Cloner - 20-11-2012 at 02:08
I have obtained hypochlorite solution from two sources. The first is household bleach. The second is a stronger solution, nominally 12.5%.
I wanted to determine the chlorine content using the following procedure:
* add 5ml sample to 45 ml water and 5ml glacial acetic acid
* add 3ml of a 0.1M KI solution
* titrate using 0.1M sodium thiosulfate solution
The observations:
household bleach and iodide solution at first produce a brown cloud, this dissipates after seconds. Iodine color appears after titrating a few ml, and
disappears at a point, at which the titration is completed.
The other bleach solution immediately forms a brown color that does not go away and reappear before the titration is completed.
I wonder what is the meaning of the iodine color disappearing when titrating household bleach. Could there be some impurity in there?
woelen - 20-11-2012 at 02:36
The disappearance and reappearance of iodine is what I expect.
You only use a small amount of iodine, relative to the amount of bleach. When the iodide is added, then immediately, the iodide is oxidized to iodine.
But as there is a large excess amount of bleach, the iodine is further oxidized to iodate (or even periodate). Iodate and periodate ions are
colorless, so this explains the disappearance of the brown color of iodine.
When you are titrating with thiosulfate, then first the hypochlorite is reduced and when this is used up, then the (per)iodate are reduced. First to
iodine (this is the reappearance of the brown color) and finally all iodine is reduced to iodide as well and the liquid becomes colorless again.
The role of the iodide hence is as an endpoint indicator for the titration. Without the iodide it would be impossible to see when you have added
enough thiosulfate such that the hypochlorite is just used up.
What I do not udnerstand is that with the more concentrated bleach you do not see this behavior. The only thing I can imagine is that the supposed to
be more concentrated bleach is not concentrated at all (maybe it is very old and has decomposed).
Did you need more thiosulfate for the supposedly concentrated bleach?
Another issue might be lower alkalinity of the more concentrated bleach. But before more information can be given, I would like to read more about the
results of both titrations.
Cloner - 20-11-2012 at 15:19
I must be more detailed:
the iodine color appeared and vanished in seconds, but after addition of a small fraction of the total thiosulfate, the iodine color did appear. That
is to say, the titration involved a clear liquid (a few ml thiosulphate), a brown liquid (the rest of the titration) and then finally disappearance of
iodine as expected.
You mention the strength of the bleach: the result of the titrations were 2.85 mmol thiosulphate for 5ml household bleach and 2.3 mmol thiosulphate
for the '12.5% bleach'. Still, this difference is not that big.
woelen - 21-11-2012 at 00:15
The '12.5%' bleach indeed is not very concentrated anymore. You need less thiosulfate for this than for the household bleach. It might be that the
bleach was not enough to fully oxidize all iodide to iodate.
Based on these observations, I however, think that the concentration of the iodide must have been higher than 0.1 M.