Wayne, I say pour conc. acid into the peroxide allowing the acid to dissolve into the peroxide, thus allowing the ionizing acid to dilute into the entire volume of peroxide on contact. Pouring peroxide into the conc.acid would cause the peroxide to "spit"
off the surface of the acid, as there will be pockets of conc. acid ionizing with smaller volumes of peroxide, without any medium to absorb the heat of solution.
Acid into water is what I was always taught for the reasoning above.
Jim
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