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Potassium hydrogen phthalate

Potassium hydrogen phthalate
Systematic name Potassium hydrogen phthalate
Other names hydrogen potassium phthalate,
phthalic acid potassium salt,
potassium biphthalate,
potassium acid phthalate,
1,2-benzenedicarboxylic acid,
monopotassium salt,
Molecular formula KHC8H4O4
Molar mass 204.22 g/mol
Appearance White or colourless solid
CAS number [877-24-7]
Density and phase 1.64 g/cm3, solid
Solubility in water soluble
Melting point ca. 295 °C (568 K)--decomposes
Boiling point n/a
Acidity (Ka) 3.9x10-6
Crystal structure  ?
MSDS External MSDS
Main hazards Irritant to eyes, skin,
and respiratory system
NFPA 704  ?
Flash point Non-flammable
R/S statement R: R36, R37, R38
S: ?
RTECS number  ?
Except where noted otherwise, data are given for
materials in their standard state (at 25 °C, 100 kPa)
Infobox disclaimer and references

Potassium hydrogen phthalate, often called simply KHP, is a white or colorless, ionic solid that is the monopotassium salt of phthalic acid. The hydrogen 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, however, slightly hygroscopic and is generally kept in a desiccator before use.[1] 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. The pH of a 0.05 mol kg−1 aqueous solution of KHP at various temperatures is given by the following table:[2]

T (°C) pH
0 4.000
5 3.998
10 3.997
15 3.998
20 4.000
25 4.005
30 4.011
35 4.018
40 4.022
45 4.027
50 4.050

KHP can be used as a buffering agent (in combination with HCL or 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.


  1. ^ MSDS for KHP
  2. ^ The Measurement of pH - Definition, Standards and Procedures
This article is licensed under the GNU Free Documentation License. It uses material from the Wikipedia article "Potassium_hydrogen_phthalate". A list of authors is available in Wikipedia.
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