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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.

Double Salts of Vanadous Thiocyanate

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.

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