Sciencemadness Discussion Board - Foaming in destilling nitric acid (bisulfate method)

Foaming in destilling nitric acid (bisulfate method)

RU_KLO - 15-8-2025 at 05:42

I tried destilling nitric acid from the bisulfate method (for the first time)

used (aprox):
150 gr. sodium bisulfate (from ph- for pools) pic 1
50 gr. potassium nitrate.

It foamed a lot. and it clogged the delibery tube (pic 2), triggering the "safety valve" (aka as flying cork, 3 meters high and 1,2 meters long - pic 03).

when cleaning I realized that the foam was like pumice stone (aereated stone) and needed a blowtorch to liquify and remove it. (boiling water was no good or too slow)

Luckily I was 2 meters away when this happened, because one can find molten salt drops 50 cm away from the launching site).

After cleaning I redo the experiment, but instead, water was added and the salts reduced by half (75 gr bisulfate, 25 gr KNO3) and a stirrer was also added.

the destilantion proceeded normaly, but the destilate was mostly water (less than 1M HNO3)

Once the brown fumes (mild) were produced, checked the flask (it was covered with aluminium foil) and there was again foam. pic 4.

When /I thing) that concentrated nitric acid was finally destilling, there was a lot of foaming. it did not reached the top, but 75% of the flask was with foam (pic 5)

I have seen videos online that this is not normal (maybe some could share the experience with this method) and the procees is very straight foward.

So, how can I do this, without /or controling the foam.

First idea (that currently is underway) is to cristallize the bisulfate, maybe it contains some surfactant that is complicating things.

The information I found online (no MSDS or datahsheet) is that it contains sodium bisulfate/sodium sulfate.

Any ideas? bigger flask - which I dont have....?

Radiums Lab - 15-8-2025 at 14:51

Is your nitrate and bisulphate free of carbonates and bicarbonates?

bnull - 15-8-2025 at 15:29

Could it be that your potassium nitrate is actually a mix of nitrate and urea? Urea decomposes above 130 °C.

RU_KLO - 15-8-2025 at 16:18

Quote: Originally posted by Radiums Lab

Is your nitrate and bisulphate free of carbonates and bicarbonates?

Will check by adding a small amount of diluted sulfuric acid (to the bisulfate) and nitric to the nitrate, and check for bubbling/gas production. if gas, Ca(OH)2 test for carbonate.

RU_KLO - 15-8-2025 at 16:21

Quote: Originally posted by bnull

Could it be that your potassium nitrate is actually a mix of nitrate and urea? Urea decomposes above 130 °C.

It was bought in a company that sells bulk salts, maybe.

How do I check? by decomposition temperature?

bnull - 15-8-2025 at 18:11

Quote: Originally posted by RU_KLO

Quote: Originally posted by bnull

Could it be that your potassium nitrate is actually a mix of nitrate and urea? Urea decomposes above 130 °C.

It was bought in a company that sells bulk salts, maybe.

How do I check? by decomposition temperature?

Add calcium hydroxide and heat it. Ammonia is evolved and can be detected by smell or by waving a filter paper wet with hydrochloric acid over the mix. To see if biuret is formed when heating would be even better but I have no idea of the effect nitrates have on the reaction.

RU_KLO - 17-8-2025 at 16:11

test performed:

a spatula tip of sodium bisulfate was added to a test tube. 2-3 ml of 4M H2SO4 added. No bubbling. It was heated and the salt dissolved in the solution. No gas was produced.
Meaning: no carbonate/bicarbonate in the bisulfate

a spatula tip of potassium nitrate was added to a test tube. 2-3 ml of 3M HNO3 added. No bubbling. It was heated and the salt dissolved in the solution. No gas was produced.
Meaning: no carbonate/bicarbonate in the nitrate.

a spatula tip of potassium nitrate was added to a test tube. 2-3 ml of 6M NaOH added. No bubbling. It was heated and the salt dissolved in the solution. No gas was produced. No smell of ammonia.
Meaning: no urea was present in the KNO3.

So maybe there is a surfactant in the bisulfate?
How to test for surfactants? (without knowing which one - if any)

bnull - 17-8-2025 at 19:30

I see no reason why one would sell a mix of sodium bisulfate and carbonate or bicarbonate; it defeats the main purpose of a "pH minus".

Dissolve some bisulfate in water, put the solution in a capped flask and shake it. If the bubbles take too long to burst, it means there is a surfactant.

woelen - 17-8-2025 at 23:45

I once made HNO3 by distilling HNO3 from KNO3 and conc. H2SO4. I also had a lot of foaming. Distilling a mix of H2SO4 and KNO3 resulted in excessive foaming, so much that it rose into the cooler and even into the receiving flask. It was horrible. In my case, the KNO3 was the issue. It was fertilizer grade. It consisted of white prills, which felt a little bit waxy. When this stuff was added to water, I obtained a solution, with quite some off-white scum on the surface. I found out that the prills were covered by some wax-like substance, probably to assure that the prills do not stick together, when stored in a cold and somewhat humid barn.

I dissolved a few 100's of grams of the fertilizer KNO3 in water and poured the hot solution with scum in it through a big coffee filter. In this way I obtained a nearly clear solution without scum. This I boiled down until I had a dry solid. When i used that, I hardly suffered from foaming in my distillation.

Where I live, in the recent past we could buy 96% KNO3/4% MgO in 25 kg bags. The prills of MgO could be hand-picked, they were dark grey, while the KNO3 was white. Nowadays, these bags of KNO3 are gone, but I still have several kilo's of it. I keep it, maybe in the future I use it for making HNO3 again.

j_sum1 - 18-8-2025 at 01:49

I have found NaNO3 less foamy than the potassium salt.
Last batch I made was from Fe(NO3)3 and it was a most pleasant experience.

macckone - 23-8-2025 at 12:21

The solution to this is to keep the level of reactants below 25% of reaction flask volume. Bubbling is a result of water and nitric acid being generated in the dry mix. Bisulfate hydrate has a pretty low melting point. And it separates into water and bisulfate with it dehydrating and distilling off nitric acid. You can dehydrated the sodium bisulfate in advance to reduce melting below 300+ C.

RU_KLO - 24-8-2025 at 11:33

Quote: Originally posted by bnull

Dissolve some bisulfate in water, put the solution in a capped flask and shake it. If the bubbles take too long to burst, it means there is a surfactant.

I did this procedure with the bisulfate, no indication (bubbles dont hold), buy when I done with the KNO3, some small bubbles were hold for longer time.

1) cristallized some KNO3 (dissolving a random amount , filtering trough cotton, and once some cristals were made, removed (the orange... solution) and dried the cristals

On testing both (putting in a test tube and shaking), original KNO3 vs filtered KNO3, no much difference was seen,

but then read: that adding methylene blue and using PP test tube, there is a qualitative test (paper attached) for surfactants.

So I took tip of the spatuta of powder methylen blue, added to each PP test tube, shook them and compare. A distintive difference, even after 10 seconds was seen. (Pic)

So the problem was not the bisulfate but the KNO3 (which wasn't like Woelen's prils, but like sugar consistency)

So will try again with the "cleaned" KNO3 if better results.

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