Sodium chlorate, NaClO₃

When chlorine is passed into a hot solution of sodium hydroxide, the hypochlorite primarily formed changes into a mixture of chlorate and chloride, both salts crystallizing out:

2NaOH+Cl₂=NaOCl+NaCl+H₂O;

3NaOCl =NaClO₃+2NaCl.

The chlorate is purified from the chloride by fractional crystallization. In Muspratt's method magnesium oxide suspended in water is substituted for sodium hydroxide, the solution being concentrated after treatment with chlorine, and sodium carbonate added to precipitate the magnesium for further use. The sodium chlorate crystallizes from the mother-liquor. The salt is also formed from potassium chlorate by double decomposition with substances such as sodium hydrogen tartrate and sodium silicofluoride. as well as by the electrolytic decomposition of sodium-chloride solution under certain conditions.

Sodium chlorate is a colourless, crystalline substance, and exhibits trimorphism, forming crystals belonging to the cubic, hexagonal, and rhombic systems. For its melting-point Retgers gives 248° C., Foote and Levy 255° C., Smits 261° C., and Carnelley 302° C. The density at 15° C. is given by Bodeker as 2.289, by Retgers as 2.490, and by Le Blanc and Rohland as 2.996. The specific heat of the solid is given by Foote and Levy as 0.281, and of the fused salt as 0.581, the latent heat of fusion being 48.4 cal.

At 15° C., 100 grams of water dissolve 91 grams. Kremers gives the solubility-table:

Solubility of Sodium Chlorate, NaClO₃.

Temperature, °C.	0	12	20	40	60	80	100
Grams NaClO3 in 100 g. water	82	98.5	99	123.5	147	175	204

The saturated solution exhibits the remarkable phenomenon of having two boiling-points, 126° C. and 255° C., a property characteristic of some other readily soluble salts, exemplified by the nitrates of sodium, potassium, silver, and thallium. It is due to an increase with rise of temperature in the vapour-pressure of the solution up to a maximum greater than the atmospheric pressure, further rise of temperature being accompanied by a diminution in the vapour-pressure of the solution as the composition of the system tends to approximate to that of the solid phase.

For the heat of formation from the elements, Thomsen gives 86.7 Cal., and Berthelot 85.4 Cal. For the heat of solution at 10° C., Berthelot gives -5.6 Cal.

References are appended to investigations of the optical properties of the crystalline forms; to such properties of the aqueous solution as vapour-pressure, density molecular depression of the freezing-point, electric conductivity, and index of refraction; and to the solubility in ethyl alcohol.

Sodium perchlorate, NaClO₄. - On heating sodium chlorate, besides the decomposition with evolution of oxygen, there is a partial conversion of the salt into perchlorate and chloride -

4NaClO₃ = 3NaClO₄+NaCl.

The salt can also be prepared by neutralizing perchloric acid with sodium hydroxide, but it is manufactured by the electrolytic oxidation of a 25 per cent, sodium-chlorate solution at 10° C., platinum electrodes and a high anode-potential being employed. This process finds application in the manufacture of potassium perchlorate, this salt being obtained from the sodium compound by the action of potassium chloride.

At ordinary temperature, sodium perchlorate crystallizes as the very deliquescent monohydrate, but above 50° C. as the anhydrous salt. Its deliquescent character hinders its technical application. The melting- point of the anhydrous salt is 482° C. On heating, it is decomposed into chloride and oxygen, a proportion of chlorate being simultaneously formed. From cryoscopic experiments with sodium sulphate as solvent, Lowenherz inferred the molecular formula to be NaClO₄.

For the heat of formation from its elements, Berthelot gives 100.3 Cal., and for the heat of solution at 10° C., 3.5 Cal. The electric conductivity has been investigated by Ostwald and by Walden.