What is the Difference Between Soda Ash & Lye?

Soda ash is contained in laundry detergents as a water softener.

Soda ash and lye are different chemical substances with different chemical properties and different applications. They should never be substituted for each other.

Soda Ash

Soda ash, also known as sodium carbonate or washing soda, possesses a chemical formula of Na₂CO₃.

Lye

Lye is known as sodium hydroxide or caustic soda, with chemical formula NaOH.

Alkalinity

Both soda ash and lye are alkaline substances, which means they will increase the pH of a water-based solution. Lye, however, is considered a strong base, whereas soda ash is not. As a consequence, lye is corrosive and will cause chemical burns to skin.

Solubility

Both soda ash and lye are water soluble. Lye, however, is more soluble and can form solutions at concentrations in excess of 50 percent lye by weight (50 grams of lye per 100 milliliters of solution). Soda ash can be dissolved only at concentrations up to about 20 percent by weight.

Applications

Lye, when combined with fat, converts fat to glycerin. This process is known as "saponification" and is the primary method by which soap is manufactured.
Soda ash acts as a water softener and is used extensively in laundry detergents. Swimming pool owners use it to raise the pH of pool water.

References

• Mallinckrodt Baker, Inc.: Material Safety Data Sheet--Sodium Hydroxide
• Missouri Univ. of Science and Technology: Strong Electrolyte--Sodium Hydroxide (a Base)
• Acros Organic, Inc: MSDS for Sodium Carbonate Decahydrate
• Elmhurst College: Soap
• Purdue University: Detergent