Calcium Chlorate, Ca(ClO₃)₂.2H₂O

To prepare the pure Calcium Chlorate, Ca(ClO₃)₂.2H₂O, pure chloric acid should be neutralised with calcium carbonate or hydroxide, and the solution evaporated in the cold over sulphuric acid. The dihydrate forms rhombic crystals which are very deliquescent and melt when heated quickly to a temperature of over 100° C. On fusion, the anhydrous salt gives up oxygen with a trace of chlorine, leaving a mixture of oxide and chloride. It is easily soluble in water and alcohol.

A certain amount of chlorate is formed by heating milk of lime saturated with chlorine, the quantity increasing with temperature. The production of chlorate by this method was begun on the commercial scale at St. Helens in 1847. Milk of lime of specific gravity 1.08 is agitated in cylindrical cast-iron vats, 10 ft. in diameter and ft. high, whilst chlorine is passed through. The temperature is kept at 60°-70° C. Several vats may be placed in series, and the process run on the counter-current principle. The end-point is reached when addition of a dilute acid causes no evolution of chlorine. The chlorate solution is used chiefly for the production of sodium and potassium chlorates.

Calcium chlorate may also be prepared by the electrolytic oxidation of the chloride. According to Tucker and Moody, the best output is obtained by the use of a 20 per cent, solution at 80° C. and an anode density of 8 amp. per sq. dcm. The platinum electrodes should be in a horizontal position and 10 cm. apart. Addition of sodium dichromate or hydroxide increases the efficiency considerably. For example, 10 per cent, of sodium hydroxide raises the efficiency from 67.4 per cent, to 83 per cent.