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I bought 20 liters of hydrogen peroxide ($\ce{H2O2}$) about two years ago and I was told that it was pure at that time. I'm aware of the instability of $\ce{H2O2}$, which easily decomposes into water and hydrogen.

The $\ce{H2O2}$ at my premises had probably been decomposed partially within this period. According to its current practical effectiveness, I guess the concentration is still above 15%, but I can not be sure.

How can I estimate its current concentration by using equipment one might have at home?

Note that I can find a small batch of known 3% $\ce{H2O2}$ solution for some kind of comparison if needed.

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    $\begingroup$ Pure hydrogen peroxide is very dangerous! Even 30% is nasty stuff. Are there any environmental authorities you can contact to safely take it away? It is a bad idea to try to guess how concentrated it might be and a do it yourself test could have very bad consequences. $\endgroup$
    – Ed V
    Commented Mar 30, 2020 at 0:41
  • $\begingroup$ This H2O2 is actually a component I'm using for etching PCB along with HCl, so I'm using it for some other purpose (I don't want to give it away). I have doubts about its purity in the first place, it's only what I'm told by the seller but it probably isn't the case. Anyhow, I'm aware of its dangers and always careful about the security measures, so you can be sure that I'll follow a safe path. $\endgroup$
    – ceremcem
    Commented Mar 30, 2020 at 0:56
  • $\begingroup$ Glad to hear it! So one simple test would be to see how much oxygen gas could be evolved for a given volume of hydrogen peroxide solution catalytically decomposed. Somewhat awkward to do at home, but possible to kluge together a setup as per high school chemistry gas experiments. $\endgroup$
    – Ed V
    Commented Mar 30, 2020 at 1:04
  • $\begingroup$ How can I be sure that the sample is fully decomposed? What material should I use for that purpose? Is shaking or gently heating (placing the H2O2 cup inside a boiled water cup) sufficient for proper decomposition? I can precisely measure the H2 emission by using syringe as a measuring flask. $\endgroup$
    – ceremcem
    Commented Mar 30, 2020 at 1:12
  • $\begingroup$ A simple catalyst, e.g., manganese dioxide or KI solution, will do the job and you don’t need total decomposition just to get an estimate. $\endgroup$
    – Ed V
    Commented Mar 30, 2020 at 1:15

2 Answers 2

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Purchase a small amount of very 'fresh' 30%, 12%, 6% and 3%. Using a pipette, place 2-3 drops of each, along with 2-3 drops of the solution in question, onto a piece of cardboard in separate locations. Add one drop of a reagent of your choosing to each and observe/compare reaction.

Peroxides tend to be very reactive. Please be safe.

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One path to safely determine the strength of $\ce{H2O2}$ is:

  • First dilute it to 1/3 strength (this is for safety when dealing with higher concentrations of hydrogen peroxide).

  • Slowly titrate against a known concentration of $\ce{NaOCl}$ until the vigorous oxygen liberation ceases.

  • One can also collect the volume of $\ce{O2}$ evolved.

Reaction equation:

$\ce{NaOCl + H2O2 -> NaCl + H2O + O2 (g)}$

So, moles of oxygen liberated equate to moles of hydrogen peroxide. Also, moles of sodium hypochlorite employed should also equate to the moles of $\ce{H2O2}$.

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    $\begingroup$ I used to do this as a lecture demo: put 30% hydrogen peroxide in a fishbowl, kill the lights in the lecture hall, and squirt in Chlorox bleach (5.25% by weight NaOCl solution). Beautiful red, but weak, forbidden singlet delta oxygen emission! Students loved this chemiluminescence demo. $\endgroup$
    – Ed V
    Commented Mar 30, 2020 at 4:04
  • $\begingroup$ The reaction you referenced is apparently a function of concentration and the pH difference between solutions. Another safety advantage of working with dilute solutions (per Step 1) is that this very interesting reaction no longer proceeds to any extent of being at all visible, so no Singlet Oxygen from dilute ingredients! $\endgroup$
    – AJKOER
    Commented Mar 30, 2020 at 12:19
  • $\begingroup$ I do not know why your answer got a downvote, because it is fine, especially if the OP only needs a reasonable estimate of the hydrogen peroxide concentration. So I am upvoting. $\endgroup$
    – Ed V
    Commented Mar 30, 2020 at 12:35

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