What is a Polyborate?

      Firstly, you need to understand what a borate is. A borate is a boron containing oxyanion which either takes the form of the trigonal BO3 or tetrahedral BO4 structure.4

bo3-and-bo4

 A: trigonal BO3. B: tetrahedral BO4.

      These two forms of Boron can be found naturally in borax, colemanite and boracite.5 In aqueous solution BO3 is found to be B(OH)3 as shown in equation 1. B(OH)3 acts as Lewis acid in solution accepting hydroxyl ions to form B(OH)4.

                                  B(OH)3 + 2H2O ⇌ B(OH)4 + H3O                                            (1)

      In concentrated solutions, B(OH)4 will react to form several different polyborate anions that exist in a temperature, pH and Boron concentration dependent equilibrium as shown in equations 2-4.67

2B(OH)3 + B(OH)4 ⇌ B3O3(OH)4+ 3H2O                                            (2)

4B(OH)3 + B(OH)4⇌ B5O6(OH) + 6H2O                                           (3)

2B(OH)3 + 2B(OH)4 ⇌ B4O5(OH)42- + 5H2O                                          (4)

      Structurally oxidoborates can be viewed as polymeric systems or discrete anions. Polyborates have many different structures depending on their formula. The three polyborates from equations 2-4 are shown below.

polyborates

      Polyborates are generally made from (BO)3 ring systems with larger polyborates being made from interconnecting (BO)3 rings. These can be prepared by the dehydration of ROB(OH)2 and B(OH)3 compounds, the RO / B ratio influences the polymerisation degree.6

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