Phase transitions of calcium gluceptate

Calcium gluceptate (calcium α-glucoheptonate, CaC₁₄H₂₆O₁₆) is commercially available as a crystalline hydrate containing 3.5 molecules of water per atom of calcium. A crystalline anhydrate was prepared by dehydration. The transition between the hydrate and the anhydrous phases is reversible, and both phases become X-ray amorphous on grinding. The crystalline phases were characterized by X-ray diffraction, and the crystalline and amorphous forms were subjected to thermal analyses (thermogravimetric analysis, differential scanning calorimetry and hot stage microscopy). Marked differences exist between the apparent water solubilities of these forms at room temperature, and at concentrations greater than the equilibrium solubility of the crystalline hydrate, solutions prepared using the crystalline anhydrate, the X-ray amorphous hydrate and the X-ray amorphous anhydrate are supersaturated, and the stable hydrate crystallizes from solution. A relative humidity (RH)-composition phase diagram shows that the crystalline hydrate and the crystalline anhydrate appear to coexist between 0 and 66% RH (25 °C). Various hypotheses for this anomalous behavior were tested. The adsorption of a small amount of water by the anhydrate seems to inhibit further water IJPtake and prevents the phase transition to the hydrate.