teodor
International Hazard
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Registered: 28-6-2019
Location: Netherlands
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About my false claim of getting 100% AcOH by reflux with (NaPO3)n
As prof. Martyn Poliakoff ("The Periodic Table of Videos") said "if you formulate a hypothesys before every experiment your experiments can never be
failure because they will either proove or disprove the hypothesys".
What really was happening is that starting with 100 ml of acetic acid with boiling point 116.0C (@ 770 mmHg atm) by reflux with Graham's salt (NaPO3)n
(n >= 3) after 6 - 12 hrs (overnight) I've got the boiling point 118.4C (more details about my measurement of boiling points I plan to say later
because this experiment is a part of series of my experiments with NaPO3).
I repeated the experiment sucessfully with 500 ml of "99.5%" acetic acid (b.p. 117.4C) raising it to 118.0C by the same setup (except the different
size of the flask and the heating mantle and obviously much higher rate of vapours).
But during the second batch I've realised that the drying is probably not the result of the action of metaphosphate. Actually it was the result of my
unusual reflux setup I've used in both experiments.
As you see I use 2 Dimroth condenser (not Graham's, sorry, only the salt is Graham's) attached sequentionally. The lower is cooled with ice-water
(~+5C) and the second with ethanol @-20 - -25C. The purpose of the second condenser is a water vapours trap (from outside). In my early experiments it
was CaCl2 tube in place of the upper one, but I replaced it with this improvised cold trap after CaCl2 was shooted off by vapours (the boiling with
(NaPO3)n is with bumping).
In both experiment I noticed that the ice was grown on the bottom part of a spiral of the second condenser. (It was quite considerable amount, the
photo below is at the beginning stage).
What is unusual here you can see from these 2 photos.
The bottom condenser:
The top condenser:
My photo skills obviously must be improved but I hope you can see that the upper part of the bottom condenser (+5C) is dry!
After performing the experiment with 500ml (117.4 -> 118.0C) I have detached the flask and replaced the bottom part with 150 ml of dry (kept over
CaCl2) benzene and the water-measurement device:
Now only the upper condenser is cooled with water, so the benzene vapours are mixing with the residue in both condensers and this residue (H2O + AcOH)
is filling the bottom part of the measurement device. At the present moment 1.85 ml liquid is collected. I am pretty sure it is mostly water, but will
think how to check that (probably by titration of AcOH part).
And let me say that now I have mixed feelling. It is obvious that in acidic media NaPO3 is resistant to hydrolisys at 114-120C. So, my primary
hypothesis was failed. But what I observe is somehow even more interesting than that.
By some reason water vapours pass through the bottom (+5C) condenser but stopped by the upper (-15 - -25C). They refuse to condense but easily are
sublimed to ice. I can't explain this effect. I have some idea that it could be the difference of the vapours density but need some proof.
What I plan to do is to repeat this experiment without NaPO3 with the same setup to check that 2 condenser (separately for liquid and solid phases) is
enough to dry solvent/water mixture like 99.5% acetic acid. And the second, I plan to do the analysis of the collected liquid which is not miscible
with the benzene in the water measurement tube.
I hope somebody can explain why water and AcOH are sharply separated in this relatively simple setup.
[Edited on 25-3-2025 by teodor]
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RU_KLO
Hazard to Others
Posts: 245
Registered: 12-10-2022
Location: Argentina
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Im guessing here:
maybe you are doing a freezing separation of acetic acid and water (there are a lot of posts here
but you are starting with almost glacial acetic acid. ( "99.5%" acetic acid)
the first condenser (5°C) just cools the vapors enough not to overdrive te second condenser (-20C) which cristallizes the water.
by overdrive I mean that the vapor temperature that touches the second condenser, is just right to allow water to freeze (and not remelt), but AA not.
some test you could make:
1) test the vapor temperature at the junction of both condenser and at the end of the last condenser
2) increase the temperature of first condenser until the vapors, remelts the ice in the second condenser.
3) decrease the temperature of first condenser (if possible) to check if no ice forms in the second condenser.
maybe this way, by having a train of different condensers with different temperatures you "made/synthetize" a fractional destilation setup.
Go SAFE, because stupidity and bad Luck exist.
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