Sciencemadness Discussion Board - Potassium Carbonate from Potassium Sulfate? K2S04 ---> K2C03

Potassium Carbonate from Potassium Sulfate? K2S04 ---> K2C03

boonga - 19-9-2011 at 19:56

Hey guys,

was wondering what would be the easiest/most convenient way to procure (in powder form) potassium carbonate starting from potassium sulfate. I possess common household ingredients along with a few others; NaOH, NaCL, HCL, H2S04, NaHC03, Na2C03, vinegar etc etc

cheers in advance,

Boong

AndersHoveland - 19-9-2011 at 21:09

There is no direct or easy way to prepare potassium carbonate from the sulfate.

One method would be to reduce it to K2S by heating it with charcoal, in the absence of air, then mix with water and finely divided calcium carbonate. The calcium sulfide will solidify out, leaving K2CO2 in solution. This method, known as the Leblanc process, was the industrial basis for preparing sodium carbonate (soda ash) before the development of the Solvay process, which is now also obsolete.

not_important - 20-9-2011 at 05:23

In water, mix K2SO4 and Ca(OH)2, gently boil for some time K2SO4 + Ca(OH)2 <=> 2 KOH + CaSO4. Let cool slightly and allow the CaSO4 to settle out; decant the KOH solution away from that.

The KOH solution will contain some CaSO4 and K2SO4. Evaporate it in a dish, silver is preferred but you can get by with nickle. As the solution concentrates the sulfates will precipitate out, when a KOH concnetration of around 50% is reached it's time to stop heating. Allow to cool while limited access to air, then dissolve in alcohol (MeOH or EtOH) and decant away from the sulfates. Treat the solution with CO2 to get K2CO3 + KHCO3, heating to about 120 C will decompose the bicarbonate.

ScienceSquirrel - 20-9-2011 at 07:20

Quote: Originally posted by AndersHoveland

There is no direct or easy way to prepare potassium carbonate from the sulfate.

One method would be to reduce it to K2S by heating it with charcoal, in the absence of air, then mix with water and finely divided calcium carbonate. The calcium sulfide will solidify out, leaving K2CO2 in solution. This method, known as the Leblanc process, was the industrial basis for preparing sodium carbonate (soda ash) before the development of the Solvay process, which is now also obsolete.

That is not quite right but the vile and disgusting Leblanc process is the way to go

bbartlog - 20-9-2011 at 08:07

Quote: Originally posted by not_important

In water, mix K2SO4 and Ca(OH)2, gently boil for some time K2SO4 + Ca(OH)2 <=> 2 KOH + CaSO4.

Are you sure about this, or are you generalizing from the similar reaction that goes from K2CO3 to KOH? The same thought had crossed my mind, but I don't think that CaSO4 is less soluble than Ca(OH)2 the way that CaCO3 is, so I'm not clear what would drive the reaction.

AndersHoveland - 20-9-2011 at 09:49

K2SO4 + Ca(OH)2 --> (2)KOH + CaSO4
would probably work.

Before the industrial use of electrolysis, the main method of preparing sodium hydroxide was to react CaO with sodium carbonate and water, the calcium carbonate eventually precipitating out.

But if it does work, one would wonder why it was not used instead of the LeBlanc process, perhaps CaO was to expensive/difficult to make because of the high roasting temperatures?