Refractive index database

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Optical constants of C8H5KO4 (Potassium hydrogen phthalate, KHP)
Moutzouris et al. 2011: n(γ) 0.450–1.55 µm

Wavelength: µm

Complex refractive index (n+ik)[ i ]

n   k   LogX   LogY   eV

Derived optical constants

Dispersion formula [ i ]



nγ; 24.5 °C (see the original publication for temperature-dependent dispersion formula).


K. Moutzouris, I. Stavrakas, D. Triantis and M. Enculescub. Temperature-dependent refractive index of potassium acid phthalate (KAP) in the visible and near-infrared, Optical Materials 33, 812-816 (2011)


[Expressions for n]   [CSV - comma separated]   [TXT - tab separated]   [Full database record]


Potassium hydrogen phthalate, C8H5KO4

Potassium hydrogen phthalate (C8H5KO4) is a white crystalline salt commonly referred to as KHP. Due to its high purity and stability, KHP is frequently utilized as a primary standard for acid-base titrations. This means it has a well-known and stable stoichiometry, which is essential for accurate volumetric analyses. Another key characteristic of KHP is its low hygroscopicity, allowing it to be easily weighed accurately without the worry of moisture absorption skewing the results. In the optics domain, as with many salts, KHP can be utilized to produce specific refractive properties when dissolved in a solvent or integrated within composite materials. Besides its laboratory applications, potassium hydrogen phthalate serves as a buffer solution and can be used in the preparation of certain pharmaceuticals.

Other names

  • Hydrogen potassium phthalate
  • Phthalic acid potassium salt
  • Potassium biphthalate
  • Potassium acid phthalate
  • 1,2-Benzenedicarboxylic acid
  • Monopotassium salt
  • KHP
  • KHPh

External links