Chrome alum

Chrome alum, also known as chromium potassium sulfate, is a chemical compound. Its chemical formula is KCr(SO4)2. It contains chromium, potassium, and sulfate ions. The chromium is in its +3 oxidation state.
Properties.
It is a dark purple solid. It is normally attached to extra water molecules. It dissolves in water.
Preparation.
It is made by reducing potassium dichromate with sulfur dioxide.
Uses.
It was used in making leather; now chromium(III) sulfate is used instead.