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Uranyl hydroxide is a
hydroxide Hydroxide is a diatomic anion with chemical formula OH−. It consists of an oxygen and hydrogen atom held together by a single covalent bond, and carries a negative electric charge. It is an important but usually minor constituent of water. It ...
of
uranium Uranium is a chemical element; it has chemical symbol, symbol U and atomic number 92. It is a silvery-grey metal in the actinide series of the periodic table. A uranium atom has 92 protons and 92 electrons, of which 6 are valence electrons. Ura ...
with the chemical formula UO2(OH)2 in the monomeric form and UO2)2(OH)4sup>2- in the dimeric; both forms may exist in normal aqueous media. In aerobic conditions, up to 5 hydroxides can bind to uranyl ( UO2)2(OH)5sup>3-). Uranyl hydroxide hydrate is precipitated as a
colloid A colloid is a mixture in which one substance consisting of microscopically dispersed insoluble particles is suspended throughout another substance. Some definitions specify that the particles must be dispersed in a liquid, while others exte ...
al
yellowcake Yellowcake (also called urania) is a type of powdered uranium concentrate obtained from leach solutions, in an intermediate step in the processing of uranium ores. It is a step in the processing of uranium after it has been mined but before ...
from oxidized uranium liquors near neutral pH.
Uranyl The uranyl ion with the chemical formula has a linear structure with short U–O bonds, indicative of the presence of multiple bonds between uranium and oxygen, with uranium in the oxidation state +6. Four or more ligands may be bound to the u ...
hydroxide was once used in glassmaking and
ceramic A ceramic is any of the various hard, brittle, heat-resistant, and corrosion-resistant materials made by shaping and then firing an inorganic, nonmetallic material, such as clay, at a high temperature. Common examples are earthenware, porcela ...
s in the colouring of the vitreous phases and the preparation of pigments for high temperature firing. The introduction of
alkaline In chemistry, an alkali (; from the Arabic word , ) is a basic salt of an alkali metal or an alkaline earth metal. An alkali can also be defined as a base that dissolves in water. A solution of a soluble base has a pH greater than 7.0. The ...
di
uranates A uranate is a ternary oxide involving the element uranium in one of the oxidation states 4, 5 or 6. A typical chemical formula is MxUyOz, where M represents a cation. The uranium atom in uranates(VI) has two short collinear U–O bonds and eith ...
(like
sodium diuranate Sodium diuranate, also known as the yellow oxide of uranium, is an inorganic chemical compound with the chemical formula . It is a sodium salt of a diuranate anion. It forms a hexahydrate . Sodium diuranate is commonly referred to by the initial ...
) into
glass Glass is an amorphous (non-crystalline solid, non-crystalline) solid. Because it is often transparency and translucency, transparent and chemically inert, glass has found widespread practical, technological, and decorative use in window pane ...
es leads to yellow by transmission, green by reflection; moreover these glasses become
dichroic In optics, a dichroic material is either one which causes visible light to be split up into distinct beams of different wavelengths (colours) (not to be confused with dispersion), or one in which light rays having different polarizations are ab ...
and
fluorescent Fluorescence is one of two kinds of photoluminescence, the emission of light by a substance that has absorbed light or other electromagnetic radiation. When exposed to ultraviolet radiation, many substances will glow (fluoresce) with color ...
under
ultraviolet Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of ...
rays. Uranyl hydroxide is
teratogenic Teratology is the study of abnormalities of physiological development in organisms during their life span. It is a sub-discipline in medical genetics which focuses on the classification of congenital abnormalities in dysmorphology caused by ...
and
radioactive Radioactive decay (also known as nuclear decay, radioactivity, radioactive disintegration, or nuclear disintegration) is the process by which an unstable atomic nucleus loses energy by radiation. A material containing unstable nuclei is conside ...
.


Formation

The formation of uranyl hydroxide hydrate can occur via hydrated uranyl fluoride UO2F2)(H2O)sub>7·4H2O which is not stable at an elevated water vapor pressure. A complete loss of fluorine is undergone and the formation of uranyl hydroxide hydrate ( UO2)4O(OH)6�5H2O) occurs. This uranyl hydroxide species is structurally similar to the uranyl hydroxide hydrate minerals schoepite and metaschoepite. X-ray diffraction data was gathered and found that this species has expanded interlayer spacing suggesting there may be additional water molecules in between uranyl layers. Unlike metaschoepite, however, this species does not form UO2(OH)2 upon dehydration.


Reactions

# O2(OH)sup>+ + H2O → UO2(OH)(H2O)sup>+ # UO2(OH)(H2O)sup>+ + H2O → UO2(OH)(H2O)2sup>+ UO2(OH)2 reacts with water in a hydration reaction to form UO2(OH)2)(H2O)sup>+ and the monohydrate form also reacted with water to form dihydrates UO2OH)(H2O)2sup>+ and trihydrates UO2OH)(H2O)3sup>+. The hydration reaction to form the monohydrate was significantly slower than if the hydroxide were replaced with acetate or nitrate. This could be due to the strongly basic (OH)− reducing the Lewis acidity of U or because the more complex acetate and nitrate anions provide more degrees of freedom. However, it was found that the formation of the dihydrate uranyl hydroxide hydrate (2) was nearly three times faster than the monohydrate (1). A mechanism for oxygen exchange between the UO22+ cations in a highly alkaline solution was proposed and investigated by Shamov et al. in the ''Journal of the American Chemical Society''. An equilibrium between O2(OH)4sup>2- and O2(OH)5sup>3- was observed followed by the formation of the stable O3(OH)3·H2Osup>3- intermediate that formed from O2(OH)5sup>3- via intramolecular water elimination.


References


The Structure of the a Form of Uranyl Hydroxide
* Alexander, C.A. (2005
"Volatilization of urania under strongly oxidizing conditions,"
''Journal of Nuclear Materials,'' 346, 312–318. {{Hydroxides Uranyl compounds Hydroxides