RU_KLO - 27-11-2024 at 04:07
I needed some potassium carbonate.
So I used KOH + CO2 (by NaHCO3 + HCl), but in ethanol.
5.6gr KOH was disolved in 50ml 95% ethanol (aprox 2M KOH solution in ethanol)
CO2 was bubbled into this solution. At start nothing happened, but with time, the solution became turbid and then rapidly a white creamy precipitate
was obtained.
Reading this:
"-Step 1: when absorption starts, reactions (2) and (3) begin concurrent. A large amount of OH- reacts to give CO3(2-) (reaction 4) and the pH is
decreased in this part. The highest amount of CO2 is absorbed at this
stage and results in reaction 5:
CO2(g) -> CO2(aq) (2)
CO2(aq) + OH- (aq) -> HCO3- (aq) (3)
HCO3- (aq) + OH- (aq) -> H2O(l) + CO3(2-)(aq) (4)
2KOH(aq) + CO2(g)-> K2CO3(aq) + H2O(l) (5)
The K2CO3 produced exists as K+ and CO3(2-) which dissociate in the absorber or absorbent.
-Step 2: since CO2 is continuously introduced into the KOH solution during the reaction, CO2 is simultaneously absorbed and affects to OH- depletion
and CO3(2-) accumulation through reaction (3) and forward reaction (4). However, concentration of CO3 (2-) still adds up and triggers domination of
the reverse reaction (4), then accelerates forward reaction (3). Hence, reaction (6) is activated. While a small amount of H+ is released from HCO3-,
which impose pH at this stage.
K2CO3(aq) + CO2(g) + H2O(l) -> 2KHCO3(aq) (6)
-Step 3: diminutive OH- concentration occurs while chemical absorption almost done. Therefore, CO2 is considered to have been physically absorbed at
its equilibrium value in water at a certain partial
pressure. KOH is a limiting reactant, the entire absorption reaction of CO2 and KOH in aqueous solution can be summarized as reaction (7), which
stands for reactions (5) and (6):
KOH(aq) + CO2(g) -> KHCO3(aq) (7)
"
Absorption of Carbon Dioxide into Potassium Hydroxide: Preliminary Study for its Application into Liquid Scintillation Counting Procedure
Nur Faiizah Aqiilah Firman1,*, Alfian Noor2, Muhammad Zakir2, Maming2, Achmad Fuad Fathurrahman3
Egypt. J. Chem. Vol. 64, No. 9 pp. 4907 - 4912 (2021)
OK, this is in H2O,
I obtained a white creamy precipitate, which 24 hs later, became a ethanol smelling pool, because of humidity, the K2CO3/KHCO3 dissolved.
But in ethanol, in which K2CO3 is insoluble, will the reaction proceed to KHCO3?
(for example, will a sample of K2CO3 transform into KHCO3 thru time because CO2 from atmosphere?)
Can K2CO3 obtained directly from KOH in ethanol or it will convert to KHCO3 like in H2O? (by bubbling excess CO2 - till it precipitates all, but no
further.)
(I know that I can heat - and I need to remove water/ethanol, to transform KHCO3 to K2CO3 - like 250C)
Thanks
[Edited on 27-11-2024 by RU_KLO]
j_sum1 - 27-11-2024 at 17:15
Interesting idea.
My instinct would be to use anhydrous ethanol and add a pile of 3A sieves to mop up water produced by reaction 5.
Then you should obtain a precipitate of K2CO3. You avoid all the intermediate aqueous compounds. I would think that the product you get would be
reasonably stable. However, I would be keen to dry it quickly and thoroughly.
BromicAcid - 27-11-2024 at 17:47
Doesn't potassium carbonate do weird things in ethanol causing phases to separate if there's water around? There's a thread around here somewhere on
that.
Be careful with a KOH/EtOH solution though, it's essentially a base bath and will react over time with air to make a polymeric mess.
Texium - 27-11-2024 at 21:46
Relevant discussion from last year: https://www.sciencemadness.org/whisper/viewthread.php?tid=15...
I wouldn’t recommend putting KOH in ethanol. Weird things do indeed happen. Methanol or isopropanol would be fine alternatives.
RU_KLO - 28-11-2024 at 04:56
thanks for the link.
Also read potassium ethoxide (wiki) - to check if this can also be produced - (an then spooked out because it says its explosive with water)
" It hydrolyzes to yield ethanol and potassium hydroxide."
so (thanksfully) no potassium ethoxide, because of hydrolisys.
I remove all the water (by heat) obtained a white powder (hygroscopic).
used 5.6gr KOH, obtained 6.2gr of white powder.
(do not know how much CO2 was added)
if:
2KOH(aq) + CO2(g)-> K2CO3(aq) + H2O(l) (5)
5.6gr + X CO2 -> 13.82gr/2 = 6.91gr
(more or less the qty I obtained (mechanical loss from scraping the watchglass)
KOH(aq) + CO2(g) -> KHCO3(aq)
5.6gr + X CO2 -> 10gr.
Which difference is a lot.
(I dont know how good this type of verification is....) Maybe there is a mix of potassium salts..
dissolved a tip in dH2O (10ml), ph 10.5 - 11 (using alkaline ph paper)
take the same amount of KOH in 10ml dH2O , ph 13+
So it seems that it is no KOH.
Added some HCl and directed the gass to another test tube with Ca(OH)2 and obtained flocculent white precipitate -> carbonate.
Is there a simple test of bicarbonate in carbonate (or the other way around)
or any test to check if I have carbonate or bicaronate and in which proportion?
Edit: after posting. found this:
Answer: One important chemical test to differentiate carbonate and bicarbonate is MgSO4 test. When carbonate salt is reacted with MgSO4, a white salt
precipited while ("bicarbonate salt on reacting with MgSO4, not only a white salt precipited but also a brisk effervescence occur.)-> this is not
correct. From the video bellow, bicarbonate does not precipitate anything.
https://www.youtube.com/watch?v=Nww1dchSRzg&ab_channel=S...
also:
There is a titration method using barium chloride to precipitate the carbonate and back titration for the bicarbonate. This will measure total
carbonate/ bicarbonate and also bicarbonate only, in order to calculate the carbonate. I am not sure if this will be sensitive enough for small amount
of carbonate, which is less than 0.23%.
Does any one have how is this done?
second edit (to avoid new post):
Done the magnesium sulfate test. A white precipitate (more than precipitate, was like milk....)
Also done the same test with sodium carbonate, same result. but with bicarbonate no precipitation. solution remain clear.
So from these test: its potassium carbonate.
Maybe, when heating to remove alcohol/water (because it was left one day, and being hygroscopic, a lot of water was absorved, till it was fully
dissolved - almost clear) transformed the KHCO3 into K2CO3 by heat.
(in some literature states that conversion starts at 50C others at 150C.)
Most water was removed at 70C (steam) and when almost all water was removed, directly on the hotplate to fully dried. Temperature was meassured 150C
but only 5-10 minutes.
Rounding up:
1) when bubbling CO2 in an alcoholic KOH solution, K2CO3 is made (and maybe small quantities of KHCO3 (if any)
2) I used 95% ethanol, but from other post, ethanol behaves strangely with ethanol. Rework should be done quick (evaporation of alcohol). Methanol,
Isopropanol should be used instead.
[Edited on 28-11-2024 by RU_KLO]
[Edited on 28-11-2024 by RU_KLO]