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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:
2018-06-07 to 2018-06-11
Reliability:
1 (reliable without restriction)
Rationale for reliability incl. deficiencies:
guideline study
Qualifier:
according to guideline
Guideline:
OECD Guideline 112 (Dissociation Constants in Water)
Version / remarks:
Adopted May 1981
Deviations:
no
Qualifier:
according to guideline
Guideline:
other: EU method A.25
Version / remarks:
COMMISSION REGULATION (EU) 2017/735 of 14 February 2017, amending COMMISSION REGULATION (EC) No 440/2008
Deviations:
no
GLP compliance:
yes (incl. QA statement)
Dissociating properties:
yes
No.:
#1
pKa:
4.74
Temp.:
20 °C
Remarks on result:
other: Run 1
No.:
#2
pKa:
4.96
Temp.:
20 °C
Remarks on result:
other: Run 2
No.:
#3
pKa:
4.84
Temp.:
20 °C
Remarks on result:
other: Run 3
No.:
#4
pKa:
4.8
Temp.:
20 °C
Remarks on result:
other: mean value of run 1 to 3
Tables 1 to 3 present the raw data generated during this test. These data were plotted (abscissa: volume base added; ordinate: measured pH) and graphically evaluted to determine the end point volume, 1/2 endpoint volume and pKa based on pH at 1/2 of the endpoint volume. See the attached figure for an example evaluation.

Included in the tabulated data below are the end point volumes for each titration and the pH at half of the end point volume (equivalent to the pKa value).

iso-Nonanoic acid (3,5,5-Trimethylhexanoic acid) was determined to have a pKa value of 4.8 (mean of 4.7, 5.0 and 4.8). The repeatability of ± 0.1 units satisfied the requirement of the test method.

Table1:            Dissociation Constant Results for Run 1

Titre (mL)

pH

 

 

 

 

0.000

5.534

3.792

9.809

0.010

5.758

3.793

9.978

0.020

6.030

3.794

10.144

0.030

6.340

3.798

10.275

0.166

6.724

3.846

10.415

0.460

7.138

3.968

10.520

0.822

7.626

4.115

10.633

1.214

8.132

4.259

10.729

1.626

8.678

4.415

10.800

2.032

9.290

4.547

10.875

2.470

9.912

4.740

10.946

2.846

10.544

4.886

10.996

3.226

11.228

5.069

11.051

3.534

11.910

5.247

11.096

3.812

12.614

5.441

11.139

4.048

13.328

5.643

11.176

4.238

14.058

5.871

11.213

4.332

14.790

6.017

11.243

4.414

15.540

6.154

11.276

4.554

16.284

6.484

11.302

4.592

17.044

6.490

11.324

4.606

17.816

6.624

11.350

4.616

18.580

6.624

11.355

4.626

19.400

6.650

11.391

4.638

20.000

6.652

11.399

5.132

 

9.252

 

5.194

 

9.377

 

5.206

 

9.415

 

5.226

 

9.455

 

5.356

 

9.632

 

 

 

 

 

End point volume, V = 4.86 mL

½ V = 2.43 mL

pH at ½ V =4.74

pKa= 4.74

 

Table2 :           Dissociation Constant Results for Run 2

Titre (mL)

pH

 

 

 

 

0

7.432

4.039

10.791

0.010

7.976

4.036

10.880

0.020

8.548

4.042

10.963

0.030

9.142

4.043

11.030

0.104

9.776

4.048

11.086

0.786

10.446

4.387

11.101

1.138

11.232

4.524

11.197

1.462

11.854

4.654

11.237

1.856

12.482

4.807

11.250

2.256

13.212

4.956

11.310

2.666

13.890

5.132

11.341

3.046

14.630

5.326

11.370

3.356

15.382

5.519

11.382

3.612

16.182

5.722

11.418

3.802

16.924

5.730

11.449

3.842

17.672

5.968

11.456

3.852

18.450

5.974

11.489

3.862

19.196

5.982

11.501

4.022

19.996

6.246

11.523

4.176

20.002

6.249

11.529

4.958

 

9.692

 

4.968

 

9.702

 

4.980

 

9.710

 

5.078

 

9.854

 

5.246

 

10.013

 

5.478

 

10.171

 

5.768

 

10.326

 

6.104

 

10.462

 

6.496

 

10.588

 

6.936

 

10.696

 

 

 

 

 

End point volume, V = 4.57 mL

½ V = 2.28 mL

pH at ½ V = 4.96

pKa = 4.96

 

Table 3:            Dissociation Constant Results for Run 3

Titre (mL)

pH

 

 

 

 

0

6.382

3.943

10.481

0.010

6.794

3.944

10.590

0.020

7.274

3.947

10.695

0.030

7.792

3.951

10.793

0.328

8.338

4.099

10.871

0.680

8.934

4.250

10.942

1.028

9.560

4.390

11.007

1.422

10.206

4.539

11.065

1.828

10.874

4.695

11.112

2.232

11.572

4.840

11.161

2.650

12.270

5.008

11.205

3.044

12.980

5.191

11.250

3.384

13.690

5.370

11.278

3.702

14.442

5.582

11.312

3.930

15.184

5.786

11.330

4.078

15.966

5.958

11.362

4.196

16.716

6.133

11.381

4.288

17.496

6.274

11.407

4.448

18.260

6.622

11.426

4.490

19.040

6.928

11.452

4.500

19.804

6.921

11.473

4.510

20.000

6.921

11.478

5.164

 

9.689

 

5.238

 

9.800

 

5.250

 

9.801

 

5.270

 

9.833

 

5.424

 

9.997

 

5.638

 

10.144

 

5.734

 

10.208

 

6.026

 

10.350

 

 

 

 

 

End point volume, V = 4.74 mL

½ V = 2.37 mL

pH at ½ V = 4.84

pKa = 4.84

 

Conclusions:
pKa (OECD 112; titration method): 4.8 at 20°C, based on acidic functionality
Executive summary:

The pKa for 3,5,5 -trimethyhexanoic acid was determined in a reliable study performed according to OECD 112 compliant with GLP, following the potentiometric titration method.

Three titration runs were performed, each consisting of at least 49 titration steps, with the following results:

Run 1: pKa 4.74 (20 +/- 1 °C)

Run 2: pKa 4.96 (20 +/- 1 °C)

Run 3: pKa 4.84 (20 +/- 1 °C)

In conclusion, iso-nonanoic acid (3,5,5 -trimethylhexanoic acid) was found to have a pKa (acidic functionality) value of 4.8 at 20ºC (mean value of three runs). The repeatability of ± 0.1 units satisfied the requirement of the test method.

Description of key information

pKa (OECD 112; titration method): 4.8 at 20°C, based on acidic functionality

Key value for chemical safety assessment

pKa at 20°C:
4.8

Additional information

The pKa for 3,5,5 -trimethyhexanoic acid was determined in a reliable study performed according to OECD 112 compliant with GLP, following the potentiometric titration method.

Three titration runs were performed, each consisting of at least 49 titration steps, with the following results:

Run 1: pKa 4.74 (20 +/- 1 °C)

Run 2: pKa 4.96 (20 +/- 1 °C)

Run 3: pKa 4.84 (20 +/- 1 °C)

In conclusion, iso-nonanoic acid (3,5,5 -trimethylhexanoic acid) was found to have a pKa (acidic functionality) value of 4.8 at 20ºC (mean value of three runs). The repeatability of ± 0.1 units satisfied the requirement of the test method.

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