Boron in Seawater
In natural seawater, boron is present at about 0.41 mM (4.4 ppm total boron) and takes two different chemical forms. The predominate form is boric acid, comprising about 70% of the total boron present. Boric acid, B(OH)3, consists of a central boron atom and three hydroxyl groups arranged in a triangle (Figure 1). The second form is borate, B(OH)4-. It consists of a central boron atom and four hydroxyl groups arranged in a tetrahedron (Figure 1). It carries a net negative charge, while boric acid is neutral. These two forms can interconvert in less than a second, so the two forms are in chemical equilibrium with each other.
Figure 1. Space-filling molecular models of borate (left) and boric acid (right). The boron is yellow, oxygen is red, and hydrogen is blue.