100g Potassium Ferricyanide and 200g Ferric Ammonium Citrate. Use 1 part Potassium Ferricyanide to 2 parts Ferric Ammonium Citrate.
The chemical names sound alarming but are fairly common place and have no significant adverse effects on us or the environment. The main iron salt, ferric ammonium citrate, is used as a food additive (potential hazards are listed in the pack) while the other, potassium ferricyanide, sounds nasty but is only likely to cause problems if mixed with strong acids.
Chemicals should be handled with care and sensible precautions should be taken