Potassium hydrogen phthalate, often called simply KHP, is an acidic salt compound.

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.

KHP can be used as a buffering agent in combination with hydrochloric acid (HCl) or sodium hydroxide (NaOH).

The pKa of KHP is 5.4, so its pH buffering range would be 4.4 to 6.4; however, due to the presence of the second acidic group that bears the potassium ion, the first pKa also contributes to the buffering range well below pH 4.0, which is why KHP is a good choice for use as a reference standard for pH 4.00.

[8] [9] KHP is also a useful standard for total organic carbon (TOC) testing.