Thermochemical properties of three isotypic polyborate cluster compounds: Na5[MB24O34(OH)12]·nH2O (M = Cr3+, Al3+, Ga3+)

Three isotypic polyborate cluster compounds for Na 5 [AlB 24 O 34 (OH) 12 ]·5.5H 2 O, Na 5 [CrB 24 O 34 (OH) 12 ]·5H 2 O and Na 5 [GaB 24 O 34 (OH) 12 ]·3H 2 O have been synthesized and characterized by EDS, XRD, FT-IR, TG techniques and chemical analysis. The molar enthalpies of solution of Na 5 [AlB 24 O 34 (OH) 12 ]·5.5H 2 O (s) and Na 5 [CrB 24 O 34 (OH) 12 ]·5H 2 O (s) in 1   mol   ·   dm −3 HCl (aq) were measured to be (58.27   ±   0.15)   kJ   ·   mol −1 and (130.99   ±   0.33)   kJ   ·   mol −1 , respectively. The molar enthalpies of solution of AlCl 3 ·6H 2 O (s) and CrCl 3 ·6H 2 O (s) in (HCl   +   H 3 BO 3   +   NaCl) (aq) were also measured, respectively. With the incorporation of the previously determined enthalpy of solution of H 3 BO 3 (s) in 1   mol   ·   dm −3 HCl (aq), together with the standard molar enthalpies of formation for AlCl 3 ·6H 2 O (s)/CrCl 3 ·6H 2 O (s), NaCl(s), H 3 BO 3 (s), HCl (aq) and H 2 O (l), the standard molar enthalpies of formation of −(21316.0   ±   19.3)   kJ   ·   mol −1 for Na 5 [CrB 24 O 34 (OH) 12 ]·5H 2 O (s) and −(21669.7   ±   19.3)   kJ   ·   mol −1 for Na 5 [AlB 24 O 34 (OH) 12 ]·5.5H 2 O (s) at T   =   298.15   K were obtained on the basis of the appropriate thermochemical cycles. The molar enthalpy of formation of Na 5 [GaB 24 O 34 (OH) 12 )]·3H 2 O was estimated as −20662.9   kJ   ·   mol −1 by using a group contribution method on the basis of these measured results.