Calculations for Iodine Solutions
Jason MacArthur
the Ripper Titration Method of determining Free SO2 in cider call for
the use of .005M Iodine solution. I was able to purchase from a local
supplier .1N Iodine solution, but I am afraid I don't understand the
relationship between Normality and Molarity well enough to convert
between the two with any confidence.
Any suggestions?
Jason MacArthur
Whetstone Cider Works
Andrew Lea
> I need a little Chemistry 101 help here. The instructions I have for
> the Ripper Titration Method of determining Free SO2 in cider call for
> the use of .005M Iodine solution. I was able to purchase from a local
> supplier .1N Iodine solution, but I am afraid I don't understand the
> relationship between Normality and Molarity well enough to convert
> between the two with any confidence.
Oh dear! Iodine titrations are one of those areas where the normality
can differ according to the reaction you're studying. Ideally you really
do need to check with the supplier what the exact molarity is. But for
sulphite titration it's two equivalents per mole so 0.1N is 0.05M.
That's probably what your solution is. For instance that's how these UK
suppliers quote it
http://www.camlab.co.uk/item.asp?itemid=28562&categoryid=799&browsecategoryid=656
But if you can get it from the horse's mouth it would be best.
BTW iodine solution is not hugely stable unless freshly made and is
itself often standardised against thiosulphate, yet another level of
complexity I'm afraid. Certainly if you are making an 0.005M solution
from 0.05M stock (10 fold dilution) you should not keep the diluted
solution longer than a day. Make up fresh from stock every time and keep
everything cool and dark.
Andrew
--
Wittenham Hill Cider Page
http://www.cider.org.uk
Jason MacArthur
On Jun 17, 11:59 am, Andrew Lea <y...@cider.org.uk> wrote:
Certainly if you are making an 0.005M solution
> from 0.05M stock (10 fold dilution) you should not keep the diluted
> solution longer than a day. Make up fresh from stock every time and keep
> everything cool and dark.
solution?
I do have thiosulphate for standardization, so I can make that
correction as needed.
Thanks Andrew, I will check with the supplier.
Jason
Andrew Lea
>
> So I take it that the .05M stock is more stable than the .005M
> solution?
It is normally accepted that strong stock analytical solutions,
correctly stored, are more stable than diluted ones (presumably because
the degradation is at some constant absolute rate which is not
proportional to the actual concentration in solution, which seems to be
especially the case where reduction / oxidation chemistry is involved,
as it is here). So standard lab practice is to discard the working
solutions and make fresh from stock each day.
Generations of analysts have done it this way from the 19th century
onwards. Of course they might all be wrong (mass self-delusion!) but I
suspect there is something in it. FWIW my own lab experience backs it up.