Oxides And Oxy-acids Of Chlorine : Chlorates And Perchlorates, Manufacture

Chlorates are manufactured either by the action of excess of chlorine on concentrated solutions of alkalies, or by the electrolysis of chlorides. Calcium chlorate is produced bypassing chlorine into hot milk of lime contained in cast-iron vats with agitating paddles

Fig: Manufacture of calcium chlorate

until the reaction is complete:

6Ca(OH)₂ + 6Cl₂ = 5CaCl₂ + Ca(ClO₃)₂ + 6H₂O.

The reaction appears to take place with the intermediate formation of free hypochlorous acid, which acts as a carrier of oxygen. The action of heat alone on calcium hypochlorite, in the absence of excess of chlorine, is mainly according to the equation:

Ca(OCl)₂ = CaCl₂ + O₂.

Alkaline hypochlorite solutions may be boiled without much decomposition, but oxygen is slowly evolved, traces of chlorites being also formed:

2KOCl = KCl + KClO₂.

The solution of calcium chlorate may be treated with potassium chloride, when the sparingly soluble potassium chlorate crystallises out, and is recrystallised. It is now usual to produce the very soluble sodium chlorate, NaClO₃. The solution of calcium salts is concentrated, cooled, and filtered from the crystals of hydrated calcium chloride which separate. Excess of sodium sulphate is then added, when all the calcium is precipitated as sulphate. On evaporation of the filtered solution, sodium chloride separates: this is removed, and on cooling sodium chlorate crystallises out.

Chlorates and perchlorates are also produced by the electrolysis of saturated sodium chloride solution at 80°, between platinum electrodes placed close together. A little potassium chromate is added. The chloride is first completely converted into chlorate; on prolonged electrolysis, this passes into perchlorate. Chlorates are used as oxidising agents (e.g., in the oxidation of aniline to aniline black), as weed killers, and in making fireworks. Perchlorates are employed in the manufacture of detonators and explosives.

The action of the free hypochlorous acid as a carrier of oxygen to the hypochlorite in the formation of chlorate may, according to Foerster (1899), be represented as follows. As long as the liquid remains alkaline, chloride and hypochlorite are produced:

1. Cl₂ + 3OH' = Cl' + OCl' + H₂O.

When all the alkali is removed, the hypochlorite ion reacts with the free hypochlorous acid, producing chlorate and chloride ions:

2. OCl' + 2HOCl = ClO₃' + 2Cl' + 2H^۰.

The hypochlorite and hydrogen ions then form hypochlorous acid, and the latter reacts again according to (2):

3. H^۰ + OCl' = HOCl.

If we multiply equation (1) by 3, (3) by 2, and add, we obtain the usual equation for the total reaction:

3Cl₂ + 6OH' = ClO₃' + 5Cl' + 3H₂O.

Protein crystallography
Chemistry guide
Oxides And Oxy-acids Of Chlorine
-C-
<<<	>>>