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Author: Subject: Concentrating H2SO4 without distillation (with flask ans some PTFE )
rockyit98
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[*] posted on 25-7-2019 at 17:01
Concentrating H2SO4 without distillation (with flask ans some PTFE )


about 2 years ago I discovered something very useful Accidentally on Purpose .
That is you can make up to 99%(I know H2O and H2SO4 Azeotrope is 98%) Concentrate H2SO4 by this simple setup.I know it seems like magic but it not.it use Graham's law of effusion (also called Graham's law of diffusion) https://en.wikipedia.org/wiki/Graham's_law coupled with Raoult's law and Dalton's law. since water is about 5 times lighter than Sulfuric Acid it will diffuse faster.
if you think i'm lying hold a pH paper above to check the Difference.

IMG_20190726_053319[1].jpg - 216kB
it uses 30-35% acid from battery suppler mix in with glass chips to prevent flash boiling .The opening of the flask is covered by 2-4 layers of plumbers PTFE tape if you use thick ones 1-2 will suffice.just try one layer first you can add more if necessary. secure them in place using CAPTON tape.

Use a sand heating to evenly and slowly heat the flask.otherwise PTFE layer will pop. i recommend flask with a big surface area opening(this is what i had in hand)
let the water escape leaving behind acid . i don't recommend pushing this to the extreme. note in hand don't boil the content in the flask just let it evaporate for safety resonances.
pros - all acid will concentrate in signal run
easy to clean just one flask
cons - this doesn't purify the acid
take bit more of time
need attention

BE CAREFUL!
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draculic acid69
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[*] posted on 26-7-2019 at 06:00


So your saying heat dilute h2so4 in a flask to concentrate it.thats not really a new thing.
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Sulaiman
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[*] posted on 26-7-2019 at 06:38


The apparatus looks 'sensitive' to overheating,
but if 99% is correct then that is very interesting,
(whenever I've tried simple boiling I had to stop well before azeotropic due to the copious quantity of fumes)
(and I've not done a true distillation as I don't want to risk my Chinese glassware and/or my vacuum pump)
how confident are you that the concentration is greater than azeotropic ?


P.S. is the ptfe tape 'separating' water and acid ?
if so where does the energy come from ?
or is it just the pressure difference across the (assumed) semipermeable membrane ?
Whatever the mechanism - good thinking - great experiment :cool:

[Edited on 26-7-2019 by Sulaiman]




CAUTION : Hobby Chemist, not Professional or even Amateur
(suffering from separation of me and my chemistry stuff)
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Tsjerk
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[*] posted on 26-7-2019 at 07:37


I would love to see this repeated by someone else!
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RedDwarf
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[*] posted on 26-7-2019 at 08:53


I can believe this might work - membrane separation due to greater permeability of the much smaller water molecule, and much more likely to work in the vapour phase with less interaction between the acid and the water molecules - but I'll need to finish building my fume cabinet before i'd want to give it a try.
Just doing a quick google search shows a number of papers / articles on membrane separation involving sulfuric acid and water including:
https://patents.google.com/patent/WO2004074811A2
https://pdfs.semanticscholar.org/4da8/1b9d5a2d1342865c68c0d1...
https://inldigitallibrary.inl.gov/sites/sti/sti/3562835.pdf :
"Pervaporation was also applied with similar results for the concentration of sulfuric
acid. Decomposition of sulfuric acid is equilibrium limited requiring a recycle stream.
Concentration by distillation involves serious corrosion issues at the vapor-liquid interface. In
this work, Nafion-112® was used successfully as a membrane to remove water from sulfuric
acid using pervaporation. "

Excellent idea!
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happyfooddance
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[*] posted on 26-7-2019 at 09:33


Yeah, this is a really good idea, thanks for this
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WGTR
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[*] posted on 27-7-2019 at 06:56


Interesting. What is the width of your tape? Does it cover the mouth of the flask in one piece? The typical hardware store ptfe tape that we have here is 12mm wide.

Do you have a part number (etc) for the tape? There are three densities of ptfe tape available from what I can see at McMaster: https://www.mcmaster.com/#ptfe/=e44a82f26bce45b3ab895274af6a...

I figure that the widest possible tape would be the best option (about 5cm in this case). When you are heating the flask, is there condensation on the inside of the ptfe film, or is it dry?


[Edited on 7-27-2019 by WGTR]




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RedDwarf
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[*] posted on 27-7-2019 at 07:12


Here in the UK there are 3 types of ptfe tape :
yellow for gas ( heavier weight, not sure if it's thicker or denser or both)
white for water
pink (I've never seen this) which is apparently even heavier than the yellow.
Like WGTR all the tape I've ever come across has been 12 mm wide but I have managed to find a 50 mm online.

Further question : How long did you have to heat to remove what volume of water?

Edit- I'm guessing that you don't want to heat so much that you have condensation on the underside of the tape as that would reduce the surface area available for transport?

[Edited on 27-7-2019 by RedDwarf]
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rockyit98
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[*] posted on 27-7-2019 at 11:26


PTFE tape stretch easily You can even use 10mm ones if have to.it just matter of how many layers.
I used 20mm .
maybe can do phosphoric acid .I don"t got any.:(
(I was meant to get 90% phosphoric acid by heating mono calcium phosphate.got no time:( )
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[*] posted on 27-7-2019 at 12:10


Quote: Originally posted by rockyit98  
PTFE tape stretch easily You can even use 10mm ones if have to.it just matter of how many layers.
I used 20mm .
maybe can do phosphoric acid .I don"t got any.:(
(I was meant to get 90% phosphoric acid by heating mono calcium phosphate.got no time:( )


i have 19mm ptfe tape, when stretched sideways it reaches maximum 6.5cm





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[*] posted on 27-7-2019 at 12:16


You might be on to something here, this is an idea most interesting. I wonder if you can find membranes even more effective then the teflon tape. My worry would be corrosion of the membrane otherwise, though.



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RedDwarf
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[*] posted on 27-7-2019 at 12:57


Without the fume cupboard I'm trying a less corrosive solvent mix (2017 Plum wine), ethanol is a smaller molecule than sulfuric acid but still significantly bigger than water. I've used strips of 12mm tape in parallel to cover neck of the flask (will try stretching tape next time if this isn't successful). Anyone got any scraps/offcuts of nafion they don't want?
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[*] posted on 27-7-2019 at 13:17


The first thing I suggest is trying this, but with water only, no acid. Measure how fast the water leaves the flask through the membrane at various temperatures and conditions.



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[*] posted on 27-7-2019 at 14:00


Quote: Originally posted by WGTR  
The first thing I suggest is trying this, but with water only, no acid. Measure how fast the water leaves the flask through the membrane at various temperatures and conditions.


I thought this too, but a problem is that you are much limited as to how high of a temp you can achieve because of water's B.P.

I have a suspicion that a little higher temp is necessary to get reasonable permeation to make this effective.
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[*] posted on 27-7-2019 at 16:07


Here is a document that explains permeabilities of different materials to water, and gives a general discussion on the matter:

Attachment: PNNL-26070.pdf (3.3MB)
This file has been downloaded 97 times

I have a small glass screw-top vial in the oven right now. I covered the opening with some 12mm ptfe. As others mentioned, it stretched easily. Also, it sticks very well to the glass, and the Kapton tape sticks very well to it. It seems to have a good seal, and I did not see any pinholes under the microscope.

The vial has been in the oven at 95°C for about an hour, and water does seem to be permeating the membrane. I would expect the film to be bulging outward like a balloon, but it seems to be laying flat, with only a slight convex curvature.





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[*] posted on 27-7-2019 at 16:29


I don't know if the pores in the ptfe tape are the mechanism or if it's capillary spaces, a little research has led me to believe it's the latter, though. From wikipedia article on Knudsen diffusion (https://en.m.wikipedia.org/wiki/Knudsen_diffusion):

"Consider the diffusion of gas molecules through very small capillary pores. If the pore diameter is smaller than the mean free path of the diffusing gas molecules and the density of the gas is low, the gas molecules collide with the pore walls more frequently than with each other. This process is known as Knudsen flow or Knudsen diffusion."

It seems to me that a frit or a short column of well-packed celite would work better than this ptfe tape deal, and a vacuum to greatly increase the mean free path. Heat will still need to be applied because of the low vapor pressure of 98% H2SO4. This is cool and I hope to do an experiment soon, unfortunately I'm hella busy at the moment. I will try and report back as soon as I do, though.
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[*] posted on 27-7-2019 at 18:59


When I get the chance tomorrow, I will try this with PTFE in different conditions with water to observe the escapement levels in different conditions. Would steam diffuse through agar or gelatin membranes? I do not see either of these options as being very useful as I imagine they would quickly disintegrate. However, a drying tube packed with celite may be a better alternative to teflon as happyfooddance said.

To both concentrate and purify sulfuric acid by this method, I suppose you could add concentrated H2O2 to produce a piranha solution, destroying most all impurities.
It could be a safer alternative to distillation.
Bumping would of course still be a problem, however.




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Abromination
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[*] posted on 27-7-2019 at 22:14


Tomorrow morning, I will slowly heat a flask of 10% acetic acid with a teflon membrane secured by a thermometer adaptor. Ideally, I will be able to get a quite concentrated solution. I doubt that the acid vapors will be able to permeate the membrane, but we shall see. I also doubt that I will be able to concentrate the acid to the glacial state, but again we will see.

I’m a bit nervous to use this setup, it is almost like heating a closed system, but it will be done very slowly and carefully.
Hopefully if the pressure does get too high the teflon will just bust, but if not the joint will be quite loose and pointed away from anything.

I also, if it works, imagine it could also be used to overcome some other azeotropic mixtures. As long as the water passes through the membrane and the solute does not, it should work relatively decently.





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[*] posted on 28-7-2019 at 11:43


Last night's trial with the wine didn't work as the seams between the tape leaked with a water/alcohol mixture escaping. I've tried again today with a single piece of tape stretched across the entire opening (4 layers of the white tape).
At 55C I lost 4g of water over 2 hours (but I couldn't be sure that this wasn't mostly during the initial phase when i was struggling to get a constant low temp).
At 95C I lost 16g of water over 90 minutes. During this run the ptfe was bulging then going flat again regularly. I can't see a leak on the tape so this may just be the bulging stretching the pores and pressure being released this way.
Third run with heating just up a little further (temp showing as 97C) but membrane continuously bulging: 32g lost over 110 minutes

[Edited on 28-7-2019 by RedDwarf]
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[*] posted on 28-7-2019 at 13:38


Quote: Originally posted by RedDwarf  
Last night's trial with the wine didn't work as the seams between the tape leaked with a water/alcohol mixture escaping. I've tried again today with a single piece of tape stretched across the entire opening (4 layers of the white tape).
At 55C I lost 4g of water over 2 hours (but I couldn't be sure that this wasn't mostly during the initial phase when i was struggling to get a constant low temp).
At 95C I lost 16g of water over 90 minutes. During this run the ptfe was bulging then going flat again regularly. I can't see a leak on the tape so this may just be the bulging stretching the pores and pressure being released this way.
Third run with heating just up a little further (temp showing as 97C) but membrane continuously bulging: 32g lost over 110 minutes

[Edited on 28-7-2019 by RedDwarf]


Do you know the concentration of ethanol before and after your second run? Even if it doesn’t work well with telfon, the idea is still there and celite packed in a drying tube may be an alternative. I am currently working with a 50mL sample of 10% acetic acid, I will have some better observations in a few minutes.

EDIT: Good results so far. I am slowly building up the temperature of my hotplate, the membrane is bulging out but the joint does not seem to be under stress. I smell little to no vinegar.

[Edited on 7-28-19 by Abromination]




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[*] posted on 28-7-2019 at 14:02


Quote: Originally posted by Abromination  


Do you know the concentration of ethanol before and after your second run?

[Edited on 7-28-19 by Abromination]


The runs today were just with water. I'm going to try isopropyl alcohol/water mix tomorrow.
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[*] posted on 28-7-2019 at 14:17


Quote: Originally posted by RedDwarf  
Quote: Originally posted by Abromination  


Do you know the concentration of ethanol before and after your second run?

[Edited on 7-28-19 by Abromination]


The runs today were just with water. I'm going to try isopropyl alcohol/water mix tomorrow.

Alright, the wine part confused me. A problem I am realizing that was actually addressed by OP is that the diffusion through the membrane depends on the weight of the two liquids. If its too close together, I imagine they would diffuse out together which would explain why I can smell acetic acid a bit now.
With this issue, it may only be useful in a few applications.

UPDATE: it appears that most of what is escaping is water, although I can smell a faint aroma of vinegar. I can feel the water vapor escaping from the membrane, which when condensed only seems like a very dilute solution of acid.

Another thing to note is that acetic acid and water do not form an azeotrope, they just have close boiling points.

[Edited on 7-28-19 by Abromination]

[Edited on 7-28-19 by Abromination]




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rockyit98
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[*] posted on 29-7-2019 at 08:47


it not the size that matter its the molecular weight!
why most one people trying to concentrate alcohol ? they must be reading between the lines of "Concentrating H2SO4 without distillation (with flask and some PTFE) ".which means they must be drunk already.

i don't think it's works, ethanol Molar mass is: 46.07 g/mol and shaped kinda linearly.acetic acid seems to work but try it any way:P.
can any one find the efficiency of that .its my bad i ran out of the stuff.:P

[Edited on 29-7-2019 by rockyit98]
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[*] posted on 29-7-2019 at 10:06


Quote: Originally posted by rockyit98  
it not the size that matter its the molecular weight!
why most one people trying to concentrate alcohol ? they must be reading between the lines of "Concentrating H2SO4 without distillation (with flask and some PTFE) ".which means they must be drunk already.

i don't think its works, ethanol Molar mass is: 46.07 g/mol and shaped kinda linearly.acetic acid seems to work but I try it any way:P.
can any one find the efficiency of that .its my bad i ran out of the stuff.:P

Yeah, I was kind of starting to think that. Not sure why I didnt edit that right away.
Anyways, acetic acid should work although there still is a decent escapement of acid through the membrane. Also, you definitely need a larger opening and SA of the membrane if you are going to release any large amount of vapor put into the system. It will pop the joint, as expected.
Is there a certain way you layer different pieces of the teflon over the flask mouth? I think lattice layers of tape could help prevent unwanted escapement, but I’m not certain. I to am almost out, I plan to look for some tape with a wider width.
It would also help to find the compound with the lowest molecular weight that can stay in. If ethanol is 46.07g/mol and acetic acid is 60.052g/mol there should be something in between, probably closer to acetic acid.




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rockyit98
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[*] posted on 29-7-2019 at 10:24


must be H2SO4 has low Vapor pressure (higher boiling point) than water.
i'm going to watch https://www.youtube.com/watch?v=-n8CBYoKU_c to get better understanding of things later.
The separation of ethanol water mixture with a PVA (https://pdfs.semanticscholar.org/8f20/1d2ba346a7ea18612265b0...)

[Edited on 29-7-2019 by rockyit98]
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