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Potassium hydrogen phthalate
CAS number 877-24-7 Yes check.svgY
Molecular formula KHC8H4O4
Molar mass 204.2212 g/mol
Appearance White or colorless solid
Density 1.64 g/cm3, solid
Melting point

~295 °C (decomposes)

Solubility in water soluble
Acidity (pKa) 5.4
MSDS External MSDS
R-phrases R36 R37 R38
Flash point Non-flammable
 Yes check.svgY (what is this?)  (verify)
Except where noted otherwise, data are given for materials in their standard state (at 25 °C, 100 kPa)
Infobox references

Potassium hydrogen phthalate, often called simply KHP, is an acidic salt compound. It has forms of white powder, colorless crystals, and a colorless solution, 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.

In water KHP dissociates completely 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.




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