Potassium Hydrogen Phthalate (KHP) is a classic primary standard substance in that it is readily available in high purity; is relatively inexpensive; is easy to dry; is neither hygroscopic, oxidized by air, nor affected by Carbon Dioxide (thus making it easy to weigh out); does not undergo decomposition during storage; has a relatively high equivalent weight (thus reducing errors when weighed to 0.1 mg precision); is readily soluble in water; and is stoichiometric and instantaneous in reactions.