Potassium hydrogen phthalate is the monopotassium salt of phthalic acid and is a common acid-base standard used for calibration of pH meters. It is a solid, air-stable compound that can be easily weighed accurately because it is not hygroscopic. It is also useful for determining the total organic carbon content of liquids, as it reacts with carbon to form potassium tert-butoxide and thereby serves as a good indicator of the amount of CO2 in the sample.
This monograph, titled Potassium Hydrogen Phthalate, Acidimetric Standard, describes a procedure for measuring the concentration of solutions based on the titration of the standard with sodium carbonate. It provides the necessary physical constants, a general description of the sample and its preparation, changes in state (approximate), aqueous solubility, density, and the assay.
In order to obtain accurate results from the titration, it is critical that extreme care is taken during all steps of the assay. The exact weight of the sample and the standard must be carefully controlled, and titrations should be run in duplicate to ensure that the assay tolerance is within 2 parts per million. In addition, the values of M (the number of meq of alkali required per gram of potassium hydrogen phthalate) obtained on the sample and the NIST SRM should agree to within 2 parts in 5000.
American Elements manufactures potassium hydrogen phthalate to many standard grades when applicable, including Mil Spec (military grade), ACS Reagent, Chemical, Food, Agricultural and Pharmaceutical Grade, as well as EP/BP and USP. This material is also available in ultrapure and submicron powder forms.