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Physical & Chemical properties

Dissociation constant

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Reference
Endpoint:
dissociation constant
Type of information:
read-across from supporting substance (structural analogue or surrogate)
Adequacy of study:
weight of evidence
Justification for type of information:
REPORTING FORMAT FOR THE ANALOGUE APPROACH

1. HYPOTHESIS FOR THE ANALOGUE APPROACH
The dissociation constant for disodium octaborate tetrahydrate as such cannot be determined because disodium octaborate terahydrate is converted into boric acid/borate upon dissolution in water: Na2B8O13•4H2O + 9H2O ? 2NaB(OH)4 + 6B(OH)3. The dissociation constant found will be the dissociation constant for boric acid in the presence of sodium ions. At low boron concentrations (B = 0.025 M) the following equilibrium is found: B(OH)3 + 2H2O ? B(OH)4- + H3O+ with pKa = 9.0 at 25 °C. In dilute aqueous solutions (B = 0.025 M) boric acid exists as undissociated boric acid B OH)3 at pH < 7, at pH > 11 the metaborate ion becomes the main species in solution. In between values (pH 7 – 11) both species are present. At higher boron concentrations (B > 0.025 M) an equilibrium is formed between B(OH)3, polynuclear complexes of B3O3(OH)4-, B4O5(OH)42-, B3O3(OH)52-, B5O6(OH)4- and B(OH)4-. In short B(OH)3 ? polynuclear anions ? B(OH)4-. In acid solutions at pH < 5, boron is mainly present as B(OH)3 and in alkaline solutions at pH > 12.5 boron is mainly present as B(OH)4-. At in between values (pH 5 – 12) polynuclear anions are found as well as B(OH)3 and B(OH)4-. The dissociation constant depends upon temperature, ionic strength and presence of group I metal ions (Na, K, Cs). In the presence of metal ions (e.g. Na, Mg, Ca) ion-pair complexes are formed, which further reduce the undissociated boric acid concentration: Mn+ + B(OH)4- ? MB(OH)4(n-1)+ These ion pair complexes are expected to be present in solutions of disodium tetraborate, disodium octaborate and buffered solutions of boric acid and boric oxide (Ingri N, 1963).

2. SOURCE AND TARGET CHEMICAL(S) (INCLUDING INFORMATION ON PURITY AND IMPURITIES)
Source: Boric acid
Target: Boric oxide

3. ANALOGUE APPROACH JUSTIFICATION
please refer to "Hypothesis for the anaolgue approach"

Reason / purpose for cross-reference:
read-across source
Dissociating properties:
yes
pKa:
8.94
Temp.:
20 °C
Remarks on result:
other: Overall standard deviation = 0.22; overall % RSD = 2.45

Test item determination 1:

Weight of test item taken (g) = 0.1547

Molecular weight of test item = 61.84

Final volume of solution made up in carbon dioxide-free distilled water (mL) = 250.

Concentration of test item solution (M) = 0.1547/61.84 x 1000/250 = 0.010006

NB in the absence of a certificate of analysis for the test item, the calcualtions of test item solution concentration the purity of the test item has been assumed to be 100 %

Volume of test item solution taken for titration = 50 mL.

Volume of 0.1 M

sodium hydroxide added

(mL)

pH of solution

pKa

0.0

6.55

N/A

0.4

8.08

9.14

0.8

8.43

9.15

1.2

8.63

9.14*

1.6

8.78

9.11*

2.0

8.91

9.9*

2.4

9.05

9.09*

2.8

9.16

9.06*

3.2

9.27

9.03*

3.6

9.37

8.97*

4.0

9.48

8.90*

4.4

9.60

8.77*

4.8

9.72

8.43*

5.2

9.84

N/A

5.6

9.99

N/A

6.0

10.16

N/A

6.4

10.37

N/A

6.8

10.69

N/A

7.2

10.99

N/A

7.6

11.27

N/A

8.0

11.43

N/A

Mean (using values marked * only) =

8.96

SD (using values marked * only) =

0.22

%RSD =

2.44

Test item determination 2:

Weight of test item taken (g) = 0.1548

Molecular weight of test item = 61.84

Final volume of solution made up in carbon dioxide-free distilled water (mL) = 250.

Concentration of test item solution (M) = 0.1548/61.84 x 1000/250 = 0.010013

Volume of test item solution taken for titration = 50 mL.

Volume of 0.1 M

sodium hydroxide added

(mL)

pH of solution

pKa

0.00

6.55

N/A

0.4

8.04

9.11

0.8

8.42

9.15

1.20

8.62

9.13*

1.6

8.77

9.10*

2.0

8.89

9.07*

2.4

9.00

9.04*

2.8

9.12

9.02*

3.2

9.22

8.98*

3.6

9.33

8.93*

4.0

9.43

8.85*

4.4

9.53

8.70*

4.8

9.64

8.35*

5.2

9.76

N/A

5.6

9.90

N/A

6.0

10.03

N/A

6.4

10.21

N/A

6.8

10.43

N/A

7.2

10.72

N/A

7.6

11.03

N/A

8.0

11.26

N/A

8.4

11.43

N/A

8.8

11.55

N/A

9.2

11.66

N/A

9.6

11.74

N/A

10.0

11.80

N/A

10.4

11.87

N/A

10.8

11.92

N/A

11.2

11.96

N/A

Mean (using values marked * only) =

8.92

SD (using values marked * only) =

0.24

%RSD =

2.65

Test item determination 3:

Weight of test item taken (g) = 0.1546

Molecular weight of test item = 61.84

Final volume of solution made up in carbon dioxide-free distilled water (mL) = 250.

Concentration of test item solution (M) = 0.1546/61.84 x 1000/250 = 0.010000

Volume of test item solution taken for titration = 50 mL

Volume of 0.1 M

sodium hydroxide added

(mL)

pH of solution

pKa

0.00

6.33

N/A

0.4

8.08

9.14

0.8

8.39

9.10

1.20

8.60

9.11

1.6

8.77

9.10

2.0

8.90

9.08

2.4

9.02

9.06

2.8

9.14

9.04

3.2

9.25

9.01

3.6

9.35

8.95

4.0

9.47

8.89

4.4

9.57

8.73

4.8

9.69

8.39

5.2

9.82

N/A

5.6

9.98

N/A

6.0

10.19

N/A

6.4

10.40

N/A

6.8

10.72

N/A

7.2

11.08

N/A

7.6

11.32

N/A

8.0

11.46

N/A

8.4

11.58

N/A

8.8

11.68

N/A

9.2

11.75

N/A

9.6

11.83

N/A

10.0

11.88

N/A

10.4

11.93

N/A

10.8

11.98

N/A

11.2

12.01

N/A

Mean (using values marked * only) =

8.94

SD (using values marked * only) =

0.22

%RSD =

2.50

Conclusions:
The dissociation constant for the test item as such cannot be determined because it is converted into boric acid/borate upon dissolution in water, therefore boric acid was analysed for dissociation constant. The mean dissociation constant (pKa) was found to be 8.94.
Executive summary:

The dissociation constant for the test item as such cannot be determined because it is converted into boric acid/borate upon dissolution in water, therefore boric acid was analysed for dissociation constant according to OECD Test Guideline 112 in a GLP study.  The mean dissociation constant (pKa) was found to be 8.94.

Description of key information

The dissociation constant for the test item as such cannot be determined because it is converted into boric acid/borate upon dissolution in water, therefore boric acid was analysed for dissociation constant according to OECD Test Guideline 112 in a GLP study.  The mean dissociation constant (pKa) was found to be 8.94.

Key value for chemical safety assessment

pKa at 20°C:
8.94

Additional information

The dissociation constant for disodium octaborate tetrahydrate as such cannot be determined because disodium octaborate terahydrate is converted into boric acid/borate upon dissolution in water:

Na2B8O13·4H2O + 9H2O ↔ 2NaB(OH)4+ 6B(OH)3.

The dissociation constant found will be the dissociation constant for boric acid in the presence of sodium ions.

 

At low boron concentrations (B ≤ 0.025 M) the following equilibrium is found:

B(OH)3+ 2H2O ↔ B(OH)4-+ H3O+with pKa = 9.0 at 25 °C.

 

In dilute aqueous solutions (B ≤ 0.025 M) boric acid exists as undissociated boric acid B OH)3at pH < 7, at pH > 11 the metaborate ion becomes the main species in solution. In between values (pH 7 – 11) both species are present.

 

At higher boron concentrations (B > 0.025 M) an equilibrium is formed between B(OH)3, polynuclear complexes of B3O3(OH)4-, B4O5(OH)42-, B3O3(OH)52-, B5O6(OH)4-and B(OH)4-. In short B(OH)3↔ polynuclear anions ↔ B(OH)4-.

 

In acid solutions at pH < 5, boron is mainly present as B(OH)3and in alkaline solutions at pH > 12.5 boron is mainly present as B(OH)4-. At in between values (pH 5 – 12) polynuclear anions are found as well as B(OH)3and B(OH)4-.

The dissociation constant depends upon temperature, ionic strength and presence of group I metal ions (Na, K, Cs).

In the presence of metal ions (e.g. Na, Mg, Ca) ion-pair complexes are formed, which further reduce the undissociated boric acid concentration:

Mn++ B(OH)4-↔ MB(OH)4(n-1)+

These ion pair complexes are expected to be present in solutions of disodium tetraborate, disodium octaborate and buffered solutions of boric acid and boric oxide (Ingri N, 1963).