JERRY MASON / SCIENCE PHOTO LIBRARY JERRY MASON / SCIENCE PHOTO LIBRARY
Colour change due to ligand displacement. Experiment illustrating a method for identifying a cobalt(II) solution. The method relies on a colour change when a cobalt complex is altered by ligand displacement. The pink solution at left contains a complex ion, hexaaquacobalt(II). If concentrated hydrochloric acid is added (centre) a blue colour is seen. The chlorine of the acid displaces the water in the cobalt complex, forming a new complex ion, tetrachlorocobaltate(II) with a characteristic blue colour. Adding water (right) reverses the reaction and the solution returns to pink.
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