The yellow (there are also red) ferrocyanide (also yellow potash, potassium ferrocyanide, potassium or Kaliumhexacyanidoferrat) is a complex potassium-iron compound in wine and is used in the so-called blue fining (see below Fine ). It reacts with copper, although iron is present and binds both to an insoluble, blue substance that settles on the ground. Since it also can form hydrogen cyanide, its use is subject to approval. Means of test solution in a small amount of wine (10 ml) can be found in a "laboratory test for iron," whether before bottling a blue fining is required. A finding of fact, then, if necessary ferrocyanide-demand may be performed only by authorized persons. Improper use can at worst, serious wine of error bitter almond emerge.