# How concentrated does sulfuric acid need to be for nitric acid synthesis (H2SO4 + nitrate salt distillation)?

So I have a home lab setup and recently found myself in need of some nitric acid. I could just buy some, but I thought it would be a useful exercise to make my own. I have plenty of nitrate salts around, as well as plenty of unused ~30% sulfuric acid (battery acid refills, fresh out of the box so there's no lead etc in it).

I know distillation of a mixture of a nitrate salt (ex. NaNO3) in sulfuric acid should yield nitric acid, but all the protocols I've found call for concentrated H2SO4. Would ~30% work? I know "concentrated" sulfuric acid usually means >90% if not fuming, but as long as I use stoichiometric ratios of the nitrate and the sulfuric acid, the reaction should still proceed fine, correct? Obviously the nitric acid will be more dilute, but I don't need very concentrated nitric acid anyway.

I realize I can distill the sulfuric aid to concentrate it, but I haven't done that before and would like to minimize the number of "dangerous" distillations I do. My vacuum pump is pretty shoddy and distillation of H2SO4 at ambient pressure requires so much heat, it just seems like an unnecessary risk if 30% would work.

• $1$ kg $30$% sulfuric acid contains $300$ g $\ce{H2SO4}$ (= about $3$ moles). It may react with $\ce{NaNO3}$ according to : $$\ce{H2SO4 + NaNO3 -> NaHSO4 + HNO3}$$ and may produce $3$ moles or $3·63 = 189$ g $\ce{HNO3}$, plus $700$ g water. The solution contains also $3$ moles $\ce{NaHSO4}$ ($360$ g). It is hard to describe the composition of the vapor that will distill out of such a mixture. It should be a mixture $\ce{HNO3 + H2O}$. In the beginning it should be nearly pure water. But slowly the vapor will contain more nitric acid. But at what temperature and how much ? Oct 19, 2020 at 12:42
• Related Oct 19, 2020 at 13:00