190 POISONS : THEIR EFFECTS AND DETECTION. [§ 236 . 
and then a small quantity of sodic hypochlorite solution, avoiding 
excess, a blue colour appears ; warming quickens the reaction : the 
blue is permanent, but turns to red with acids. If there is a smaller 
quantity than the above proportion of acid, the reaction may be still 
produced feebly after standing for some time. 
3. Ferric Chloride. —One part of phenol in 3000 parts of water can 
be detected by adding a solution of ferric chloride ; a fine violet colour 
is produced. This is also a very good test when applied to a distillate ; 
but if applied to a complex liquid, the disturbing action of neutral salts 
and other substances may be too great to make the reaction under those 
circumstances of service. 
4. Bromine. The most satisfactory test of all is treatment of the 
liquid by bromine-water. A precipitate of tri-bromo-phenol (C 6 H 3 Br 3 0) 
is rapidly or slowly formed, according to the strength of the solution ; 
in detecting very minute quantities the precipitate must be given time to 
form. According to Allen, 1 a solution containing but of carbolic 
acid gave the reaction after standing twenty-four hours. 
The properties of the precipitate are as follows :—It is crystalline, 
and under the microscope is seen to consist of fine stars of needles ; its 
smell is peculiar ; it is insoluble in water and acid liquids, but soluble 
in alkalies, ether, and absolute alcohol; a very minute quantity of 
water suffices to precipitate it from an alcoholic solution ; it is therefore 
essential to the success of the test that the watery liquid to be examined 
be either neutral or acid in reaction. 
§ 236. Tri-bromo-phenol may be used for the quantitative estimation 
of carbolic acid ; 100 parts of tri-bromo-phenol are equal to 28*4 of 
carbolic acid ; by the action of sodium amalgam, tri-bromo-phenol is 
changed back into carbolic acid. 
That bromine-water precipitates several volatile and fixed alkaloids 
from their solutions is no objection to the bromine test, for it may be 
applied to a distillation product, the bases having been previously fixed 
by sulphuric acid. Besides, the properties of tri-bromo-phenol are 
distinct enough, and therefore there is no valid objection to the test. 
It is the best hitherto discovered. There are also other reactions, such 
as that Millon s reagent strikes a red—molybdic acid, in concentrated 
sulphuric acid, a blue—and potassic dichromate, with sulphuric acid, a 
biown colour ; but to these there are objections. Again, we have the 
Euchlorine test, in which the procedure is as follows :—A test tube is 
taken, and concentrated hydrochloric acid is allowed to act therein upon 
potassic chlorate. After the gas has been evolved for from 30 to 40 
seconds, the liquid is diluted with 1^ volume of water, the gas removed 
by blowing through a tube, and solution of strong ammonia poured in 
so as to form a layer on the top ; after blowing out the white fumes of 
1 Commercial Organic Analysis , vol. i. p. 306. 
