Sciencemadness Discussion Board

Sulfuric acid purification problems

Sir_Gawain - 15-11-2023 at 15:51

While I usually buy the Walmart Kleen-Out brand drain opener, I decided to try the Ace Hardware Rooto stuff because the msds seemed to indicate a higher purity. Instead of the nice clear liquid I was expecting, it was cloudy with some white precipitate.



IMG_0064.jpeg - 1.4MB
After running it through a filter it came out clear:

IMG_0062.jpeg - 1.7MB

I added some hydrogen peroxide and brought it to a simmer. I couldn’t heat it for long because it got dark. I did notice that it turned yellow but reverted to clear as it cooled. After it cooled I measured the density and found it was only about 94%. I guess it wasn’t boiled long enough. I tried again today and this time I boiled it longer. But when it cooled, a precipitate formed.

IMG_0063.jpeg - 1.4MB
What’s going on? The only thing I can think of is that there’s a water soluble substance that precipitated when the acid got more concentrated.

Rainwater - 15-11-2023 at 16:43

Quote: Originally posted by Sir_Gawain  

What’s going on? The only thing I can think of is that there’s a water soluble substance that precipitated when the acid got more concentrated.

nailed it.
you can try to collect and analyze the precipitate to see if it will interfere with your chemistry, or distill it to remove the contamination.

Sir_Gawain - 15-11-2023 at 16:59

Distilling sulfuric acid is not something I feel comfortable doing. I’m afraid my condenser’s going to explode.
I have no idea what the precipitate is; it can’t be organic or it would’ve been destroyed by the heat and peroxide. I do remember reading somewhere here that someone had a crystalline material appear after distilling drain opener, so maybe it’s the same thing.
Edit: I just checked the density and it’s about 97% sulfuric acid.

[Edited on 16-11-2023 by Sir_Gawain]

AvBaeyer - 15-11-2023 at 18:57

Some time ago I mentioned a problem that I had with Rooto sulfuric acid. I always wash my hexane with sulfuric acid to remove some initial impurities before distillation. I have always used sulfuric acid from Duda for this purpose. However, I had some of the Rooto acid to use up so I used it instead. Shaking Rooto acid with the tech hexane gave an emulsion that took some time to separate which had never happened with the Duda acid. To get to the point, I suspect that the Rooto acid contains some sort of detergent like material which is what you are seeing as a contaminant. There may be no practical way to get rid of it.

Just my long winded 2 cents worth.

AvB

Sir_Gawain - 15-11-2023 at 20:01

Was your acid cloudy to begin with?

Achilles - 16-11-2023 at 01:13

it is normal in every sulfuric acid you buy it will turn yellow if heated and clear when cool, the only way to get rid of it is distillation.. extremely slow and the distillate will get water from the air so its complicated, unless you need it for tritrations it still work well just for nitric acid youll get a 80% since it will oxidize the impurities and get the water from it

Rainwater - 16-11-2023 at 01:32

You can try and freeze it out using an ice bath.
Melting point is about 10c.

Sir_Gawain - 16-11-2023 at 05:11

Quote: Originally posted by Rainwater  
You can try and freeze it out using an ice bath.
Melting point is about 10c.

I had no idea the melting point was so high. I've cooled sulfuric acid in a freezer to -18C and it's never frozen. Do you think that 2% of water lowers it that much?

[Edited on 16-11-2023 by Sir_Gawain]

Sulaiman - 16-11-2023 at 06:44

Just a couple of quick notes regarding distillation of sulphuric acid:

The scariest bit of chemistry I've done for a long time.
Bumping is a major hazard - boiling hot concentrated sulphuric acid will destroy anything organic, skin, eyes etc.
No chance of remedial actions no matter how quickly you act.

If you do distill, then collect and keep all of the fractions... all useful.

No need for peroxide or other additives.

Cheap Chinese borosilicate glassware is more than adequate.

.......................................
I've had success and failures (bumping) using boiling stones,
If I do another distillation I will try air via an aquarium air pump, drying tube and a glass tube drawn to a capillary to produce a stream of tiny bubbles.
In Malaysia I can buy Reagent or Analyytical grade acids via a home shopping sites, so I will probably not distill sulphuric acid again.

yobbo II - 16-11-2023 at 06:48



If you want to know more with great detail get some (good soul) to get you this

https://pubs.acs.org/doi/pdf/10.1021/ja01141a010#

Yob

Tsjerk - 16-11-2023 at 07:39

Quote: Originally posted by Sir_Gawain  
Quote: Originally posted by Rainwater  
You can try and freeze it out using an ice bath.
Melting point is about 10c.

I had no idea the melting point was so high. I've cooled sulfuric acid in a freezer to -18C and it's never frozen. Do you think that 2% of water lowers it that much?

[Edited on 16-11-2023 by Sir_Gawain]


Yes, that little bit of water drops the freezing point significantly.

leau - 16-11-2023 at 08:45

Four more documents including that mentioned up thread are in the attached archive :cool:

Attachment: SulfuricAcid.zip (5.5MB)
This file has been downloaded 140 times


Sir_Gawain - 16-11-2023 at 08:55

Thanks, leau! I wonder, would be possible to purify sulfuric acid by freezing it? While it probably wouldn't raise the concentration much, it might exclude soluble contaminants.

Sir_Gawain - 16-11-2023 at 11:25

Ok, so I scraped the precipitate off of my filter and it’s not soluble in water.It actually settled out pretty quickly, which makes sense given water’s lower density and viscosity than sulfuric acid.

IMG_0065.jpeg - 1.3MB

Rainwater - 16-11-2023 at 11:49

Flame test.
Found this while googleing
Screenshot_20231116_144509_Samsung Notes.jpg - 119kB
solubility of CaSO4 in H2SO4
Fits the observations so far.
Insoluble in water
White powder
Test:
It shouldnt react with any mineral acid, sulfuric, nitric, HCl
Heat a sample red hot. Let cool, crush into powder
add a few drops of water.
If it sets rock hard you for CaSO4.

[Edited on 16-11-2023 by Rainwater]

Sir_Gawain - 16-11-2023 at 11:58

Calcium sulfate was also my first guess. Are calcium compounds used at any point during the manufacture? Perhaps some CaO was used to clean up and got in it.
I could try dissolving some CaSO4 in sulfuric acid and seeing if it behaves the same way.

unionised - 16-11-2023 at 14:15

Quote: Originally posted by Rainwater  

Heat a sample red hot. Let cool, crush into powder
add a few drops of water.
If it sets rock hard you for CaSO4.

[Edited on 16-11-2023 by Rainwater]


If you are trying to make gypsum plaster you don't heat it to red heat.
That will convert it to anhydrous stuff and that doesn't react with water.
You need to heat it somewhere about 200 or 250C, I think.



Sir_Gawain - 16-11-2023 at 14:32

I’ve filtered about a liter of it and don’t have nearly enough to try this test.

Sulaiman - 16-11-2023 at 14:45

Quote: Originally posted by Rainwater  
Flame test.

Sir_Gawain - 17-11-2023 at 11:09

Quote: Originally posted by Sulaiman  
Quote: Originally posted by Rainwater  
Flame test.

Next time I filter some I'll try it. By the way, 24 hours later and the precipitate is almost completely dissolved into the water. It's gotta be calcium sulfate.

Cathoderay - 17-11-2023 at 18:49

Manufacturers mix additives to sulfuric drain cleaners (inhibitors) so that it is less corrosive to metal pipes (cast iron, etc.). It isn't so much a impurity of manufacture situation, although for drain cleaning use the impurities in the raw acid don't matter much.
The inhibitors could be organic or inorganic.

Sir_Gawain - 17-11-2023 at 19:40

Any organic inhibitors would have been destroyed by boiling with hydrogen peroxide. I don't know of any inorganic inhibitors, but that's probably what this is.

Sir_Gawain - 19-11-2023 at 18:06

To test for calcium sulfate, I added a solution of sodium carbonate to the beaker I had dissolved the precipitate in. A brown solid immediately formed.

IMG_0079.jpeg - 846kB
Not what I expected.

Cathoderay - 19-11-2023 at 19:56

Seems like "Out of the frying pan and into the fire."
That seems like that is a lot or precipitate, much more than the amount of inhibitor I would expect.

What exactly did the MSDS say?

How much hydrogen peroxide (compared to the acid) and what was the concentration?
Maybe you didn't completely destroy the organic component.

Are you sure the filter was clean?
The sodium carbonate pure?

Calculating the percentage based on the density only works if it is only acid and water.

Calcium sulfate might work as an inhibitor.

Sir_Gawain - 19-11-2023 at 21:18

1. There's not as much there as it looks like; it's really low density.

2. The MSDS says 93% sulfuric acid and 7% water. It makes no reference to any inhibitors.

3. I added about 25 mL of 34% hydrogen peroxide and boiled it for over 30 minutes.

4. I ran some piranha solution through it before filtering the sulfuric acid. It's fairly new and I haven't used it for anything that could get stuck in it.

5. The sodium carbonate was washing soda from the grocery store, which is pretty pure.

6. Calcium sulfate is my best guess, but judging by the brown color that's not all. Maybe iron?

Cathoderay - 20-11-2023 at 09:51

OK. That type of precipitate is called "flocculent". That could be something organic if you didn't use enough peroxide or cook it long enough.

The MSDS is obviously not accurate. Most of the time info on a MSDS is stated approximately because the exact formula is proprietary (company secret) and the exact
amount of contents are not not important for safety. The sheet would often state something like Sulfuric acid 92 - 94%, Water 6 - 8%. That sheet should state that there is something else there, if the inhibitor is very safe and only there in a very small amount maybe they don't have to include it. Giving a range also makes it easier on the manufacturer because a MSDS is somewhat of a legal document and by giving a range they don't have to guarantee an exact percentage or test for it.

You didn't say how much acid there was that you added the hydrogen peroxide. In chemistry ratios are important. In your original post you said you stopped boiling because the color go dark, why did you do that?

I don't know how much peroxide is enough. Just how pure the sodium carbonate is would be another question. Someone else might have an idea of what that brown stuff is.

fx-991ex - 20-11-2023 at 12:44

$$H2SO4 + Na2CO3 \rightarrow Na2SO4 + CO2 + H2O$$
Could the carbonate react with the acid to make sodium sulfate?
I guess the acid was removed completly and replaced with water from the filtering/precipitate, just making sure.

[Edited on 20-11-2023 by fx-991ex]

Sir_Gawain - 20-11-2023 at 12:54

Quote: Originally posted by Cathoderay  

You didn't say how much acid there was that you added the hydrogen peroxide. In chemistry ratios are important. In your original post you said you stopped boiling because the color go dark, why did you do that?

The peroxide was added to about 250 mL of sulfuric acid. I'm sorry if I was unclear; I meant it was night time, not that the acid got dark.

Cathoderay - 21-11-2023 at 09:53

I thought I posted something last night but it isn't here now. I must have clicked the wrong thing.
To figure out what that stuff is it is important to rule out organic inhibitors.
Reading between the lines of this NileRed video, it seems like you added enough peroxide. Maybe you didn't boil it long enough.
https://www.youtube.com/watch?v=4DUGRWjdNLI
Try boiling it some more and maybe added more peroxide.

A flame test is a good idea if there is something like calcium sulfate in it. Don't add anything with sodium beforehand because the bright sodium flame can mask other things.
This Wikipedia link has a good video link in it.
https://en.wikipedia.org/wiki/Flame_test
You may want to neutralize some of the high acidity before doing tests. The best thing would be ammonium hydroxide since that will not add any metals that you might want to test for later.

I wouldn't be surprised if the acid the cleaner was made from was the cheapest they could find, not laboratory grade. It may even have been "once used" by another chemical plant to clean vessels of slight water and contamination. The cleaner manufacturer then added the inhibitor and maybe a little more water. After all the intended use is to remove muck from drain pipes, then into the sewer.

I'm running out of ideas, maybe someone else can add to this.

Sir_Gawain - 21-11-2023 at 19:35

I don’t think it’s organic. Over 300*C piranha for 30 minutes probably would destroy any organic material. Next time I concentrate some, I’ll boil it longer.

Keras - 22-11-2023 at 01:33

Speaking of, I wonder if anyone has already recycled sulphuric acid used for diethyl ether synthesis. Since the reaction forms water and some other products, but the sulphuric acid being mainly catalytic should remain more or less untouched, it would be nice to be able to recover it. However, barring distillation, I don’t really see how to do it.

Sir_Gawain - 22-11-2023 at 04:49

If you use pure ethanol, all the by products should be organic and you can purify it by boiling it down. If you have hydrogen peroxide, add some.

Cathoderay - 22-11-2023 at 11:23

I noticed in the comments of that NileRed video that someone said they tried the peroxide method with several different brands of drain cleaner. The person said that three were found that did not work with that method. The brands were not listed.

BTW, not to nit-pick (but nit-picking kind of goes along with science), but did you measure the temperature? Didn't you say you simmered it in your original post?

Sir_Gawain - 22-11-2023 at 12:43

Judging by the fact that the sulfuric acid was slightly boiling and letting off a huge cloud of acid vapor, I'd say it was around 300 C. The hotplate I used can melt lead, and when it was dark the first time I could actually see it glowing.

akmetal - 23-11-2023 at 02:25

I just bought 30% sulpheric from Rileys auto parts, I havent made it to the sulpheric distillation yet but the distillation curve is very wide and I suspect that its going to be very easy to boil off most of the water with only maybe 4 stages of distillation (theoretical stages using rashing rings).

Seems like alot of work to try to use some multi component source, this is a good time to use that water asperator or eductor on the outlet of the condenser?

akmetal - 23-11-2023 at 02:28

Quote: Originally posted by Sir_Gawain  
Judging by the fact that the sulfuric acid was slightly boiling and letting off a huge cloud of acid vapor, I'd say it was around 300 C. The hotplate I used can melt lead, and when it was dark the first time I could actually see it glowing.


Do you guys monitor the top and bottom still temps, I built my own top side temp probes and watch on a raspberry pi and my heat source has a display.


Rainwater - 23-11-2023 at 05:09

The battery acid source is very pure sulfuric acid around %30 consentration
Very expensive to, here it is over $30 per quart.
If your distilling anyway, why not use the cheap drain cleaner?

When consentrating H2SO4, i have found that using a simple distillation is more
than sufficient. Ive never needed to add packing. The water that comes over has
traces of acid, but after distilling a full litter of battery acid, it took less than 0.5g of
baking soda to bring the ph of the distillate over 7. Sulfuric is easy to seperate.

Using a vacuum distillation will lower the boiling temperature, and help with any
leaks in the appratus. Also, adding a thin capillary bubbler will help with bumping.

Take extream safty precautions. Boiling sulfuric acid gives no second chances.
One drop will scar you for life, half a liter will shorten that life so you dont have to
worry about the scars

[Edited on 23-11-2023 by Rainwater]

akmetal - 24-11-2023 at 00:43

Quote: Originally posted by Rainwater  
The battery acid source is very pure sulfuric acid around %30 consentration
Very expensive to, here it is over $30 per quart.
If your distilling anyway, why not use the cheap drain cleaner?

When consentrating H2SO4, i have found that using a simple distillation is more
than sufficient. Ive never needed to add packing. The water that comes over has
traces of acid, but after distilling a full litter of battery acid, it took less than 0.5g of
baking soda to bring the ph of the distillate over 7. Sulfuric is easy to seperate.

Using a vacuum distillation will lower the boiling temperature, and help with any
leaks in the appratus. Also, adding a thin capillary bubbler will help with bumping.

Take extream safty precautions. Boiling sulfuric acid gives no second chances.
One drop will scar you for life, half a liter will shorten that life so you dont have to
worry about the scars

[Edited on 23-11-2023 by Rainwater]


Which one is worse 98% sulpheric or 98% nitric?

Sir_Gawain - 24-11-2023 at 10:40

Distilling nitric acid is waaaaaay easier! Pure nitric acid has a boiling point lower than water. Even boiling nitric acid is far less dangerous than hot sulfuric.

Rainwater - 25-11-2023 at 05:30

Without trying to sound like im belittling the danger factor, nitric acid is easier to distille.
Boiling acid, is just that.
Nothing safe about it, but can be done safely as long as you take proper precautions.

I try to setup my appratus in such a way, if the entire thing shattered and liquids went everywhere,
it only requires me to put the on/off switch into the off position and wait for stuff to cool.
Ive rigged most my stuff to be worked remotely so i can be at a minimum safe distance to make
adjustments.
when things go bad, they go bad way over there, far away form people, places, or
things i care about

akmetal - 25-11-2023 at 13:16

Quote: Originally posted by Rainwater  
Without trying to sound like im belittling the danger factor, nitric acid is easier to distille.
Boiling acid, is just that.
Nothing safe about it, but can be done safely as long as you take proper precautions.

I try to setup my appratus in such a way, if the entire thing shattered and liquids went everywhere,
it only requires me to put the on/off switch into the off position and wait for stuff to cool.
Ive rigged most my stuff to be worked remotely so i can be at a minimum safe distance to make
adjustments.
when things go bad, they go bad way over there, far away form people, places, or
things i care about


Thats a good idea, I have a switch panel with each section on a switch but its under the apparatus which I need to move it. I have a blow down fan that blows down the whole room in seconds.

Its probably fair to say that while nitric is "better/easier" you dont want to be breathing in either one. In order to get pure nitric you have to break the azeotrope though. Has anyone ever used molecular sieve to do a final clean up of nitric to get it super pure? I built a rough one at the very end of the condenser on second stage distillation but still a long way from testing it.

ave369 - 25-1-2024 at 23:34

Quote: Originally posted by Sir_Gawain  
Distilling sulfuric acid is not something I feel comfortable doing. I’m afraid my condenser’s going to explode.
[Edited on 16-11-2023 by Sir_Gawain]


Try my old condenser saving trick. There should be two condensers: one air-cooled precooler and one water-cooled main condenser. In the precooler, sulfuric acid vapors get precooled to a temperature that is comfortable for the water-cooled condenser. Concentrated vapors condense in the precooler and flow to the main condenser as a liquid.

Sir_Gawain - 26-1-2024 at 12:09

If I had a good quality Pyrex condenser I would try it, but I only have a cheap Chinese one. Tom from Extractions&Ire distilled sulfuric acid using a similar condenser, air cooled, and it still broke. I haven't ran into any purity problems with my sulfuric acid yet, so I don't think distillation is worth the risk.