Sciencemadness Discussion Board

Sulfuric acid from distillation issues

zwalker900 - 17-9-2019 at 10:09

Hi everyone!
This is my first post so I hope I am asking this in the correct place.

I have been attempting to obtain pure azeotropic SA via distillation from a couple different sources.

1. Super Start Battery Acid (~30% initial concentration MSDS)
2. Kleen-Out Drain Cleaner (90-93% initial concentration MSDS)

When distilling the battery acid, I boiled off as much of the water content as I could, then added it to my boiling flask. At This point, the acid is VERY yellow (Not sure why because I did not think battery acid has any inhibitors?). I then distill the acid. The first few fractions are obviously discarded as the are mostly water. Once I see that the density is no longer increasing, I begin collecting.

The weirdest thing is occurring every time I do this process though. The distilled acid has a slight yellow tint, and even has tiny suspended black impurities.

The same thing occurs when distilling the drain cleaner; however, obviously in this process I don't do any "preboiling". The yellow tint is less and there are less impurities when compared to using the battery acid, but it's still there.

If anyone has any suggestions it would be greatly appreciated.

mackolol - 17-9-2019 at 10:54

These are probably impurities, as both drain cleaner and battery acid are rather not highest grade.
When you boil the acid, it would react with those and they distill along.

zwalker900 - 17-9-2019 at 11:13

Any suggestions on removing these? I have seen a lot of other people that have used this method without any issues.

12thealchemist - 17-9-2019 at 11:24

The simplest solution would be to simply redistil. An alternative method I've seen for dealing with impurities like this is to add hydrogen peroxide (CAREFULLY) and let the resulting piranha solution destroy the impurities. Then distill/boil to purity/concentrate.

zwalker900 - 17-9-2019 at 11:40

I suppose re-distilling is probably the best approach. I have not tried the h2o2 method before simply because obtaining decent concentrations isn't easy

12thealchemist - 17-9-2019 at 11:58

I believe hydrogen peroxide can be concentrated by boiling it down. I'm not sure how concentrated you can get it, but I'm told that works. Alternatively, piranha will work even at 50% acid concentration, and given that the impurities will be fairly small due to the first distillation, using dilute peroxide will probably work well enough to at least remove the colour.

zwalker900 - 17-9-2019 at 12:12

In my experience, the boiling of store-bought h2o2 doesn't work particularly well because it decomposes fairly quickly as you near boiling point. I suppose one could try vacuum distillation? or instead of actually boiling it, just raise the temperature enough to increase the vapor pressure of the water and blow a fan across it? Idk, either way I think I will try the re-distilling.

[Edited on 17-9-2019 by zwalker900]

Praxichys - 17-9-2019 at 17:59

You're just looking to create an oxidizing environment to destroy the junk in the acid. H2O2 is convenient because it disappears after doing its work, but there is nothing preventing you from using another oxidizing species like dichromate or persulfate. Just be aware that dichromate will leave behind CrO3.

Don't try this with perchlorate, chlorate, or permanganate since explosive species will form.

Nitrate may also work but will probably take a while owing to the loss of HNO3 before the temperature gets high enough for rapid oxidation.

happyfooddance - 17-9-2019 at 18:02

Is your acid still hot?

PartVIII - 17-9-2019 at 18:08

The yellow-colored impurity almost certainly won't affect any reaction. If you plan to use it in a reaction which involves washing the organic layer with water, then the color will probably come out with the aqueous layer.

If you do plan to use it for a reaction, then you should titrate your sulfuric acid or get an accurate density measurement to determine its purity.

As a side note: if you want to purify H2O2, then exploit its melting point (-0.43C). Freeze 3% and collect what melts first. This process is called Fractional Freezing.

zwalker900 - 18-9-2019 at 06:25

I plan on using this sulfuric in two ways:

1. To produce nitric acid via distillation (In this circumstance the yellow impurities certainly are trivial)

2. To use in conjunction with said nitric to produce ETN.

Would it matter that the sulfuric isn't perfectly pure for this reaction anyway? I understand this is more of an "Energetic Materials" question but at least this thread gives the context.

And no the yellow color is unrelated to temperature.

[Edited on 18-9-2019 by zwalker900]

[Edited on 18-9-2019 by zwalker900]

Mush - 18-9-2019 at 10:19

Purifying Sulfuric Acid Drain Cleaner

Recovery, Concentration and Purification of Mineral Acids

Mineral acids are very important in chemical procedures. Their uses vary from process to process but include reaction participants, catalysts, aids (e.g. sulfuric acid for drying gasses or during extractive distillation), and cleaning agents. Through our experience with corrosion-resistant materials, we have process know-how in mineral acid handling including:

Absorption of Hydrogen Chloride
Sulfuric Acid Concentration
Nitric Acid Concentration
Sulfuric Acid Dilution

Sulfuric acid purification process WO1998043716A3

A process for purifying an aqueous sulfuric acid solution includes cooling an aqueous sulfuric acid solution to at or near its freezing point to form a slurry of a solid phase and a liquid phase. The slurry has an acid-rich region and an acid-poor region. The acid-rich region is separated from the acid-poor region on the basis of density.

[Edited on 18-9-2019 by Mush]

Pumukli - 19-9-2019 at 05:31

Eons ago I also concentrated sulfuric acid from battery acid (which is 37 m/m % here).
My acid also got yellow and the small, black "dots", some sort of solid impurity was surely there!

For some time (several months) I did not used this acid and when I opened the bottle again and poured some acid into a beaker I noticed two things: the yellow discoloration was turned into a "rustic" looking brown hue, similar to old whisky/whatever. The black dots disappeared.

I think with time the concentrated acid oxidized the impurities further. Maybe those black dots are actually carbon from dehydrated organic contaminants, e.g. dust/micro fibers from clothes, etc.

Btw. the uncleaned acid was good for everything I used it for. :)

PirateDocBrown - 19-9-2019 at 15:58

You can readily get 25% + H2O2 at many pool and spa stores. I used it for years to sanitize my hot tub.

Used by hydroponics people for cleaning, too. Try a store for that, also.

[Edited on 9/20/19 by PirateDocBrown]

monolithic - 19-9-2019 at 16:15

Quote: Originally posted by zwalker900  
In my experience, the boiling of store-bought h2o2 doesn't work particularly well because it decomposes fairly quickly as you near boiling point. I suppose one could try vacuum distillation? or instead of actually boiling it, just raise the temperature enough to increase the vapor pressure of the water and blow a fan across it? Idk, either way I think I will try the re-distilling.

[Edited on 17-9-2019 by zwalker900]

This is how I've seen it done with success -- low and slow over a hot plate in very clean glassware. Obviously you want to keep a close eye on the volume as hydrogen peroxide can explode if it's highly concentrated. There's a decent video on YouTube demonstrating this concentration.

Apparently freezing also works.

[Edited on 9-20-2019 by monolithic]

XeonTheMGPony - 19-9-2019 at 19:48

Health care stores that sell all the fake crap like homeopathy usually have 30 to 35% peroxide.

So ask there too.

C6(NO2)5CH2CH(CH3)N(NO2)2 - 20-9-2019 at 15:01

Quote: Originally posted by PartVIII  

As a side note: if you want to purify H2O2, then exploit its melting point (-0.43C). Freeze 3% and collect what melts first. This process is called Fractional Freezing.

On a side note, this definitely works for slight increases in concentration, but I haven't been able to overcome one big problem for big leaps in concentration. Freezing 3% H2O2 at around -20C will give 1 part of 30% peroxide solution to 9 parts of ice. Mixtures of peroxide and water freeze at a much lower temperature than either neat hydrogen peroxide or water. all the concentrated peroxide gets absorbed into the ice. It is similar to getting fabric or dirt slightly damp with water; The liquid phase sticks to the solid, and will not settle to the bottom under normal gravity. It can't be pressed out by hand in my experience, and it definitely can't be filtered. If anybody has made this process work, then I'd love to hear how.

It might be worth it to try evaporation, or maybe an alternative medicine store. The bummer with evaporation is that hydrogen peroxide is harmful to inhale (the permissible exposure limit is 1 ppm, similar to chlorine and nitrogen dioxide), so you can't do it inside without a fume hood, and dust will add more impurities and cause decomposition, so you can't do it outside.

As far as the acid? That's odd that it has a tint after distillation. I'd assume some organic material distilled over first, and was not entirely cleaned out.

I think I'd try the sulphuric acid for nitric acid production and even a very small volume test nitration, and if it works and has a good yield you know the impurity is OK. If not, re-distill with the cleanest equipment you can muster.

XeonTheMGPony - 21-9-2019 at 05:30

I've distilled allot of different drain cleaners and never had this issue, You may have some contaminants in your distillation condenser and receiver.

As for purity, I will use peroxide if it is particularly nasty, I add in 100ml @ 35% then heat with a Vigreux column till it starts fuming, by this point it is crystal clear.

I then set up for a slow distillation (Slower it is the more pure it will be, but a lot more time and energy is used.

First fraction will be weak (Save this it is useful for cleaning and much other stuff)

Save from then on out when it will instantly melt paper.

Save the tailings (Stuff left inside the flask) as this is ideal stuff for making Sulfur Trioxide with.

When you do manage to make Sulfur Trioxide add this to your concentrated fraction to further increase its strength

As suggested Hydroponics shops and "Alternative" medicine shops will have it in the 30 and up percent range

ManyInterests - 3-12-2021 at 10:34

I've had some issues like this, but somewhat differently... while boiling the acid instead of distillation.

So I use my 35% H2O2/hydrogen peroxide added en masse to my drain cleaner and I heat it to around 60C with as strong stirring as possible. After around 30 minutes the acid still fizzes slightly, but is otherwise completely clear.

So I remove the stir bar (and thermometer) from my boiling flask and crank up the heat. The problem is that while heating, it goes from the crystal clear color it was to a very dark brown. Almost like the impurities were playing a disappearing act and came back once heat was applied.

It could be the brand of drain opener I am using, but I am confused. I am trying to concentrate and purify more acid right now, but it looks like I will need to add more peroxide in order to clean it up even more just in case...

macckone - 4-12-2021 at 01:03


You are probably getting partial oxidation and cleavage in the first part yielding a clear product, then when you heat it, you are getting tar which coagluates.
If you add more hydrogen peroxide it will likely oxidize further to CO2.
You could also have inorganics which are turning to oxides in hot sulfuric acid.
So it could be separate impurities.

ManyInterests - 4-12-2021 at 06:31

Yeah. This is what likely happened.

I cooled down the acid (it would not drop below 95 C for some reason) I monitored the temperature as I added pipettes worth of H2O2 there to try to clear it up. It worked, but it was still off-green at the end.

I added so much of my hydrogen peroxide that my supply nearly ran out. I figured the off-green is a result of the peroxide mixing with the acid. I had to boil it off. I added so much that the volume of my flask was well above 500ml, and I boiled for a long time to get the amount less than the 500ml (my starting volume of sulfuric acid). There is still some very mild discoloration, but I can live with that since I believe it is quite concentrated by now. Maybe not 98%, but close enough.

There is also some something precipitate at the bottom. I think it's the boiling chips that got ground down.

Edit: OK I am not sure what that white percipitate is, but a bit of it is also in suspension. The acid is THICK. This is unquestionably the most concentrated acid I made. I will need to store it seperately from the previous acid runs.

Edit2: I think the reason why I had a problem with this acid concentration/purification is because I mixed the drain cleaner from two different brand. One of those brands (which is thankfully used up) was an absolute beast to clean up. I used three different brands and I think I know which brand to buy and which to avoid now.

[Edited on 4-12-2021 by ManyInterests]

[Edited on 4-12-2021 by ManyInterests]