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allowed to crystallise, deposits yellow tabular crystals of a compound of potassium, iron, carbon, and nitrogen, originally known as yellow prussiate of potash; the iron in this exists in combination with the carbon and nitrogen in an acid state, forming the radicle known as ferrocyanogen, Fe Co Ne The chemical name of the yellow prussiate of potash is potassium ferrocyanide, K4 Fe Co N. Besides the yellow prussiate there is another, the red prussiate or potassium ferricyanide, K, Fe Co N These two compounds differ from one another in their colour and in the reactions which they give with iron salts. With ferrous salts the ferrocyanide gives a bluish-white precipitate of ferrous ferrocyanide; while with ferric salts a deep blue (Prussian blue) precipitate of ferric ferrocyanide is obtained With ferrous salts the ferricyanide gives a deep blue precipitate of ferrous ferricyanide (Turnbull's blue); with ferric salts no precipitate is obtained, but the colour of the solution becomes a little darker. The production of Prussian blue is a most characteristic reaction of iron, no other metal is capable of producing it, and very minute traces of iron in a solution can be detected by adding a few drops of a solution of potassium ferrocyanide.
It has been ascertained by various observers that the precipitates obtained by adding solutions of the prussiates to solutions of iron salts contain potaşsium as an essential part of their composition, and it is difficult, although possible, to rid them of this potassium. Thus the bluish-white precipitate is really potassium ferrous ferrocyanide, K, Fe Fe C. No; Prussian blue is potassium ferric ferrocyanide, K, Fe, 2 Fe Co N; Turnbull's blue is potassium ferro-ferricyanide, K Fe Fe Co No
Prussian blue and Turnbull's blue have exactly the same composition, but their constitution is different; the one being a ferrocyanide and the other a ferricyanide. Skraup,* Reindel, Kekule, f and other authorities consider that they are identical ; but that they are different is proved by the fact that when the alkali is eliminated from them the residual blues are of different composition, Prussian blue having the composition Fe, C18 N18 and Turnbull's blue the composition Fe, C12 N12 $; then the shade of the two blues is different, Prussian blue is a greenishblue, while Turnbull's blue is a violet-blue.
COMMERCIAL PRUSSIAN BLUE. In commerce several varieties of Prussian blue are sold under the names of —
Skraup, Liebig's Annalen, clxxi., p. 371. + Reindel, Jour. Prak. Chemie, cii., p. 38. #Kekule, Lehrbuch. Organ. Chem.
& Williamson, Mem. Soc. Chem., iii., p. 125; Reynolds, Jour. Chem. Soc., li., p. 644. See also E. J. Parry on “Composition of Commercial Prussian Blues,” Analyst, September, 1896.
1, Chinese blue ; 2, Prussian blue; 3, soluble blue; 4, Antwerp blue; 5, Brunswick blue, and others of less importance. There are several synonyms, such as Berlin blue and Paris blue. Turnbull's blue is a name rarely met with now. No special distinction is made between a blue obtained from the red prussiate or a blue obtained from the yellow prussiate of potash.
CHINESE BLUE.—This is the name given to the best qualities of Prussian blue, and in the manufacture of which every care is taken to obtain a product of good colour. Chinese blue is especiallharácterised by being in pieces or powder having a fine bronze lustre, the pieces break with a peculiar conchoidal fracture, and the fractured surfaces show the lustre or bloom; it is completely soluble in oxalic acid. Chinese blue is largely used by calico-printers and dyers. It is a blue of a greenish shade.
Chinese blue is made as ollows:-1 cwt. of ferrous sulphate (green copperas), as free from insoluble oxide as possible, is dissolved in cold water, and to it is added 10 lbs. of sulphuric acid. This solution must be made as required, as it soon begins to oxidise, and to deposit oxide of iron, while the liquor will then not make good Chinese blue. One cwt. of yellow prussiate of potash is dissolved in water.
The solutions should-be made as dilute as possible, not less than 30 to 5 gallons of water for each cwt. of material; even weaker solutions are preferable, as these yield finer precipitates than strong solutions, and so facilitate the production of the lustre on the finished blue.
On mixing the solutions a bluish-white precipitate is obtained, which is allowed to settle, and the clear liquor poured off; to the residual blue is added—first, a thin cream of 20 lbs. of bleaching powder with water, which is thoroughly mixed with the precipitate, and then some hydrochloric acid, and the blue colour gradually develops. It is allowed to settle, the top liquor run off, and the blue well washed with water, and drained on a filter; the wet mass is then pressed into drying-pans, and slowly dried in the dark, at a temperature not exceeding 120° to 130° F.
It is important that the oxidation of the precipitate first obtained be done by purely chemical means, and not by the agency of the oxygen of the air; in the former case a pure blue colour is obtained, while in the latter case oxide of iron is mixed with the blue, and materially influences the tint of the pigment. The best oxidiser and the cheapest is bleaching powder; nitric acid may be used, but it is more costly and not more efficient than bleaching powder. It will be found best to
add the oxidiser in small quantities, as when used all at once there is generally an escape of chlorine, owing to this body being evolved rather more rapidly than the blue can take it up, which not only increases the cost of production, but also deleteriously affects the workmen's lungs. No part of the blue should be allowed to come in contact with the atmosphere before it is fully. oxidised. The slower the colour is dried, the better and finer is the lustre of the finished product.
Instead of adding the bleaching powder after preciating, it may be added to the iron solution to oxidise that to the ferric condition ; the blue obtained is not of so green a tint, being, if anything, a little more violet. *
If a blue with a violet tint is required it may be made by dissolving 1 cwt. of copperas in water and adding first 10 lbs. of sulphuric acid, and then a solution 1cwt. of the red prussiate of potash. The precipitate is collected, washed with water, and dried as before.
Chinese blue is mostly sold in the form of small cubical lumps, about 1 to 2 inches in diameter, but it is also sold in the form of fine powder. In grinding the blue great precaution is required to exclude particles of iron, as the production of a spark will ignite the dry powdered Chinese or Prussian blue and reduce it to a mass of red oxide of iron.
A sample of Chinese blue examined by the author had the following composition :Water, .
4:487 per cent. Oxide of iron,
PRUSSIAN BLUE.—The commoner makes of blue are sold under the names of Prussian blue, Berlin blue, paste blue, &c., in two forms, dry and pulp or paste. Some makes of these blues have a green shade, and others a violet shade or tint, owing to slight differences in the method of making.
Green-tint Blues.—There are several ways of making these blues. 1. Dissolving 1 cwt. each of yellow prussiate of potash and copperas in about 50 gallons of water, mixing the two solutions, allowing to settle, pouring off the clear top-liquor, washing the colour with water, then throwing it on to the filter and allowing it to be exposed to the air until it has acquired the desired blue ; to facilitate this the blue should be turned over from time to time so as to expose fresh surfaces to the action of the air.
A very fine and bronzy Chinese blue can be thus made :-- Dissolve 33 lbs. of copperas and 20 lbs. of sulphuric acid in sufficient water; add (1) a solution of 29 lbs. of potassium ferrocyanide, (2) a boiling solution of 4 lbs. of chromic acid; heat the mixture to the boil, filter off the blue, wash it well, and dry slowly
This method is not a good one as it leads to the production of oxide of iron in the colour, which affects the shade of the colour; it may be got rid of by treating the wet colour with hydrochloric acid, but this adds to the expense of making.
2. The blue is precipitated as before, but the colour is developed by adding bleaching powder. This process is identical with that used in making Chinese blue, but is less carefully carried out.
3. One cwt. of copperas, 20 lbs. of alum, and 10 lbs. of sulphuric acid are dissolved in water and a solution of 1 cwt. of yellow prussiate of potash added; the mixed solutions are allowed to stand for 2 or 3 hours, when they are finished as in No.1. The addition of alum makes the shade of blue lighter and, when dry, much easier to grind; the proportion of alum added varies with different makers; at one time comparatively large quantities were added, but the tendency has been of late years to reduce the proportion. When a solution of yellow prussiate is added to one of alum there is no immediate precipitate, but on standing for about an hour a bluish-white precipitate falls down, the nature of which is somewhat uncertain,
In the early days of Prussian-blue making blue-makers usually made their own prussiate, which, owing to the rough mode of preparation, yielded liquors containing a good many impurities, such as cyanides, sulphides, carbonates, and other salts of iron. The use of this crude liquor necessitated the use of much acid to prevent them from precipitating the iron in other forms than Prussian blue; as the makers objected to the use of much acid they added alum instead, but, as a result, they got a paler blue. Blue makers rarely use such crude leys now, as the refined crystal prussiate is cheap and makes better blues with less trouble.
Violet-tint Blues.—These are sometimes known as Paris blue. They are made by dissolving 1 cwt. each of the red prussiate of potash and copperas in water, adding the two solutions together, allowing the precipitated blue to settle, pouring off the topliquor, washing the residue with water, filtering, and drying the blue.
When sold as pulp blues or paste blues the blue is simply allowed to drain on the filter and not dried ; such pulp colours contain from 25 to 30 per cent. of dry colour, as a general rule, although as little as 17 per cent. has been found in some makes; such pulp colours should always be bought under a guarantee of the quantity of actual dry colour they contain.
SOLUBLE BLUE.-While Chinese and Prussian blues are ordinarily insoluble in water and acids, yet a variety of the blue
can be made which is soluble in water; so far as chemical composition is concerned this soluble blue does not differ from the insoluble varieties. Many recipes have been published for its preparation, of which the following are the principal :
1. Prepare a solution of 100 lbs. of perchloride of iron and 10 lbs. of Glauber's salt. A solution of 217 lbs. of yellow prussiate of potash and 10 lbs. of Glauber's salt is also prepared. The iron solution is poured into the prussiate solution, whereby a blue precipitate is obtained; this is collected on a filter and washed with water until the wash-waters are tinged with blue; it is then dried. In making soluble blue in this way it is important to pour the iron solution into the potash solution, and to keep the latter in excess. The object of adding the Glauber's salts is to ensure the complete precipitation of the blue by taking advantage of the fact that, while soluble in water, it is not soluble in saline solutions, so that by having the liquor saline there is a more complete precipitation of the blue.
2. Dissolve 72 lbs. of copperas in hot water, and pour this solution into a hot solution of 110 lbs. of red prussiate of potash, and boil the mixture for two hours, filter, wash until the washwaters have a blue colour, then dry the residual blue.
3. Take 100 lbs. of Prussian blue, mix well with about 100 gallons of water, and add 30 lbs of yellow prussiate of potash, boil well for 3 to 4 hours, drain on a filter, wash as before, and dry.
4. Dissolve separately in water 100 lbs. of yellow prussiate of potash, and 80 lbs. of copperas, add the two solutions together, and boil for 1 hour; then add 20 lbs. of nitric acid and 10 lbs. of sulphuric acid, and boil 1 hour longer; then filter, wash, and dry as before.
Soluble blue is not made on so large a scale now as formerly. It is used mostly for making blue ink and for painting velvets, for which purposes it has been replaced by the aniline blues.
ANTWERP BLUE.—This blue has practically gone out of use, its place having been taken by the Brunswick blue described below. Antwerp blue is made as follows :-20 lbs. of copperas, 10 lbs. of alum, and 10 lbs. of zinc sulphate are dissolved in 50 to 60 gallons of water, and to this solution is added one of 40 lbs. of the red or yellow prussiate of potash, dissolved in 50 to 60 gallons of water. The blue is finished in the ordinary way.
Antwerp blue is paler in colour than Prussian blue, and is probably a mixture of the ferrocyanides of iron, zinc, and alumina. Its properties are almost identical with those of Prussian blue.