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I've been using same bottle of hydrogen peroxide and same bottle of 50 % of sodium hydroxide to make sodium peroxide for a month now. You could see the presence of sodium peroxide in the solution because it turned yellow, but it won't turn yellow now and I don't know why, but hydrogen peroxide is not efficient oxidizer for my current use. Where is the problem?

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    $\begingroup$ If the result is suddenly different the conditions are suddenly different. We should ask you what is different and not vice versa. Perhaps slow decay of H2O2, or absorption of CO2 by hydroxide. $\endgroup$
    – Poutnik
    Oct 12 at 20:24
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Poutnik is right, what likely happened is one or both of the reagents lost its potency through chemical reactions to which they are susceptible.

In the case of the sodium hydroxide, if it reacted with carbon dioxide from the air (it is best stored and handled under a CO2-free atmosphere), the resulting carbonate is no longer a strong enough base to react with the hydrogen peroxide (this is an acid-base reaction, not a redox one). If the sodium hydroxide is/was as strong as 50%, you may be able to see a sign of this contamination: sodium carbonate is not as water-soluble and might be seen as cloudiness or a white precipitate.

If the hydrogen peroxide decomposed, that is that as far as forming sodium peroxide goes. Decomposition of hydrogen peroxide, unlike carbonation of sodium hydroxide, cannot be prevented by using an inert atmosphere, and it has no visible effect on the solution until, as you describe, it's called on to perform. If there was no apparent outward change in your reagents, I would therefore suspect hydrogen peroxide decomposition first.

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