Barium carbonyl ferrocyanide, Ba₃[Fe(CN)₅.CO]₂

Barium carbonyl ferrocyanide, Ba₃[Fe(CN)₅.CO]₂.11H₂O, is readily prepared by decomposing the copper salt with a soluble barium salt, or by neutralising the free acid with barium hydroxide. It loses 10 molecules of water at 19° C. in vacuo, and becomes anhydrous at 130° C. The heats of solution are: -

[Ba₃[Fe(CN)₅CO]₂] + Aq. = Ba₃[Fe(CN)₅CO]₂Aq. + 16,900 calories,
[Ba₃[Fe(CN)₅CO]₂.11H₂O] + Aq. = Ba₃[Fe(CN)₅CO]₂Aq.- 6,900 calories,

whence the heat of hydration of the anhydrous salt is 23,800 calories at 17.5° C.