Sodium pentaborate

Sodium pentaborate, more properly disodium decaborate, is a chemical compound of sodium, boron, and oxygen; a salt with elemental formula NaB5O8, Na2B10O16, or Na2O*5B2O3. It is a colorless crystalline solid, soluble in water.

The compound is often encountered or traded as hydrates NaB5O8*nH2O, Na2B10O16*2nH2O, or Na2O*5B2O3*2nH2O for n = 2, 4, 5,  or other values. This formula is often misleading as some of the water molecules are actually hydroxyl groups covalently attached to boron atoms.

The compound is used in agriculture as a boron supplement in fertilizer with various trade names such as Solubor and Aquabor. It has also been tested as an additive to improve plasma electrolytic oxidation of magnesium alloys.

The name "sodium pentaborate" has also been used for a distinct compound with formula Na3B5O8*nH2O, better called trisodium pentaborate.

Dihydrate
Sodium pentaborate "dihydrate" has the elemental formula NaH4B5O10, which can be parsed as NaB5O8*2H2O or Na2O*5B3O3*4H2O, however the correct formula seems to be either Na+[B5O6(OH)4]- or Na+[B5O7(OH)2]-*H2O. The latter seems more likely, since under thermogravimetric analysis the material starts to decompose at about 130 °C with partial loss of water.

It can be prepared by reacting water solutions of sodium carbonate Na2CO3 and boric acid B(OH)3 in mole ratio 1:10 and evaporating the resulting solution at 40 °C. It belongs to the monoclinic crystal system with symmetry group P21/c (C2h5) and parameters a = 1110.3 pm, b = 1643.7 pm, c = 1356.4 pm, α = 89.960°, β = 112.850° and γ = 89.9°, formulas per cell Z = 4.