Sciencemadness Discussion Board

Distilling oddness

aga - 17-1-2016 at 08:03

Today i tried distilling some of this 76% agricultural H3PO4 in order to purify it - this is a Green liquid, so there must be 'other' stuff in there.

The setup was a 250ml RBF, stillhead, 300mm liebig.

At 100 C water happily came over.

At 109 C a small (~2cm long, ~0.5cm high) mist formed at the top of the condenser and periodically 'exploded' down the condenser tube with a 'pop' sound.

This continued for quite a long time.

I stopped the distillation as i have never seen that phenomenon before, and was a little concerned about the 'pop' becoming a 'BOOM'.

Has anyone seen this kind of thing before ?

morsagh - 17-1-2016 at 08:17

Because phosphoric acid near up to its boiling point turn to mixture of polyphosphoric acids. I find it easier to produce Ca3(PO4)2 and than by mixing it with sulphuric acid and filtration get pure phosphoric acid.

blogfast25 - 17-1-2016 at 08:18

Hmmm. No, I can't say I've ever encountered that. Strange...

How was boiling behaviour?

aga - 17-1-2016 at 10:15

Boiling was normal and continuous, which is what made it seem really odd.

Could have understood it if the pot were bumping & puking.

The aim is to purify it, so morsagh's suggestion sounds like a better plan than distilling.

UC235 - 17-1-2016 at 12:03

Phosphoric acid cannot be distilled. At extreme temperatures, it can lose water to form polyphosphoric acid, but I suspect this is not possible in a flask. There is a prepub (I think) thread where fusion of purified monoammonium phosphate loses at least some of it's ammonia and becomes much more acidic than the starting product. This may be something to look into.

aga - 17-1-2016 at 13:12

A problem is the sheer weight of results when searching for 'phosphoric acid'.

Many begin with stuff i do not have, like red phosphorous.

Currently i have a few gallons of this green stuff that claims to be 76% phosphoric acid.

All i need is about 100mls of actual phosphoric acid to make tripotassium phosphate to rip the last 4% water from some ethanol before said ethanol gets chlorinated on it's way to become trichloroacetic acid.

Obviously a way to 'clean' or even 'de-green' it would also be of use in future.

Whilst searching i came accross a reference using Corn Starch to remove water from alcohols, albeit in the vapour phase:-

http://onlinelibrary.wiley.com/doi/10.1002/bit.260350607/abs...

Definitely worth a try tomorrow at test-tube scale on the liquid.

aga - 17-1-2016 at 14:47

Quote: Originally posted by UC235  
Phosphoric acid cannot be distilled. At extreme temperatures, it can lose water to form polyphosphoric acid, but I suspect this is not possible in a flask.

From the few things i found (today) about the pyro- and poly- phosphoric acid situation, it appears to be reversible with the addition of water.

Once poly'd does it stay poly independant of concentration in water ?

[Edited on 17-1-2016 by aga]

S.C. Wack - 17-1-2016 at 15:41

It depends on whether the water is boiling.

Would be interesting if the additive was making phosphine somehow.

The best method to purify phosphoric acid in a caveman way is likely through a salt. Add baking soda or something.

Deathunter88 - 17-1-2016 at 16:16

Quote: Originally posted by aga  
mist formed at the top of the condenser and periodically 'exploded' down the condenser tube with a 'pop' sound.

Has anyone seen this kind of thing before ?


Yes, I have seen this happen before when distilling ethanol. The ethanol in the distillation flask was boiling normally, but there would popping sounds as the vapor raced down the condenser. If you put your hand at the tubing connector on the vacuum adapter outlet you could fell periodic jets of air, coinciding with the pops. I don't know why though.

Hawkguy - 17-1-2016 at 16:30

As everyone else sorta said, H3PO4 will just be dehydrated to HPO4, meta - Phosphoric Acid, at which point it'll kinda just attack your glass and you'll be sad. HPO4 is still immensely useful though, so if you have already made it, don't discard it. The stuff is nifty for making white phosphorus, and Sulfur Trioxide.


EDIT: Gj finding a source of the stuff like that. The only H3PO4 I knew of OTC as some grout/drain bullshit was taken off the shelves over two years ago and nothing really took its place.

[Edited on 18-1-2016 by Hawkguy]

aga - 18-1-2016 at 04:19

During cleanup no damage was seen to the RBF, still head or condenser.

The acid must have gotten Hot as it was a sticky treacle so thick that the stirbars (i use 'em as boiling chips) would not move until the biggest Nd magnet was used, and then they moved very slowly.

Adding water left the thick layer untouched until it was stirred by hand.
The resistance to being stirred was incredible !

blogfast25 - 18-1-2016 at 05:51

Quote: Originally posted by Hawkguy  
As everyone else sorta said, H3PO4 will just be dehydrated to HPO4, meta - Phosphoric Acid, at which point it'll kinda just attack your glass and you'll be sad.


It's HPO3 actually.

And the idea that a common Brønsted–Lowry acid can attack glass is just a chemical urban myth that gets bandied around a lot without evidential basis. Just because someone somewhere claims it doesn't make it true.

The only acids that attack glass are HF-based because they attack silicates to SiF4.

S.C. Wack - 18-1-2016 at 17:03

Quote: Originally posted by blogfast25  
And the idea that a common Brønsted–Lowry acid can attack glass is just a chemical urban myth that gets bandied around a lot without evidential basis. Just because someone somewhere claims it doesn't make it true.

The only acids that attack glass are HF-based because they attack silicates to SiF4.


Have you looked for evidence? In the right place? Not that this is evidence, just sayin this language from Corning on the "Care and Safe Handling of Laboratory Glassware" is typical: "Glass will be chemically attacked by hydrofluoric acid, hot phosphoric acid and strong hot alkalis, so it should never be used to contain or to process these materials." and who are you really to tell us not to believe it, without experimentation?

blogfast25 - 18-1-2016 at 17:12

Quote: Originally posted by S.C. Wack  
and who are you really to tell us not to believe it, without experimentation?


And who are you really to tell me to believe it, without experimentation?
Quote: Originally posted by aga  
During cleanup no damage was seen to the RBF, still head or condenser.



That IS evidence, albeit not necessarily conclusive evidence.

[Edited on 19-1-2016 by blogfast25]

Detonationology - 18-1-2016 at 17:35

http://www.corrosion-doctors.org/Household/Glass.htm
Quote:

Only a few chemicals aggressively attack glass -- hydrofluoric acid, concentrated phosphoric acid (when hot, or when it contains fluorides), hot concentrated alkali solutions and superheated water. Hydrofluoric acid is the most powerful of this group; it attacks any type of silicate glass. Other acids attack only slightly; the degree of attack can be measured in laboratory tests but such corrosion is rarely significant in service for acids other than hydrofluoric and phosphoric.


http://www.qvf.com/glass-equipment/borosilicate-glass/composition-corrosion-resistance.html
Quote:

There are only a few chemicals which can cause noticeable corrosion of the glass surface namely hydrofluoric acid, concentrated phosphoric acid and strong caustic solutions at elevated temperatures.



http://www.duran-group.com/en/about-duran/duran-properties/chemical-properties.html
Quote:

Only hydrofluoric acid, concentrated phosphoric acid and strong alkali cause appreciable surface removal of the glass (glass corrosion) at elevated temperatures (>100 °C).

macckone - 18-1-2016 at 21:06

Sodium hydroxide definetly attacks glass at high temp. Of course then it is forming sodium silicate. Of course hydrofluoric acid does as well. The mechanism for phosphoric acid attack leaves me stumped. With soda glass, dissolving the sodium and calcium hydroxides out will damage the glass but will that happen with borosilicate glass?

gsd - 19-1-2016 at 01:07

Quote: Originally posted by blogfast25  

And the idea that a common Brønsted–Lowry acid can attack glass is just a chemical urban myth that gets bandied around a lot without evidential basis. Just because someone somewhere claims it doesn't make it true.


http://onlinelibrary.wiley.com/doi/10.1111/j.1151-2916.1983....

"A borosilicate glass was resistant to orthophosphoric acid at 95°C, whereas considerable corrosion occurred at higher temperatures. Tenacious crystal formation provided a barrier against further attack, but also produced stress resulting in spontaneous glass breakage. The acid-glass reaction is attributed to acid dehydration and consequent transformation to aggressive pyrophosphate."

This should be evidence enough.

gsd

aga - 19-1-2016 at 01:07

Half an hour of boiling this acid in this glass seems to have left it unaffected.

Perhaps there is a reaction, just so incredibly slow as to render it irrelvant in a home lab setting.