Platinum(IV) bromide
Platinum(IV) bromide is the chemical compound composed of platinum and bromine with the formula PtBr4.
A solution of platinum(IV) bromide and gold(III) bromide in water can be used to test for the presence of caesium by putting a drop of the reagent and a drop of the unknown solution onto a piece of filter paper. A gray or black stain indicates a positive result; however, the mechanism for this reaction is unknown .