page_title: Borate
section_title: Boric acid
hierarchical_section_title: Borate / Boric acid
context_page_description: Borates are boron-oxygen compounds, which form boron oxyanions. These can be trigonal or tetrahedral in structure, or more loosely can consist of chemical mixtures which contain borate anions of either description. The element boron most often occurs in nature as borates, such as borate minerals and borosilicates.
context_section_description: All borates can be considered derivatives of boric acid, B(OH)₃. Boric acid is a weak proton donor (pKₐ ~ 9) in the sense of Brønsted acid, but is a Lewis acid, i.e., it can accept an electron pair. In water, it behaves as a Lewis acid, accepting the electron pair of a hydroxyl ion produced by the water autoprotolysis.
  B(OH)₃ is acidic because of its reaction with OH⁻ from water, forming the tetrahydroxyborate complex [B(OH)₄]⁻ and releasing the corresponding proton left by the water autoprotolysis:
  B(OH)₃ + 2 H₂O ⇌ [B(OH)₄]⁻ + [H₃O]⁺  (pKₐ = 8.98)
  In the presence of cis-vicinal diols, such as mannitol, sorbitol, glucose and glycerol, the acidity of the boric acid solution is increased, and the pKₐ can be lowered to about 4 if enough mannitol is added. 
  With different mannitol concentrations, the pK of B(OH)₃ extends on 5 orders of magnitude (from 9 to 4). Greenwood and Earnshawn (1997) refer to a pK value of 5.15, while a pK value of 3.80 is also reported in the Vogel's book. 
  The formation of the complex (more exactly, in fact an ester) between one B(OH)₃ molecule and two mannitol (C₆H₁₄O₆) molecules (sometimes referred as mannitoborate, the conjugated base of the mannitoboric acid, pKₐ = 3.80), releases three water molecules and one proton in water as follows: 
   ⇌  + 3 H₂O + H⁺
  (pKₐ ranging from 4 to 9, depending on the mannitol concentration)
  The solution obtained after the complexation/esterification reaction – involving also the release of a proton, from there, the ancient name of mannitoboric acid – is then sufficiently acid to be titrated by a strong base as NaOH. The equivalence point can then be determined by potentiometric titration using an automated titrator in order to assay the borate content present in aqueous solution. This method is often used to determine the boron content in the water of the primary circuit of light-water reactor, in which boric acid is added as a neutron moderator to control the reactivity of the core.
caption: The structure of the tetrahydroxyborate anion.
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