The different hydrates of calcium chloride are presented, as well as their behavior regarding solubility and hygroscopicity.
Under standard conditions the hexahydrate of calcium chloride Antarcticite is the stable form. The salt has got a high solubility in water which increases with increasing temperatures. The dehydration steps to the calcium chloride tetrahydrate and to sinjarite take place at temperatures of 30 °C and 45 °C, respectively.
The hydration/dehydration and crystallization/deliquescence processes in the CaCl2-H2O system can occur by either changing relative humidity or temperature. The deliquescence relative humidity decreases with increasing temperature. At room temperature (20ºC), the hexahydrate [Antarcticite]] is the stable phase, having a deliquescence humidity of about 30% RH. Lowering the relative humidity at the same temperature, calcium chloride tetrahydrate forms at values below 18% RH, while dehydration to the dihydrate (sinjarite) occurs at 9% RH, while the latter changes to the monohydrate at 6% RH.
|Phase transition||Deliquescence or equilibrium humidity at 20°C|
|Antarcticite-Calcium chloride tetrahydrate||18.5 %|
|Calcium chloride tetrahydrate-Sinjarite||9 %|
|Sinjarite-Calcium chloride monohydrate||6 %|