The next portion of the distillate consists of absolute acid, and when it measures 625 grains the receiver is again changed. The residue in the retort consists wholly of eresylic acid and still higher homologues of carbolic acid. The 625 grains measure of absolute acid contains variable proportions of carbolic and cresylie acid. These may be approximately estimated by determining its solidify ing poiut, which should be between 15° and 24° (59° and 75° F.). Having ascertained this temperature, a mixture of pure carbolic and cresylie acids is made io such proportions as to have the same solidifying point. This must be adjusted by trial ; or a series of standard specimens may be prepared. The exact point of solidification can be more sharply read if a minute fragment of crystallized carbolic acid be added to induce the commencement of the change of state ; or the sample may he solidified, and the liquefying point noted.
As excess of cresylic acid prevents solidification, in some cases a second fractional distillation should be resorted to, the process being arrested when the thermometer rises to O.
The same principle as the above is adopted for the purification of carbolic acid on a large scale. Crusylic acid boiling at a considerably higher temperature than carbolic, a tolerably perfect separation of the two acids can be effected by repeated fractional distillation.
Cresylie acid being itself a liquid, the presence of it in carbolic acid tends to lower the melting , • t so that the inferior grades of carbolic acid aro liquid at ordinary temperatures. Another cause. for their fluidity is the presence of water, which is taken up by the coal-tar acids to a greater extent than is commonly supposed. The best test fcr the freedom of carbolic from cresylic acid is to drive off any water by boiling the sample, in a tube fora few minutes, and to ascertain the solidifying point of the remainder. When pure, carbolic acid solidifies at about 42° (108° F.).
In addition to cresylic acid, which is a normal constituent of erude carbolic acid, the commercial article is frequently largely adulterated with neutral coal-tar oils (naphthalene oils), which are of very little value as disinfectants. If purified by solution in alkali, carbolic acid cannot contain more than traces of these bodies; hence their presence in quantity is always due either to imperfect manufacture or to their subsequent addition as adulterants. Specimens of so-called " earbolic acid" are sometimes sold containing only a few percentages of real coal-tar acida. These products
are probably the refuse neutral oils resulting from the treatment of the anthracene oils with soda.
The presence of neutral coal-oils in carbolic acid may be detected, and their quantity estimated with sufficient aoeuracy by the following simple process.
Into a graduated tube pour 100 fluid grains of the sample to be tested, and add gradually— noting the effect produeed—twice its volume of a solution of eaustic soda (free from aliimioa), containing 9 per cent. of real hydrate of soda. A solution of this strength will have a density of about 1'090, or 18° Tw. The tube is then closed and well agitated. The coal-tar acids will be completely dissolved by the alkaline liquid, whilst on standing any neutral oils used as adulterants will form a separate layer above or below the other according as the admixture consisted of the light or the heavy oil of tar. By the volume occupied by the oily stratum the extent of the adulteration is at once indicated. After noticing whether the tar-oil is "light" or "heavy," a volume of petroleum spirit (commercial " benzolene ") equal to that of the sample taken, may be advantageously added. Its employment facilitates the separation of the oily stratum and renders the reading off of its volume more easy and accurate. Of course, the volume of petroleum spirit used must be deducted from that of the total oily layer.
Carbolic acid has met with numerous applications, and the number of its uses is continually extending.
When heated with oxalic acid it yields the colouring matter known as aurin, or roadie acid.
By the action of nitric acid, carbolic acid yields picric acid, which has met with extensive application as a dye and an explosive agent.
When acted on by sulphuric acid, carbolic acid forms sulpho-phenic acid, the salts of which have been extensively employed in medicine.
By combining carbolic acid with soda, and acting on the heated product with carbonic acid gas, salicylic acid is generated, and is now manufactured extensively, by this reaction.
The purer varieties of carbolic acid are largely employed in medicine. It is used both internally and externally, and is employed in the form of ointment, spray, and in various other ways.
The valuable antiseptic properties possessed by carbolic and cresylic adds have caused them to be very widely used for disinfectant purposes. For this purpose they have been employed in the liquid state, in the form of soap, and as disinfecting powders. The preparation of the latter is now extensively carried on.