Properties of the Ammine Copper Complexes

Problem The coloring of various complex compounds is suitable to experimentally show, that a specific rather than an arbitrary amount of ligands per central ion is bonded. For instance, through the successive additions of ammonia solution to copper sulfate solution the deep violet colored solution of the tetra ammine copper complex is finally achieved. With this, it is possible to demonstrate that a specific amount of ammonia solution is necessary to get the violet colored ammine complex, any further additions do not lead to other changes in color. 

Material Beakers, test tubes, plastic pipettes, measuring cylinders (100 ml), scales copper sulfate hydrate, 2-M hydrochloric acid, ammonia solution (25% solution, diluted 1 50). 

Procedure Weigh 1 g copper sulfate, add 40 ml hydrochloric acid and dissolve. Label 10 test tubes from 1 to 10 and fill each with 3 ml of this green colored solution. Use test tube 1 as reference. Add ammonia solution to test tubes 2-10. Starting with the addition of 0.5 ml in test tube 2, with 1.0 ml to test tube 3, increase the volume of ammonia solution by 0.5 ml per test tube. Finally, by adding water, make the volume of all test tubes equal. 

Observation A gradual color change from green to dark violet can be seen from test tubes 2 to 5. The intensity of the violet color remains constant from test tube 5 until test tube 10. 

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