Prussian Blues

Several of The Prussian Blues are known, namely: α soluble Prussian blue, 4Fe^•••K[Fe^••(CN)₆].7H₂O. This compound is obtained when a solution of potassium ferrocyanide interacts with rather less than one equivalent of a ferric salt. It occurs as a deep blue precipitate, which when washed with alcohol and dried yields a beautiful blue powder exhibiting a bronze cast by reflected light. It is soluble in pure water, but is precipitated on the addition of salts. Aqueous solutions of oxalic acid dissolve it, whilst alkalies decompose it, precipitating ferric hydroxide. A similar blue-coloured precipitate, known as Turnbull's blue (the soluble variety), is formed when rather less than one equivalent of a ferrous salt interacts with potassium ferricyanide. According to Skraup and Hofmann, the two pre- , cipitates are identical, but this is denied by Muller.

β soluble Prussian blue is produced when molecular proportions of a ferrous salt and potassium ferrocyanide are brought together in neutral solution and oxidised with hydrogen peroxide. It is rapidly decomposed by ammonia, but is not soluble in oxalic acid solution. It is isomeric with the γ variety, and the formula Fe^•••K[Fe(CN)₆].H₂O is attributed to it.

γ soluble Prussian blue is obtained when molecular proportions of a ferrous salt and potassium ferrocyanide are brought together in cold, acid solution and oxidised with nitric acid or hydrogen peroxide. Hofmann regards it as consisting of two molecules of KFe[Fe(CN)₆] + Aq. and suggests the asymmetrical formula

Isomeric with this salt is: Williamson's violet or Williamson's blue, KFe^•••[Fe(CN)₆] + H₂O, obtained by suspending Everitt's salt in one per cent, sulphuric acid and adding nitric acid or hydrogen peroxide. A ferric salt may be used as oxidising agent. The compound is insoluble in water, but forms a suspension which appears dark purple by reflected light, but greenish blue by transmitted light. It is insoluble in oxalic acid and is not decomposed by dilute mineral acids or by digestion with ferric chloride, but 4 per cent, ammonium hydroxide converts it after some hours into ferric hydroxide and alkali ferrocyanide. Hofmann and Resenscheck suggest the symmetrical constitutional formula: -



the water also being constitutional in some way.

When ferric ammonium alum and potassium ferricyanide are acted upon with hydrogen peroxide in the presence of free hydrochloric acid, a blue substance, Fe₇(CN)₁₈.10H₂O, is obtained, having properties similar to Williamson's violet, but less stable towards dilute ammonium hydroxide.

Insoluble Prussian blue, Fe^•••₄[Fe(CN)₆]₃ + Aq., is obtained when potassium ferrocyanide is treated with an excess of a ferric salt, the ratio, in the case of ferric chloride, FeCl₃/K₄Fe(CN)₆, being greater than 1.33. The same product is obtained when a soluble Prussian blue is acted upon by a ferric salt, and is stated to be identical with insoluble Turnbull's blue, which is the salt obtained by addition of excess ferrous salt to potassium ferricyanide and washing in full exposure to air; but this is denied by Muller, who gives it the formula KFe^••Fe^•••₃[Fe(CN)₆]₃.

The substance is insoluble in water and in dilute mineral acids, but dissolves in oxalic acid, yielding a blue solution at one time largely used for ink. When dried, the salt contains 9-10 molecules of water, and if heated strongly in air it burns, leaving a residue of ferric oxide. Alkalies decompose it, precipitating ferric hydroxide, soluble ferrocyanides passing into solution: -

Fe₄[Fe(CN)₆]₃ + 12NaOH = 4Fe(OH)₃ + 8Na₄[Fe(CN)₆],

whilst hot concentrated hydrochloric acid effects its solution, hydrogen ferrocyanide eventually separating out. Heated with concentrated sulphuric acid, the cyanogen group is decomposed, the nitrogen yielding ammonium sulphate. When fused with a mixture of ammonium nitrate and sulphate, sulphate of iron is produced.

Prussian blue was discovered accidentally in 1704 by Diesbach, and is highly valued as a pigment on account of its remarkable intensity of colour. It was manufactured in Great Britain in 1770, and sold at 2 guineas per lb. One pound of Prussian blue will perceptibly tinge some 600 lbs. of white lead. The pigment is sometimes prepared commercially by the direct method of adding a ferric salt to a solution of potassium ferrocyanide; but it is more usual to adopt an indirect method, namely to add a ferrous salt to potassium ferrocyanide and subsequently to oxidise the white precipitated mass of ferrous potassium ferrocyanide. Chemically it consists of a variable mixture of some or all of the Prussian blues already described.