Search

Non-stoichiometric cerium oxides were prepared in NaOH-H 2 O mixtures of various compositions at room temperature from hydrated cerium(III) nitrate and cerium(IV) sulphate as precursors. The resulting products were characterized by their compositions (chemical, thermogravimetric and energy-dispersive spectrometry analysis) and by their morphology (X-ray diffraction, granulometry and electron microscopy). The compounds obtained by bubbling oxygen in cerium(III) nitrate solutions are amorphous. Their oxidation states are close to 3.8 and their average grain size is 7.6 μm. In contrast, those obtained from cerium(IV) sulphate solutions are better crystallized and have lower oxidation states. These compounds are formed of grains of about 9.2 μm.

Experimental study of the solubility of Mn(II) and Mn(III) was carried out by polarography in water+hydroxide mixtures at 25°C. This method also permitted determination of the standard potential of the Mn(II)/Mn(III) electrochemical system as a function of the composition of the medium. These measurements were supplemented by a voltammetric study carried out with a carbon-paste electrode. The results obtained, combined with published thermochemical data, permitted the construction of potential-composition diagrams (analogous to diagrams of Pourbaix for aqueous solutions), thus identifying the properties of manganese in water+sodium hydroxide and water+potassium hydroxide mixtures.