I have a reference sheet for inorganic analysis which says that the confirmatory test for ferrous ions is as follows:
Make a solution of the given salt (whose cation is to identified) and add potassium ferrocyanide [ potassium hexacyanidoferrate(II) ] solution to it. If a dark blue precipitate is formed, then the presence of ferrous ions in the given salt is confirmed.
What is the chemical composition of this dark blue precipitate? Or is there a flaw in the test that I have specified? If it is flawed, then what is the appropriate test?