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 also used as a primary standard for calibrating pH meters because, besides the properties just mentioned, its pH in solution is very stable.
A commonly used primary standard for titration with sodium hydroxide solution is the weak acid potassium hydrogen phthalate or KHP (C8H5O4K). In this reaction as well, one mole of KHP completely reacts with one mole of NaOH. ... The data from the titration is then used to calculate the molarity of the NaOH.