Sciencemadness Discussion Board

Testing concentration of H2O2 after fractional freezing

kt5000 - 7-11-2013 at 00:09

I'm tinkering with concentrating H2O2 from 3% off-the-shelf H2O2. It sounds like the easiest/safest way to do it is fractional freezing, and that may yield around 30-50%.

After removing some water ice, is there a way for me to test the concentration of the remaining H2O2 solution? Aside from sticking my finger in it and seeing if it gets bleached for 3 days :P

woelen - 7-11-2013 at 00:59

If you are happy with a rough estimate, you could measure the amount of gas, produced from the H2O2. Take a well-defined small volume of your original 3% solution of H2O2 (e.g. 1 ml) and mix this with 5 ml of water. Suck this solution into a syringe. Then suck in a few ml of a solution of KMnO4 or a suspension of MnO2 and IMMEDIATELY cap the tip of the syringe. Swirl and measure how much oxygen you get.

Repeat the same procedure with your enriched H2O2 taking the same well-defined small volume. The ratio of volume of oxygen from the two experiments tells you something about the concentration of the enriched H2O2.

This procedure does not require a precise absolute measurement. As long as you can take the same volume of liquid repeatedly you are OK. An option could be to use a thin flexible PVC tube of 4 mm diameter and draw a mark on that at 5 cm or so from the open end and then suck liquid in that piece of PVC tube up to the mark.

[Edited on 7-11-13 by woelen]

kt5000 - 7-11-2013 at 17:29

A rough estimate would be just fine. I'm looking at concentrations 5-10 times the original 3% solution, so it should be fairly obvious.

I don't have MnO2 or KMnO4 onhand though. Might something else like NaOH or NaHCO3 do the trick?

kt5000 - 7-11-2013 at 17:42

On further reading, it looks like the MnO2 just catalyzes the decomp.. I was unclear on the role it was playing.

confused - 7-11-2013 at 21:24

yeast would also work in place of MnO2 or KmnO4

or you could just take apart an alkaline battery for the MnO2

kt5000 - 7-11-2013 at 22:17

I can :) I wasn't sure if that MnO2 would be pure enough. I'll give that a shot, thanks!

kt5000 - 7-11-2013 at 22:19

Would it matter if it was a brand new or older alkaline battery?

confused - 7-11-2013 at 22:51

a new battery would be better, but an old battery should have enough to work as well and if im not wrong the mn2o3 in old batteries should also work as a catalyst


same principle, just a much larger battery
http://www.youtube.com/watch?v=knc1lSupAwQ

[Edited on 8-11-2013 by confused]

madcedar - 7-11-2013 at 22:55

Just do the test with bleach. A little care is needed as this reaction is spontaneous.
NaClO + H2O2 → H2O + NaCl + O2↑

kt5000 - 8-11-2013 at 00:34

I popped open a AA battery tonight and scraped out the MnO2/Mn2O3. I'll test the NaClO method as well. Thanks again.

woelen - 8-11-2013 at 00:51

The test with bleach is not really useful, there are side reactions and the result strongly depends on the concentration of the bleach, which is highly variable and usually unknown. The use of MnO2 or a tiny pinch of KMnO4 is that it catalyzes decomposition of H2O2 and the only reaction is H2O2 decomposing to O2 and H2O. The precise amount of MnO2 does not matter at all.

kmno4 - 8-11-2013 at 04:29

Quote: Originally posted by woelen  
If you are happy with a rough estimate, you could measure the amount of gas, produced from the H2O2. Take a well-defined small volume of your original 3% solution of H2O2 (e.g. 1 ml) and mix this with 5 ml of water. Suck this solution into a syringe. Then suck in a few ml of a solution of KMnO4 or a suspension of MnO2 (...)

I can confirm this methode: very simple and - what is important - accurate way for estimation H2O2 content.
I use more tricky setup: solution of H2O2 in the flask + [small plastic vessel with MnO2 + small magnet] inside flask, held with another magnet outside the flask. Flask is stoppered and conected with measuring cylinder (of course upsaid-down and filled with water) with PCV pipe. Turning outside magnet causes the vessel also turning and MnO2 poures out to H2O2 solution.
Volume of O2 is measured and reaclculated to H2O2.
You can measure 1% - 40 % H2O2 concentrations in this way.
I use 250 cm3 cylinder, amount of MnO2 should be small (~10mg, 100 mg max because reaction may be to violent).
KMnO4 is not so good as MnO2, because it can decompose itself to O2 under these conditions and gives too much O2.

"MnO2" from batteries is very good. You can test is with your H2O2.
Caution:
Reaction of conc. H2O2 with MnO2 is VERY violent, and original solution must be diluted to max. 5 % with water before measurement.

[Edited on 8-11-2013 by kmno4]

kt5000 - 8-11-2013 at 09:31

Quote: Originally posted by kmno4  

Caution:
Reaction of conc. H2O2 with MnO2 is VERY violent, and original solution must be diluted to max. 5 % with water before measurement.


woelen trying to blow me up, hehe :)