The Chemical Gazette 334, 15.9.1856
By G. E. Habich.
Paris blue is equally well adapted for a body colour as for mixtures, from its being readily ground up and possessing a considerable covering-power. With chrome-yellow it gives the admired green cinnabar, or the so-called sap-green. The author recommends the following processes for the preparation of this colour, as being cheaper than the old methods: —
1. Preparation with Chlorine (nitromuriatic acid).
As in the old processes, the white precipitate is first prepared with ferrocyanide of potassium and sulphate of iron. The latter must as far as possible be free from the persalt. For this purpose metallic iron is kept in the tubs in which the solution of sulphate of iron is set to clear, and this also precipitates any copper that may be in the solution. It is also necessary that the precipitation should be effected in the solution of ferrocyanide of potassium whilst still hot, in order to avoid as far as possible the absorption of oxygen, and the premature blue coloration of the precipitate produced thereby. For the same reason the filtration of the white precipitate is effected at once and as quickly as possible. It is only the blue produced by the action of chlorine, nitric acid, &c. upon the white precipitate that has the intensity of colour required by the colour manufacturers, whilst the precipitate which has become blue in the air, even when the intermixed hydrated peroxide of iron has been extracted from it by muriatic acid, always furnishes a colour with less body, which is never sufficient for the manufacture of green cinnabar.
When to 100 lbs. of ferrocyanide of potassium 90 lbs. of sulphate of iron are employed, a drop of the solution of iron no longer produces a precipitate in a filtered portion of the fluid; the white precipitate has then carried a quantity of ferrocyanide of potassium mechanically mixed with it to the bottom, which can be extracted by washing with water. This quantity of the expensive material would thus be partially lost, and the best way to avoid this loss is the following. The iron solution is added, with constant stirring, until no further precipitate is produced, and then a ninth part of the quantity of solution employed. If the mixture be now stirred for a quarter of an hour, there is a certainty that all the intermixed ferrocyanide of potassium is decomposed.
To give a blue colour to this precipitate, which is allowed to drain until it forms a thick paste, byt the agency of chlorine, a mixture of muriatic and nitric acids, prepared on the previous day, is employed. The quantity to be mixed must of course depend on the quantity of anhydrous acid contained in the commercial acids. The mixture is made so that it may contain 54 parts by weight of anhydrous nitric acid, and 36½ parts of anhydrous muriatic acid. To colour the white precipitate, such a quantity of this mixture is employed that for every 100 parts of ferrocyanide of potassium made use of in the precipitation, there may be 10 7/10 parts of anhydrous nitric acid in the mixture. Thus if we have a nitric acid of 30°B. (=spec. grav. 1.256 according to the tables in Graham's manual), and a muriatic acid of 23°B. (spec. grav. 1.185), the former will contain, according to Ure's tables, 35 4/10 per cent. of nitric acid, and the latter 37½ per cent. of muriatic acid. The mixture would be prepared with 100 lbs. of this nitric acid (containing 35 4/10 lbs. of anhydrous acid), and 64 2/10 lbs. of the muriatic acid (containing 23 9/10 lbs. of anhydrous acid); and of this mixture 49 lbs. would be sufficient to colour the precipitate from 100 lbs. of ferrocyanide of potassium. This quantity is gradually added, with conatant stirring, to the white precipitate contained in wooden tubs.
To determine whether the proper colour is attained, a little of the colour precipitate is put into a glass and a drop of the acid mixture is added to it. Of this sample a drop is rubbed upon white paper, and compared with a sample of the colour as it exists in the tub. If the addition of acid has increased the intensity of the colour, too little of the acid mixture has been used, and a further quantity must be added. But if the sample has acquired a greenish tinge, etiher the exact quantity or too much of the mixture has been used. To determine this, a fresh sample is put into the glass, and a drop of the white precipitate suspended in water is put in. If the intensity of the colour be increased, too much of the acid mixture has been employed, and this error is repaired by the addition of small quantities of the white precipitate until the highest degree of intensity is attained. The colour is then washed, &c. in the usual way.
2. Process with Perchloride of Iron.
In this the blue coloration of the white precipitate produced by ferrocyanide of potassium and sulphate of iron is effected by a solution of perchloride of iron; this is converted into protochloride of iron, which then serves in place of an addition of sulphate of iron. To prepare the perchloride of iron, an iron ore must be obtained as free as possible from alumina and lime, but it is of no consequence whether it be a red or a brown ironstone. If an iron ore of this kind cannot be procured, the residue from the manufacture of oil of vitriol, well known under the names of caput mortuum, colcothar, English rouge, &c., may be employed. The disposable oxide of iron from either of these sources is put in a finely powdered state into a wooden tub lined with thin sheet lead, and the ordinary muriatic acid (such as is furnished b the soda manufacturers) is poured over it. The mixture is allowed to stand for several days, with frequent stirring, and the supernatant fluid, which must have become saturated with oxide of iron, is drawn off into another vessel, in which it may become perfectly clear. This solution of perchloride of iron is kept constantly ready for the production of the blue colour.
A precipitate is prepared with ferrocyanide of potassium and sulphate of iron in the way already described; this is filtered, and the pasty residue is heated to boiling in a copper pot, when it is emptied into a tub standing under the crane of the kettle, and mixed with the perchloride of iron, stirring constantly until the highest intensity of colour is obtained. In this operation there is no occasion to be so careful as in the production of the blue colour with nitromuriatic acid, as an excess of perchloride of iron is not injurious to the purity of the colour, Perchloride of iron is therefore added until a slight excess of it is present, that is to say, until a filtered portion of the fluid gives a blue precipitate with a drop of solution of ferrocyanide of potassium. When this point is attained, the fluid is either filtered off, or the colour is merely allowed to settle, and the clear fluid is drawn off. This fluid, as already mentioned, is principally a solution of protochloride of iron. To convert it entirely into this, it is poured upon fragments of old iron, when it will soon serve in place of sulphate of iron for the precipitation of the ferrocyanide of potassium, a great advantage in this process of manufacture.
3 Process with Perchloride of Manganese.
The commercial value of the ores of manganese depends upon the amount of peroxide of manganese which they contain. The ordinary ores however generally contain a considerable quantity of oxide of manganese, which can be extracted from them by cold muriatic acid; and this will of course increase their value, and at the same time furnish a new material for this manufacture. The process is exactly the same as with perchloride of iron; but as the supernatant solution of proto chloride of manganese is of no particular value, the employment of superfluous perchloride in the production of the blue colour must be carefully avoided; and it is necessary, during the gradual addition of this agent, to determine the exact point at which the greatest intensity of colour is attained by the comparison of samples. In consequence of the ready decomposability of the perchloride of manganese, this comparison is the only way by which the object can be attained. The other manipulations are as usual.
The residue of the ore, after treatment with muriatic acid, is of course carefully washed and dried before it is sold as peroxide of mauganese.
4. Process with Chromic Acid.
This method, like the preceding, is only to be recommended under certain circumstances, as the chrome-salts produced are not of much value. 10 parts by weight of red chromate of potash are dissolved in about 10 times their weight of hot water, and after cooling mixed with 13½ parts by weight of English sulphuric acid. This mixture is preserved for use in well-closed carboys. When it is to be employed, the precipitate from ferrocyanide of potassium and sulphate of iron is boiled, and the solution of chromic acid is added to it until the greatest intensity of colour is produced.
In the preparation of Paris blue in many manufactories, the ferro cyanide of potassium is simply precipitated by the addition of sulphate of iron as long as a precipitate is formed, the mixture ren dered blue by the action of the air, and washed. By this means a considerable loss of the valuable ferrocyanide of potassium takes place. It is established by exact chemical experiments, that 50 per cent. of the ferrocyanide of potassium employed in the precipitation of a protosalt of iron enters with the whole of the potash into the white precipitate, and that the greater part of this dissolves again when the precipitate is rendered blue by the action of the air, and is thus lost during washing. This loss is avoided, at least for the most part, when the blue is produced by any of the above methods, and no loss need be sustained if the filtrates from the white precipitate be collected and precipitated with sulphate of iron.
— Polytechn. Journ., cxxxviii. p. 295.