Determination of the average ligand number and the equilibrium constant for the complexation reaction of Co2+ and SCN– in water-organic solvent mixtures: A physical chemistry laboratory experiment for undergraduate students

In the present study, the suitability of the complexation reaction between Co2+ and SCN– in water-organic solvent mixtures for the development of a laboratory experiment for undergraduate students was examined. From the measurement of absorbances of a series of solutions containing equal quantities of the metal (Co2+) but different quantities of the ligand (SCN–), the average ligand number and the equilibrium constant for the complex formation reaction are estimated using a Bent–French method. The students derive an expression for the relation between the experimentally determined values of the equilibrium constants and the stability constants of two adjacent complexes and estimate these stability constants. The experimental conditions and the used instrumentation drastically affect the measured average ligand number and equilibrium constant, and the calculated stability constants. However, the results are illustrative, suitable for educational purposes and reproducible by the various groups of students. The instructions for the students and the example data processing are provided.