Boric acid

Dissociation constant

Link to relevant study record(s)

Reference

Endpoint:

dissociation constant

Type of information:

experimental study

Adequacy of study:

key study

Study period:

2010-06-02 - 2010-06-24

Reliability:

1 (reliable without restriction)

Rationale for reliability incl. deficiencies:

guideline study

Reason / purpose for cross-reference:

reference to same study

Reason / purpose for cross-reference:

reference to same study

Reason / purpose for cross-reference:

reference to same study

Reason / purpose for cross-reference:

reference to same study

Reason / purpose for cross-reference:

reference to same study

Reason / purpose for cross-reference:

reference to same study

Qualifier:

according to guideline

Guideline:

OECD Guideline 112 (Dissociation Constants in Water)

Deviations:

not specified

GLP compliance:

yes

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 test substance was analysed for dissociation constant according to OECD Test Guideline 112. The mean dissociation constant (pKa) was found to be 8.94.

Executive summary:

The dissociation constant of the test-item was determined in a GLP-conform study according to OECD duideline 112. The mean dissociation constant (pKa) was found to be 8.94.

Description of key information

A study conducted according to OECD guideline 112 gave a dissociation constant for boric acid of 8.94 at 20 ºC. The dissociation behaviour is also described in several publications.
Boric acid is a Lewis acid (hydroxide ion acceptor) rather than a Brønsted acid (proton donator). For this purpose the formula for boric acid is best written as B(OH)3.
pKa = 9.0 at 25 ºC for boric acid in dilute solutions only (B ≤ 0.025 M).
At higher boron concentrations, polynuclear complexes are formed and several dissociation/formation constants apply.

Key value for chemical safety assessment

pKa at 20°C:

8.94

Additional information

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

B(OH)₃+ 2H₂O«[B(OH)₄]^-+ H₃O⁺

pKa = 9.0 at 25 °C

pKa = 8.94 at 20 °C

Although at these concentrations, boric acid exists as undissociated boric acid B(OH)₃at pH < 5, whereas at pH > 12.5 the metaborate ion -[B(OH)₄]^-- becomes the main species in solution. Both species are present at pH 5-12.5 at concentrations B ≤ 0.025 M.

At higher boron concentrations (B > 0.025 M) an equilibrium is formed between B(OH)₃, polynuclear complexes of B₃O₃(OH)₄^-, B₄O₅(OH)₄^2-, B₃O₃(OH)₅^2-, B₅O₆(OH)₄^-and B(OH)₄^-. In short:

B(OH)₃ «polynuclear anions«B(OH)₄^-.

Again, pH<5, boron is mainly present at B(OH)₃and in alkaline solution 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)).