Sciencemadness Discussion Board

K2CO2 to KOH

Random - 29-5-2010 at 07:03

Is there an easy way to make KOH from K2CO3?

Lambda-Eyde - 29-5-2010 at 07:23

Adding Ca(OH)<sub>2</sub> should precipitate CaCO<sub>3</sub> and leave KOH in solution... I think.

not_important - 29-5-2010 at 08:01

Yup - that's the method used perhaps into the early 20th century. If you need solid and/or high purity KOH then some extra work is needed.

Formatik - 29-5-2010 at 09:09

Quote: Originally posted by Lambda-Eyde  
Adding Ca(OH)<sub>2</sub> should precipitate CaCO<sub>3</sub> and leave KOH in solution... I think.


I made some KOH like this in the primordial chemicals thread. There is also some other method that uses boiling down in iron to drive the reaction.

Random - 29-5-2010 at 12:57

But then, is there an easy way to make ca(oh)2? As I think the decomposition temperature of caco3 is like 800 °C to make calcium oxide, and it looks like a hard way to do it with such high temperature (only if someone knows how to get that temperature the fastest and easy way).
Chemicals are very hard to get at the place where I live so I am trying using the most simplest chemicals to get the harder to get ones :D

Thanks for the primordial chemicals thread tip, I'll check it too. :)

not_important - 29-5-2010 at 17:21

It's either use Ca(OH)2, or convert the K2CO3 to KCl, melt it and electrolysis that to get metallic potassium, and store that in a beaker next to a beaker of water in a container with a pinhole leak to let out H2. You could convert the K2CO3 to K2SO4 or KNO3, then electrolysis a solution of that using ion selective membranes to keep the acid and alkali formed separate.

If you know anyone with a ceramics kiln, they can make CaO for you. Cone 014 is roughly 840 C, cone 01 is 1140 C, cone 1 around 1150 and the edge of yellow heat, cone 10 is white hot 1300 C. Low fire earthenwares, terra cotta, and red bricks are fired at cone 07 or below, so almost any kiln will be able to reach the needed temperature.






JohnWW - 30-5-2010 at 00:10

A problem with the whole procedure, with both the Ca(OH)2 reagent and desired KOH product by metathesis, is that both of these hydroxides are liable to absorb CO2 from the air, and (especially KOH) are deliquescent, absorbing from air large quantities of H2O vapor (in which KOH is very soluble). You would have to store the Ca(OH)2 hermetically sealed from CO2 and H2O vapor (dry air or N2 from which CO2 and H2O have been removed), and same with the KOH product if you want to evaporate off all the water from its solution and obtain it as a dry solid.

[Edited on 30-5-10 by JohnWW]

12AX7 - 30-5-2010 at 02:07

The old way: load blocks of limestone into a kiln with layers of charcoal. Stoke it up, get it to temperature, then unload.

Tim

watson.fawkes - 30-5-2010 at 08:33

Quote: Originally posted by 12AX7  
The old way: load blocks of limestone into a kiln with layers of charcoal.
A small addendum. Practical lime kilns of that era required that the limestone material be reasonably well sized. Pieces too large would either be insufficiently reacted or would tend to form dead-burnt clinkers, which is essentially a surface passivization by the formation of a glassine layer on the outside. Limestone fines (i.e. dust) would be product unrecoverable from the ash. The target size was around 1-2 cm, as I recall.

starman - 4-6-2010 at 18:07

Quote: Originally posted by Random  
But then, is there an easy way to make ca(oh)2? As I think the decomposition temperature of caco3 is like 800 °C to make calcium oxide, and it looks like a hard way to do it with such high temperature (only if someone knows how to get that temperature the fastest and easy way).
Chemicals are very hard to get at the place where I live so I am trying using the most simplest chemicals to get the harder to get ones :D

Thanks for the primordial chemicals thread tip, I'll check it too. :)


You do realise that calcium hydroxide is builders lime?Used universally for mortar and available anywhere you buy cement.

blogfast25 - 6-6-2010 at 08:04

Make 'synthetic slaked lime' by reacting CaCl2 with NaOH from concentrated solutions. By filtering off what is mainly NaCl brine and washing with a bit of water you obtain a slurry high in Ca(OH)2. Add some water to it, to keep as a slurry without too much CO2 absorption.

Assuming you knew with how much CaCl2 you start off with, you also know how much Ca(OH)2 your slurry now contains. Add the equivalent of K2CO3 and boil for a few minutes to form KOH and insoluble CaCO3. Filter. Really strong solutions of (impure) KOH can thus be obtained. I've done it.

The overall reaction is basically:

CaCl2 + 2 NaOH + K2CO3 ---> 2 KOH + 2 NaCl + CaCO3