There are two processes required to stop bleeding from a cut. One is coagulation of blood, and second is contraction of cut blood vessels to reduce the flow. As Potassium is one of the component of plasma - it do not help much to clot the blood or to contracts the blood vessels - if used as dilute solution, and stronger solutions create much irritation. It is established fact that higher the charge on cation higher will be its efficiency to coagulate the colloidal solution. Blood is a colloid, and blood cells are suspended in the plasma - blood serum. Ferric salts help coagulation of blood by helping polymerisation of blood protein.