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When preparing aqueous solutions of sulfuric acid through dilution of the pure acid, it is well known that we should add acid to water and never the reverse.

To prepare a solution of hydrogen peroxide and sulfuric acid it appears that the standard operating procedure is the opposite: the hydrogen peroxide is added to the acid, and never the reverse.

Given the similarity between water and hydrogen peroxide, this is surprising. Like water, hydrogen peroxide is less dense than sulfuric acid, has a lower boiling point, and a higher volumetric heat capacity. Therefore, all the reasons why adding acid to water is supposed to be safer than adding water to acid would seem to indicate adding acid to hydrogen peroxide.

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    $\begingroup$ There is no similarity between H20 and H2O2 except that both are made by O and H only. There is nothing surprising here. What you have listed are marginal reasons. The main reason (on which your list works on) is the partial heat of mixing. And there is no reason to expect the same. $\endgroup$
    – Alchimista
    Aug 8, 2021 at 19:22
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    $\begingroup$ @Alchimista As 30% H2O2 still contains 70% of water, there is still similarity. I suppose it must have something to do with the overall H2O2 concentration and perhaps also relative H2O2 stability alone and in H2SO4. Adding the acid to concentrated H2O2 may be more vulnerable to sudden H2O2 decomposition, while adding H2O2 to the acid never leads to high H2O2 concentration but very locally and temporarily. So adding H2O2 to acid may be the choice of the lesser evil. Note that I have no personal experience with "Piranha solution". $\endgroup$
    – Poutnik
    Aug 9, 2021 at 6:49
  • $\begingroup$ @Poutnik all right but the premise is false. 30% x in water isn't water. Now for the precise reason I don't know as well. I just wanted to say that in principle the order of mixing 30% H2SO4 and pure water might be reversed as well (though it does not pose much problem, just talking about of heat). $\endgroup$
    – Alchimista
    Aug 9, 2021 at 8:57
  • $\begingroup$ In either case, I'd be extremely cautious, since boiling, effervescing piranha acid, evolving nascent oxygen, might be somewhat hazardous. ehs.harvard.edu/sites/default/files/… $\endgroup$ Aug 15, 2021 at 4:14

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One possibility is that with either ordering the solution becomes hot enough to destabilize the hydrogen peroxide, and the resulting risk of violent decomposition apparently outweighs the "acid spattering" risk that you have adding just water to acid. So the decision is made to accept the "add water to acid" spattering in exchange for lowering the concentration of heated hydrogen peroxide, and also doing the addition slowly (recommended anyway for diluting strong acids) to minimize that.

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