A LANTHANIDE LANTHOLOGY (.pdf) - Davidson Physics

OXALATESAddition of oxalic acid, or an alkali oxalate or ammonium oxalate, to a neutral or slightlyacidic solution of a trivalent lanthanide will precipitate a hydrated oxalate. The deca-hydrate isobtained with La - Ho (and Y), an isomorphous series, whereas another series, the hexa-hydrate,tends to form for Er - Lu, the heavier lanthanides. In strongly acidic solutions the other anionpresent may be included in the solid brought down, chloro-oxalate from HCl for example, or aninsoluble acid salt be produced.The high insolubility of the hydrated oxalate in near-neutral solutions is used in thecommercial-scale preparation of high-purity derivatives of the lanthanides and yttrium.Calcination of the precipitate produces a pure oxide. This process is also used for the gravimetricanalysis of lanthanides. The oxalate is preferred to the carbonate because transition metals ionstend to be rejected and remain in solution. Physical occlusion of contaminant ions though can be aproblem.Thermal decomposition[1] of the light-Ln oxalates involves initially the steady loss of waterof hydration, from ambient up to≈ 250°C, but intermediate hydrates are not seen. With the heavylanthanides an intermediate dihydrate can be formed. The anhydrous oxalates are not stable and thefirst clearly identifiable species is the dioxomonocarbonate, Ln 2 O 2 CO 3 , containing the stable(LnO) n grouping seen in many Ln compounds.Ln 2 (C 2 O 4 ) 3 .nH 2 O => Ln 2 (C 2 O 4 ) 3 =>[? Ln 2 (CO 3 ) 3 => Ln 2 O(CO 3 ) 2 ?] =>=> Ln 2 O 2 CO 3 => Ln 2 O 3The temperatures corresponding to the various transitions depend on the surroundings, thepresence of CO 2 delays decomposition. In addition localized reducing conditions can cause spotformation of carbon. This can give a gray cast to the resulting oxide product if the C is not burnt offduring the final stages of the decomposition.Ln 2 (C 2 O 4 ) 3 => Ln 2 O 2 CO 3 + 2CO 2 + 3CO2CO => CO 2 + CWith cerium, oxidation of Ce(III) to Ce(IV) starts at low temperatures, ≈300 °C,because of the formation of the oxide, CeO 2 , no intermediate stages are seen.and,[1] Thermal Analysis of the Oxalate Hexahydrates and Decahydrates of Yttrium and the Lanthanide Elements,M.J. Fuller and J. Pinkstone, J. Less-Common Metals, 1980, 70, 12717