Praseodymium dioxide, PrO₂

Praseodymium dioxide, PrO₂, may be prepared by heating the nitrate to 440° or by fusing the nitrate with nitre at 400° until decomposition is complete. It is a black solid of density 5.978 at 20°. According to Brauner, the pure oxide is not of the hydrogen peroxide type, but contains quadrivalent praseodymium. It sets free chlorine from hydrochloric acid, iodine from hydriodic acid, and ozonised oxygen with oxyacids, it oxidises cerous salts from ceric, manganous salts to permanganic acid, and gives a violet coloration with a solution of strychnine in sulphuric acid. It is reduced by hydrogen peroxide in acid solution. It oxidises ferrous and stannous salts, but a part of the available oxygen is always lost (von Scheele).

When praseodymium hydroxide, carbonate, nitrate, oxalate, etc., are heated, dark brown oxide residues are obtained which are intermediate in composition between Pr₂O₃ and PrO₂, and which contain less oxygen in proportion as the temperature at which they are produced increases. Intermediate oxides of the formulae Pr₄O₇, Pr₅O₉, and Pr₆O₁₁ have been described, but their individuality is doubtful. Brauner considers the oxide Pr₅O₉ to be a complex oxide formed by the combination of Pr₉O₃ with PrO₂ (Pr₅O₉ = Pr₂O₃.3PrO₂).

The formation of praseodymium dioxide in a mixture of cerium earths is favourably influenced by the presence of ceria, which probably acts as an oxygen-carrier, but is adversely affected by the presence of lanthana or neodymia.