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Hydrated Palladium Dioxide, PdO2

Hydrated Palladium Dioxide, PdO2.xH2O, may be obtained, more or less contaminated with alkali, by decomposing a solution of potassium palladate with a slight excess of potassium hydroxide.

The brown precipitate is washed with hot water and dried over concentrated sulphuric acid. The product thus obtained has or;e or two molecules of combined water according to circumstances.

Mailfert prepared palladium dioxide free from alkali by the action of ozone in the cold on palladous chloride in dilute alkaline solution. The product, however, contained traces of basic salt. The reaction probably takes place according to the following equation:

Na2PdCl4 + 2NaOH + O3 + H2O = 4NaCl + O2 + PdO2.2H2O.

Leidie and Quennessen prepared the dioxide by treatment of palladium sponge with sodium peroxide, but unfortunately were unable to isolate it.

It is best prepared by electrolytic oxidation of palladous nitrate in acid solution. This is effected by cooling a concentrated solution to 8° C., and effecting electrolysis with a current density of 0.5 amperes/cm2. At first the sesquioxide, Pd2O3.xH2O, is formed, but by continued electrolysis the dioxide is ultimately obtained. This, curiously enough, is not a result of direct oxidation, but one of decomposition followed by oxidation, the sesquioxide splitting up into the dioxide and monoxide, the latter dissolving in the free acid and undergoing oxidation again.

Palladium dioxide obtained by the method of Berzelius is a bright brown precipitate which gradually becomes dark brown in the cold but black on boiling with water or treatment with dilute acids. The water content varies considerably, but when dried over sulphuric acid to the point at which it becomes powdery it retains two molecules of water. Prolonged drying may reduce this to one molecule, which, however, is held very tenaciously. Palladium dioxide is not a very stable substance. When freshly prepared it is decomposed by boiling with water, yielding the monoxide. It is reduced by hydrogen at the ordinary temperature, evolving so much heat in the process that the mass becomes incandescent.

Hydrogen peroxide reduces it both in acid and in alkaline solution. It oxidises organic substances when boiled with their solutions.

Palladium dioxide cannot exist in air at temperatures above 200° C. as it decomposes according to the equation:

PdO2 = PdO + O.

The reverse reaction takes place too slowly to be observed.

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