Sodium periodates

Several periodates of sodium have been described, derived respectively from the anhydrous acid, HIO₄, the mono- hydrate, H₃IO₅, and the dihydrate, H₅IO₆. The normal salt, NaIO₄, has been prepared in the anhydrous form, and also as dihydrate, NaIO₄,2H₂O, and trihydrate, NaIO₄,3H₂O. The anhydrous salt forms quadratic crystals of density 3.865 at 16° C. They are isomorphous with the corresponding salts of ammonium, potassium, and rubidium. The density of the trihydrate at 18° C. is 3.219.

Disodium periodate, Na₂H₃IO₆, can be prepared by the action of chlorine on a mixture of sodium iodate and hydroxide, or by the oxidation of iodine with sodium peroxide. It is soluble with difficulty in both cold and hot water. The table gives determinations of solubility made by Rosenheim and Loewenthal -

Solubility of Sodium Periodate

Temperature, °C	0	25	40	100
Grams of Na2H3IO6 in 100 g. of solution	0.104	0.157	0.187	0.434

A cold, aqueous solution of sodium hydroxide converts this salt into the periodate of the formula Na₃H₂IO₆. These two substances are the only periodates of sodium known to exist in aqueous solution. The optical properties and electric conductivity in aqueous solution of the disodium salt have been investigated.

Other periodates have been described by Walden and by Muller.