NITROBENZENE. 
§§ 242-244.] 
T 95 
Any excess of bromine liberated in the first reaction above that necessary for the 
second, will exist in the free state, and from the amount of bromine which remains 
freo the quantity of carbolic acid can be calculated, always provided the stiength of 
the bromine solution is first known. The volumetric part of the analysis, therefore, 
merely amounts to the determination of free bromine, which is best found by causing 
it to react on potassium iodide, and ascertaining the amount of free iodine by titra¬ 
tion with a standard solution of sodium thiosulphate. In other words, titrate in 
this way the standard alkaline bromine solution, using as an indicator starch paste 
until the blue colour disappears. Another method of indicating the end of the 
reaction is by the use of strips of paper first soaked in starch solution, and dried, and 
then the same papers moistened with zinc iodide, and again dried ; the least excess 
of bromine sets free iodine, and strikes a blue colour. 
Colorimetric Method of Estimation. —A very simple and ever-ready way of 
approximately estimating minute quantities of the phenols consists in shaking up 
10 grms. of the sample with water, allowing any tar or insoluble impurities to sub¬ 
side. Ten c.c. of the clear fluid are then taken, and half a c.c. of a 5 per cent, solu¬ 
tion of ferric chloride added. The colour produced is imitated by a standard solution 
of carbolic acid, and a similar amount of the reagent, on the usual principles of colori¬ 
metric analysis. 
§ 242. Carbolic Acid Powders. —Siliceous carbolic acid powders are placed in 
a retort and distilled. Towards the end the heat may be raised to approaching red¬ 
ness. The distillate separates into two portions—the one aqueous, the other con¬ 
sisting of the acids—and the volume may be read off, if the distillate be received in 
a graduated receiver. Carbolic acid powders, having lime as a basis, may be dis¬ 
tilled in the same way, after first decomposing with sulphuric acid. The estimation 
of the neutral tar oils in the distillate is easily performed by shaking the distillate 
with caustic soda solution, which dissolves completely the tar acids. The volume of 
the oils may be directly read off if the receiver is a graduated tube. Allen 1 has 
suggested the addition of a known volume of petroleum to the distillate, which dis¬ 
solves the tar oils, and easily separates, and thus the volume may be more accurately 
determined, a correction being of course made by subtracting the volume of petroleum 
first added. 
§ 243. Carbolic Acid Soap. —A convenient quantity of soap is carefully weighed, 
and dissolved in a solution of caustic soda by means of heat. A saturated solution 
of salt is next added, sufficient to precipitate entirely the soap, which is filtered off ; 
the filtrate is acidified with hydrochloric acid, and bromine-water added. The preci¬ 
pitated tri-bromo-phenol is first melted by heat, then allowed to cool, and the mass 
removed from the liquid, dried, and weighed. 
X.—Nitro-benzene. 
§ 244.—Nitro-benzene is the product resulting from the action of 
strong nitric acid on benzene. Its chemical formula is C 6 H 5 N0 2 . When 
pure, it is of a pale yellow colour, of a density of 1-186, and boils at from 
205° to 210°. It may be obtained in prismatic crystals by exposure to 
a temperature of 3°. Its smell is exactly the same as that from the oil 
or essence of bitter almonds ; and it is from this circumstance, under 
the name of “ essence of mirbane,” much used in the preparation of 
perfumes and flavouring agents. 
In commerce there are three kinds of nitro-benzene—the purest, with 
the characters given above ; a heavier nitro-benzene, boiling at 210° to 
1 Op. cit., vol. i. p. 310. 
