Why does NaOH + H2O2 = crystals?

September 14, 2009 8:10 AM

Chemistry Filter: Why are crystals forming when we mix together sodium hydroxide (NaOH) and hydrogen peroxide (H2O2)?

The details:
- When we make a solution of 5M NaOH and 3.75% H2O2 (cold and at RT), it causes crystals to form, and resembles a gel. Heating this to ~40C causes the crystals to disappear, but it recrystallizes upon cooling.
- A solution of 5M NaOH and 1.5% H2O2 crystallizes, as does a solution of 5M NaOH and 0.75% H2O2. When the latter solution is boiled for a few minutes, the mixture does not recrystallize upon cooling (however, tests for presence of H2O2 are negative; pH still 14)
- A solution of .1M NaOH and 8% H2O2 does not crystallize (not ideal, because of the low concentration of NaOH)
- Adding H2O2 to NaOH solution produces heat, so crystal formation reaction may be exothermic; however, diluting NaOH in water is an exothermic reaction.

The internet seems to think NaOH and H2O2 should not react upon mixing. So can anyone explain what is happening (chemistry details would be great)? Bonus points if anyone has any suggestions for ways to make the 5M NaOH and 3.75% H2O2 more stable at RT, as this would be the ideal.

posted by dormouse to Science & Nature (1 answer total)

Sodium hydroxide certainly deprotonates hydrogen peroxide (only once). The pKa of peroxide is about 12, lower than that of water (15.7). This is a standard method for the production of epoxides next to electron withdrawing groups. Maybe sodium hydroperoxide is just less soluble and crashing out?

That said, peroxides can decompose pretty readily, so who knows what you're seeing.
posted by Dr.Enormous at 8:37 AM on September 14, 2009