Analytical Chemistry of Cadmium

Zinc forms a number of complexes but they are not as numerous as those of the 3d transition metals. They are generally less stable than those of the preceding metals. The complexes may be cationic, anionic or neutral e.g. the complex with 8-hydroxyquinoline encountered above. 

The tetrahedral tetraamminezinc(II) ion can be precipitated by the large tetrafluoroborate(III) anion. The complex can be analysed by dissolving in a known excess of HCl, back titrating the excess with standard alkali. 

Recrystallise 3 g of NH4BF4 (Sec.4.4.1). Dissolve 2 g of zinc sulphate heptahydrate in the minimum volume of water. Add cone, ammonia until the precipitate initially formed has completely dissolved. Add a solution of the recrystallised fluoroborate to the zinc salt solution with stirring. Allow the precipitate to stand until crystallisation seems complete. Filter the complex precipitated under suction. Wash on the filter with small volumes of acetone. Dry by continued suction while pressing between filter paper. Weigh your product and calculate the % yield based on zinc. 

Weigh out accurately about 0.25 g of your preparation into a conical flask. Add 50.0 cm of 0.1 M HCl from a burette followed by a few drops of methyl red indicator. Titrate with standard 0.1 M NaOH until the colour changes to yellow. Repeat to obtain concordant results. Calculate the average % purity of your preparation. 

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