Calcium chloride

Author: Amelie Stahlbuhk
back to Chloride

Abstract

The different hydrate stages 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

Solubility

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.

Hygroscopicity

Figure 2: Deliquescence behaviour of calcium chloride in the temperature range from -45 to 80 °C. The water activity a_w 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. At room temperature, Antarcticite is the phase present, having a deliquescence humidity of about 30% RH. The deliquescence behavior depends on the temperature as the deliquescence humidity decreases with increasing temperature. Changing the relative humidity at 20 °C, calcium chloride tetrahydrate forms at values below 18% RH. Dehydration to the dihydrate (sinjarite) and to the monohydrate occurs at 9% RH and 6% RH, respectively.

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 %

concentration given as amount of substance (mol) of the dissolved substance per kilogram solvent; unit: mol•kg-1.

the relative humidity, above which a material, e.g. a salt, absorbes humidity from the air and dissolves.

relative humidity

A phase is a physically distinctive form of matter and is characterized by having relatively uniform chemical and physical properties.