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OXIDES OF THE ALKALI AND ALKALINE EARTH METALS WILLIAM H. SCHECHTER and JACOB KLEINBERG University of Kansas, Lawrence, Kansas
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INTRODUCTION Of the higher oxides of the alkali and alkaline earth metals, only the peroxides of sodium and barium had any extensive use before World War 11. Since the beginning of the war, however, potassium superoxide, KO2, has been prepared for the Navy in large quantities and finds use as a source of oxygen in breathing apparatus for fire-fighting and rescue work aboard ship. The oxide is used in canistersin a closed-system apparatus where its reaction with carbon dioxide and moisture from the breath releases sufficient oxygen to supply the wearer's needs. Since oxides higher than monoxides and peroxides are no longer laboratory curiosities and since there are interesting relationships among the three types of oxides known, this report is concerned with some of the less familiar aspects of the methods of preparation, chemical properties, and structures of the oxides of the alkali and alkaline earth metals. he divided into three classes~h~
higher than peroxide. Then the remaining solution is catalytically decomposed and the amount of oxygen liberated gives a measure of the peroxide oxygen in solution. These two measurements provide sufficient data for a quantitative calculation of the composition of the original substance. THE MONOXIDES
The chemistry of,the alkaline earth monoxides is too well known to warrant ,extended discussion. It will suffice to say that they may readily be prepared by the thermal decomposition of their carbonates. Unlike the alkaline earth monoxides, the corresponding alkali metal compounds are not easily obtained. It appears, moreover, that with increasing size of the alkali metal atom the preparation of the monoxide hecomes increasingly difficult. The combustion of the metal in excess oxygen gives the monoxide only in the case of lithium, and even in this reaction a trace of the peroxide is formed. Sodium monoxide may he prepared
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. ..... -... -..---. - -. ,- - .~- .-* > --~" be considered to he salts of the weak acid water. These the group mhy be made by direct oxidation in the presoxides react with water to &re, in most cases, highly ence of excess metal (6) or by the action of the,metal the corresponding nitrate ionized hydroxides. The members of the second type of oxides, the peroxides, are derivatives of the weak 5K KNOp 3Kn0 '/SN* and contain the OZ3, of the acid hydrogen peroxide .. By definition, peroxides are corn- THE PEROXIDES structure -:&o:-. When a concentrated aqueous solution of lithium pounds whiLh &e a solution of f;ydrogen peroxide when treated with a strong acid. Since they are salts of hydroxide is treated with 30 per cent hydrogen peroxide the weak acid hydrogen peroxide, the peroxides are ex- and then with alcohol, a compound of the formula is precipitated. The drying of this tensively hydrolyzed; their strongly alkaline solutions Liz02.HzO2.3H20 are active oxidizing agents. The least familiar class is substance in uacuo yields pure lithium peroxide (7). that group of oxides referred to in the recent literature Similarly, the peroxides of calcium, strontium, and haras superoxides, a name suggested by Bray and East- ium may be precipitated as octahydrates, MOr8H20, man (1). The superoxides are salts of the unstable from which stable peroxides are obtained by dehydraacid Hog, which decomposes in the following manner tion (8). I t has been impossible, however, to obtain pure magnesium peroxide, and beryllium shows a ten(2). dency to form the monoxide only. 2HO2 HOl 0% Potassium peroxide is formed by the action of the theoretical quantity of nitrous oxide on the metal (4). The superoxide salts react similarly in water. Upon careful addition of peroxides to ice water, no This substance is unstable toward oxygen, being oxygen is liberated until the solution is decomposed rapidly converted to the superoxide. The reaction of dry air with sodium a t approximately catalytically. Use is made of this fact in the two-step analysis for superoxides devised by Kraus and Par- 300°C. is used as the commercial method for the promenter (3). In this method the unknown material is duction of sodium peroxide; the product obtained conadded to ice water and oxygen is evolved from the oxide tains about 95 per cent sodium peroxide (0). Oxidation ~
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JUNE, 1947 of calcium monoxide with oxygen a t high pressure and temperature produces no peroxide, and only a 15 per cent yield of strontium peroxide is obtained by the oxidation of the monoxide a t 400°C. and 98 atmos pheres pressure (10). On the other hand, barium monoxide is readily converted to the peroxide by heating in air to 400°C. This peroxide is readily decomposed at low pressures or a t temperatures above 400°, and before the advent of liquid oxygen, this reaction was the basis of Brin's process for the manufacture of oxygen. Several of the alkali metal peroxides have been made by treatment of the metal with oxygen in liquid ammonia. Kraus and Whyte (11) state that the highest oxidation product of sodium in liquid ammonia a t -33°C. is the peroxide. The first step in the rapid oxidation of potassium, rubidium, and cesium in liquid ammonia is the formation of the peroxide (6). Attempts to obtain good yields of calcium peroxide in this manner have been unsuccessful, although the preparation of the strontium compound has been reported (18). From this discussion it is readily apparent that there is a'direct relationship between the size of the atom of an alkaline earth metal and its ability to form a peroxide. The peroxide of the comparatively large barium atom is readily obtained, while those of the smaller strontium and calcium atoms are less readily formed. Magnesium peroxide, thus far, has been obtainable only in the form of the impure hydrate, whereas it has proved impossible to make the peroxide of beryllium, the member of the group whose atom is smallest. THE SUPEROXIDES Attempts to prepare the superoxides of lithium and sodium have so far proved fruitless. The highest oxide of potassium, KO2, may be formed by the combustion of the metal in an excess of oxygen (4), and also by the rapid oxidation of the metal in liquid ammonia (5, 11, 13); the former method serves as the commercial source of the superoxide. Rubidium ahd cesium behave c similarly to potassium. The superoxides of the alkali metals are fairly stable, having melting points in the neighborhood of 400°C. The potassium compound loses oxygen reversibly to give asubstance, the analysis of which corresponds to the formulaKzOa(11). The literature on the alkaline earth metal superoxides is meager. Traube and Schulze (14) reported the preparation of CaOl (8.7 per cent in CaOz) and BaOl (about 8 per cent) by the action of 30 per cent hydrogen peroxide on the corresponding hydroxides. Presumably, the low yields are due to decomposition of some of the superoxide formed by water from the hydrogen peroxide solution. The superoxides of the other members of the group have not been prepared. Until recently, the alkali metal superoxides have been considered to be tetroxides of the formula MpOl.
The belief prevailed that these substinces were analogous to the tetrasulfides and contained t h e o h (tetroxide) anion. However, the discovery of the three-electron bond (16) led to the idea that the alkali metal oxides might contain the 0 2 - (superoxide) ion with the structure :0'-0:; in which both a single (two-eleetron) t bond and a three-electron bond between the two oxygen atoms are involved. The results of magnetic susceptibilitf measurements on potassium superoxide (16) lend credence to the suggestion just given, since they show a for the superoxide ion corresponding t o one unnaired electron. whereas the tetroxide ion would be diamagnetic. ~uhhermore,X-ray studies on KOz (17) show a crystal structure consisting of potassium and superoxide ions arranged in a simple cubic lattice; the interatomic distance in the superoxide ion is in good agreement with that expected for a single bond plus s three-electron bond (I). Several investigators (3, 4, 5, 11) have reported alkali metal oxides of the empirical formula M ~ O S Magnetic and X-ray data on tKe rubidium and cesium compounds prove that these contain both the peroxide and the superoxide ion (17) and that their structure may therefore be represented by the formula MzO2.2MOZ. Magnetic studies on impure CaOl (18) demonstrate the existence of the Oz- ion in this compound, thus proving the similarity of the highest oxides of the alkali and alkaline earth metals.
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LITERATURE CITED PAULING, L., ('The Nature of the Chemical Bond," 2nd Ed., Cornell University Press, Ithaoa, 1940, p. 272. LATIMER, W. M.,'ANDJ. H. HILDEBRAND, "Reference Book of Inorganic Chemistry," The Maamillan Company, New York, 1940, p. 34. KRAUS,C. A., AND E. F. P-ENTER, J. Am. Chem. Soe., 56, 2384 (1934).
HOLT,W., AND W. E. SIMS,J. Chm.Spc., 65,432 (1894). RENGADE, E., Ann. china. phys., (8) 11, 348 (1907). Ref. (2), p. 44. DEFORCRAND, R., Compt. rend., 130, 1465 (1900). RIESENFELD, E. H., AND W. NOTTEBOHM, Z. anorg. Chm., 89,405 (1914).
MELWR,J. W., AND G. D. PARKES,"Mellor's Modern Inorganic Chemistry," Longmans, Green and Company, New York, 1939, p. 553. FISCHER, 'F., AND H. PLOETZE, 2. anorg. Chem., 75, 10 (1912); 75, 30 (1912).
KRAUS,C. A,, AND E. F. WHYTE, J. Am. Chem. Sac., 48,1781 f192fi). (12) E M ~ L E H. U ~J., , AND J. S. ANDERSON, "Modern Aspects of \----,-
Inorganic Chemistry," D. Van Nostrand Company. Ino., 1938, p. 341. JOANNIS, A,, Compt. rend., 116,1370 (1893). TRAUBE, W., AND W. SCWLZE,Be?., 54,1626 (1921). PAULING, L., J. Am. C h m . Soc., 53,3225 (1931). NEUMIW.E. W.. J. Chem. Phus.. " , 2.31 ,~ (1934). , ~ ~ . HE&S, A.,AND W. KLEMM, Z. anwg. Chem., 241,97 (1939); 242, 201 (1939).
EARLICE, P., Z. a w g . C h m . , 252, 370 (1944).
