Chromium(III) phosphate
Chromium phosphate describes inorganic compounds with the chemical formula, where n = 0, 4, or 6. All are deeply colored solids. Anhydrous is green. The hexahydrate is violet.
Synthesis
Chromium phosphate is prepared by treating a phosphoric acid solution of chromium(VI) oxide with hydrazine.Hexahydrated chromium(III) phosphate
Hexahydrate chromium phosphate,, is prepared by reducing chromium trioxide,, with ethanol in the presence of orthophosphoric acid,, at temperatures ranging from −24 °C to +80 °C.Mesoporous phase
Gel-like chromium phosphate is prepared through the reduction of ammonium dichromate,, using ethanol,, and nitric acid,. This process is done in the presence of ammonium dihydrogen phosphate and urea at an elevated temperature where tetradecyltrimethylammonium bromide is used as structure directing agent.Films
Preparation of textured chromium phosphate is carried out by mixing equimolar solutions of aqueous chromium nitrate and diammonium phosphate in a dish placed in a sealed chamber with the low temperature ammonia vapor catalyst diffusing into the solution at a constant rate. After 24h, the resulting purple film grows out from the liquid through the hydrolysis and polycondensation occurring in the reaction environment at the air/liquid and film/liquid boundary. Surface tension makes the film compact making it easy to insert a microscope slide and lift the film from underneath the solution surface. Once obtained the solution is washed with deionized water and ethanol, then dried in a vacuum.Amorphous phase
The preparation of anhydrous chromium phosphate begins by grinding a mixture of 75 mol% of chromium oxide,, and 25 mol% of pure ammonium hydrogen phosphate,. This mixture is pressed into pellets and heated under air pressure at 400 °C for 24h in order to remove ammonia and water. After this, a heating sequence of 450 °C, 700 °C, 800 °C and 850 °C occurs. The pellet mixture is gradually cooled thereafter.Physical properties
Crystal structure
Chromium phosphate can exist as two isomorphs. Its β-isoform is orthorhombic with the Cmcm space group. The structure consists of infinite chains of trans edge-sharing octahedra, which run parallel to the c-axis, and are linked by tetrahedra. Above 1175 °C, β-CrPO4 converts to α-CrPO4. α-CrPO4 is orthorhombic as well, with the Imma space group. The structure consists of an infinite network of linked polyhedra with a CrO6 octahedron and a PO4 tetrahedron sharing a common edge. The site form edge-sharing Cr/Cr pairs and share two corners with the four Cr octahedra.Magnetic properties
The magnetic properties of the β-CrPO4 are a result of the cation-cation distances along the octahedral chains which give rise to strong direct-exchange interactions and even metal-metal bonding. Neutron diffraction studies reveal that the spiral moments in β-CrPO4 are collinear and anti-ferromagnetically coupled along the chains in the 001 planes, at low temperature.Observations from a diffraction study has shown that at low temperature, the α-CrPO4 octahedra units build up an infinite, three-dimensional network expected to provide strong Cr-O-Cr magnetic superexchange linkages with exchange pathway through the phosphate group. These linkages give the structure its anti-ferromagnetic characteristic which results in the anti-parallel magnetic spins in the plane that is perpendicular to the chains of the octahedral CrO6.
Chemical properties
Ion exchange
At a high temperature and pH ranging from 283-383K and pH 4-7 respectively, equilibrated KOH/HCl solution, insoluble solid and aqueous cation solution yield a sorption reaction. Studies reveal that catalyzes the adsorption of divalent cations onto its amorphous surface through the cation exchange mechanism. The mechanism suggests that the H+ ions are liberated from the solid to aqueous phase as the cations become hydrolyzed and adsorb onto the catalyst surface. Thus, a decrease in the pH of the reaction is used as a direct indicator of the rate of adsorption in the reaction:A plot of the Kurbatov equation is used to relate the release of H+ ion to the equilibrium constant of the reaction:
where Kd represents the distribution coefficient, and n is the slope of the straight line giving an indication of the H+/Mz+ stoichiometry of the exchange reaction. Under similar conditions, the selectivity of for dative cations follows the sequence:. Increases in temperature and pH enhances the ion exchange reaction.
Chromium phosphate is also used to catalyze cation exchange in sorption reactions. This catalysis is widely used in the reduction of metal toxicity during environmental clean-ups. This has been applied in decreasing the concentration of lead in aquatic habitat and drinking water.