Aqueous Chemistry
Boric acid, B(OH)3, while sometimes indicated as dissociating in aqueous solution,
- B(OH)3 BH2O3– + H+ (Ka = 5.794 x 10−10 mol dm-3, pKa = 9.237)
may also be represented as acidic due to its hydrolysis reaction with water molecules, forming tetrahydroxyborate and releasing a proton:
- B(OH)3 + 2H2O B(OH)−
4 + H3O+ (Ka = 5.8x10−10 mol dm-3, pKa = 9.24)
In the presence of cis-diols such as mannitol, glucose, sorbitol and glycerol the pK is lowered to about 4.
Under acid conditions boric acid may undergo condensation reactions to form polymeric oxyanions. The equilibrium
- 4 - + 2 H+ 2− + 7 H2O
illustrates the process. The tetrameric anion is present in the mineral borax, as an octahydrate, Na2B4O5(OH)4.8H2O. This compound can be obtained in high purity and so can be used to make a standard solution in titrimetric analysis.
Read more about this topic: Borate
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