title: Sodium pentaborate
text: # Sodium pentaborate
  
  Sodium pentaborate, more properly disodium decaborate, is a chemical compound of sodium, boron, and oxygen; a salt with elemental formula $NaB_5O_8,$ $Na_2B_{10}O_{16},$ or $Na_2O·5B_2O_3.$ It is a colorless crystalline solid, soluble in water.
  
  The compound is often encountered or traded as hydrates $NaB_5O_8·nH_2O,$ $Na_2B_{10}O_{16}·2nH_2O,$ or $Na_2O·5B_2O_3·2nH_2O$ 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 $Na_3B_5O_8·nH_2O,$ better called trisodium pentaborate.
  
  ## Structure and preparation
  
  ### Dihydrate
  
  Sodium pentaborate "dihydrate" has the elemental formula $NaH_4B_5O_{10},$ which can be parsed as $NaB_5O_8·2H_2O$ or $Na_2O·5B_3O_3·4H_2O,$ however the correct formula seems to be either $Na^+[B_5O_6(OH)_4]^−$ or $Na^+[B_5O_7(OH)_2]^−·H_2O.$ 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 $Na_2CO_3$ 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 $P2_1/c$ $(C_{2h}^5)$ 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.
  
  