Abstract
A potentiometric pF-stat method has been used for the investigation the kinetics of lead fluoride crystal growth on seeds (α-PbF2) at 25°C. The kinetics was studied as a function of ionic strength, pH and lead-to-fluoride ions activity ratio, which were changed only by changing Pb(NO3)2 and KF concentrations. Depending on the excess of lead(II) nitrate or potassium fluoride in supersaturated solution two different crystal phases grow: (a) In solutions with large excess of Pb(NO3)2 and corresponding pH=4.35−5.00, a large needle-like crystals with some sponge-like precipitate are grown. The effective order of crystallization of this mixture is 1.55, indicating preferentially one-dimensional growth of needles. (b) In solutions with KF in excess, and consequently with pH=5.0–7.0, however, voluminous bipyramids of orthorhomic α-PbF2 crystals are grown. The effective order of crystallization of this known phase is 4.0, i.e. presumably an exponential rate law is valid.
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