Form II of Maleic Acid. It proved impossible to obtain polymorph II of maleic acid using either the method reported by Day et al.18 or recrystallization of a 2:1 stoichiometry using acetone as a solvent. Even during the measurement of the ternary phase diagram it was never observed. Ostwald
et al. recently demonstrated the crystallization of Form II from aqueous solutions at elevated pressure. It is evident that at the pressure above 0.5 GPa it is the stable phase but that it reverts to form I when the pressure is released