Sodium pentaborate

Not to be confused with Trisodium 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.

Disodium decaborate

Names
Other names Sodium pentaborate
Identifiers
CAS Number	12007-92-0 N trihydrate: 12631-71-9
3D model (JSmol)	Interactive image trihydrate: Interactive image
ChemSpider	52085559 N
ECHA InfoCard	100.031.371
EC Number	234-522-7
PubChem CID	131634859
UNII	XQG9Q75WDN N
CompTox Dashboard (EPA)	DTXSID50894254
InChI InChI=1S/B5O8.Na/c6-1-8-3-11-4-9-2(7)10-5(12-3)13-4;/q-1;+1 Key: OLBJLOUJSZVXOT-UHFFFAOYSA-N Key: HLIXBZYPVKDOBP-UHFFFAOYSA-N trihydrate: InChI=1S/B5O8.Na.3H2O/c6-1-8-3-11-4-9-2(7)10-5(12-3)13-4;;;;/h;;3*1H2/q-1;+1;;;
SMILES B(=O)OB1OB2OB(OB(O2)O1)[O-].[Na+] trihydrate: O.O.O.[Na+].[O-]B1OB2OB(OB=O)OB(O1)O2
Properties
Chemical formula	Na+[B5O6(OH)4]− or Na+[B5O7(OH)2]−·H2O
Molar mass	241.06 g·mol−1
Appearance	Colorless crystalline solid
Solubility in water	Soluble
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

The compound is often encountered or traded as hydrates NaB₅O₈·nH₂O, Na₂B₁₀O₁₆·2nH₂O, or Na₂O·5B₂O₃·2nH₂O for n = 2,^[1] 4,^[2][3] 5,^[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^[5] and Aquabor.^[3] It has also been tested as an additive to improve plasma electrolytic oxidation of magnesium alloys.^[4] It is also used in nuclear reactors as a neutron absorber/poison.^[6]

The name "sodium pentaborate" has also been used for a distinct compound with formula Na₃B₅O₈·nH₂O, better called trisodium pentaborate.^[7]

Structure and preparation

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. ^[1]

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 (C_2h⁵) 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.^[1]