76 
TEST FOR IODIDE OF POTASSIUM. 
the purity, of the article, for on adding sulphuric acid and 
starch to a solution of pure iodide of potassium, the blue 
colour is not produced ; but if the specimen contain iodate 
of potash or free iodine, the blue colour will be developed." 
This subject had been previously brought under our 
notice. It appears that some iodide of potassium was 
recently condemned by a purchaser as bad, the manufac- 
turer being told, to his astonishment, that it contained no 
iodine, the test described in the Pharmacopoeia having failed 
to indicate the presence of that body. It was found, how- 
ever, that the only peculiarity in the salt consisted in its 
being very pure. On adding sulphuric acid to a solution 
of pure iodide of potassium, water is decomposed together 
with the salt, yielding sulphate of potash and hydriodic acid, 
as represented in the following diagram : — 
Iodide of Potassium 
Water 
Sulphuric Acid 
Hydriodic acid (H I) is a colourless compound which 
does not affect the colour of starch, and therefore, on adding 
starch to the above mixture the blue iodide of starch is not 
formed. But hydriodic acid is a very unstable compound ; 
it speedily undergoes decomposition, iodine being set free, 
which colours the solution and gives the characteristic blue 
colour with starch. If, therefore, instead of adding the 
sulphuric acid and starch together, the acid be added first, 
and the mixture allowed to stand for some minutes before 
the addition of the starch, the presence of the iodine will 
be made manifest. It might be supposed that the same 
effect would ensue if the acid and starch were added simul- 
taneously, and the mixture allowed to stand for some time, 
but experience shows that this method of applying the test 
will not answer. 
It is evident from what, has been stated, that the Phar- 
macopoeia test is a bad one. Sulphuric acid is not the best 
