It may be purified by redistillation over barium and silver nitrates, followed by treatment of the distillate with a stream of ozonized air.
The composition of the distillate is determinate (by Avogadro's law) if the molecular weights and vapour pressure of the components at the temperature of distillation be known.
The boiling point, being determined by the character of the constituents of the oil, necessarily varies greatly in different oils, as do the amounts of distillate obtained from them at specified temperatures.
On standing, the distillate separates into two layers, an aqueous and an oily layer, the oil floating on or sinking through the water according to its specific gravity.
If a more dilute acid than this be distilled, water passes over in excess and the residue in the retort reaches the above composition and boiling point; on distillation of a stronger acid, excess of acid passes into the distillate and the boiling point rises until the values of the constant boiling mixture are reached.
The crude solid product from the tar distillate is digested with carbon bisulphide to dissolve the pyrene, the solution filtered and the solvent evaporated.
Benzene is manufactured from the low-boiling fractions of the coal-tar distillate '(see' Coal-Tar).
The first portion of the distillate brings over the gases dissolved in the water, ammonia and other volatile impurities, and is consequently rejected; scarcely two-fifths of the entire quantity of water can be safely used as pure distilled water.
The distillate is freed from vanadium by digestion with sodium amalgam.
In order to separate the distillate into various fractions, and to remove as much of it as possible free from condensed steam, it is now usual to employ condensing appliances of special form with outlets for running off the different fractions.