Constant composition method
- quantitative data about growth/dissolution rates under controlled pH conditions
The experimental equipment was developed by members of our team (Lakshtanov et al., 2011) to study the kinetics of growth and dissolution of minerals, in particular calcite, at constant pH. Each set up consists of a thermostated 50 mL reactor vessel, with a pH electrode, a meter, an overhead propeller and two syringe tubes connected to a pump. A computer monitors pH and controls the volume of solutions injected into the reaction vessel. In the case of calcite growth, the working solution could be slightly supersaturated with respect to calcite, containing CaCl2 and NaHCO3 in 0.1 M NaCl medium for keeping ionic strength constant. Calcite precipitation starts when a weighed amount of calcite seed is added to the system. As precipitation continues, pH decreases. This triggers the injection of CaCl2 and NaCO3 by the pH dependent titration system, until the pH threshold of 8.3 is reached. The volume of each titrant added (mL/s) is recorded over the course of the experiment. It is proportional to growth rate (mol/m2·s). Growth rate of calcite can then be determined and compared for different values of saturation index and in presence of various additives such as ions or organic molecules.
Solution and solid samples can be collected during the reaction, for example for analysis by AAS (Atomic Absorption Spectroscopy), XPS (X-ray Photoelectron Spectroscopy) and SEM (Scanning Electron Microscopy).