Potassium hydrogen phthalate

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. The pH of a 0.05 mol kg−1 aqueous solution of KHP at various temperatures is given by the following table:

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.