He mentioned as an important exception the case of ferric ferrocyanide, which, when dissolved in oxalic acid, transmits the rays in great abundance, though the same rays be absorbed both by ferrocyanides and by ferric salts.
The most modern and the most generally accepted method is volumetric, and is based on the reaction between zinc chloride and potassium ferrocyanide, by which insoluble zinc ferrocyanide and soluble potassium chloride are formed; the presence of the slightest excess of potassium ferrocyanide is shown by a brownish tint being imparted by the solution to a drop of uranium nitrate.
They are soluble in water and give characteristic precipitates with platinic and auric chlorides, and with potassium ferrocyanide.
It may also be prepared by heating formic and oxalic acids (or their salts) with concentrated sulphuric acid (in the case of oxalic acid, an equal volume of carbon dioxide is produced); and by heating potassium ferrocyanide with a large excess of concentrated sulphuric acid, K 4 Fe(CN) 6 -i-6H2S04+6H20=2K2S04+FeS04+3(NH4)2S04+6C0.
The soluble salts are removed by lixiviation, and the residue is boiled with lime to form the soluble calcium ferrocyanide, which is finally converted into the potassium salt by potassium chloride or carbonate.