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A method is described for the measurement of the active oxygen present in a mixture of ignited rare. earth oxides. The procedure is superior to the classical Bunsen method in that no difficulty is encountered in the analysis of high ceria mixtures. Data are given for the analysis of pure cerium dioxide, of some cerium dioxidelanthanum sesquioxide mixtures, and of pure praseodymium oxide.Pi GESERAL, gravimetric methods of analysis are based upon I a separation followed by a final weighing of the desired constituent in a pure compound of known composition. However, in the analysis of rare earth materials, separation of the individual earths is impractical and one is faced with the necessity of weighing a mixture which may contain elements of the entire group. Certainly the oxalates of cerium, praseodymium, and terbium yield upon ignition in air oxides containing more oxygen than can be accounted for by sesquioxide formation ( I ) , and there is some indication of a higher oxide of neodymium (6). The situation is furt,lier complicated by the possible influence of certain other rare earths upon the higher oxides formed by the elements mentioned (2,S, 5 ) .The evaluat,ion of the true rare earth metal content of a mixture of such oxides would. be greatly simplified by a method n-hich permitted direct measurement of the so-called "active oxygen", the oxygen present in excess of that in the sesquioxide. Correction of the weight of the oxide mixture would then permit calculation of an accurate metal content.Such an analytical method would be useful also in obtaining data regarding the composition of rare earth oxides formed under various conditions, both in the pure state and in mixtures. Such studies have been made in this laboratory using the method described below and will form the subject. of a later paper.The method of Bunsen (4) has found wide application for the determination of higher-valent cations in compounds similar to the rare earth metal oxides. Hydrochloric acid is added to the compound under investigation and the chlorine liberated by the reaction is absorbed in a potassiumiodide solution. The liberated iodine is then titrated with thiosulfate solution in the usual manner.Such a procedure is not feasible, however, if an appreciable amount of cerium is present in the mixture under investigation. Cerium dioxide, once formed, is extremely inert to all but the strongest reducing agents and even extended contact with concent,rated hydrochloric acid results in very little attack. EXPERIMENTALSeveral modifications of the Bunsen method were attempted, using both arsenious oxide and sodium oxalate as reducing agents. The reaction rate between the oxides proved to be too slow to be useful, while the conditions necessary to complete the reaction with sodium oxalate were so vigorous that appreciable decomposition of the oxalate in the sulfuric acid medium occurred.A simple procedure using potassium iodide and a modified Bunsen apparatus was finally developed which gave satisfactory results.The apparatus consisted of a...

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