Analysis of Nucleation Rate Measurements in Precipitation Processes

For precipitation processes, measurement of the nucleation rate as a function of supersaturation is not so easy because the formation of the precipitate is fast and often hard to control due to the high supersaturation. For a number of studies on both ionic and molecular precipitating compounds, the experimental methods are discussed. From the experimentally determined nucleation rates, the probable nucleation mechanism, homogeneous or heterogeneous, is derived and compared to the theoretically expected mechanism. For all ionic compounds and for one molecular compound considered in this work, theoretically homogeneous nucleation is achievable. However, only for two ionic compounds, barium sulfate and boehmite, the measurements possibly indicated a homogeneous nucleation mechanism. For the other studies, including all molecular compounds, a heterogeneous mechanism was more likely. The difference between a theoretically expected and experimentally observed mechanism may be explained by not only the use of less suitable experimental methods and measurement techniques but also by secondary processes, such as agglomeration and transformation, taking place in the precipitated suspension. On the basis of these findings, an experimental procedure is suggested to standardize and to improve nucleation rate measurements in precipitation.