A study of in situ trace precipitation of copper ferrocyanide on reactive polystyrene sulfonate resinates

Abstract

The in situ precipitation of copper(II) ferrocyanide on binary M n+Cu 2+ polystyrene sulfonate cation exchanger resinate beds was studied. The resin mole fraction of copper(II), X cu 2+ , was varied from 1.00 to 0.001 with the remaining exchange sites filled by an accompanying, nonreactive metal ion, M n+ = Li +, K +, Mg 2+, or La 3+. Potassium ferrocyanide solutions from 4.7 × 10 −7 to 4.7 × 10 −5 M were passed through 0.200 g resinate packed in the standardized minicolumns. The percentage precipitate yield of total ferrocyanide was determined as a function of X Cu 2+ , accompanying ion, solution concentration, and cross-linking and particle size of the resin. Scanning electron microscopy was used to study precipitate distribution on the beads as a function X Cu 2+ in Mg 2+Cu 2+ resinates. The dramatic differences found between precipitation limits—in terms X Cu 2+ —and virtual solubility products for different binary resinates correlated with charge, size, and hydration energy of the accompanying counterion M n+ . Relative tendencies toward ion pair formation with the Fe(CN) 6 4− species and the nature of the ion pair are of crucial importance. At low X Cu 2+ and solution concentration of [Fe(CN) 6] 4− precipitate “islands” are formed on otherwise “clean” resin beads suggesting that nucleation occurs first at a few favored locations with initial growth confined to those areas. A simple model consistent with our results explains in situ precipitation on ion exchange resin beds under trace conditions.