Chemistry in Air

• Sodium hydroxide, also known as lye or caustic soda, and potassium hydroxide, also known as caustic potash, are both white solids. They are hygroscopic, meaning they will absorb water from the air.
  
  Sodium hydroxide will form sodium carbonate (Na₂CO₃, washing soda) by absorption of carbon dioxide (CO₂):
  
  2 NaOH + CO₂ ' Na₂CO₃ + H₂O
  
  Potassium hydroxide, however, will form potassium bicarbonate (KHCO₃):
  
  KOH + CO₂ ' KHCO₃
  
  

Solubility

• Sodium and potassium hydroxide are strong bases. At room temperature, they exhibit almost identical solubilities in water - 111 grams of sodium hydroxide will dissolve in 100 milliliters of water vs. 110 grams of potassium hydroxide.
  
  

Applications

• Both hydroxides can be used in the saponification of fats to form soap. Soaps prepared from potassium hydroxide tend to exhibit greater solubility and are commonly referred to as "soft" soaps.
  
  

Manufacture

• Sodium and potassium hydroxides are prepared commercially by electrolysis of sodium chloride (NaCl, table salt) or potassium chloride (KCl):
  
  2 NaCl + H₂O ' H₂ + Cl₂ + NaOH
  2 KCl + H₂O ' H₂ + Cl₂ + KOH
  
  

Differences

• In many applications, potassium and sodium hydroxide can be freely interchanged. The primary difference between them is cost. In ton quantities, potassium hydroxide is about three times more expensive than sodium hydroxide.