As thus carried out the distillation yields:— Gallons Percent by weight.
Ammonitteal liquor, about .. • • .. 50 = First light oils .. • 20 = Second light oils .. • 20 = Creasote oils .. .. 250 = 22 Anthraocne oils .. 50 = 4 4 tons = 67100 The proportions of the various products obtained vary considerably with the character of the tax and the manner in which the details of the operation are conducted.
The still and condenser employed in the above operation will be more appropriately described in detail in the section treatiug of coal-tar distillation, but it may be said generally that the appa ratite; used consists of a vertical cylinder of cast or wrought iron, having a movable top and bottom, both of which are convex upwards. A pipe near the top of the still admits the tar from the tank, and a largo cock at the bottom suffices to run off the pitch at the end of the operation. The size and shape of stills used varies somewhat widely, but a desirable size is one which will work off a chargo of 1200 gallons of tar in the course of ten or twelve hours, thus avoiding night work. Such a still will be about 7 ft. in diameter, and nearly the same in height. The condenser is usually a 4-inch socket pipe arranged in a rectangular tank. About 140 or 150 ft. of condensing length is sufficient for a still of the above size.
In a well-known works, in which the manufacture of carbolic acid is of primary importance, 20 tons of gas-tar are worked on ; the first 200 gallons, consisting of "light benzols," &c., are of no use for the extraction of carbolic acid. When that measure of distillate has passed over, the shoots are changed, and the next 600 gallons are collected separately. This portion of the distillate cor responds closely with the " ereasote oils" of the process previously described, and it is from these products that carbolic acid is always obtained.
In some cases these eila are redistilled, and the portion passing over between 150° and 200° C. used for the manufacture of carbolic acid. More frequently they are treated directly in the following manner : Two hundred gallons of the oil having a density of 1.000 to are treated with alkali. 30 gallons of caustic soda solution of 1.34 specific gravity (= 68° Tw.) are diluted to 150 gallons
with water, the liquid thus obtained added to the oil, and the whole well agitated together for two hours. By this treatment the carbolic and cresylic acids become dissolved in the alkaline liquid, while the naphthalene and other hydrocarbons and oils of neutral character are left unacted on. The mixture is next allowed to settle for about four hours, when the alkaline solution is drawn off and neutralized with sulphuric acid. This causes a separation of the carbolic and cresylie acids from the aqueous liquid, on the top of which they form an oily layer. Time having been allowed for to separate thoroughly, it is skimmed off and allowed to settle in tanks for several days, when it is ready for tasking.
It is sometimes stated that hydrochloric acid may be substituted for the aulphurie, but this cannot be done with advantage.
The caustic soda employed must bo free from nitrates (which are commonly present in somo varieties), or on neutralization the nitric acid set free will act violently on the carbolic acid, produ cing awkward consequences.
The crude carbolic acid obtained as above contains a considerable but variable proportion of cresyl e acid, and sometimes of still higher homologues. When to be used simply as a disin fectant, the crude product is sufficiently pure, but for certain medical and other purposes a superior preparation is required.
The proportion of cresylic acid contained in crude carbolic acid may be ascertained approxi mately by the following process, which is also very similar to the method of purification adopted on a large scale.
One thousand fluid grains of the sample are placed in a retort and distilled, the liquid which passes over being collected in graduated tubes. Water first condenses, followed by an oily liquid. When 100 fluid grains of the latter have been collected the receiver is changed. The volume of water is then read off. if the oily liquid floats on the surface, it contains light neutral oils. It should be heavier than water, in which case it may be regarded as hydrous acid containing about 50 per cent. of real carbolic acid.