Sciencemadness Discussion Board

Weird Sulphury smell noticed with Sulphuric acid!

Radiums Lab - 19-9-2025 at 13:09

I tried to dissolve some metallic iron in dilute sulphuric acid which I stole from my home's inverter battery(Lead acid one).

Concentration of the acid is around 15%-30%

I did this around 3 time in small quantities and noticed the same smell again and again.

I made a strip of lead acetate and brought it near the mouth of the beaker but no darkening(i.e no H2S was produced)
It was rotten egg smell but not spicy like SO2.

What could this gas be?
Separate Gloves were used and disposed after using lead acetate.
Thanks for reading through this post.

bnull - 19-9-2025 at 13:17

Where did the iron come from?

Radiums Lab - 19-9-2025 at 13:19

Iron nails.

bnull - 19-9-2025 at 13:22

Then the nails are to blame. There are lots of things in iron nails (which are never high quality iron to begin with and tend to leave graphite or some carbonaceous crud floating on the surface of the liquid), and there is invariably an ugly smell when dissolving them in hydrochloric acid. Have you tried other acids?

Edit: Have you tested other metals with the abducted sulfuric acid?

[Edited on 19-9-2025 by bnull]

Radiums Lab - 19-9-2025 at 13:51

There was no carbon crud after the nail dissolved.
I once wanted to make iron sulphate hepta hydrate in a little bigger quantity,
I used a different Iron source(Iron bars used to make house fittings etc.) and got a lot of carbon crud and the same smell but after filtering the solution and testing for iron hydroxide I got a pretty pure dark green ppt. which slowly turned yellow.

@bnull recently in some other post I think you mentioned some gaseous sulphur containing acid or something which smells bad. Is that compound being produced?
Sorry I forgot it’s name.

No I didn’t test with other acids because I wanted to open my conc. HCl, HNO3, H2SO4 or HF bottles after my exam, which will get over by the May of next year.

[Edited on 19-9-2025 by Radiums Lab]

bnull - 19-9-2025 at 15:34

Quote:
I once wanted to make iron sulphate hepta hydrate in a little bigger quantity,
I used a different Iron source(Iron bars used to make house fittings etc.) and got a lot of carbon crud and the same smell but after filtering the solution and testing for iron hydroxide I got a pretty pure dark green ppt. which slowly turned yellow.

The green precipitate is one of those iron layered hydroxides. Next time, boil both solutions prior to mixing. You'll be in for a nice surprise.

If you want to make ferrous sulfate, make a saturated solution of copper sulfate, add more solid copper sulfate, a few nails (thin 1 inch long nails are good) and boil the mixture. You need to rub them a few times when the initial reaction subsides. The solution goes from the nice blue to almost colorless. Decant the solution, transfer it for a suitable flask with very little air over the solution and let it cool. Ferrous sulfate crystallizes on the bottom. Or add ammonium sulfate to precipitate the double salt. It contains some copper sulfate but that won't be a problem for most of the time.
Quote:
@bnull recently in some other post I think you mentioned some gaseous sulphur containing acid or something which smells bad. Is that compound being produced?
Sorry I forgot it’s name.

I don't remember that. The only smelly sulfur derivatives that come to mind are sulfhydric acid, sulfur dioxide, and the mercaptans. If the action of sulfuric acid on iron nails generates any mercaptan, I have no idea. But I'd suppose that what you smelled was a mixture of gases.

Sulaiman - 19-9-2025 at 23:06

I believe that all nails leave the factory with some type of anti-rust coating/finishing,
from the lubricating/cooling oils used by the manufacturing machinery to metalic coatings such as zinc.
if your nails were not rusty then there was probably a coating of some kind.

when bubbles burst they explode tiny fragments of the bubbles into the air,
so traces of almost anything in the beaker, volatile or not, will be aerosolised,
and get up your nose, in your mouth, eyes etc.
(no, not the eyes, becauses we allways wear eye protection, right?)

I find that iron dust has a particular smell/taste.
I guess it reacts with my saliva and/or mucus.
I remember there was a discussion of the taste of metals.

so, there are too many variables.
if it really interests you then you will need to eliminate as many variables as practicable.
eg1 a different source of iron
eg2 partially dissolve the nails, recover, rinse and dry them.
add the now coating-free nails to fresh acid.

just to make life more fun, your battery acid will contain lead, antimony, calcium etc.

too many variables for my limited knowledge.

Radiums Lab - 19-9-2025 at 23:35

I will repeat the exp. if I find a small peice of TMT bar. They catch rust quite easily.

Radiums Lab - 22-9-2025 at 11:56

@bnull can we remove sulfhydric acid since its H2S? Yea it was same acid mentioned in other post.

[Edited on 22-9-2025 by Radiums Lab]

bnull - 22-9-2025 at 15:19

Yep, I was just listing the smelly sulfur derivatives.

Here's something that may help you. Take a couple of small test tubes, put sulfuric acid and iron nails in one (stopper with one hole) and water in the other (stoppered with a cotton ball). Run a rubber or silicon tubing from the acid tube to the water tube so the latter acts as a scrubber. If, as @Sulaiman supposes (I suppose he supposes), the smell is due to an aerosol containing iron ions acting on your nose cells, the scrubber will remove it; if not, then a gas is the culprit. What gas? No idea.

I'm probably the only one to call H2S sulfhydric acid.

[Edited on 23-9-2025 by bnull]

Sulaiman - 22-9-2025 at 17:09

although iron particles are a suspect, that is just one possibility,
any coating is probably more suspect, also any contaminants in the acid.

If you want to post a few nails to me I would repeat your experiment
(using my local nails may give different products)
my analysis would be mostly based on smells,
because my lab is still mostly in boxes :(

I should be home in a few days, I have some malaysian nails and some british nails
- I'll give it a go just out of curiosity.
please show what type of nails you are using.

U2U me for address if you want.

Radiums Lab - 22-9-2025 at 19:52

Thank you for the idea guys. I'll try with TMT bars. If smell is noted I am forced to open the acid

Radiums Lab - 22-9-2025 at 19:56

Acid means new H2SO4 bottle(SQ grade). If theres same smell I will send the Fe to @Sulaiman.

MrDoctor - 22-9-2025 at 20:36

i can recall once when i stripped the zinc off a large piece of galvanized steel to feed into acid to make iron sulfate, it produced a strong sulfide smell as it bubbled away in HCl, i have no idea where it would come from, but, you can detect H2S so easily when it is mixed with lighter gases or just has some mechanism for dispersion. Its not crazy strong, its just there.

Radiums Lab - 22-9-2025 at 21:27

@MrDoctor I tested for H2S but the result was negative. But my observations are almost same as yours

Radiums Lab - 22-9-2025 at 21:33

If I am not wrong, you tried to make FeSO4 using HCl and Fe? How. Or did you remove Zn using HCl?

Sulaiman - 23-9-2025 at 02:11

a plug of cotton wool or similar in the top of your test tube would 'catch' aerosols,
passing only gasses

Texium - 23-9-2025 at 09:11

I also have noticed unpleasant smells when dissolving iron nails in sulfuric acid. I probably asked a similar question here like 10 years ago. At the time, I believe it was suggested that phosphorus, rather than sulfur, could be at least partially to blame for the odor, as trace amounts of phosphine gas.

Radiums Lab - 23-9-2025 at 14:43

@Texium if its phosphine gas where is P coming from? The impurities in acid or the nails.

Radiums Lab - 23-9-2025 at 15:07

I checked other thread and the research is wrong there H2S is not liberated.I have JEE mains on jan.

[Edited on 23-9-2025 by Radiums Lab]

Radiums Lab - 23-9-2025 at 15:11

My syllabus is far from completion so i'll have to shut experimenting for quite a long time.

[Edited on 23-9-2025 by Radiums Lab]

bnull - 4-10-2025 at 02:47

Update: there is no foul smell when attacking iron with citric acid. Room temperature citric acid solution, boiling solution, boiling solution with added ammonium sulfate to increase acidity, none of them produced the stench. There was only a nice lemon smell, like those citrus cleaning products. It is not conclusive but it seems to me that the smell comes from the action of aerosol on the nose cells. I was too lazy to stuff some cotton on the mouth of the test tube.

[Edited on 4-10-2025 by bnull]

Radiums Lab - 4-10-2025 at 22:11

@bnull thanks for the input.

Texium - 5-10-2025 at 06:03

Quote: Originally posted by Radiums Lab  
@Texium if its phosphine gas where is P coming from? The impurities in acid or the nails.
The nails

woelen - 5-10-2025 at 23:39

If you want to try this kind of experiments, then you need really pure acid. I also noticed this kind of smell with different metal pieces in different acids. A few years ago, I decided to try this with reagent grade HCl, diluted to around 10% and added several small M3 locking rings, made of steel. These rings were clean, direct from their plastic blister package, not handled at all. So, no grease from hands or stuff from outside on used rings. On heating, I was greeted with a strong smell, somewhat resembling the smell of technical grade calcium carbide, when added to water.

Because I used reagent grade acid, I am quite sure that the smell is from the rings. Maybe the iron contains a little phosphorus (ferro-phosphorus) and possibly also other non-metal elements. Even a tiny fraction of such elements gives a strong smell (a few PPM of gases like PH3, H2S in otherwise pure HCl gives a clearly noticeable smell).

I have done similar experiments with zinc plates (for construction purposes outside). From these I also obtained a strong smell. Different from the iron rings, but also quite strong and actually quite nasty. I really think that the gas mix from that zinc is quite toxic.

I also did an experiment with aluminium granules, sold to me as 99.99% pure aluminium for element collectors, from eBay. I again used the reagent grade HCl. The H2-gas from this aluminium was free of any smell. I also did the expeirment with little magnesium balls from Amazon.

https://www.amazon.nl/-/en/Magnesium-Metal-99-99-Material-Ma...

With these, I also had no real smell. The reaction was very vigorous, and I had a strong stinging sensation (more of a tingle in the nose than a real smell). This most likely is from the acid itself. It becomes very hot when a few of these beads are added to 10% HCl and a lot of steam was produced as well.

I did a final experiment with high purity lab grade zinc (Merck, 99.99% Zn-granules, coarse powder, old stuff). With these I also had a fairly vigorous reaction, but no smell. This was in strong contrast with the plates of zinc for construction purposes.

---------------------

Al in all, my conclusion is that this kind of weird smells is from impurities in the metals, especially in technical grade metals, used for construction. Sulfides, phosphides, or more complicated sulfur- or phosphorus organic compounds.

Another remark I can place. With H2SO4 you can get smelly things, even with reagent grade acid. Zinc is capable of reducing a tiny fraction of the acid to H2S and you can smell that. If I add my Merck 99.99% zinc to very pure dilute H2SO4, then I notice a very weak rotten egg odor, but it is definitely there. I think that this is from reduction of the acid. I, however, did not smell this with the Mg-beads.

pesco - 10-1-2026 at 16:07

Pretty much all grades of steel contain some S and P either as impurity or added deliberately. Sulfur content generally improves machinability and phosphorus and tensile strength. There are more benefits and drawbacks of P and S content, but this is not metallurgy forum :D

I also have dissolved scrap steel and various acids and the smell was always present. Never bothered to check what exactly was is causing the smell as dissolution was always outside, just under some sort of canopy to protect from rain.
Always assumed that culprit are compounds of P and/or S.
Even if they were PH3 or H2S the amount was negligible and they had no chance to accumulate when reaction was run outdoors.