Sciencemadness Discussion Board

Potassium carbonate synthesis

Akira990 - 14-1-2015 at 15:29

Hi, i have little problem with synthesis of potassium carbonate, let me explain it.. I have KOH (potassium hydroxide) and i made KOH solution (around 20g of KOH in 100ml dH2O) so only thing left is to bubble CO2 through that solution...

(Correct me if i am wrong)

KOH + CO2 = KHCO3

KHCO3 + KOH = K2CO3 + H2O

KOH which reacts with CO2 will turn to KHCO3 and KHCO3 will react with rest of KOH to produce K2CO3 and water...

So my problem here is i use ph papers (1-14 value) to measure ph level, it is not so precise so i do not rly know how much of CO2 is enough (when is it done) and i have only K2CO3 left and water ofc...

My thoughts about this, evaporate water and see if what left is soluble in ethanol (i have 95% ethanol)

K2CO3 is insoluble in ethanol so i will know what i left with...

But before i do that, i have beaker and i dont want to damage it (KOH is strong base which reacts with glass) if u understand what i mean..

BTW sry for my English, i know my grammar is horible..



[Edited on 15-1-2015 by Bert]

HgDinis25 - 14-1-2015 at 16:14

This is a more complete version of what actually happens:
CO2(g) <--> CO2(aq)
CO2(aq) + H2O(l) <--> H2CO3(aq)
H2CO3(aq) + H2O(l) <--> H3O+(aq) + HCO3-(aq)

At this point, the Bicarbonate ion will start to reacti with Potassium Hydroxide:
KOH(s) --> K+(aq) + HO-(aq)
HO-(aq) + H3O+(aq) --> 2H2O(l)

As you can see, you end up with a solution of HCO3- ions and Potassium ions. Like you said, the Bicarbonate ions can ionize further and get neutralized:
HCO3-(aq) + H2O(l) <--> H3O+(aq) + CO32-(aq)
HO-(aq) + H3O+(aq) --> 2H2O(l)

So, as you can see, Carbonate ions are formed. So, when you start, because there is a lot of Hydroxide ions, you'll get mostly Carbonate ions, not Bicarbonate ions. As you bubble more and more CO2, the Hydroxide quantity will decrease, allowing more Bicarbonate ions to "survive". At this moment your reaction is done. However, if you overshot this point, there won't be Hydroxide ions to react with the Hydronium ions formed by the ionization of CO2. Thus, it will react to form more and more Bicarbonate, destroying your Carbonate ions.

Carefull controll of how much CO2 gets in is a good indicator of when the reaction has ended...

Akira990 - 14-1-2015 at 16:53

Thx for detailed answer,

I dont mind ending with bicarbonate, correct me if i am wrong but KHCO3 can be decomposed by heating to K2CO3 between 100 and 120 C ?

I mentioned i measure ph value with ph papers but i didnt mention source of my CO2 (sodium bi / carbonate + HCl) so controlling flow of CO2 in my KOH solution without some proper equipment for that measuring is very hard for me, if u understand what i mean... But if KHCO3 is so easily decomposed to K2CO3 than i dont rly mind with what i am left i will obtain K2CO3 either way..

HgDinis25 - 14-1-2015 at 17:05

Quote: Originally posted by Akira990  
Thx for detailed answer,

I dont mind ending with bicarbonate, correct me if i am wrong but KHCO3 can be decomposed by heating to K2CO3 between 100 and 120 C ?

I mentioned i measure ph value with ph papers but i didnt mention source of my CO2 (sodium bi / carbonate + HCl) so controlling flow of CO2 in my KOH solution without some proper equipment for that measuring is very hard for me, if u understand what i mean... But if KHCO3 is so easily decomposed to K2CO3 than i dont rly mind with what i am left i will obtain K2CO3 either way..


Yes, Potassium Bicarbonate decomposes around that temperature to produce Water, Carbon Dioxide and Potassium Carbonate. If you have no problem going through this then you don't need to worry about overshoting.

Akira990 - 14-1-2015 at 19:42

I am only worried cause my ph papers show 14 (there is no 13, only 10, 12...)

If i am left with KHCO3 than ph value should be lower, lower enough to be noticed by color change on ph papers but (there is always but) if i am left with K2CO3 solution than ph difference might not be enough to be noticed on ph papers, that is only reason i am worried cause i might didnt bubble enough CO2 through solution (i also left it on open air for some time and KOH should absorb CO2 even from air but we speaking here about very low percent which might not affect solution at all)

But i guess best way to test this is by evaporating water and testing what i am left with... after heating on 120 C for some time (if i am left with KHCO3 it will decompose to K2CO3) i will test solubility with ethanol and i will know for sure...

Again thx for your help..

macckone - 14-1-2015 at 20:33

One method to control the carbon dioxide is to use a balloon to capture the carbon dioxide not absorbed by the solution. With a slow drip into your gas generator, you can keep the balloon partially filled. Then you can use stoichiometry to determine your carbon dioxide levels.

Antiswat - 15-1-2015 at 06:20

practically you can use stainless steel, if its decent alloy (look up chemical compatability charts) it will have minimal wear from hydroxides, even well heated for years (several millimetres over 1 year??)

as for when you know that its totally done, if you have the time you can just let it stand somewhere to drag CO2 straight out of the air, CO2 being formed by ofcourse humans breathing, cars etc etc..
i guess bubbling air through solution of KOH would perhaps even work, perhaps put a small candle nearby?

Hawkguy - 15-1-2015 at 07:19

There must be an easier to make the carbonate since Ive tried the CO2 method, it takes awhile, too long, and its hard to gauge your progress.

Amos - 15-1-2015 at 13:29

You could combine sodium bicarbonate with potassium hydroxide in solution to form a solution of both carbonates, and then try to fractionally crystallize out the sodium carbonate, which is much less soluble in ice-cold water (7g/100mL Na2CO3 vs. 105g/100mL K2CO3).

[Edited on 1-15-2015 by No Tears Only Dreams Now]

CO2 source as dry ice?

Khemi - 24-10-2018 at 07:30

I was thinking of using up some dry ice I have slowly disappearing. It will either be a CO2 source to bubble through a KOH dH2O sol. or what if I just add some to the solution?? When dropped into the solution only small pieces floating on top of the solution will give off waste CO2. If a chunk is added and restrained to the bottom of the beaker perhaps life will be complete and I'll have Potassium Carbonate. Sounds feasible?

DavidJR - 24-10-2018 at 07:39

I see no reason for that not to work.

Ubya - 24-10-2018 at 08:57

Quote: Originally posted by Khemi  
I was thinking of using up some dry ice I have slowly disappearing. It will either be a CO2 source to bubble through a KOH dH2O sol. or what if I just add some to the solution?? When dropped into the solution only small pieces floating on top of the solution will give off waste CO2. If a chunk is added and restrained to the bottom of the beaker perhaps life will be complete and I'll have Potassium Carbonate. Sounds feasible?


it will work, but in an inefficient way, a big chunk of dry ice dropped in water (or KOH solution in this case) sublimates making big bubbles of CO2, big bubbles=relatively small surface area compared to its volume, the bigger the surface area the better the interaction with the solution. it would be better to put your dry ice chunks in a separate container connected to your KOH solution with a tube and a bubbler stone or a fine point pipette, the smaller bubbles will react better and more efficiently

symboom - 24-10-2018 at 09:35

Solvay process may be the answer if you have calcium carbonate potassium chloride
Overall process
2 KCl + CaCO3 → K2CO3 + CaCl2
This case baking soda could supplement for CO2 production
And ammonia is added to the mix there you have it

Urea could produce both

Or
Decomposition of potassium tartrate


[Edited on 24-10-2018 by symboom]

Ubya - 24-10-2018 at 09:55

Quote: Originally posted by symboom  
Solvay process may be the answer if you have calcium carbonate potassium chloride
Overall process
2 KCl + CaCO3 → K2CO3 + CaCl2
This case baking soda could supplement for CO2 production
And ammonia is added to the mix there you have it


[Edited on 24-10-2018 by symboom]

it's not that simple...
it's not a one pot reaction just throw ammonia, CO2, CaCO3 and KOH, and then the fact that potassium carbonate is really solubile in water (when sodium carbonate it's not) makes this process not usefull for K2CO3

CO2 source as dry ice?

Khemi - 24-10-2018 at 10:38

I was thinking of using up some dry ice I have slowly disappearing. It will either be a CO2 source to bubble through a KOH dH2O sol. or what if I just add some to the solution?? When dropped into the solution only small pieces floating on top of the solution will give off waste CO2. If a chunk is added and restrained to the bottom of the beaker perhaps life will be complete and I'll have Potassium Carbonate. Sounds feasible?

Ubya - 24-10-2018 at 12:45

Quote: Originally posted by Khemi  
I was thinking of using up some dry ice I have slowly disappearing. It will either be a CO2 source to bubble through a KOH dH2O sol. or what if I just add some to the solution?? When dropped into the solution only small pieces floating on top of the solution will give off waste CO2. If a chunk is added and restrained to the bottom of the beaker perhaps life will be complete and I'll have Potassium Carbonate. Sounds feasible?


read the answers, it should work, but it would be way better if you bubble co2 in solution from a generator (dry ice in a separate flask) and use a bubble stone or fine pipette to make tiny bubbles

macckone - 24-10-2018 at 20:37

bubbling carbon dioxide into potassium hydroxide will produce a mix of potassium bicarbonate and carbonate. The bicarbonate can then be destroyed with heat.

Alternatively, potassium acetate can be decomposed to the carbonate yielding acetic acid and acetone as byproducts.