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Author: Subject: Sulfuric acid purification problems
Sir_Gawain
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[*] posted on 15-11-2023 at 15:51
Sulfuric acid purification problems


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.




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[*] posted on 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.




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Sir_Gawain
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[*] posted on 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]




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[*] posted on 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.

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Sir_Gawain
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[*] posted on 15-11-2023 at 20:01


Was your acid cloudy to begin with?



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[*] posted on 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
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[*] posted on 16-11-2023 at 01:32


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




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[*] posted on 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]




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[*] posted on 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.




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[*] posted on 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#

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[*] posted on 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.
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[*] posted on 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 141 times

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Sir_Gawain
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[*] posted on 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.



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Sir_Gawain
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[*] posted on 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




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[*] posted on 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]




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Sir_Gawain
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[*] posted on 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.




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[*] posted on 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.


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[*] posted on 16-11-2023 at 14:32


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



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[*] posted on 16-11-2023 at 14:45


Quote: Originally posted by Rainwater  
Flame test.




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[*] posted on 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.




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[*] posted on 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.
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[*] posted on 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.



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[*] posted on 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.




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[*] posted on 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.
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[*] posted on 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?




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