Calcium Bicarbonate

As has already been seen from McCoy and Smith's determinations of the solubility of calcium carbonate, calcium hydrogen carbonate as a solid phase only becomes stable under a pressure of about 15 atmospheres. It is evident, therefore, that under ordinary conditions it must be a very unstable compound.

By precipitating calcium chloride with potassium, or, preferably, ammonium bicarbonate at 0° C., filtering, washing with ice-cold water saturated with carbon dioxide, and then with ice-cold alcohol and ether, and drying at 0° C., a compound is formed of composition corresponding to the formula CaCO₃.1.75H₂CO₃. The fine white powder first obtained rapidly becomes pasty and evolves a considerable quantity of gas if the temperature is allowed to rise.

If a solution of calcium bicarbonate which is just neutral to phenolphthalein is analysed, its composition corresponds to the formula CaCO₃.1.8H₂CO₃, but this is not necessarily any indication of the composition of the bicarbonate.

It is the presence of the acid carbonate of calcium which is the main cause of the temporary hardness of water.