Weak Electrolyte Definition and Examples

How Weak Electrolytes Work

Acetic acid is an example of a weak electrolyte even though it is highly soluble in water.
Acetic acid is an example of a weak electrolyte even though it is highly soluble in water. ELLA MARU STUDIO / SCIENCE PHOTO LIBRARY / Getty Images

A weak electrolyte is an electrolyte that does not completely dissociate in aqueous solution. The solution will contain both ions and molecules of the electrolyte. Weak electrolytes only partially ionize in water (usually 1% to 10%), while strong electrolytes completely ionize (100%). 

Weak Electrolyte Examples

HC2H3O2 (acetic acid), H2CO3 (carbonic acid), NH3 (ammonia), and H3PO4 (phosphoric acid) are all examples of weak electrolytes. Weak acids and weak bases are weak electrolytes. In contrast, strong acids, strong bases, and salts are strong electrolytes. Note a salt may have low solubility in water, yet still be a strong electrolyte because the amount that does dissolve completely ionizes in water.

Acetic Acid as a Weak Electrolyte

Whether or not a substance dissolves in water is not the determining factor in its strength as an electrolyte. In other words, dissociation and dissolution are not the same thing.

For example, acetic acid (the acid found in vinegar) is extremely soluble in water. However, most of the acetic acid remains intact as its original molecule rather than its ionized form, ethanoate (CH3COO-). An equilibrium reaction plays a big role in this. Acetic acid dissolves in water an ionizes into ethanoate and the hydronium ion, but the equilibrium position is to the left (reactants are favored). In other words, when ethanoate and hydronium form, they readily return to acetic acid and water:

CH3COOH + H2O ⇆ CH3COO- + H3O+

The small amount of product (ethanoate) makes acetic acid a weak electrolyte rather than a strong electrolyte.