# Reacting NaOH in solution with O3

I use common air to produce ozone. However the gas after the ozone generator does contain $\ce{NO2}$ gas, which I have to remove for the subsequent steps. I tried to remove the $\ce{NO2}$ with a stainless steel gas wash bottle filled with distilled water. This works fine, however the distilled water will subsequently get charged with $\ce{NO2}$ and turn to dilute nitric acid.

After a few days of operation, I have changed the water and tested its acidity. The $\mathrm{pH}$ was around 0.4 - it was possible to etch a copper pipe with it. So the formation of this acid is not very good for the stainless steel gas wash bottle. I have decided to add 2 more stainless steel gas wash bottles and fill the first one with $\ce{H2O + NaOH}$, in order to neutralize the $\ce{HNO3}$. The other two bottles are for capturing the remaining traces of $\ce{NO2}$.

Bottle 1: 5 liter $\ce{H2O}$ + $\pu{15g}$ $\ce{NaOH}$
Bottle 2: 5 liter $\ce{H2O}$
Bottle 3: 5 liter $\ce{H2O}$

Now I have done some ozone measurements with and without these gas wash bottles. The result was as follows:

• It seemed as if $\ce{O3}$ would pass through pure $\ce{H2O}$ without any reaction once the solution is saturated and the saturation seems to happen almost immediately.

• However the solution with the $\ce{NaOH}$ seems to react the first 2-3 minutes and then suddenly stop reacting, which is actually good, because it does not use up ozone, however I have no clue what is happening in detail.

Here are some charts from the measurement. Upper chart: test of the $\ce{H2O + NaOH}$ solution. Lower chart: test of the $\ce{H2O}$. Note: the lower the value in the chart, the higher the ozone concentration.

The duration of each test was about 5 minutes. I did measure the $\mathrm{pH}$ of the $\ce{NaOH}$ containing solution and it basically did not change due to the ozone treatment. (It was around 12.35 before and after ozone bubbling...)

Any idea what is happening there and why?

$\ce{OH^- + O3 -> HO2^- + O2}$
Such an oxygen transfer is favored by strongly basic solutions because $\ce{HO2^-}$ is a weaker base ($\mathrm{p}K_\mathrm{a}$ hydrogen peroxide = 11.75) than $\ce{OH^-}$, as well as ozone being a more powerful oxidizer than diatomic oxygen.