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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.

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