Zinc, Phosphide. Fragments of pure distilled zinc are introduced into a tubulated stoneware retort, so as to occupy about one fourth of its capacity; the retort is placed in an ordinary furnace, and a current of dry carbonic acid is passed into it through the neck. Over the tubulure is placed a crucible cover, so as to close the orifice incompletely, and allow the carbonic acid, after traversing the retort, to escape. When the zinc enters into ebullition, small fragments of previously-dried phosphorus are successively thrown in through the tubulure. The cover is removed and returned after each addition to prevent loss of phosphorus. From time to time it is necessary to break the crust of phosphorus formed, in order to expose a new layer of metal to the action of the phosphorus. The operation is terminated by increasing the heat strongly—a precaution that is indispensable, in order to separate as completely as possible the zinc phosphide from the metallic button of nearly pure zinc, which collects at the bottom of the retort. Further, the zinc phosphide
should be reduced to very fine powder, and the fragments which resist the action of the metal, however slightly, should be reserved for another operation. The product pulverises the more readily in proportion as it approaches more closely the formula P2Zn3. In this state it resembles iron reduced by hydrogen, and only thus should it be used by pharmaceutists. It is completely soluble in hydrochloric acid.[274]
[274] From ‘Formulæ for New Medicaments,’ adopted by the Paris Pharmaceutical Society.
Zinc, Sulphate of. ZnSO4. Syn. White copperas*; White vitriol*; Zinci sulphus (B. P., Ph. L., E., & D.), L. Prep. 1. (Ph. L.) Granulated zinc, 5 oz.; diluted sulphuric acid, 1 quart; dissolve, filter. Evaporate to a pellicle, and set it aside to crystallise.
2. (Ph. D.) Zinc (laminated or granulated), 4 oz.; sulphuric acid, 3 fl. oz.; water, 1 pint; mix in a porcelain capsule, and, when gas ceases to be evolved, boil for 10 minutes, filter through calico, and evaporate the filtrate to dryness; dissolve the dry salt in water, 1 pint; frequently agitate the solution, when cold, during 6 hours, with prepared chalk, 1⁄4 oz.; next filter, acidulate the filtrate with nitric acid and dilute sulphuric acid, of each 1 fl. dr.; evaporate until a pellicle forms on the surface, and set it aside to crystallise; dry the crystals on bibulous paper, without heat, and preserve them in a bottle. More crystals may be obtained from the mother-liquor.
3. (B. P.) Pour 12 fl. oz. of sulphuric acid, previously mixed with 4 pints of distilled water, on 16 oz. of granulated zinc contained in a porcelain basin, and when effervescence has nearly ceased, aid the action by a gentle heat. Filter the fluid into a gallon bottle, and add, gradually, with constant agitation, solution of chlorine until the fluid acquires a permanent odour of chlorine. Add now, with continued agitation, 1⁄2 oz. or q. s. of carbonate of zinc, until a brown precipitate appears; let it settle, filter the solution, evaporate till a pellicle forms on the surface, and set aside to crystallise. Dry the crystals by exposure to air on filtering paper placed on porous tiles. More crystals may be obtained by again evaporating the mother-liquor.
4. The common sulphate of zinc of commerce frequently contains copper, cadmium, lead, iron, and manganese, and nearly always one or more of them. By digesting its concentrated solution for some time with metallic zinc, it may be freed from copper, lead, and cadmium, for these metals are all reduced and precipitated in a metallic state; or, the acidulated solution may be treated with sulphuretted hydrogen as long as any precipitate forms. In order to separate the iron, chlorine gas may be passed into the solution, by which the iron is converted into the ferrous chloride; if this solution be exposed to the air for a
length of time, it absorbs oxygen, and oxide of iron is deposited as a yellow powder, from which the solution must be filtered. When the sulphate contains manganese, which is not very often the case, the solution must be boiled up a few times with purified charcoal, filtered, and evaporated. (‘Jour. für prakt. Chem.’) The product of each of the above formulæ is nearly chemically pure.
5. (Commercial.) The crude sulphate of zinc (white copperas, or white vitriol, of the shops) is prepared by roasting native sulphide of zinc (zinc blende) in a reverberatory furnace, exposing the calcined mass to the air and humidity for some time, then lixiviating it, and evaporating the resulting solution until it forms a white semi-crystalline mass on cooling.
Prop. Pure sulphate of zinc forms inodorous, colourless, transparent, quadrangular prisms, closely resembling in appearance those of Epsom salt, which effloresce slightly in the air, and contains 7 equiv. of water; it has a slightly acidulous and very styptic metallic taste; the crystals dissolve in 21⁄2 parts of cold and in less than their own weight of boiling water; they are insoluble in alcohol. The crude sulphate of zinc of commerce (white vitriol) occurs in irregular granular masses, which somewhat resemble loaf sugar.