Question

How would you decide between magnesium chloride hexahydrate and zinc chloride if they are both unknown using optical spectroscopy and/or conductivity and/or EDTA titration?

Answer 1

We can find the magnesium in a solution by eriochrome black T, pH will be 10 if Mg is present, We can also find by preparing pH 10 ammonia buffer solution, eriochrome black T changes its colour from red to pale blue. This is how we can decide between magnesium chloride hexahydrate and zinc chloride. If we add ammonia solution the Zn⁺² forms white precipitate by following reaction

Zn⁺² + ammonia + water gives Zinc chloride and NH₄⁺ and further adding ammonia its colour changes to colourless due to formation of Tetraammine zinc ion.

So both forms white precipitate on adding ammonia but zinc further looses its white colour on adding ammonia and magnesium does not loose, this is how we can decide between both.