uranium-oxygen system are complex. The most important oxidation states of uranium are uranium(IV) and uranium(VI), and their two corresponding oxides are, respectively, uranium dioxide () and uranium trioxide (). Other uranium oxides such as uranium monoxide (UO), diuranium pentoxide (), and uranium peroxide () also exist. The most common forms of uranium oxide are triuranium octoxide () and . Both oxide forms are solids that have low solubility in water and are relatively stable over a wide range of environmental conditions. Triuranium octoxide is (depending on conditions) the most stable compound of uranium and is the form most commonly found in nature. Uranium dioxide is the form in which uranium is most commonly used as a nuclear reactor fuel. At ambient temperatures, will gradually convert to . Because of their stability, uranium oxides are generally considered the preferred chemical form for storage or disposal. Aqueous chemistry Salts of many oxidation states of uranium are water-soluble and may be studied in aqueous solutions. The most common ionic forms are (brown-red), (green), (unstable), and (yellow), for U(III), U(IV), U(V), and U(VI), respectively. A few solid and semi-metallic compounds such as UO and US exist for the formal oxidation state uranium(II), but no simple ions are known to exist in solution for that state. Ions of liberate hydrogen from water and are therefore considered to be highly unstable. The ion represents the uranium(VI) state and is known to form compounds such as uranyl carbonate, uranyl chloride and uranyl sulfate. also forms complexes with various organic chelating agents, the most commonly encountered of which is uranyl acetate. Unlike the uranyl salts of uranium and polyatomic ion uranium-oxide cationic forms, the uranates, salts containing a polyatomic uranium-oxide anion, are generally not water-soluble. Carbonates The interactions of carbonate anions with uranium(VI) cause the Pourbaix diagram to change greatly when the medium is changed from water to a carbonate containing solution. While the vast majority of carbonates are insoluble in water (students are often taught that all carbonates other than those of alkali metals are insoluble in water), uranium carbonates are often soluble in water. This is because a U(VI) cation is able to bind two terminal oxides and three or more carbonates to form anionic complexes. Effects of pH The uranium fraction diagrams in the presence of carbonate illustrate this further: when the pH of a uranium(VI) solution increases, the uranium is converted to a hydrated