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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:
19 October 2011 until 16 November 2011
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
other: Guideline-conform study under GLP without restrictions.
Reference:
Composition 0
Qualifier:
according to
Guideline:
OECD Guideline 112 (Dissociation Constants in Water)
Deviations:
no
GLP compliance:
yes (incl. certificate)
Test material information:
Composition 1
Dissociating properties:
yes
No.:
#1
pKa:
5.89
Temp.:
25 °C
No.:
#2
pKa:
2.66
Temp.:
25 °C

Neutralisation curves

see attached

Determination of dissociation constants

Determination 1 - Calculation of First Dissociation Constant

Volume of 0.1M HCl (ml)

pH

Stoicheiometric Concentrations

{H+}

pKa1

[NH3+]

[NH2]

0.0

8.247

0

0.010

 

 

0.5

7.034

0.001

0.009

0.00000

6.08

1.0

6.495

0.002

0.008

0.00000

5.89

1.5

6.173

0.003

0.007

0.00000

5.81

2.0

6.019

0.004

0.006

0.00000

5.84

2.5

5.871

0.005

0.005

0.00000

5.87

3.0

5.691

0.006

0.004

0.00000

5.87

3.5

5.499

0.007

0.003

0.00000

5.87

4.0

5.218

0.008

0.002

0.00001

5.82

4.5

4.694

0.009

0.001

0.00002

5.64

5.0

3.873

0.010

0

 

 

Mean pKa1(using the inner 5 of the set of 9 values)      =         5.85

Determination 1 - Calculation of Second Dissociation Constant

Volume of 0.1M HCl (ml)

pH

Stoicheiometric Concentrations

{H+}

pKa2

[NH3+]

[NH2]

5.0

3.873

0

0.010

 

 

5.5

3.481

0.001

0.009

0.00033

2.34

6.0

3.259

0.002

0.008

0.00055

2.49

6.5

3.100

0.003

0.007

0.00079

2.55

7.0

2.974

0.004

0.006

0.00106

2.59

7.5

2.869

0.005

0.005

0.00135

2.63

8.0

2.779

0.006

0.004

0.00166

2.66

8.5

2.703

0.007

0.003

0.00198

2.71

9.0

2.636

0.008

0.002

0.00231

2.76

9.5

2.575

0.009

0.001

0.00266

2.81

10.0

2.525

0.010

0

 

 

Mean pKa2(using the inner 5 of the set of 9 values)      =         2.63

Determination 2 - Calculation of First Dissociation Constant

Volume of 0.1M HCl (ml)

pH

Stoicheiometric Concentrations

{H+}

pKa1

[NH3+]

[NH2]

0.0

9.486

0

0.010

 

 

0.5

8.114

0.001

0.009

0.00000

7.16

1.0

6.917

0.002

0.008

0.00000

6.32

1.5

6.197

0.003

0.007

0.00000

5.83

2.0

6.115

0.004

0.006

0.00000

5.94

2.5

5.994

0.005

0.005

0.00000

5.99

3.0

5.839

0.006

0.004

0.00000

6.02

3.5

5.626

0.007

0.003

0.00000

5.99

4.0

5.338

0.008

0.002

0.00000

5.94

4.5

4.804

0.009

0.001

0.00002

5.75

5.0

3.953

0.010

0

 

 

Mean pKa1(using the inner 5 of the set of 9 values)      =         5.95

Determination 2 - Calculation of Second Dissociation Constant

Volume of 0.1M HCl (ml)

pH

Stoicheiometric Concentrations

{H+}

pKa2

[NH3+]

[NH2]

5.0

3.953

0

0.010

 

 

5.5

3.550

0.001

0.009

0.00028

2.44

6.0

3.326

0.002

0.008

0.00047

2.58

6.5

3.164

0.003

0.007

0.00069

2.64

7.0

3.034

0.004

0.006

0.00092

2.68

7.5

2.933

0.005

0.005

0.00117

2.73

8.0

2.842

0.006

0.004

0.00144

2.77

8.5

2.754

0.007

0.003

0.00176

2.80

9.0

2.683

0.008

0.002

0.00207

2.85

9.5

2.616

0.009

0.001

0.00242

2.90

10.0

2.556

0.010

0

 

 

Mean pKa2(using the inner 5 of the set of 9 values)      =         2.72

Determination 3 - Calculation of First Dissociation Constant

Volume of 0.1M HCl (ml)

pH

Stoicheiometric Concentrations

{H+}

pKa1

[NH3+]

[NH2]

0.0

8.390

0

0.010

 

 

0.5

7.045

0.001

0.009

0.00000

6.09

1.0

6.631

0.002

0.008

0.00000

6.03

1.5

6.351

0.003

0.007

0.00000

5.98

2.0

6.154

0.004

0.006

0.00000

5.98

2.5

5.954

0.005

0.005

0.00000

5.95

3.0

5.755

0.006

0.004

0.00000

5.93

3.5

5.533

0.007

0.003

0.00000

5.90

4.0

5.219

0.008

0.002

0.00001

5.82

4.5

4.647

0.009

0.001

0.00002

5.60

5.0

3.818

0.010

0

 

 

Mean pKa1(using the inner 5 of the set of 9 values)      =         5.96

Table

Determination 3 - Calculation of Second Dissociation Constant

Volume of 0.1M HCl (ml)

pH

Stoicheiometric Concentrations

{H+}

pKa2

[NH3+]

[NH2]

5.0

3.818

0

0.010

 

 

5.5

3.425

0.001

0.009

0.00038

2.25

6.0

3.199

0.002

0.008

0.00063

2.40

6.5

3.034

0.003

0.007

0.00092

2.45

7.0

2.905

0.004

0.006

0.00124

2.49

7.5

2.796

0.005

0.005

0.00160

2.51

8.0

2.703

0.006

0.004

0.00198

2.53

8.5

2.620

0.007

0.003

0.00240

2.55

9.0

2.552

0.008

0.002

0.00281

2.59

9.5

2.496

0.009

0.001

0.00319

2.64

10.0

2.447

0.010

0

 

 

Mean pKa2(using the inner 5 of the set of 9 values)      =         2.51

Determination 4 - Calculation of First Dissociation Constant

Volume of 0.1M HCl (ml)

pH

Stoicheiometric Concentrations

{H+}

pKa1

[NH3+]

[NH2]

0.0

9.028

0

0.010

 

 

0.5

7.354

0.001

0.009

0.00000

6.40

1.0

6.657

0.002

0.008

0.00000

6.06

1.5

6.037

0.003

0.007

0.00000

5.67

2.0

5.930

0.004

0.006

0.00000

5.75

2.5

5.831

0.005

0.005

0.00000

5.83

3.0

5.710

0.006

0.004

0.00000

5.89

3.5

5.563

0.007

0.003

0.00000

5.93

4.0

5.342

0.008

0.002

0.00000

5.94

4.5

4.951

0.009

0.001

0.00001

5.90

5.0

4.139

0.010

0

 

 

Mean pKa1(using the inner 5 of the set of 9 values)      =         5.81

Determination 4 - Calculation of Second Dissociation Constant

Volume of 0.1M HCl (ml)

pH

Stoicheiometric Concentrations

{H+}

pKa2

[NH3+]

[NH2]

5.0

4.139

0

0.010

 

 

5.5

3.625

0.001

0.009

0.00024

2.54

6.0

3.367

0.002

0.008

0.00043

2.64

6.5

3.197

0.003

0.007

0.00064

2.69

7.0

3.066

0.004

0.006

0.00086

2.73

7.5

2.959

0.005

0.005

0.00110

2.77

8.0

2.866

0.006

0.004

0.00136

2.80

8.5

2.783

0.007

0.003

0.00165

2.84

9.0

2.715

0.008

0.002

0.00193

2.90

9.5

2.644

0.009

0.001

0.00227

2.96

10.0

2.574

0.010

0

 

 

Mean pKa2(using the inner 5 of the set of 9 values)      =         2.77

The mean dissociation constants determined for the two amine functional groups during the four determinations at 25°C are shown in the following table.

Table 6.14

Determination

Dissociation Constant

First

Second

1

5.85

2.63

2

5.95

2.72

3

5.96

2.51

4

5.81

2.77

Mean

5.89

2.66

Conclusions:
The dissociation constants of the two amine functional groups at 25°C were determined by potentiometric titration to be 5.89 and 2.66.
Executive summary:

The purpose of the study was to determine the dissociation contants of test item 2-methyl-p-phenylenediamine via OECD Guideline 112 (Dissociation Constants in Water).

A set of five determinations were carried out prior to the four determinations which have been used to calculate the dissociation constants. The data yielded by the previous five determinations was not suitable for use in calculating the dissociation constants because of poor replication. The time between the preparation of the samples and titration was not controlled and was thought to be the source of the variation in the data between replicates. Therefore, another 4 determinations were carried out, and this time the titrations were carried out immediately after the preparation of each sample. The data was more consistent between the determinations and was used to calculate the dissociation constants.The inner five of the set of nine values of dissociation constant were used to calculate the mean dissociation constant values for each determination. This was justified by the fact that any errors resulting from the presence of impurities and inaccuracies in adding the titrant are likely to be more evident in the outer values.

The dissociation constants of the two amine functional groups at 25°C were determined by potentiometric titration to be 5.89 and 2.66.

Description of key information

The dissociation constants of the two amine functional groups at 25°C were determined by potentiometric titration to be 5.89 and 2.66.

Key value for chemical safety assessment

pKa at 20°C:
5.89

Additional information