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EC number: 701-311-0 | CAS number: -
Dissociation constant
Administrative data
Link to relevant study record(s)
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.
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. It is anticipated that if EC 701 -311 -0 could be isolated from the chemical matrix in which it is made, it would exhibit essentially identical behavior in this test, since the majority equilibrium boron containing species in water will be boric acid itself, especially if the initial pH of the testing water is <8 to start.
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)42-, B3O3(OH)52-, B5O6(OH)4-and B(OH)4-. In short:
B(OH)3 «polynuclear anions«B(OH)4-.
Again, at pH<5, boron is mainly present as 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)).
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