PRUSSIAN BLUE

and PRUSSIATE OF POTASH, are two important articles of chemical manufacture, which must be considered together. The first is called by English chemists, Ferrocyanodide of iron , the Cyanure ferroso-ferrique of Berzelius; Eisenblausaures eisenoxyd , or eisencyanür + eisencyanid , Germ.; the second is called Ferrocyanodide of pota ium , the Cyanure ferroso-pota i que of Berzelius; Eisencyanür-kalium , cyaneisen + cyan kalium , or Blausaures eisenoxydul-kali , Germ. Pru ian blue ( Berliner-blau , Germ.), is a chemical compound of iron and cyanogen. When organic matters abounding in nitrogen, as dried blood, horns, hair, skins, or hoofs of animals, are triturated along with potash in a strongly ignited iron pot, a dark gray ma is obtained, that affords to water the liquor originally called lixivium sanguinis , or blood-lye, which, by evaporation, yields lemon-coloured crystals in large rectangular tables, bevelled at the edges. This salt is called in commerce, pru iate of potash, and has for its ultimate constituents, pota ium, iron, oxygen, and hydrogen, (the latter two in such proportions as to form water), and the peculiar compound Cyanogen , the blaustoff of the Germans. These crystals consist, in 100 parts, of pota ium 37·02, iron 12·82, cyanogen 37·40, water 12·76; or, cyanide of pota ium 61·96, cyanide of iron 25·28, and water 12·76. They may be represented also by the following composition: 44·58 of pota a, 38·82 of hydrocyanic or pru ic acid, and 16·60 of oxide of iron, in 100 parts; but the first appears to be their true chemical constitution. Dry ferrocyanodide of pota ium, is a compound of, one atom of cyanide of iron, 54 = (28 + 26), and 2 atoms of cyanide of pota ium, 132, = ( 26 × 2 + 40 × 2 ); the sum being 186; hydrogen being 1·0 in the scale of equivalents. The crystals of pru iate of potash are nearly transparent, soft, of a sweetish saline and somewhat bitterish taste, soluble in 4 parts of water at 52° F., and in 1 part of boiling water, but insoluble in alcohol. They are permanent in the air at ordinary temperatures, but in a moderately warm stove-room they part with 12 3 ⁄ 4 per cent. of water, without losing their form or coherence, and become thereby a white friable anhydrous ferrocyanodide of pota ium, consisting of 42·44 pota ium, 42·87 cyanogen, and 14·69 iron, in 100 parts. This salt is an excellent reagent for distinguishing metals from each other, as the following Table of the precipitates which it throws down from their saline solutions will show :— | Metallic solutions. | Colour of precipitate. | | Antimony | white. | | Bismuth | white. | | Cadmium | white, a little yellowish. | | Cerium (protoxide) | white, soluble in acids. | | Cobalt | green, soon turning reddish-gray. | | Copper (protoxide) | white, changing to red. | | Co Do. r (peroxide) | brown-red. | | Iron (protoxide) | white, rapidly turning blue. | | I Do. (peroxide) | dark blue. | | Lead | white, with a yellowish cast. | | Manganese (protoxide) | white, turning quickly peach or blood-red. | | Manganese (deutoxide) | greenish-gray. | | Mercury (protoxide) | white. | | Me Do. ry (peroxide) | white, turning blue. | | Molybdenum | dark brown. | | Nickel (oxide) | white, turning greenish. | | Palladium (protoxide) | green (gelatinous). | | Silver | white, turning brown in the light. | | Tantalum | yellow, dark burned colour. | | Tin (protoxide) | white, (gelatinous). | | Do. (peroxide) | yellow, per do. | | Uranium | red-brown. | | Zinc | white. | No precipitations ensue with solutions of the alkaline or earthy salts, except that of yttria, which is white; nor with those of gold, platinum, rhodium, iridium, osmium, (in concentrated solutions,) tellurium, chromium, tungstenium. All the precipitates by the ferrocyanodide of iron, are double compounds of cyanide of iron with cyanide of cyanide of pota ium and the peculiar metallic oxide present in the solution. The precipitate from the sulphate of copper has a fine brown colour, and has been used as a pigment; but it is somewhat transparent, and therefore does not cover well. The precipitate from the peroxide salts of iron is a very intense pru ian blue, called on the continent, Paris blue. It may be regarded as a compound of pru iate of protoxide and pru iate of peroxide of iron; or as a double cyanide of the protoxide and peroxide of iron, as the denomination cyanure ferroso-ferrique denotes. In numbers, its composition may be therefore stated thus: pru ic or hydrocyanic acid, 48·48; protoxide of iron, 20·73; peroxide of iron, 30·79; or cyanogen, 46·71; iron, 37·36; water, 15·93; which represent its constitution when it is formed by precipitation with the pru iate of potash or a salt of iron that contains no protoxide. If the iron be but partially peroxidized in the salt, it will afford a precipitate, at first pale blue, which turns dark blue in the air, consisting of a mixture of pru iate of protoxide and pru iate of peroxide. In fact, the white cyanide of iron (the pru iate of the pure protoxide), when exposed to the air in a moist condition, becomes, as above stated, dark blue; yet the new combination formed in this case through absorption of oxygen, is e entially different from that resulting from the precipitation by the peroxide of iron, since it contains an exce of the peroxide in addition to the usual two cyanides of iron. It has been therefore called basic pru ian blue, and, from its di olving in pure water, soluble pru ian blue. Both kinds of pru ian blue agree in being void of taste and smell, in attracting humidity from the air when they are artificially dried, and being decomposed at a heat above 348° F. The neutral or insoluble pru ian blue is not affected by alcohol; the basic, when di olved in water, is not precipitated by that liquid. Neither is acted upon by dilute acids; but they form with concentrated sulphuric acid a white pasty ma , by strong sulphuric acid at a boiling heat, and by strong nitric acid at common temperatures; but they are hardly affected by the muriatic. They become green with chlorine, but resume their blue colour when treated with dis oxidizing reagents. When pru ian blue is digested in warm water along with potash, soda, or lime, peroxide of iron is separated, and a ferropru iate of potash, soda, or lime remains in solution. If the pru ian blue has been previously purified by boiling in dilute muriatic acid, and washing with water, it will afford by this treatment a solution of ferrocyanodide of pota ium, from which by evaporation this salt may be obtained in its purest crystalline state. When the powdered pru ian blue is diffused in boiling water, and digested with red oxide of mercury, it parts with all its oxide of iron, and forms a solution of bi-cyanodide, improperly called pru iate of mercury; consisting of 79·33 mercury, and 20·67 cyanogen; or, upon the hydrogen equivalent scale, of 200 mercury, and 52 = (26 × 2) cyanogen. When this salt is gently ignited, it affords gaseous cyanogen. Hydrocyanic or pru ic acid, which consists of 1 atom of cyanogen = 26, + 1 of hydrogen = 1, is prepared by distilling the mercurial bi-cyanide in a gla retort with the saturating quantity of dilute muriatic acid. Pru ic acid may also be obtained by precipitating the mercury by sulphuretted hydrogen gas from the solution of its cyanide; as also by distilling the ferrocyanide of pota ium along with dilute sulphuric acid. Pru ic acid is a very volatile light fluid, eminently poisonous, and is spontaneously decomposed by keeping, especially when somewhat concentrated. Having expounded the chemical constitution of pru ian blue and pru iate of potash, I shall now treat of their manufacture upon the commercial scale . 1. Of blood-lye , the phlogisticated alkali of Scheele. Among the animal substances used for the preparation of this lixivium, blood deserves the preference, where it can be had cheap enough. It must be evaporated to perfect dryne , reduced to powder, and sifted. Hoofs, parings of horns, hides, old woollen rags, and other animal offals, are, however, generally had recourse to, as condensing most azotized matter in the smallest bulk. Dried funguses have been also prescribed. These animal matters may either be first carbonized in cast-iron cylinders, as for the manufacture of sal ammoniac (which see), and the residual charcoal may be then taken for making the ferropru iate; or the dry animal matters may be directly employed. The latter proce is apt to be exceedingly offensive to the workmen and neighbourhood, from the nauseous vapours that are exhaled in it. Eight pounds of horn (hoofs), or ten pounds of dry blood, afford upon an average one pound of charcoal. This must be mixed well with good pearlash, (freed previously from most of the sulphate of pota a, with which it is always contaminated,) either in the dry way, or by soaking the bruised charcoal with a strong solution of the alkali; the proportion being one part of carbonate of pota a to from 1 1 ⁄ 2 to 2 parts of charcoal, or to about eight parts of hard animal matter. Gautier has proposed to calcine three parts of dry blood with one of nitre; with what advantage to the manufacturer, I cannot discover. IMG:4147767755307473660_illo1042.png:Calcination pot The pot for calcining the mixture of animal and alkaline matter is egg-shaped as represented at a , fig. 928 , and is considerably narrowed at the neck e , to facilitate the closing of the mouth with a lid i . It is made of cast iron, about two inches thick in the belly and bottom; this strength being requisite because the chemical action of the materials wears the metal fast away. It should be built into the furnace in a direction sloping downwards, (more than is shown in the figure,) and have a strong knob b , projecting from its bottom to support it upon the back wall, while its shoulder is embraced at the arms c , c , by the brickwork in front. The interior of the furnace is so formed as to leave but a space of a few inches round the pot, in order to make the flame play closely over its whole surface. The fire-door f , and the draught-hole z , of the ash-pit, are placed in the posterior part of the furnace, in order that the workmen may not be incommoded by the heat. The smoke vent o , i ues through the arched top h of the furnace, towards the front, and is thence led backwards by a flue to the main chimney of the factory. d is an iron or stone shelf, inserted before the mouth of the pot, to prevent lo in shovelling out the semi-liquid paste. The pot may be half filled with the materials. The calcining proce is different, according as the animal substances are fresh or carbonized. In the first case, the pot must remain open, to allow of diligent stirring of its contents, with a slightly bent flat iron bar or scoop, and of introducing more of the mixture as the intumescence subsides, during a period of five or six hours, till the nauseous vapours cease to rise, till the flame becomes smaller and brighter, and till a smell of ammonia be perceived. At this time, the heat should be increased, the mouth of the pot should be shut, and opened only once every half hour, for the purpose of working the ma with the iron paddle. When on opening the mouth of the pot, and stirring the pasty mixture, no more flame rises, the proce is finished. If the animal ingredients are employed in a carbonized state, the pot must be shut as soon as its contents are brought to ignition by a briskly urged fire, and opened for a few seconds only every quarter of an hour, during the action of stirring. At first, a body of flame bursts forth every time that the lid is removed; but by degrees this ceases, and the mixture soon agglomerates, and then softens into a paste. Though the fire be steadily kept up, the flame becomes le and le each time that the pot is opened; and when it ceases, the proce is at an end. The operation, with a ma of 50 pounds of charcoal and 50 pounds of purified pearlash, lasts about 12 hours, the first time that the furnace is kindled; but when the pot has been previously brought to a state of ignition, it takes only 7 or 8 hours. In a well-appointed factory, the fire should be invariably maintained at the proper pitch, and the pots should be worked with relays of operatives. The molten ma is now to be scooped out with an appropriate iron shovel, having a long shank, and caused to cool in small portions, as quickly as po ible; but not by throwing it into water, as has sometimes been prescribed; for in this way a good deal of the cyanogen is converted into ammonia. If it be heaped up and kept hot in contact with air, some of the ferrocyanide is also decomposed, with diminution of the product. The crude ma is to be then put into a pan with cold water, di olved by the application of a moderate heat, and filtered through cloths. The charcoal which remains upon the filter po e es the properties of decolouring syrups, vinegars, ., and of destroying smells in a pre-eminent degree. It may also serve, when mixed with fresh animal coal, for another calcining operation. As the iron requisite for the formation of the ferrocyanide is in general derived from the sides of the pot, this is apt to wear out into holes, especially at its under side, where the heat is greatest. In this event, it may be taken out of the furnace, patched up with iron-rust cement, and re-inserted with the sound side undermost. The erosion of the pot may be obviated in some measure by mixing iron borings or cinder (hammerschlag) with the other materials, to the amount of one or two hundredths of the potash. The above lixivium is not a solution of pure ferropru iate; it contains not a little cyanide of pota ium, which in the course of the proce had not absorbed the proper dose of iron to form a ferrocyanide; it contains also more or le carbonate of potash, with phosphate, sulphate, hydrogenated sulphuret, muriate, and sulpho-cyanide of the same base, as well as phosphate of lime; substances derived partly from the impure potash, and partly from the incinerated animal matters. Formerly that very complex impure solution was employed directly for the precipitation of pru ian blue; but now, in all well regulated works, it is converted by evaporation and cooling into crystallized ferropru iate of potash. The mother-water is again evaporated and crystallized, whereby a somewhat inferior ferropru iate is obtained. Before evaporating the lye, however, it is advisable to add as much solution of green sulphate of iron to it, as will re-di olve the white precipitate of cyanide of iron which first falls, and thereby convert the cyanide of pota ium, which is present in the liquor, into ferrocyanide of pota ium. The commercial pru iate of potash may be rendered chemically pure by making its crystals effloresce in a stove, fusing them with a gentle heat in a gla retort, di olving the ma in water, neutralizing any carbonate and cyanide of potash that may be present with acetic acid, then precipitating the ferropru iate of potash by the addition of a sufficient quantity of alcohol, and finally crystallizing the precipitated salt twice over in water. The sulphate of pota a may be decomposed by acetate of baryta, and the resulting acetate of pota a removed by alcohol. 2. The precipitation of pru ian blue. —Green sulphate of iron is always employed by the manufacturer, on account of its cheapne , for mixing with solution of the ferropru iate, in forming pru ian blue, though the red sulphate, nitrate, or muriate of iron would afford a much richer blue pigment. Whatever salt of iron be preferred, should be carefully freed from any cupreous impregnation, as this would give the pure blue a dirty brownish cast. The green sulphate of iron is the most advantageous precipitant, on account of its affording protoxide, to convert into ferrocyanide any cyanide of pota ium that may happen to be present in the uncrystallized lixivium. The carbonate of potash in that lixivium might be saturated with sulphuric acid before adding the solution of sulphate of iron; but it is more commonly done by adding a certain portion of alum; in which case, alumina falls along with the pru ian blue; and though it renders it somewhat paler, yet it proportionally increases its weight; whilst the acid of the alum saturates the carbonate of potash, and prevents its throwing down iron-oxide, to degrade by its brown-red tint the tone of the blue. For every pound of pearlash used in the calcination, from two to three pounds of alum are employed in the precipitation. When a rich blue is wished for, the free alkali in the pru ian lye may be partly saturated with sulphuric acid, before adding the mingled solutions of copperas and alum. One part of the sulphate of iron is generally allowed for 15 or 20 parts of dried blood, and 2 or 3 of horn-shavings or hoofs. But the proportion will depend very much upon the manipulations; which, if skilfully conducted, will produce more of the cyanides of iron, and require more copperas to neutralize them. The mixed solutions of alum and copperas should be progre ively added to the lye as long as they produce any precipitate. This is not at first a fine blue, but a greenish gray, in consequence of the admixture of some white cyanide of iron; it becomes gradually blue by the absorption of oxygen from the air, which is favoured by agitation of the liquor. Whenever the colour seems to be as beautiful as it is likely to become, the liquor is to be run off by a spigot or cock from the bottom of the precipitation vats, into flat cisterns, to settle. The clear supernatant fluid, which is chiefly a solution of sulphate of potash, is then drawn off by a syphon; more water is run on with agitation to wash it, which after settling is again drawn off; and whenever the washings become tastele , the sediment is thrown upon filter sieves, and exposed to dry, first in the air of a stove, but finally upon slabs of chalk or Paris plaster. But for several purposes, pru ian blue may be best employed in the fresh pasty state, as it then spreads more evenly over paper and other surfaces. A good article is known by the following tests: it feels light in the hand, adheres to the tongue, has a dark lively blue colour, and gives a smooth deep trace; it should not effervesce with acids, as when adulterated with chalk; nor become pasty with boiling water, as when adulterated with starch. The Paris blue, prepared without alum, with a peroxide salt of iron, displays, when rubbed, a copper-red lustre, like indigo. Pru ian blue, degraded in its colour by an admixture of free oxide of iron, may be improved by digestion in dilute sulphuric or muriatic acid, washing, and drying. Its relative richne in the real ferropru iate of iron may be estimated by the quantity of potash or soda which a given quantity of it requires to destroy its blue colour. Sulphuretted hydrogen pa ed through pru ian blue diffused in water, whitens it; while pru ic acid is eliminated, sulphur is thrown down, and the sesquicyanide of iron is converted into the single cyanide. Iron and tin operate in the same way. When pru ian blue is made with two atoms of ferrocyanide of pota ium, instead of one, it becomes soluble in water. For the mode of applying this pigment in dyeing, see Calico-printing . Sesquiferrocyanate of potash , is prepared by pa ing chlorine gas through a solution of ferrocyanide of pota ium, till it becomes red, and ceases to precipitate the peroxide salts of iron. The liquor yields, by evaporation, prismatic crystals, of a ruby-red transparency. They are soluble in 38 parts of water, and consist of 40·42 parts of sesquicyanide of iron, and 59·58 of cyanide of pota ium. The solution of this salt precipitates the following metals, as stated in the table :— | Bismuth | pale yellow. | | Cadmium | yellow. | | Cobalt | dark brown red. | | Copper (protoxide) | red brown. | | Co Do. r (peroxide) | yellow green. | | Iron, protoxide salts of | blue. | | Manganese | brown. | | Mercury (protoxide) | red brown. | | Mercury (peroxide) | yellow. | | Molybdenum | red brown. | | Nickel | yellow green. | | Silver | red brown. | | Tin (protoxide) | white. | | Uranium | red brown. | | Zinc | orange yellow. | PUMICE-STONE ( Pierre-ponce , Fr.; Bimstein , Germ.); is a spongy, vitreous-looking mineral, consisting of fibres of a silky lustre, interlaced with each other in all directions. It floats upon water, is harsh to the touch, having in ma a mean sp. grav. of 0·914; though brittle, it is hard enough to scratch gla and most metals. Its colour is usually grayish white; but it is sometimes bluish, greenish, reddish, or brownish. It fuses without addition at the blowpipe into a white enamel. According to Klaproth, it is composed of, silica, 77·5; alumina, 17·5; oxide of iron, 2; pota a and soda, 3; in 100 parts. The acids have hardly any action upon pumice-stone. It is used for polishing ivory, wood, marble, metals, gla , .; as also skins and parchment. Pumice-stone is usually reckoned to be a volcanic product, resulting, probably, from the action of fire upon obsidians. The chief localities of this mineral are, the islands of Lipari, Ponza, Ischia, and Vulcano. It is also found in the neighbourhood of Andernach, upon the banks of the Rhine, in Teneriffe, Iceland, Auvergne, . It is sometimes so spongy as to be of specific gravity 0·37.