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 pH and pF titration with fluoride ion selective electrode. Two processes are taking place according to the precipitation diagram: I) neutralisation process at high concentrations of precipitation components in the ratio 1:3 and with the attained pH at 4.0 +- 0.3, leading to the formation of pure LaF3 (s) ; II) ionic solubility at La(NO3)3 concentrations below (1.0 +- 0.05)x10-3 mol/dm3 with large hydrated lanthanum ions in solution having pH at 6.0 +- 0.3 (hydrolysis), leading to the fresh precipitated lanthanum fluoride-hydroxide containing large amount of water, which is hard to filter and to dry, and is not suitable for the preparation of pure LaF3. The steep increase in particle size (from submicron to micron size range, as determined by dynamic light scattering measurements) of systems at the solid/solution concentration limit is accompanied with the increase in pH of the solution from 4.6 to 5.9, deviding the concentration and pH region I and II. In the former case I, the pF-stat crystal growth led to the formation of pure hexagonal LaF3 (s), as was determined by powder X-ray diffraction analysis (XRD) of dried solid. The variety of morphologies was evidenced as a result of growth and aggregation pattern of lanthanum fluoride particles from submicron to micron size depending on pH, supersaturation and time. Formation of dendrites (aggregates of monodisperse particles (300 nm), anisotropic fibers and ribbon-like multichains, reminiscent of liquid crystals or inorganic polymers, and finally large (several hundred microns) polycrystals like hexagonal prisms and trigonal bipyramids, was characterised by dynamic light scattering and polarising microscopy.

Lanthanum fluoride/hydroxide ; Inorganic polymers ; Particle aggregation and growth ; Polarization microscopy ; Precipitation diagram.

Rad u časopisui navedenom u Pravilniku iz NN 2/97.