Thermodynamic properties of microporous crystals for two hydrated aluminoborates, K2[Al(B5O10)] · 4H2O and (NH4)2 [Al(B5O10)] · 4H2O

Two pure hydrated aluminoborates with microporous structure, K 2 [Al(B 5 O 10 )]   ·   4H 2 O and (NH 4 ) 2 [Al(B 5 O 10 )]   ·   4H 2 O, have been synthesized and characterized by XRD, FT-IR, DTA-TG techniques and chemical analysis. The molar enthalpies of solution of K 2 [Al(B 5 O 10 )]   ·   4H 2 O (s) and (NH 4 ) 2 [Al(B 5 O 10 )]   ·   4H 2 O (s) in 1   mol   ·   dm −3 HCl (aq) were measured to be −(88.65   ±   0.25)   kJ   ·   mol −1 and −(93.37   ±   0.25)   kJ   ·   mol −1 , respectively. The molar enthalpies of solution of AlCl 3 ·6H 2 O (s) in (HCl   +   H 3 BO 3   +   KCl) (aq) and in (HCl   +   H 3 BO 3   +   NH 4 Cl) (aq) were 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 use of the standard molar enthalpies of formation for AlCl 3 ·6H 2 O (s), KCl (s)/NH 4 Cl (s), H 3 BO 3 (s), HCl (aq), and H 2 O (l), the standard molar enthalpy of formation of −(6029.8   ±   4.1)   kJ   ·   mol −1 for K 2 [Al(B 5 O 10 )]   ·   4H 2 O (s) and −(5775.0   ±   4.1)   kJ   ·   mol −1 for (NH 4 ) 2 [Al(B 5 O 10 )]   ·   4H 2 O (s) at T   =   298.15   K were obtained on the basis of the appropriate thermochemical cycles. Moreover, Δ f H m o ([Al(B 5 O 10 )] 2− ) of −4355.3   kJ   ·   mol −1 has also been estimated by a group contribution method, which can be used to predict the standard molar enthalpies of formation of other aluminoborates containing [Al(B 5 O 10 )] 2− anionic framework.