Potassium hydrogen phthalate, often called simply KHP, is a white or colorless, ionic solid that is the monopotassium salt of phthalic acid. The hydrogen 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, however, slightly hygroscopic and is generally kept in a desiccator before use. 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.
In water KHP dissociates completely 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 HCl or 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.