Potassium carbonate

Potassium carbonate, sometimes referred to in historical literature as potash, is an inorganic compound with chemical formula K₂CO₃. It is a basic and somewhat hygroscopic crystalline solid, readily forming a solution that feels slippery to the touch.

Properties

Chemical

Potassium carbonate reacts with acids to release carbon dioxide.

Physical

Potassium carbonate is a white deliquescent solid. It is soluble in water, but not ethanol. K₂CO₃ decomposes at 891°C.

Availability

Potassium carbonate can be purchased online cheaply.

Preparation

Historically, potassium carbonate was obtained through the leaching of wood ash, producing a yellowish powder referred to as lye, which consisted of varying portions of potassium and sodium carbonates and a variety of other impurities. This mixture, while unsuitable for analytical chemistry, can still find some applications today, such as in food preparation (such as in traditional Japanese ramen) and soap-making. Sodium carbonate can be removed, however, by concentrating a solution of the wood lye and cooling it to near freezing. Sodium carbonate is much less soluble than potassium carbonate, and will easily crystallize out. Some of the other impurities precipitate from the evaporating solution even earlier, as brown goop that can be filtered out.

A more modern method of preparing potassium carbonate, but still a crude one, is by burning a mixture of finely ground potassium nitrate and carbon (or potassium nitrate and sugar), usually in the form of coke or charcoal. To ensure that all potassium nitrate reacts, an excess of carbon should be used, and the two reactants mixed very well. This reaction produces large, bright magenta or fuchsia flames and thick white smoke, and should only be performed outside. The end product of this reaction can be leached with water, crystallized by evaporation or boiling, and washed in hot acetone. Care must be taken when working with the reaction product as it may contain potassium cyanide.

Projects

Potassium carbonate is useful for the production of potassium salts in the lab, and can serve as a substitute for potassium hydroxide for many applications. One unique application for potassium carbonate is the salting-out of ethanol or methanol. When added in the correct proportion to a solution of ethanol and water and dissolved, potassium carbonate causes the solution to separate into two phases, one of which is nearly pure ethanol. This principle may also be useful in distillations to break the naturally occurring azeotrope of ethanol and water.

Handling

Safety

Potassium carbonate has low toxicity.

Storage

Should be stored in closed bottles.

Disposal

As it is non-toxic, it can be safely dumped in the ground.