The composition of tungsten(VI) species extracted with trioctylamine from aqueous oxalate solution

The extraction behavior of tungsten(VI) from aqueous oxalate solution with trioctylamine (TOA, R3N) and the composition of the extracted species have been studied. The chloroform solutions of extracts were analyzed by gel permeation chromatography (GPC). Two TOA polytungstates(VI) and TOA oxalatotungstate(VI) were isolated from the extracts obtained from the aqueous solution of mole ratios (C2O42-/WO42-) of 1.06 and 4.78, respectively, by using preparative GPC. The compositional formulas of the isolated species are estimated to be (R3NH)2W5O16. (R3NH)HW3O10 and (R3NH)2WO2(C2O4)2 on the basis of the data of chemical and elemental analyses of these species. These formulas are also supported by the infrared spectra and the thermal analysis of these complexes. Moreover, the composition of TOA oxalatotungstate(VI) has been investigated by several methods ; the mole ratio of R3NH+ to oxalatotungstate(VI) is determined to be 2:1. These species were also subjected to GPC and vapor pressure osmometric method (VPO) in order to estimate their molecular weight. The values determined by GPC are in agreement with those calculated from their compositional formulas. The molecular weights determined by VPO for (R3NH)2W5O16 and (R3NH)2WO2(C2O4)2 agree very closely with their formula weights. However, that obtained for (R3NH)HW3O10 goes to 2.10 x 103, corresponding to the formula weight of dimer. The measurements of molecular weight suggest that TOA polytungstate(VI) species form aggregates such as dimers and trimers in benzene and chloroform. (author)