The precipitate shows certain differences in composition according to the exact method by which it is obtained. A different result is obtained when the solution of the ferric salt is poured into that of a ferrocyanide, to that produced by the reverse procedure, the precipitate produced in the one case has properties which it does not possess in the other. Similar differences often occur in precipitating pigments.
If the solution of the ferrocyanide is poured into the iron solution, the latter being in excess, an insoluble precipitate is formed; but if the iron solution is poured into the ferrocyanide solution of which an excess is present, the blue precipitate is formed, but it is soluble in pure water, though not in water containing salts. Thus when this precipitate is separated from the liquid and washed, the washings are at first quite colourless and remain so whilst salts are present, but when these have been completely washed away, a fine blue solution of the ferric ferrocyanide is formed, from which the addition of a salt solution again separates the dissolved Prussian blue.
Before proceeding to the manufacture of Prussian blue on the large scale, it is necessary to consider the behaviour of ferrous salts towards yellow prussiate, since these are commonly used instead of ferric salts. When a solution of potassium ferrocyanide is mixed with a solution of a ferrous salt, there is formed a pure white precipitate of potassium ferrous ferrocyanide, K₂FeFe(CN)₆. In order to obtain a completely white precipitate, it is necessary that the ferrous salt should be completely free from ferric salts and that the solutions should contain no dissolved oxygen; the solutions would therefore have to be boiled before mixing. If one of the solutions contains the smallest quantity of oxygen a bluish precipitate is formed. When the white precipitate of potassium ferrous ferrocyanide is exposed to the air it immediately acquires at the surface a dark blue colour, and is completely changed to Prussian blue by sufficiently long contact with air.
In accordance with this behaviour of ferrous and ferric salts different methods may be used to obtain Prussian blue. Either the process is commenced with a ferric salt from which a precipitate of Prussian blue is at once obtained, or the solution of a ferrous salt is converted into ferric salt by a powerful oxidising agent, such as chlorine or nitric acid, or a solution of a ferrous salt is precipitated with yellow prussiate and the white or bluish precipitate, which consists chiefly of potassium ferrous ferrocyanide, converted into ferric ferrocyanide or Prussian blue by treatment with nitric acid.
Prussian blue is also known, as we have said, as Chinese blue, Berlin blue and Paris blue; Brunswick blue or mineral blue is another form. Generally the product under the name of Chinese or Paris blue, which has an intense dark blue colour, is quite pure, whilst under the names of Prussian blue and Berlin blue mixtures with starch or alumina are included, which consequently have a more or less pale colour. The pigments known as mineral or Brunswick blue are the least valuable, they have generally a less deep colour and are quite without the metallic lustre which distinguishes Chinese blue and the better varieties of Prussian blue. Mineral blue often contains very large quantities of alumina, chalk, or even barytes; the addition of the last is to be regarded as irrational, since it makes the pigment conspicuously heavy.
Chinese Blue.—Pure Chinese blue appears in the form of deep blue masses, characterised by a peculiar metallic lustre, which is especially marked when the surface of a lump is rubbed with the finger nail. This metallic lustre is accompanied by a copper-red shimmer which is similar to that exhibited by fine indigo. In large pieces pure Chinese blue appears very dark by artificial light; it possesses enormous strength of colour. The following test serves for the recognition of the complete purity of Chinese blue: A small quantity is powdered and rubbed in a thin layer on white paper; if the metallic lustre shows in undiminished strength the blue may be regarded as quite pure, for Chinese blue quickly loses this property by additions of other substances.
From what has been said above concerning the different behaviour of ferrous and ferric salts, the simplest method of making Chinese blue would be to precipitate the solution of a ferric salt with yellow prussiate, keeping the ferric salt in excess, to wash and dry the precipitate. Ferric salts are, however, dearer than ferrous salts, so that the latter are generally used.
In making Chinese blue from a ferrous salt ferrous sulphate or green vitriol is generally taken. This salt is dissolved in water; it is advisable to add a small quantity of sulphuric acid, since the water will contain carbonates, which produce ferrous carbonate. Air oxidises the latter to ferric hydroxide, which is brown, and if present even in small quantity will spoil the shade of the blue. This addition of sulphuric acid is of special importance when the white precipitate produced by the prussiate solution is to be oxidised by the air.
A solution of yellow prussiate is added to the iron solution in such quantity that a very small excess of iron salt is left; there results a white or rather, since the ferrous sulphate always contains small quantities of ferric oxide, a pale blue precipitate. This is allowed to settle, the liquid drawn off as completely as possible from the precipitate, and nitric acid then added, or a solution of bleaching powder, followed by the amount of sulphuric acid required to decompose it. The nitric acid or the chlorine liberated from the bleaching powder speedily effects the oxidation, changing the pale blue colour to the deep blue of Chinese blue. The precipitate should be left for several days in contact with the liquid in order to complete the oxidation; it is then well washed and dried. Pieces of definite shape and size bearing the trade mark of the firm are often pressed from the blue when it has acquired a stiff consistency.
There are many formulæ for the preparation of Chinese blue, differing in regard to the quantities of green vitriol, yellow prussiate, and nitric acid or bleaching powder to be used. It is, however, clear that only one formula can be correct, that in which the quantities of materials are equivalent, since the reactions take place between equivalent quantities. In practice the equivalent quantities would not be weighed to the tenth of a gramme—there would be no object in such a proceeding, since in works chemically pure substances are not used; but manufacturers should work according to equivalent quantities in order to use up their materials as completely as possible. No increase in labour is involved, the same labour weighs one or another quantity.