engleski

Potentiometric and DLS investigation of the precipitation of lanthanum fluoride

The precipitation processes were characterised in aqueous solution of La(NO3)3 - KF at 25 oC using light scattering method and potentiometric titration with fluoride ion selective electrode. Two processes are taking place:i)neutralisation at large concentration of the components in the ratio 1/3, with the attained pH in the range 4.0+-0.3, leading to the formation of pure LaF3(s);II)ionic solubility below about 10-3 mol/dm3 La(NO3)3, with large hydrated lanthanum ions in solution with pH 6.0+-0.3 and fresh lanthanum fluoride/hydroxide precipitated, which is hard to filter and to dry, not suitable for preparation of pure LaF3. The size of particles (dynamic light scattering method) at the solid/solution limit is much greater (at least for an order of magnitude) in the case II as compared to the case I, where pF-stat growth led to the formation of pure hexagonal LaF3, as was determined by XRD analysis. The activity solubility products in water at 25 oC are 1.3x10-17 (lit.value 7x10-17), and in 50 per cent ethanol 3.7x10-27, were determined potentiometrically. The investigation of micellar medium of non-ionic surfactant on the growth pattern is in progress.

hydrodynamic diameter; lanthanum fluoride/hydroxide; pF-stat growth; polarizing microscopic features; solubility product.

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