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Miscellaneous: Potassium carbonate extraction

Potassium carbonate is one of the compounds present in wood ash. Since K2CO3 is soluble in water, while most of the other compounds in wood ash aren't, it can easily be extracted.

potassium carbonate extraction

For this experiment one should prepare several larger containers, a scale, wood ash (in this case, beech ash), and some form of sieve, which is used to separate larger pieces of leftover wood and coal from the rest of the ash.

potassium carbonate extraction

In this case, after all larger particles have been removed, the mass of the ash was approximately 7700 grams.

potassium carbonate extraction

Double the amount of hot water (with regard to volume) was then added to the wood ash. The mixture was then stirred with a steel rod for about fifteen minutes.

potassium carbonate extraction

Afterwards, the mixture was filtered through a gauze, which took quite some time, since there was a lot of ash. Gloves were worn during the handling of the mixture, because of the high alkalinity, which is damaging to the skin.

potassium carbonate extraction

The result of the filtration can be seen above - approximately 7.5 liters of a clear brown colored solution. The universal pH indicator shows that the solution is very alkaline, which is in accordance with the hydrolysis of the carbonate ion.

potassium carbonate extraction

The clear solution from the plastic bottles was poured in a larger pot made from stainless steel which was heated with a gas burner until most of the water evaporated. In this case, this process took several hours.

potassium carbonate extraction

The end product was a dense light brown solid which hardened upon cooling. Afterwards the obtained solid was broken into smaller pieces which were weighed. The total mass was 381 grams. When one takes into account the starting mass of the wood ash, it turns out that the ash contained aproximately 4.9% of potassium carbonate, which is close to the values mostly mentioned in litarature (5-10%).

Clearly the yield can have different values, depending on the type of the wood ash used, on the amount of potassium carbonate which was still present in the ash after the filtration, on the amount of water and other compounds present in the end product, etc. The product can be purified by recrystallization or used directly in certain syntheses (for example, the synthesis of potassium nitrate), whereby the final product of the synthesis can be additionaly purified if necessary.


Note: Later, the product of this extraction was used for the synthesis of potassium nitrate, whereby it was determined that the 381 grams of the product obtained by this extraction, actually contained 331.7 grams of pure potassium carbonate. Therefore, the wood ash contained a bit less K2CO3 than was first determined - approximately 4.3% instead of 4.9%.

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