Visualizing the kinetics of Calcium Carbonate precipitation

This video shows an experiment which visualizes the kinetics of calcium carbonate precipitation. The video is accelerated as the actual experiment lasts about 10 minutes. Two solutions are prepared: the first contains 1 mol/L Calcium Chloride (CaCl2) and the second 1 mol/L Sodium Carbonate (Na2CO3). About 300 drops/L universal pH indicator (Sigma Aldrich pH3-10) is added to each solution, which makes the CaCl2 solution turn yellow (pH 6.5) and the Na2CO3 solution pink (pH 11). First the Erlenmeyer is filled with 100 mL of the CaCl2 solution, followed by 100 mL of the NaCO3 solution. As soon as the Na2CO3 solution is added an immediate reaction is observed. As the mixed solution is highly oversaturated, rapid crystal nucleation takes place, resulting in large number of very tiny particles which make the solution turbid and turn it instantly into a gel. Next, white zones gradually spread out downwards through the gel which illustrate the precipitation processes of continuous nucleation, crystal growth and agglomeration. At some stage the crystals or crystal aggregates become so big that they start to settle from the suspension, leaving a clearer liquid above. Towards the end of the precipitation process, particularly if there is slightly more calcium than carbonate and carbonate is depleting from solution pH drops and turns (locally) back to neutral. As a result the chemical speciation of dissolved inorganic carbon shifts from carbonate (CO32-) through bicarbonate (HCO3-) to carbonic acid (H2CO3) and some of the carbonic acid escapes the solution as gaseous carbon dioxide. Movie is made by Leon van Paassen, Delft University of Technology and Artan Hajrullahu, ETH Zurich during the E4D Summerschool 2016 on "Sand as an (in-)finite resource".

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