Potassium hydrogen phthalate

Potassium hydrogen phthalate, often called simply KHP, is an acidic salt compound. It forms white powder, colorless crystals, a colorless solution, and an ionic solid that is the monopotassium salt of phthalic acid. KHP is slightly acidic, and it is often used as a primary standard for acid-base titrations because it is solid and air-stable, making it easy to weigh accurately. It is not hygroscopic. It is also used as a primary standard for calibrating pH meters because, besides the properties just mentioned, its pH in solution is very stable.

KHP dissociates completely in water, giving the potassium cation (K⁺) and hydrogen phthalate anion (HP⁻ or Hphthalate⁻). As a weak acid hydrogen phthalate reacts reversibly with water to give hydronium (H₃O⁺) and phthalate ions.

KHP can be used as a buffering agent (in combination with hydrochloric acid (HCl) or sodium hydroxide (NaOH) depending on which side of pH 4.0 the buffer is to be) but should not be used as a buffer for decarboxylation reactions, as these will degrade the KHP and mop up the conjugation groups.