DETECTION OF IODIDE IN BROMIDE OF POTASSIUM. 
323 
be developed ; if a very small quantity, as for instance the two- or three-mil¬ 
lionth part, then a few seconds elapse before the blue colour makes its appear¬ 
ance ; this test admits of exceeding delicacy. 
My attention was now directed to a sample of bromide of potassium said to 
be pure, or free from iodide. The peculiar appearance of this salt led me to 
suspect the presence of an alkali, or that the combination was not quite perfect, 
instead of the well-defined cubes so characteristic of well-combined bromide of 
potassium. The opacity and crystalline form is one of the best indications of its 
purity and skilful manipulation, and the presence of iodide does not affect the 
crystalline form or opacity of the bromide. Farther, when iodide of potassium 
is prepared by many of the processes recommended, it has a predisposition to 
decompose and the crystals become yellow or pink, although almost pure, and 
form iodate, which has never been found to be the case with bromide. I have 
furnished an interesting specimen of bromide of potassium crystallized under 
peculiar circumstances, the geometrical form being apparently that of a fiat 
parallelopiped, but still representing the internal structure of a cube. It will 
be observed, as in iodide of potassium, the plates forming butterflies’ wings are 
not thicker than bank-note paper. I here furnish some well-defined opaque 
cubes of bromide with five perfect faces, as well illustrating the marked differ¬ 
ence between the bromide and iodide. 
To a solution of the above salt starch and chlorine were added, without the 
slightest development of iodine. A minute quantity of nitric or hydrobromic 
acid was then added to the solution previous to the application of this test.* 
This addition caused the instantaneous development of iodine cn the application 
of starch and chlorine. A few drops of solution of potassa were then added 
to the salt that I had prepared, which indicated iodine with starch and chlorine. 
Since this addition of potassa, the evidence of iodine could no longer be traced 
on the application of starch and chlorine. Having obtained samples of bromide 
of potassium from various manufacturers, I found, without exception, that they 
all indicated iodine on the addition of starch and chlorine. Four samples of 
French bromide of potassium indicated a percentage of iodine on the application 
of each or every one of the tests here employed ; also one sample obtained in 
London gave similar results. Hence it would appear from the above statement 
that starch cannot be relied upon for detecting iodine in alkaline solutions, but 
is of the greatest value in detecting minute traces of iodine in neutral or acid 
solutions, as it readily developes its presence when almost every other known 
test fails to do so. I then proceeded to examine the bromine I had operated 
upon, and on the application of various tests, I readily detected iodine, and also 
in five samples of bromine of French manufacture. One of the most favour¬ 
able conditions for examining the bromine for iodine I found to be bromide of 
ammonium. A sample of the bromine was supersaturated with sulphide of am¬ 
monium, the solution was then heated until the excess of ammonia was expelled, 
and the sulphur precipitated or separated.f If the solution be now concen¬ 
trated by evaporation, the iodine can be readily traced by starch and chlorine. 
It seems, therefore, more than probable that the processes adopted by the manu¬ 
facturer for separating iodine from the bromine, either by chlorine or sulphate 
of copper and sulphate of iron, and its subsequent purification by ether and 
potash, fail to remove the traces which are developed by starch and chlorine, 
etc.]; 
In the present state of the bromine market, therefore, chlorine and bisulphide 
* I readily obtained hydrobromic acid by decomposing bromide of zinc with oxalic acid. 
f If the bromide contains sufficient iodine it will render the sulphur ductile, and it can 
be traced by starch and chlorine. 
J A liquid containing of its weight of iodine receives a blue tinge from a solution 
of starch. 
