Uranium compounds

Template:Short description Template:Use dmy dates Uranium compounds are compounds formed by the element uranium (U). Although uranium is a radioactive actinide, its compounds are well studied due to its long half-life and its applications. It usually forms in the +4 and +6 oxidation states, although it can also form in other oxidation states.

Oxidation states and oxides

Oxides

Calcined uranium yellowcake, as produced in many large mills, contains a distribution of uranium oxidation species in various forms ranging from most oxidized to least oxidized. Particles with short residence times in a calciner will generally be less oxidized than those with long retention times or particles recovered in the stack scrubber. Uranium content is usually referenced to Template:Chem/link, which dates to the days of the Manhattan Project when Template:Chem/link was used as an analytical chemistry reporting standard.Template:Sfn

Phase relationships in the 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 (Template:Chem/link) and uranium trioxide (Template:Chem/link).Template:Sfn Other uranium oxides such as uranium monoxide (UO), diuranium pentoxide (Template:Chem/link), and uranium peroxide (Template:Chem/link) also exist.

The most common forms of uranium oxide are triuranium octoxide (Template:Chem/link) and Template:Chem/link.Template:Sfn 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.Template:Sfn At ambient temperatures, Template:Chem/link will gradually convert to Template:Chem/link. Because of their stability, uranium oxides are generally considered the preferred chemical form for storage or disposal.Template:Sfn

Aqueous chemistry

File:U Oxstufen.jpg

Uranium in its oxidation states III, IV, V, VI

Salts of many oxidation states of uranium are water-soluble and may be studied in aqueous solutions. The most common ionic forms are Template:Chem/link (brown-red), Template:Chem/link (green), Template:Chem/link (unstable), and Template:Chem/link (yellow), for U(III), U(IV), U(V), and U(VI), respectively.Template:Sfn 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 Template:Chem/link liberate hydrogen from water and are therefore considered to be highly unstable. The Template:Chem/link ion represents the uranium(VI) state and is known to form compounds such as uranyl carbonate, uranyl chloride and uranyl sulfate. Template:Chem/link also forms complexes with various organic chelating agents, the most commonly encountered of which is uranyl acetate.Template:Sfn

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.

Pourbaix diagrams

A graph of potential vs. pH showing stability regions of various uranium compounds	A graph of potential vs. pH showing stability regions of various uranium compounds
Uranium in a non-complexing aqueous medium (e.g. perchloric acid/sodium hydroxide).	Uranium in carbonate solution
A graph of potential vs. pH showing stability regions of various uranium compounds	A graph of potential vs. pH showing stability regions of various uranium compounds
Relative concentrations of the different chemical forms of uranium in a non-complexing aqueous medium (e.g. perchloric acid/sodium hydroxide).	Relative concentrations of the different chemical forms of uranium in an aqueous carbonate solution.

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 uranium oxide hydroxide and at high pHs it becomes an anionic hydroxide complex.

When carbonate is added, uranium is converted to a series of carbonate complexes if the pH is increased. One effect of these reactions is increased solubility of uranium in the pH range 6 to 8, a fact that has a direct bearing on the long term stability of spent uranium dioxide nuclear fuels.

Hydrides, carbides and nitrides

Uranium metal heated to reacts with hydrogen to form uranium hydride. Even higher temperatures will reversibly remove the hydrogen. This property makes uranium hydrides convenient starting materials to create reactive uranium powder along with various uranium carbide, nitride, and halide compounds.Template:Sfn Two crystal modifications of uranium hydride exist: an α form that is obtained at low temperatures and a β form that is created when the formation temperature is above 250 °C.Template:Sfn

Uranium carbides and uranium nitrides are both relatively inert semimetallic compounds that are minimally soluble in acids, react with water, and can ignite in air to form Template:Chem/link.Template:Sfn Carbides of uranium include uranium monocarbide (U C), uranium dicarbide (Template:Chem/link), and diuranium tricarbide (Template:Chem/link). Both UC and Template:Chem/link are formed by adding carbon to molten uranium or by exposing the metal to carbon monoxide at high temperatures. Stable below 1800 °C, Template:Chem/link is prepared by subjecting a heated mixture of UC and Template:Chem/link to mechanical stress.Template:Sfn Uranium nitrides obtained by direct exposure of the metal to nitrogen include uranium mononitride (UN), uranium dinitride (Template:Chem/link), and diuranium trinitride (Template:Chem/link).Template:Sfn

Halides

Snow-like substance in a sealed glass ampoule.

Uranium hexafluoride is the feedstock used to separate uranium-235 from natural uranium.

All uranium fluorides are created using uranium tetrafluoride (Template:Chem/link); Template:Chem/link itself is prepared by hydrofluorination of uranium dioxide.Template:Sfn Reduction of Template:Chem/link with hydrogen at 1000 °C produces uranium trifluoride (Template:Chem/link). Under the right conditions of temperature and pressure, the reaction of solid Template:Chem/link with gaseous uranium hexafluoride (Template:Chem/link) can form the intermediate fluorides of Template:Chem/link, Template:Chem/link, and Template:Chem/link.Template:Sfn

At room temperatures, Template:Chem/link has a high vapor pressure, making it useful in the gaseous diffusion process to separate the rare uranium-235 from the common uranium-238 isotope. This compound can be prepared from uranium dioxide and uranium hydride by the following process:Template:Sfn

Template:Chem/link + 4 HF → Template:Chem/link + 2 Template:Chem/link (500 °C, endothermic)

Template:Chem/link + Template:Chem/link → Template:Chem/link (350 °C, endothermic)

The resulting Template:Chem/link, a white solid, is highly reactive (by fluorination), easily sublimes (emitting a vapor that behaves as a nearly ideal gas), and is the most volatile compound of uranium known to exist.Template:Sfn

One method of preparing uranium tetrachloride (Template:Chem/link) is to directly combine chlorine with either uranium metal or uranium hydride. The reduction of Template:Chem/link by hydrogen produces uranium trichloride (Template:Chem/link) while the higher chlorides of uranium are prepared by reaction with additional chlorine.Template:Sfn All uranium chlorides react with water and air.

Bromides and iodides of uranium are formed by direct reaction of, respectively, bromine and iodine with uranium or by adding Template:Chem/link to those element's acids.Template:Sfn Known examples include: Template:Chem/link, Template:Chem/link, Template:Chem/link, and Template:Chem/link. Template:Chem/link has never been prepared. Uranium oxyhalides are water-soluble and include Template:Chem/link, Template:Chem/link, Template:Chem/link, and Template:Chem/link. Stability of the oxyhalides decrease as the atomic weight of the component halide increases.Template:Sfn

See also

• Neodymium compounds
• Protactinium compounds
• Neptunium compounds