M. C. Lea on the detection of Iodine. 109 
Art. XX.—On the detection of Iodine ; by M. Carry LEA. 
WHERE iodine exists in the form of hydriodie acid, or the 
iodid of a base, two methods are commonly employed to put it 
into a condition to be detected by the starch test. One of these 
is by the action of nitric acid, the other by chlorine or bromine 
water. The latter is the more delicate, but has the disadvantage 
that if the chlorine or bromine be added in excess, the reaction 
is missed. 
It occurred to me while engaged in testing for iodine, that the 
facility with which that body is eliminated from its hydrogen | 
and metallic combinations by chromic acid would make the latter 
substance a valuable means of bringing about the starch reac- 
tion, and a few experiments completely confirmed this view. 
If, for example, we take an extremely dilute solution of iodid 
of potassium, such that the addition of nitric acid and starch pro- 
duces no perceptible effect, the further addition of a single drop 
of very dilute solution of bichromate of potash will instantly 
bring about the characteristic reaction. 
When chlorhydric acid is substituted for nitric, the effect of 
the bichromate is (as was to be expected) still more marked. 
The test has then the full delicacy at least of the chlorine test, 
with this great advantage, that an excess of the reagent does 
not prevent the reaction. 
As to the delicacy of this test, the following observations 
were made. 
With solutions of iodid of potassium up to one hundred thou- 
sandth (1: 100,000) the precipitate was abundant, becoming less 
lue and more tawny as the dilution increased. Beyond this 
point the distinctness rapidly fell off. The indications were ob- 
Servable at one-four-hundred-thousandth. With a solution of 
one-eight-hundred-thousandth it was doubtful whether any effect 
was evident though still it was thought that a darkening was 
produced. 
The experiment can be made in two ways, according to the 
result desired. 
If it is wished to observe the effect of the chromic acid in in- 
creasing the delicacy of the indication, add the acid and starch 
to the very dilute solution of iodid, and then when the extreme 
dilution is such that no reaction appears, a drop of solution of 
bichromate instantly produces it. _ 
But in employing the reagent in the search for iodine, add 
the starch to the liquid to be tested, stir it up, add a drop of 
dilute solution of bichromate, enough to communicate a ale 
pe color, and finally add a few drops of chlorhydric acid, 
h haracteristic precipitate, 
. 
€ test is then the production of the c 
