Vanadium Ferrocyanides

Solutions of vanadates, when treated with potassium ferrocyanide, yield a precipitate of doubtful composition. The compound is insoluble in mineral acids of high concentration, and a method involving its precipitation has recently been suggested for the estimation of vanadium in steels.

When vanadous sulphate, V₂(SO₄)₃, in solution, is treated with barium thiocyanate, a green solution is obtained which is believed to contain vanadous thiocyanate, V(SCN)₃, but attempts to isolate this compound have not met with success. Its double salts with the alkali thiocyanates have, however, been obtained in well-defined crystals when an alkali thiocyanate is substituted for the barium salt. The following are known: -

Ammonium vanadous thiocyanate, 3NH₄SCN.V(SCN)₃.4H₂O; dark green crystals.
Sodium vanadous thiocyanate, 3NaSCN.V(SCN)₃.12H₂O; red tablets or leaflets.
Potassium vanadous thiocyanate, 3KSCN.V(SCN)₃.4H₂O; bright red crystals.

These compounds probably do not contain a complex [V(SCN)₆] anion; if they do, it must be extremely unstable in dilute solution, because (a) their aqueous solutions, which are brown at first, assume the green colour due to the V^••• ion; (b) addition of an alkali precipitates vanadous hydroxide, V(OH)₃; (c) acidified solutions on the addition of ferric salts give the blood-red colour characteristic of the (SCN)' ion; (d) depression of freezing-point data bear out the foregoing facts. Hexa-methylenetetramine aquo-vanadous pentathiocyanate, (C₆H₁₂N₄.H)₃ [V(SCN)₅.(OH)₂].H₂O, and a basic pyridine vanadous tetrathiocyanate, (C₅H₅N.H)₃.[V(SCN)₄(OH)₂], have, however, been prepared from ammonium vanadous thiocyanate.