Thermochemistry of hydrated ammonium borates

The enthalpies of solution of (NH4)2B4O7·4H2O (cr) and NH4B5O8·4H2O (cr) in approximately 1 mol dm−3 aqueous hydrochloric acid solution, as well as the enthalpy of solution of NH4Cl (cr) in aqueous (hydrochloric acid + boric acid) have been measured by using a microcalorimeter. From these results and other auxiliary data, the standard molar enthalpies of formation ΔfHm0 of (NH4)2B4O7·4H2O (cr) and NH4B5O8·4H2O (cr) have been derived to be (−4313.74 ± 3.26) and (−4703.56 ± 4.03) kJ mol−1 at 298.15 K, respectively. Some other thermodynamic properties of the title borates, such as the standard molar entropies of formation and the standard molar entropies have also been calculated.