Re: Need your help

Here is how you determine H2O2 concentration in an organic solvent:

1) Make or buy 0.100 M Na2S2O3 in water. If you use nice fresh bottle
with Na2S2O3.5H2O large crystals, you don't need re-normalise it using
a standard, just use balances and volumetric flask.
2) Make a solution of starch - rice or corn or potato pure starch
suspended in large volume of water, bring to boil and boiled gently
until starch dissolves. Cool to room temperature, adjust with water so
that the solution is not a gel but a thick syrupy liquid. You use this
solution as an indicator.
3) Make acetic acid-water 1:1 mixture. Put about 10-20 mL of this mix
into a titration flask with a stopper. You can use a round-bottom flask
with magnetic stirbar instead. Buble the acetic acid-water mixture with
nitrogen for few minutes to remove most of oxygen. Weigh out
approximately 1-2g of potassium iodide and add it to your diluted
acetic acid under nitrogen.
4) Measure exact volume of your sample of peroxide solution in organic
solvent and add it to the AcOH+H2O+KI mixture under nitrogen. Let it
sit or stirr in a closed flask for about 30 minutes. (you can leave it
for 1 hour - but not overnight)
5) Dilute your mixture with about threefold volume of water, add few
drops of your starch indicator solution. (The mix should turn
deep-blue-violet). Titrate this mix with 0.1M Na2S2O3 until the blue
color disapears and the mix is colorless again. (when you see the color
disapearing, slow the rate of thiosulfate addition and go drop-by-drop.
6) From volume of consumed thiosulfate calculate the amount of
peroxide: 1mL of 0.1M thiosulfate is 0.1mmol of of thiosulfate. This
corresponds to 0.05 mmol of iodine I2. This corresponds to 0.05 mmol of
H202.
7) You can do the sample preparation/titration 2-3 times and average
for a better precision.

Background oxidation of KI by air is not very fast but it is better to
prepare the sample with KI+acetic acid under nitrogen. If you want to
compare the background effect, you can titrate a blank KI+AcOH mixture
(without a peroxide solution).

Starch solution grows mold and microbes quicky - you can store it in
the fridge but it is better to make fresh indicator frequently.
Thiosulfate solution is reasonably stable but you can put it under
nitrogen in fridge if you are not using it and plan to store it for
long time.

This procedure also works for determining concentration of tBuOOH in
organic solvent.

.

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