Potassium Carbonate
Formula: K₂CO₃ — Pearl ash, potash
Appearance: White granular powder; strongly hygroscopic
Hazard: Irritant · Corrosive · Strong base
Properties
Strong base that hydrolyzes completely in water to give a strongly alkaline solution (pH ~11 at 5%). Highly hygroscopic — absorbs moisture rapidly from air, clumping and eventually dissolving; store in sealed containers. Unlike sodium carbonate, does not form a stable monohydrate under normal conditions. Dissolves readily in water with moderate heat release. Reacts with acids to release CO₂. Precipitates metal hydroxides and carbonates from their salt solutions. Does not form an insoluble calcium salt (unlike sodium carbonate), making potassium carbonate useful in some applications where calcium sensitivity matters.
Historical Context
Potassium carbonate was the original “potash” — obtained by leaching wood ash through water and evaporating the extract to a dry residue. The name “potash” literally means pot-ash, and the word “potassium” itself derives from it. Ash from hardwoods (oak, beech, hickory) yields primarily potassium carbonate; softer wood ash and coastal plants yield more sodium carbonate (soda ash).
For centuries, potash production was a major North American export industry. Settlers cleared forests for farmland and the ash was a valuable cash commodity, shipped to Europe for glassmaking, textile finishing, and soap production. The potash trade was so important to colonial New England that some historians credit it as a factor enabling settlement — farms produced both food and potash from the same forest-clearing effort.
In Europe, glass chemistry relied on potash — potash glass (K₂O-SiO₂ glass) has different properties from soda glass (Na₂O-SiO₂) and was preferred for certain optical and decorative applications. Bohemian crystal glass, famous for its clarity and suitability for cutting, is a potash-lime glass.
Experiments
Soft Soap Making: Combine potassium carbonate with animal fat or vegetable oil and heat with water. The alkaline carbonate saponifies the triglycerides to form potassium soap — a soft, water-soluble soap (as opposed to hard sodium soap). This is essentially how soap was made for thousands of years using wood-ash lye. The product is a translucent paste that lathers well.
Alkaline pH Demonstration: Dissolve a small amount of potassium carbonate in water and test with pH paper or red cabbage indicator — the solution turns distinctly alkaline (pH 11–12). Compare with sodium carbonate, sodium bicarbonate, and sodium hydroxide to build a picture of base strength.
Water Softening: Potassium carbonate precipitates calcium and magnesium as their carbonates from hard water, softening it. Add a solution of calcium chloride (representing hard water) to potassium carbonate solution — white calcium carbonate precipitates immediately.
Experiments using this chemical:
- pH Landscape — High-pH end of the acid-base spectrum
- Hard Water Chemistry — Calcium carbonate precipitation, water softening
Safety
Moderate hazard — strongly alkaline; irritant/corrosive to skin and eyes.
Wear gloves and eye protection. Solutions are corrosive to skin and mucous membranes at higher concentrations. The hygroscopic nature means powder can absorb moisture and become increasingly concentrated — reseal containers promptly. Neutralize spills with dilute acid or vinegar.
Incompatible with: Strong acids (vigorous CO₂ evolution, heat); magnesium (hydrogen gas evolved slowly); reactive metals; strong oxidizers