Heats of Solution, Transition, and Formation of Three Crystalline Forms of Metaboric Acid

The three crystalline forms of metaboric acid HBO₂ were prepared, purified, and analyzed. Heats of solution in water or of reaction with sodium hydroxide solution were compared with those of orthoboric acid H₃BO₃(c). The best values for the heats of transition at 25 °C are: (c,I) to (c,II), 2.33±0.23 kcal/mole; (c,II) to (c,III), 1.30±0.05 kcal/mole; (c,I) to (c,III), 3.63±0.24 kcal/mole. The following heats of formation at 25 °C were derived: -192.77 ± 0.35 kcal/mole for the cubic HBO₂(c,I), -190.43 ±0.34 kcal/mole for the monoclinic HBO₂ (c,II), and -189.13 ± 0.34 kcal/mole for the orthorhombic HBO₂(c,III).