitrogen). By using the Marvin calculation software it can be estimated that TEPA 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). One equivalence point was observed, which corresponded to pK_a values as follows:

 

pK_a= 9.33 ± 0.03

 

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 = 9.33 ± 0.03 at 20 °C (OECD Guideline 112)

Key value for chemical safety assessment

pKa at 20°C:

9.33

Additional information

A dissociation constant of 9.33 ± 0.03 at 20 °C was established in a test according to OECD 112. However, the test substance incorporates five 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 that the value given above is 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 and assign specific dissociation constants.