Paraffinum Durum, B.P., Hard Paraffin.
Hard paraffin, or paraffin wax (Paraffinum, U.S.P.: Paraffin), is a mixture of the harder members of the paraffin series of hydrocarbons, having the general formula CnH2n+2, ranging from C21H44 to C30H62. Hydrocarbons, however, other than those of the paraffin series, are present in notable proportions. It is chiefly obtained from the crude tarry oil produced by the destructive distillation of shale. The oil is redistilled, the distillate shaken with sulphuric acid to remove basic bodies and afterwards with solution of sodium hydroxide to remove acid substances and phenols. The product is washed, and on redistillation yields various kinds of burning and lubricating oils and finally a thick oil which deposits paraffin on cooling. The crude paraffin is separated by pressure and dissolved in naphtha, crystallised from the solution by refrigeration, and again separated by pressure, and is then melted, and filtered through animal charcoal to decolourise it. Hard paraffin occurs as a colourless, translucent, wax-like, odourless, tasteless solid, slightly unctuous to the touch. The B.P. specific gratify is given as 0.820 to 0.940 (U.S.P., 0.890 to 0.905 at 25°); melting-point, 54.4° to 57.2° (U.S.P., 51.6° to 57.2°). The melting-point and hardness increase as the molecular weight increases. It expands considerably on melting and in the molten state is a colourless oil; when heated strongly in the air it burns with a luminous flame, though not readily, liberating carbon, but ]caving no fixed residue. It is characterised by a marked indifference to most reagents. Concentrated sulphuric or nitric acids have no effect in the cold. If 5 decigrams be heated and 1 decigram of powdered fuchsia added to the melted substance, the latter should not assume a pink or red colour (absence of stearic acid).
Insoluble in water, slightly soluble in absolute alcohol and in ether; soluble in benzene, chloroform, carbon bisulphide, volatile oils, or warm fixed oils.
Uses.—Hard paraffin is employed principally in the preparation of ointment bases, for use with medicaments that are not required to be absorbed (see Unguentum Paraffini). Hard paraffin of melting-point 43.3° to 46°, sterilised by heat, is used in plastic operations., especially to correct nasal deformities. Paraffin of lower melting-point than the above is used for embedding substances for cutting microscopic sections. A hard paraffin (white) is prepared by purifying ozokerite, a natural mineral wax found in Galicia. It has a melting-point 54.5°, and is sold under the name "Ceresin." When artificially coloured it resembles yellow beeswax and is sold as "Yellow Ceresin."
PREPARATIONS.
- Massa Paraffini, B.P.C.—PARAFFIN MASS.
- Hard paraffin, 2; soft paraffin, white, 3. Used as a pill excipient for silver nitrate or oxide, potassium permanganate, gold chloride, etc.
- Unguentum Paraffini, B.P.—PARAFFIN OINTMENT.
- Hard paraffin, 30; soft paraffin 70. Melt the hard and soft paraffins together, remove from the source of heat, and stir constantly until cold. When paraffin ointment is used as the basis of white ointments, the white variety of soft paraffin should. be employed, and the yellow variety hen used as the basis of coloured ointments. The proportions of hard and soft paraffins may be modified to meet climatic conditions. This preparation is used as a basis for ointments that are required to exert a superficial effect only, not to be absorbed.
