Ferrate ( FeO42-
) is an hexavalent iron complex.
Chemically it is a lot like permanganate, except that it is
slightly milder. This is also its main use, being able to selectively oxydize secundary alcohols.
In solution Ferrate is very dark purple, and must be kept under vacuum, since it reacts with water to form rust.
diPotassium ferrate is also purplish, but even when it is already brown (rust), it can still contain quite some ferrate.
Potassium ferrate is prepared by adding iron(iii)chloride through a chilled potassium hypochlorine solution.
(prepared by bubbling chlorine (g) through a chilled saturated solution of potassiumhydroxide)
Wash (very carefully) with chilled water and ethanol and immediately put it in an exicator.
It requires some handy work, and might require some practice.
There is a turn of the century (1903ish) German article about this.