Potassium Hydrogen Phthalate

Potassium Hydrogen Phthalate
Potassium Acid Phthalate, KHP
Chemical Formula HOCO(C6H4)COOK
Formula Weight 204.22
Equivalent Weight 204.22 (Molar = Normal)
CAS No. 877-24-7

Potassium Hydrogen Phthalate (KHP) is a classic primary standard substance in that it is readily available in high purity; is relatively inexpensive; is easy to dry; is neither hygroscopic, oxidized by air, nor affected by Carbon Dioxide (thus making it easy to weigh out); does not undergo decomposition during storage; has a relatively high equivalent weight (thus reducing errors when weighed to 0.1 mg precision); is readily soluble in water; and is stoichiometric and instantaneous in reactions.

RICCA CHEMICAL COMPANY uses large quantities of KHP in the manufacture and quality control of its products. KHP is used in the manufacture of buffers, organic carbon standards, acidimetric standards and other products. It is also used in the quality control of Potassium Hydroxide and Sodium Hydroxide solutions, and in the standardization of Perchloric Acid in Glacial Acetic Acid. A typical acidimetric reaction is:

KHPhthalate + KOH → K2Phthalate + H2O

Phenolphthalein Indicator is most commonly used in titrations where KHP is used as an acidimetric standard to standardize solutions of bases. When KHP is used as a basimetric standard (e.g., in standardization of Perchloric Acid in Glacial Acetic Acid), Methyl Violet or Crystal Violet indicator is commonly used.