Trisodium pentaborate is a salt that can form as a monohydrate with formula Na3B5O8(OH)2·H2O, which contains the anion dihydroxydopentaborate [B5O8(OH)2]3−. Its formula may be incorrectly given as Na3B5O8·2H2O, or as 3Na2O5B2O3·4H2O. It may also be called simply sodium pentaborate, but this name is more properly reserved for the compound with elemental formula Na2O·5B2O3·nH2O.
Structure Trisodium pentaborate monohydrate crystallizes as thin plates in the orthorhombic crystal system, with symmetry group Pbca and cell parameters a = 880.4 pm, b = 1837.1 pm, c = 1092.4 pm, formulas per cell Z = 8.
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