Editing Boric acid (section)

===With vicinal diols===
The acidity of boric acid solutions is considerably increased in the presence of [[cis–trans isomerism|cis]]-[[diol#Vicinal diols|vicinal diols]] ([[organic compound]]s containing similarly oriented [[hydroxyl]] groups in adjacent [[carbon]] atoms, {{chem2|(R1,R2)\dC(OH)\sC(OH)\d(R3,R4)}}) such as [[glycerol]] and [[mannitol]].<ref name=vogelP357/><ref name=GandE/><ref name=NIST1969/><ref name="Belcher1960"/>

The tetrahydroxyborate anion formed in the dissolution spontaneously reacts with these diols to form relatively stable anion esters containing one or two five-member {{chem2|\sB\sO\sC\sC\sO\s}} rings. For example, the reaction with mannitol {{chem2|H(HCOH)6H}}, whose two middle hydroxyls are in ''cis'' orientation, can be written as:

: {{chem2|B(OH)3}} + {{chem2|H2O}} {{Eqm}} {{chem2|[B(OH)4]-}} + {{chem2|H+}}

: {{chem2|[B(OH)4]-}} + {{chem2|H(HCOH)6H}} {{Eqm}} {{chem2|[B(OH)2(H(HCOH)2(HCO\s)2(HCOH)2H)]-}} + 2 {{chem2|H2O}}

: {{chem2|[B(OH)2(H(HCOH)2(HCO\s)2(HCOH)2H)]-}} + {{chem2|H(HCOH)6H}} {{Eqm}} {{chem2|[B(H(HCOH)2(HCO\s)2(HCOH)2H)2]-}} + 2 {{chem2|H2O}}

Giving the overall reaction:

: {{chem2|B(OH)3}} + 2 {{chem2|H(HCOH)6H}} {{Eqm}} {{chem2|[B(H(HCOH)2(HCO\s)2(HCOH)2H)2]-}} + 3 {{chem2|H2O}} + {{chem2|H+}}

The stability of these mannitoborate ester anions shifts the equilibrium to the right, thereby increasing the solution's acidity by five orders of magnitude compared to that of pure boric oxide. This lowers the p''K''<sub>a</sub> from 9 to below 4 for a sufficient concentration of mannitol.<ref name=vogelP357/><ref name=GandE/><ref name=NIST1969/><ref name="Belcher1960"/> The resulting solution is referred to as mannitoboric acid.

The addition of mannitol to an initially neutral solution containing boric acid or simple borates lowers its [[pH]] enough for it to be titrated by a strong base such as NaOH, including with an automated potentiometric [[titrator]]. This property is used in analytical chemistry to determine the borate content of aqueous solutions, for example to monitor the depletion of boric acid by [[neutron]]s in the water of the primary circuit of [[light-water reactor]] when the compound is added as a [[neutron poison]] during refueling operations.<ref name=GandE/>