Potassium hydrogen phthalate

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"KHPh" redirects here. For radio station KHPH in Hawaii, see List of radio stations in Hawaii.
Potassium hydrogen phthalate
Potassium hydrogen phthalate Potassium hydrogen phthalate
CAS number 877-24-7 YesY
PubChem 13413
ChemSpider 12839 YesY
Jmol-3D images Image 1
Molecular formula C8H5KO4
Molar mass 204.22 g mol−1
Appearance White or colorless solid
Density 1.636 g/cm3
Melting point ~295 °C (decomposes)
Solubility in water 80 g/l (20 °C)[1]
Solubility slightly soluble in alcohol
Acidity (pKa) 5.4
MSDS External MSDS
R-phrases R36 R37 R38
Main hazards Irritant to eyes, skin,
and respiratory system
Flash point Non-flammable
Infobox references

Potassium hydrogen phthalate, often called simply KHP, is an acidic salt compound. It forms white powder, colorless crystals, a colorless solution, and an ionic solid that is the monopotassium salt of phthalic acid. 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 not hygroscopic.[2][3][4] 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.

KHP dissociates completely in water, giving the potassium cation (K+) and hydrogen phthalate anion (HP- or Hphthalate-). As a weak acid hydrogen phthalate reacts reversibly with water to give hydronium (H3O+) and phthalate ions.

HP- + H2O is in equilibrium with P2- + H3O+

KHP can be used as a buffering agent (in combination with hydrochloric acid (HCl) or sodium hydroxide (NaOH) depending on which side of pH 4.0 the buffer is to be) but should not be used as a buffer for decarboxylation reactions, as these will degrade the KHP and mop up the conjugation groups.

KHP is also a useful standard for total organic carbon (TOC) testing. Most TOC analyzers are based on the oxidation of organics to carbon dioxide and water, with subsequent quantitation of the carbon dioxide. Many TOC analyzers suggest testing their instruments with two standards: one typically easy for the instrument to oxidize (KHP), and one more difficult to oxidize. For the latter, benzoquinone is suggested.


  1. ^ http://www.merckmillipore.com/INTL/en/product/pharmaceutical-ingredients/potassium-hydrogen-phthalate,MDA_CHEM-104874
  2. ^ Hendrixson, W. S. (1920). "Further Work on Potassium Hydrogen Phthalate as a Standard in Volumetric Analysis". J Am Chem Soc 42: 724–727. doi:10.1021/ja01449a008. 
  3. ^ "Potassium Hydrogen Phthalate". Arlington, TX: Ricca Chemical Company. Retrieved 2012-10-03. 
  4. ^ "The Standardization Of NaOH and KHP Assay". Clark College. Retrieved 2012-10-03.