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.
Potassium hydrogen phthalate is used as a primary standard for acid- base titrations as well as for calibrating pH meters. It acts as a buffer in pH determination. It finds application as a useful standard for total organic carbon testing.
Potassium hydrogen phthalate makes for a good primary standard because it is stable, high in purity, non-hydroscopic, highly soluble, non-toxic, high in molecular weight, cheap, and available
