TEST FOR IODIDE OF POTASSIUM. 
77 
substance to use for decomposing the iodide of potassium 
and setting free the iodine. Nitric acid might be used with 
better success, especially if it contain nitrous acid, as it gene- 
rally does ; but chlorine water, or gaseous chlorine, will be 
found to be better than either, the whole of the iodine being 
thus instantly set free. The best method of applying the 
test is to make a weak solution of the salt to be tested in 
distilled water, to add to this some cold solution of starch, 
to allow some chlorine gas to flow from the mouth of a 
bottle on to the surface of it, and so slightly agitate the 
liquid. The blue compound will thus be instantly deve- 
loped. It is not necessary that the chlorine used for this 
purpose should be pure ; the gas occupying the upper part 
of a bottle containing chlorine water, will answer the pur- 
pose perfectly well, the stopper being removed, and the 
bottle partly inverted over the edge of the test-glass, so that 
the heavy gas may flow out. This is the most delicate 
way of testing for the presence of iodine, and should be 
adopted where the quantity to be detected is very small ; 
but in applying the test to iodide of potassium, delicacy of 
indication is not required, as iodine is, or ought to be, the 
the most abundant constituent of the salt. Chlorine water, 
or reddened nitric acid, might, therefore, be used in this 
case with the same effect as gaseous chlorine. 
Our correspondent is correct in saying that the Pharma- 
copoeia test does not indicate the purity, but the impurity of 
the salt. The test could not have been intended for the 
detection of impurities, but merely to show that iodine 
is present ; yet it has happened, unfortunately, that when 
applied according to the instructions of the Pharmacopoeia, 
it fails to afford even this indication, if the salt be perfectly 
pure. Perfectly pure iodide of potassium, [however, is 
rarely met with in commerce. The process by which the 
salt is usually made by manufacturers, consists in heating 
together a solution of caustic potash and iodine, in which 
case iodate of potash is necessarily produced at the same 
time as the iodide ; and although the salt is subsequently 
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