Thermochemistry of two lead borates; Pb(BO2)2·H2O and PbB4O7·4H2O

Two pure hydrated lead borates, Pb(BO2)2·H2O and PbB4O7·4H2O, have been characterized by XRD, FT-IR, DTA-TG techniques and chemical analysis. The molar enthalpies of solution of Pb(BO2)2·H2O and PbB4O7·4H2O in 1 mol dm−3 HNO3(aq) were measured to be (−35.00 ± 0.18) kJ mol−1 and (35.37 ± 0.14) kJ mol−1, respectively. The molar enthalpy of solution of H3BO3(s) in 1 mol dm−3 HNO3(aq) was measured to be (21.19 ± 0.18) kJ mol−1. The molar enthalpy of solution of PbO(s) in (HNO3 + H3BO3)(aq) was measured to be −(61.84 ± 0.10) kJ mol−1. From these data and with incorporation of the enthalpies of formation of PbO(s), H3BO3(s) and H2O(l), the standard molar enthalpies of formation of −(1820.5 ± 1.8) kJ mol−1 for Pb(BO2)2·H2O and −(4038.1 ± 3.4) kJ mol−1 for PbB4O7·4H2O were obtained on the basis of the appropriate thermochemical cycles.