Calcium chloride

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Author: Amelie Stahlbuhk
back to Chloride


The different hydrates of calcium chloride are presented, as well as their behavior regarding solubility and hygroscopicity.

Hydrate stages

Calcium chloride monohydrate: CaCl2•H2O
Sinjarite: CaCl2•2H2O
Calcium chloride tetrahydrate: CaCl2•4H2O
Antarcticite: CaCl2•6H2O


Figure 1: Solubility of calcium chloride in water. The molality m [n(CaCl2•xH2O)•kg(H2O)-1] is plotted against the temperature.

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.


Figure 2: Deliquescence behaviour of calcium chloride in the temperature range from -45 to 80 °C. The water activity aw is plotted against the temperature.

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.

Table 1: Deliquescence and equilibrium humidities at 20 °C.
Phase transition Deliquescence or equilibrium humidity at 20°C
Antarcticite-solution 33.3 %
Antarcticite-Calcium chloride tetrahydrate 18.5 %
Calcium chloride tetrahydrate-Sinjarite 9 %
Sinjarite-Calcium chloride monohydrate 6 %