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Dissociation constant

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Endpoint:
dissociation constant
Type of information:
experimental study
Adequacy of study:
disregarded due to major methodological deficiencies
Study period:
October 6, 2003- October 7, 2003
Reliability:
3 (not reliable)
Rationale for reliability incl. deficiencies:
other: see 'Remark'
Remarks:
Benzene, ethenyl-, ar-bromo derivs. is a neutral organic material that does not ionise. The value reported (PKa 6.72) is believed to probably represent that of the water which Benzene, ethenyl-, ar-bromo derivs. was placed in to perform the study. This can be confirmed when reviewing Table 4 (summarised as table 2) of the study report that exhibits a lot of noise in regards to titrating the test substance and supports that a very, very weak titration process took place. This is further demonstrated when looking at Figures 1 (Phosphorus acid) and 2 (4-Nitrophenol) and comparing them to Figure 3 (the test substance). Therefore, the PKa of Benzene, ethenyl-, ar-bromo derivs. reported in the study is very likely representative of water, not the actual test substance. In demonstrating that Benzene, ethenyl-, ar-bromo derivs. PKa value reported in the dissociation constant study is false and probably representative of water, the pH issue is not applicable with regards to this test substance because the test substance does not ionise. As the method indicates that the “test method measurements should be made on ionisable substances only in the neutral form by the use of an appropriate buffer with a pH of at least one pH unit below or above the dissociation constant(s)”, performing the Kow study again, specifically to include a pH, is not warranted as the substance is not ionisable. On this basis, the study is disregarded.
Qualifier:
according to guideline
Guideline:
OECD Guideline 112 (Dissociation Constants in Water)
GLP compliance:
yes
Dissociating properties:
yes
pKa:
6.72
Temp.:
20 °C
Remarks on result:
other: SD = 0.0855 (CV = 1.27%)

The pKa values for phosphoric acid were 2.38 and 7.06 at 25 °C. The pKa value for 4 -nitrophenol was 6.99 at 22.1 °C. The reference substance data were in good agreement with the literature values.

The pKa estimate obtained for DBS in the preliminary trial was 5.41. The preliminary results also indicated that a relatively weak NaOH solution would be required. Therefore, a 0.001 N NaOH solution was prepared and standardized for use in the definitive trial.

The calculated pK value for DBS is summarized below:

Table 1: pK Values of DBS at 20 °C

Sample
Number

DBS
Concentration
(mg/L)

Equivalence
Point
(pH)

Titrant
Volume
(mL)

pKa1

Mean
pKa

Standard
Deviation

298C-134-2

3

7.846

2.8

6.775

6.72

0.0855

298C-134-3

3

7.810

2.9

6.76

(CV = 1.27%)

298C-134-4

3

7.717

4.2

6.62

Antilog (-pKa) = 1.91 x 10-7

1 Results were generated using Excel 2000 in the full precision mode. Manual calculations may differ

slightly.

 

 

Table 2: Determination of dissociation constant

Volume

pH

% Ionised

% Unionised

pKa

0.00

6.126

0

1

0.25

6.303

0.089286

0.91071429

7.3116

0.50

6.414

0.178571

0.82142857

7.076758

0.75

6.498

0.267857

0.73214286

6.934693

1.00

6.726

0.357143

0.64285714

6.981273

1.50

6.815

0.535714

0.46428571

6.752852

1.70

6.898

0.607143

0.39285714

6.708944

1.90

7.012

0.678571

0.32142857

6.687489

2.10

7.102

0.75

0.25

6.624879

2.30

7.221

0.821429

0.17857143

6.558242

2.75

7.856

0.982143

0.01785714

6.115637

Mean

6.775237

Standard Deviation

0.327251

% RSD

4.830104

% ionised = volume/equivalence point volume

% unionised = 1 - % ionised

pKa = pH – log(% ionised/% unionised)

Equivalence point = change in pH/change in volume is greatest

Conclusions:
The pKa value of DBS was determined to be 6.72 (SD = 0.0855, CV = 1.27%) at 20°C.
Executive summary:

In a study performed in line with OCED Guideline 112 and in compliance with GLP, the pKa value of dibromostyrene was determined to be 6.72 (SD = 0.0855, CV = 1.27%) at 20 ºC.

Endpoint:
dissociation constant
Type of information:
(Q)SAR
Adequacy of study:
supporting study
Study period:
2013-04-27
Reliability:
2 (reliable with restrictions)
Rationale for reliability incl. deficiencies:
other: see 'Remark'
Remarks:
The dissociation constant of the substance was assessed using a representative structure of it's most abundant form, dibromostyrene. The difference between the different constituents is the degree of bromination (mono-, di- or tri-). As dibromostyrene accounts for more than 80 % of the substance, it was considered that the dibromostyrene would be the most appropriate structure to use in predicting the dissociating properties of the substance. The estimation was performed with a validated QSAR model.
Justification for type of information:
QSAR prediction: migrated from IUCLID 5.6
Qualifier:
according to guideline
Guideline:
other: other guideline: REACH Guidance on QSARs R.6, May/July 2008
Principles of method if other than guideline:
The pKalc model calculates the acidic and basic pKa values for organic compounds (usually within an error of 0.25 pKa units). The model can be used for any organic compound including aromatics, mono and polyheteroaromatics and small peptides. The applied logarithm adapted from Hammett and Taft encompases all necessary electronic, steric and other effects and is based on an extended database of nearly thousands of equations.
GLP compliance:
no
Dissociating properties:
no

Dibromostyrene does not contain any ionisable groups (basic or acidic).

Conclusions:
Dibromostyrene (a representative structure of the reaction mass) does not include any ionisable functional groups.
Executive summary:

Evaluation of dibromostyrene (a representative structure of the reaction mass) using PALLAS 3.6.2.1: pKalc (CompuDrug Chemistry Ltd.) software indicated that dibromostyrene contains not dissociating groups.

Endpoint:
dissociation constant
Data waiving:
other justification
Justification for data waiving:
other:

Description of key information

In accordance with Annex XI, point 2 (Testing is not technically feasible) and in accordance with the ECHA guidance document, Guidance on information requirements and chemical safety assessment chapter R.7a: Endpoint specific guidance (November 2012) v. 2.0, testing for dissociating properties  is technically not possible as a consequence of the properties of the substance. The three main constituents of the multi-constituent substance, dibromostyrene, monobromostyrene and tribromostyrene, cannot dissociate due to a lack of relevant functional groups; a study to determine the dissociation constant is therefore irrelevant.

Key value for chemical safety assessment

Additional information